Structural Biochemistry/Volume 1

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Introduction

Physics is the scientific study of physical phenomena and the interaction between matter and energy. Generally speaking, it is the examination and inquiry of the behavior of nature. As one of the oldest branches of academia, physics is intertwined with and helps explain the fundamental nature of the living and nonliving universe.

Thermodynamics

First law

The "first law" of thermodynamics is simply that energy is a conserved quantity (i.e. energy is neither created nor destroyed but changes from one form to another). Although there are many different, but equivalent statements of the first law, the most basic is:

$dU=dQ+dW$

dU = infinitesimal change in internal energy,

dQ = infinitesimal heat flow and

dW = infinitesimal work.

In words, the first law states that

The heat supplied is equal to the increase in internal energy of the system plus the work done by the system. Energy is conserved if heat is taken into account.

Entropy

Thermodynamics allows us to predict the initial and final states of a system. In other words, it’s an extremely useful tool in predicting the equilibrium position in chemical systems. The first law of thermodynamics states that energy is always conserved between the initial and final states. However, the first law does not provide information about the extent of a chemical reaction, or the equilibrium concentrations. The second law of thermodynamics introduces the concept of entropy S, a value that is a function of a system’s state. Entropy helps predict the equilibrium because the equilibrium concentrations of a system correspond to the maximum entropy of a system; entropy can act as a driving force of a reaction. Therefore, the second law of thermodynamics states that:

"Thermodynamic equilibrium in an isolated system is reached when the system's entropy is maximized"

Mathematically, entropy is expressed as

$dS\equiv {\frac {dq}{T}}$

for a reversible process in a closed system, where dq is heat energy and T is temperature. A reversible process is where the system is always very close to the equilibrium. Perturbations to the system must be small enough that the system and the surroundings can return to the initial state. The change in entropy, a more useful value, is defined as:

$\Delta S=S_{2}-S_{1}=\int _{1}^{2}{\frac {\mathrm {d} q_{rev}}{T}}$

An example of increasing entropy can be seen by watching ice melt in a closed container held at 25^oC. Since the temperature is constant, it can be seen that the total thermal energy of molecules in liquid water is greater than the thermal energy of molecules in ice. Therefore, ΔS is positive, which implies that entropy has increased in order to obtain equilibrium.

Entropy is conceptually more difficult to understand than other state functions such as temperature or energy. Furthermore, entropy is a macroscopic property; one molecule does not exhibit entropy. Consider a closed container with a partition down the center of the container where one side of the partition is composed of gas A and the other side of the partition is empty. If the partition is removed, there is a very small probability that gas A will stay on one side of the container. The most probable distribution of the A molecules is that gas A will be evenly distributed throughout the container. This distribution can be viewed as the equilibrium thermodynamic state, which happens to be the most probable state and the state with the most disorder.

Enthalpy

Enthalpy is a measurement of heat transfer done at a constant pressure and is often denoted as $H$. Mathematically, enthalpy is expressed as

$H=E+PV$

where E is the internal energy of the system, P is internal pressure, and V is the volume. PV accounts for the energy used in expansion work.

The change in enthalpy is then expressed as

$\Delta H=\Delta E+P\Delta V$

where pressure remains constant. $\Delta H$ is then, essentially, the internal energy corrected for work.

Reaction enthalpies are assigned to chemical reactions to denote the amount of energy that is transferred into or out of the system in exchange with the surroundings. A reaction that releases heat is defined as exothermic, where $\Delta H$ is negative. A reaction that requires an input of heat is defined as endothermic, where $\Delta H$ is positive.

Gibbs Free Energy

Free energy is a measurement of the tendency of processes to occur spontaneously. It depends upon 3 different quantities: change in entropy, change in enthalpy, and temperature. The Gibbs Free Energy is the enthalpy of the system subtracted by the temperature and the entropy of the system, G = H - TS. A positive change in free energy (such as an endergonic reaction) is thermodynamically unfavorable, whereas a negative change in free energy (such as an exergonic reaction) is thermodynamically favorable. In biological systems, many reactions that have positive free energy are coupled simultaneously to reactions that have negative free energy. Example is the synthesis of glutamine from the expense of hydrolysis of ATP.

Endergonic Reactions

Endergonic reactions are reactions that have a positive free Gibbs energy. These reactions are not thermodynamically favored and the substrates are not readily going to form products. Energy must be out into the system to drive these reactions.

Exergonic Reactions

Exergonic reactions are thermodynamically favored. These types of reactions have a negative free Gibbs energy an are readily able to form products. However, although these reactions are energetically favorable, it does not mean the reaction will occur at a reasonable rate. This is due to high activation energy barriers. To reduce these barriers, the introduction of a catalysis is needed.

Charges: The Stern-Gerlach Experiment

This iconic experiment in the realm of quantum mechanics describes the nature of the charge an atom can have. This idea is pertinent to biochemistry because of the nature of how monomers, dimers, etc. bind to metal centers (for examples, iron is the metal center that hemoglobin uses as its coordination center). These protein structures bind to metal centers by coordination complexes that are in correspondence with the oxidation state of the metal ion. The oxidation state of the metal ion is influenced by the spin charge of the metal ion and it because of the Stern-Gerlach experiment that explains the nature of charge in an atom. The Stern-Gerlach experiment is an iconic quantum mechanical experiment that physically showed the spin charges of a silver atom. The electrons of the silver atoms was emitted between two vertically oriented magnets. These magnets provided the magnetic field to separate the electrons based on their apparent spin. The result of this experiment were two spins, one going up and one going down, dictated as + 1/2 and -1/2. The two beams of directionally spun electrons were then put through a second set of horizontally oriented magnets. The result of this addition were again two beams of directionally different spun electrons except in the horizontal orientation. The last part of this experiment was sending one of the beams from the horizontally oriented spun electrons through another set of magnetic oriented in the same way as the first set. The resulting beams from this last set of magnetics is the most important realization. The main idea of the implementation with the third magnet is that two beams emerge, just as experienced with the first set of magnetics. Because of the beams of electrons shown are of the same spin nature, this in turns shows that the two beams of electrons from both magnetics after being oriented differently, are eigenvalues of each other, meaning they are of the same operator. This whole experiment displays the importance of charge and how it can be used to describe the nature of quantum behavior and the nature of coordination ligand binding.

Foundation of Biochemistry

Physics is one of the foundations of biochemistry; it deals with different types of energy and forces. Types of forces:

1. Ionic force — charge-charge interaction
2. Dipole interactions — deals with electronegativity (trend from increasing order: P, H, S, C, Br, Cl, N, O, F) and consists of hydrophobic interactions.
3. Van der Waals — molecular repulsion
4. Hydrogen bonds

Further reading

1. Nelson, David (2005). Principles of Biochemistry (4th Ed. ed.). Sara Tenney. ISBN 0-7167-4339-6. {{cite book}}: |edition= has extra text (help)
2. Levine, Ira N. (2002). Physical Chemistry (5th Ed. ed.). McGraw-Hill. ISBN 0-07-231808. {{cite book}}: |edition= has extra text (help); Check |isbn= value: length (help)

Introduction

Thermodynamics is the study of energy and its interconversion. It is the branch of physics that studies temperature, heat, and other macroscopic properties.

The science of thermodynamics began in the nineteenth century with the need to describe the operation of steam engines and to set limits of what they were capable of doing. Thus, the name itself denotes the power developed from heat, which used the steam engine as the initial example. Observations of engines were then generalized, which later became the first and second law of thermodynamics.

Thermodynamics also limit a cell's growth to a narrow temperature range. So, the heat increases the molecular movement within the proteins. The species grows within a specific thermal range since its proteins have evolved to endure that range. Proteins tend to denature or function very slow for growth if it is outside the range. [Microbiology]

In chemistry, thermodynamics predicts the spontaneity of a process rather than the kinetics of the process. It takes into account only the final and the initial states and does not require knowledge of the pathway between reactants and products. It is a state function. It is the study of heat in chemical reactions and the change of physical state in correspondence to the laws of thermodynamics.

A system defines the parameters of an object or a process that is being studied and everything else in this universe is considered the "surrounding." A system is considered to be in thermodynamics in equilibrium with another system, if both the systems have the same temperature. These two things are separated by this boundary that maybe imaginative. There are four main types of boundaries: fixed, real, movable, and imaginary This provides a volume for the system in which things such as work, heat, and matter between the system and the surroundings can be studied upon. .

The principles of thermodynamics are incorporated into the three basic laws. As an old joke summarizes them, the first law says you can't win, the second law says you can't break even and the third law says you can't leave the game.

Reference

1. Zumdahl, Chemistry Seventh Edition

2.Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

Definition

The Zeroth law (also called equilibrium law) states that "if objects A and B are separate in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other" (Jewett, Serway. Physics for Scientists and Engineers - 6th ed).

The Zeroth Law of thermodynamics focuses on the thermal equilibrium of two connected bodies in the same system. This thermal equilibrium is made apparent when an object of higher temperature transfers heat to an object that is of lower temperature. Eventually, both bodies reach the same temperature where the change in heat between the two is no longer measurable. They reach a constant temperature which exists between the two starting temperatures.

To take this definition one step further, let's consider a hypothetical situation of three systems, where system A is in contact with system B only, and system B is only in contact with system C, and we assume that A,B, and C are in thermal equilibrium then all three objects have the same temperature

History

This law was named and further studied by Ralph H. Fowler, a British physician and astronomer who contributed widely to physical chemistry and statistics. The Zeroth Law was developed long after the first three thermodynamic laws had been established. Scientists did not realize the law's extreme importance long after its discovery. Because the law was basic and laid the foundation for the rest of the thermodynamic laws, it was called the Zeroth Law rather than the Fourth Thermodynamic Law.

This law allows us to make quantitative measurements about the temperature of one system by relating it to the temperature of other systems.

Introduction

The First Law of Thermodynamics is an expression of the principle of the conservation of energy. It states that the total energy of a system and its surroundings is constant, and energy can be transformed, but may not be created or destroyed.

The First Law can be applied to living organisms by thinking of them as a system. A system cannot output more energy than it contains without an external source of more energy. Once the potential energy locked in carbohydrates and other energy sources are converted into kinetic energy (energy in use or motion) by the organism, the organism will not obtain more until energy is imputed again. This is important to understand since the variety of tasks that are performed by cells, ribosomes, proteins, etc. are only possible through the intake and transformation of pre-assembled molecules into energy.

The general equation that describes this theory is:

$dU=\delta Q-\delta W\,$

where dU is the change in internal energy, dq is the infinitesimal heat exchanged, and dw is the infinitesimal work performed. Work and heat are not state variables where as internal energy is. In this equation, it is demonstrated that only heat and work can lead to a change in the internal energy of a system, which is defined as the total of all kinetic and potential energy of everything within a closed system. This equation may also be described in words as: although energy assumes many forms, the total quantity of energy is constant; and when energy disappears in one form, it appears simultaneously in other forms.

In addition to work and external potential and kinetic energy, the generalization of the law of conservation of mechanical energy was made possible by the recognition of heat and internal energy as forms of energy. As a matter of fact, examples such as surface energy, electrical energy, and magnetic energy can all serve as extensions to the generalization stated above. The validity of this generalization was supported by overwhelming amount of evidence, which has raised its stature to a law of science, known as the First Law of Thermodynamics.

It is important to look at how the internal energy of system changes under constant pressure and temperature conditions because many chemical reactions take place under these specified conditions. Using the definition of the internal energy and assuming that only expansion work is done, one may write:

$dU=\delta Q-P*\delta V\,$

Expansion of the terms gives:

ΔU = U₂ - U₁ = Q_p + W = Q_p - P*ΔV = Q_p - P*(V₂ - V₁)

Finally, rearrangement for Q_p gives:

Q = (U₂ -P*V₂) - (U₁ - P*V₁)

Since U, P, and V are all state variables, we may define Q_p, the heat transferred at constant pressure, as a new state function called the enthalpy, H. We can represent this new state function as:

H = U + PV

Where H, U, and V are molar or unit-mass values. U denotes the internal energy of a system, P denotes pressure, and V denotes volume.

When heat is added to a system, the internal energy of the system will increase, which, in turn, increases enthalpy. Work done by the surroundings on that system will produce similar (positive work) results. Conversely, heat lost to the surroundings or work done by the system are given a negative sign for enthalpy and work. In other words, any change in the energy of a system must result in a corresponding change in the surroundings. Therefore, energy is transferred from the system to the surroundings (reset of the universe), or from the surroundings to the system. And thus, energy can neither be created nor destroyed.

The sphere of influence of the process is divided into two parts when the First Law of Thermodynamics is applied to a given process. The region in which the process occurs is set apart as the system; everything with which the system interacts is its surroundings. A system may be of any size; its boundaries may be real or imaginary, rigid or flexible. A system usually consists of a single substance; however, complex systems consist multiple substances may also be found in scientific and engineering applications. In any event, the equations of thermodynamics are written with reference to a well-defined system. Attention is often focused on the particular process of interest and on the equipment and material involved in the process directly. However, the First Law of Thermodynamics applies to the system and its surroundings; not to the system alone.

Conclusion

The first law of thermodynamics is that energy can neither be created or destroyed.

General Form

• accumulation = input - output
• input = rate of energy (potential, kinetic, internal) is inputted into the system + rate of heat entering the system
• output = rate of heat leaving the system + rate at which it leaves as work
• accumulation = rate of energy in the system

References

2.Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

3. Berg, Jeremy (2012). Biochemistry. Freeman. ISBN 9781429229364. {{cite book}}: Text "coauthors+ John L. Tymoczko, Lubert Stryer" ignored (help)

Overview

The first law of thermodynamics states that energy is conserved, however, it only describes the transformations observed, and it doesn’t impose any restriction on the process direction. Nevertheless, such a restriction has been observed and proved to be exited in all thermodynamic applications. The need of a law describing this phenomenon gives rise to the second law of thermodynamics.

The Second Law of Thermodynamics states that the entropy of a closed system is constantly increasing with respect to time.
It is often said jokingly that the first law states that one cannot win and that the second law states that one cannot even break even.

The second law of thermodynamics may be expressed in two related statements as follows:

Statement 1: It is impossible to operate a system in such a way that heat absorbed by the system is completely converted into work done by the system.

Statement 2: It is impossible for a process to consist solely in the transfer of heat from one temperature level to a higher one.

Statement 1 is not contradictory to the first law of thermodynamics. Statement 1 does not imply that heat cannot be converted to work done by the system; it only implies that either the system or the surroundings have to be changed or modified when such a process takes place. As a corollary, any continuous production of work from heat is proved to be impossible. To compress the gas back to its initial state, energy must be drawn from the surroundings in the form of work; heat is transferred to the surroundings to maintain constant temperature at the same time. The amount of work gained from expansion is required by the reverse process described above, thus the production of net work is impossible.

Heat Conversion to Work

A thermodynamic theorem, the Carnot’s theorem is generated based on statement 2. The Carnot’s theorem states that no engine can have a higher thermal efficiency than that of a Carnot engine. Since a Carnot engine is reversible, it is able to transfer heat form one temperature level to a higher one. Although such an engine does not exist in the real world, it is the most efficient engine based on the laws of thermodynamics.

The study of heat engines, devices that are able to convert heat to work in a cyclical fashion, often serves as the classical approach to the second law. This macroscopic viewpoint of properties was able to be conducted without any knowledge of either the structure or the behavior of molecules. Any heat engines consist the following cycles: absorption of heat into the system at a relatively high temperature, rejection of heat to the surroundings at a relatively low temperature, and production of work. The two temperature levels are often referred to as heat reservoirs; the higher temperature level as the hot reservoir, and the lower temperature level as the cold reservoir. In thermodynamic applications, the working fluid, a liquid or a gas, connects the hot and the cold reservoirs in the sense that it absorbs heat from the hot reservoir, produces work, discards heat to the cold reservoir, and returns to its initial state to get ready for a new cycle.

Reversible Carnot Cycle

Spontaneous Processes and Entropy

Some processes proceed spontaneously in one direction, but not in the other. This process is similar to a gas diffusing to fill its container, but never really collecting at one end. These processes are said to be spontaneous; in other words, they occur without outside intervention.

Entropy, denoted by the symbol S, is the thermodynamic property that describes the spontaneity of a process. It is a macroscopic property of randomness or disorder, and is also a function that describes the number of arrangements (positions and/or energy levels) that are available to a system existing in a given state.

The Second Law of Thermodynamics can be explained with a simple example. Consider throwing a deck of ordered playing cards into the air and picking them all up at random. It is very improbable to pick up the cards in their original order because the probability for this to happen is so minute that we never really observe it. Entropy can be closely associated with probability. The more ways a particular state can be achieved, the greater the likelihood (probability) of finding that state. However, this does not mean that it is impossible for the cards to be put back in their original order or for the gas to be only at one end of the container. It merely is improbable.

There is a natural tendency toward disorder in closed systems because the state that has the highest probability of existing is where the system will be at equilibrium. The equilibrium state, such as within a container of gas, is referred to as the disordered state (where gas molecules are equally dispersed). The equilibrium state is the same as the disordered state because the gas molecules will occupy the largest volume possible, meaning they are all equally spaced out.

The change in entropy of the universe can be represented as

ΔS_univ=ΔS_sys+ΔS_surr

where ΔS_sys and ΔS_surr represents the changes in entropy that occur in the system and the surrounding respectively.

If ΔS_univ is positive, the entropy of the system increases, and the process is spontaneous in the direction written. A negative value for ΔS_univ indicates that the process is spontaneous in the opposite direction. The system is at equilibrium, and the process has no tendency to occur if ΔS_univ is equal to zero.

In some cases, particularly biological systems, it is hard to see how the entropy of universe is increasing. For example, when a leaf use carbon dioxide and nutrients to produce cellulose, the randomness and consequently entropy is decreasing. However, this process does not have any contradiction with second law of thermodynamic because it accompanies with increasing heat in the environment which increases the entropy.

===Phase changes and Entropy ===

Entropy has to do with the freedom of particle motion. As a result, when entropy increases in the system, it can cause a phase change from solid state to liquid state, and even to the gas state. In the solid state, the particles' movements are restricted and they have less freedom to move around in a fixed area. However, in a liquid state, particles have more freedom to move around. Therefore, in a gas state, the particles have so much greater freedom to move around. Consequently, entropy increases as one goes solid to liquid and to gas state.

When such disorder occurs, the energy of motion becomes more dispersed. For instance,when a salt is dissolved in liquid water, there would be more ions and solvent molecules interacting with each other. As a result, the solution's energy of motion is more dispersed. The greater the freedom the particles have, the more energy of motion they would dispersed.

                                solid ---> liquid ----> gas
less freedom for particles interaction --------> much greater freedom for particles interaction
                fixed energy of motion --------> dispersed energy of motion

Therefore, the change in phase states and the freedom of motion of particles can help to determine whether a reaction is spontaneous or not.

===The number of Microstates and Entropy ===

As stated in the previous section about the freedom of motion and the dispersed energy of motion, they are the two factors that can determine the direction of the spontaneous reaction. Silberberg defines microstates as "the quantized states of the whole system of a gas molecules". Microstates is about a gas molecules' state when it reacts with other molecules in the system. Consequently, there would be an increase in the energy of motion because the molecules vibrate and rotate around one another. In addition, there are different microstates for different conditions in the system. In thermodynamic terms, microstates can be related to entropy, the state of disorder, because the number of microstates is the number of ways that the thermal energy can be dispersed in the system. The equation is

                                       S= k ln W
     k (the Boltzmann constant)= R (gas constant)/ Avogadro's number= 1.38 x 10^ -23 J/K
     W= the number of microstates
     S is entropy

Therefore, entropy depends on the number of microstates.

           small number of microstates -----> much greater number of microstates
                           low entropy -----> high entropy

The Effect of Temperature on Spontaneity

Entropy changes in the surroundings ΔS_surr are primarily determined by heat flow.

The sign of ΔS_surr depends on the direction of the heat flow. In an exothermic process, the resulting energy flow increases the random motions in the surroundings, increasing the entropy of the surroundings (ΔS_surr is positive). Similarly, the tendency for systems to undergo changes that lower its energy can be explained by the fact that when a system at constant temperature moves to a lower energy state, the energy it gives up is transferred to the surroundings, leading to an increase in entropy there.

The magnitude of ΔS_surr depends on the temperature. At high temperatures, atoms in the surroundings are in rapid motion. A given quantity of energy transferred to the surroundings do not make a large percent change in their motions. Thus, the impact of the transfer of a given quantity of energy as heat to or from the surroundings is greater at lower temperatures, where the randomness of the surroundings experience a greater percent change. In other words, ΔS_surr depends directly on the quantity of heat transferred and inversely on temperature.

References

4.Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

5. Silberberg, Martin S.(2010). Principles of General Chemistry (2nd Edition).McGraw Hill Publishing Company. ISBN978-0-07-351108-05 Structural Biochemistry/Carnot Cycle/

Introduction

The Third Law of Thermodynamics is a physical law regarding the role of entropy in nature. This law, along with the first two Laws of Thermodynamics are absolute, in that everything in the observable universe obeys these laws and, like time and gravity, nothing is exempt from them. The Third Law of Thermodynamics states that:

"As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value."

This minimum value, however, is not necessarily zero, although it is almost always zero in a perfect, pure crystalline substance. A perfect, pure crystal is one where the atomic and molecular arrangement is perfectly symmetrical and evenly distributed throughout the substance, and where each molecule is identical to one another. On the Kelvin scale, zero degrees Kelvin is the lowest mathematically possible temperature in the universe, corresponding to about -273.15° Celsius or -459.7 Fahrenheit.

In actuality, it is impossible for any real system to reach absolute zero, in part due to the Second Law of Thermodynamics, which says that heat transfer cannot occur spontaneously from a colder body to a hotter body. Thus, any system must draw energy from nearby systems as the system approaches absolute zero. Because it must draw energy, obtaining absolute zero is physically impossible and is a mathematical limit of the universe.

Entropy

The melting of ice is a common example of increasing entropy.

As a result of the Third Law of Thermodynamics, the concept that the entropy of a system, ΔS, reaches a constant (or 0 Kelvin for a perfect crystal) at a temperature of absolute zero is introduced. This is important because it provides a bottom foundation from which entropy can be measured. For any isothermal process that involves only substances in internal equilibrium, the entropy change goes to zero as the temperature approaches zero. Entropy quantifies the disorder of a system and is used to predict how a system will spontaneously change. Since entropy is proportional to the temperature of a system, once the temperature reaches its lowest point (absolute zero), the entropy, too, approaches zero. This idea can be visualized by drawing an example from water. Water at its most physically free state, water vapor, is fairly unaffected by intermolecular forces, and thus the molecules move and disperse freely and randomly throughout the atmosphere. Water in this form has very high entropy (randomness). As the temperature drops below 100° C and the gas condenses into liquid, the intermolecular forces come into play to a larger degree and thus the molecules move less freely. Water in this form has lost some entropy. As the liquid water approaches 0° C and freezes into solid ice, the intermolecular forces become extremely strong and the molecules can no longer move freely, but can only vibrate within the ice crystals. The entropy in this form is extremely low. As the water is cooled more and the temperature becomes closer and closer to absolute zero, the molecules would completely cease motion and the entropy at this state would have zero entropy.

Overview

In Thermodynamics, the total energy of a specific system is called the Internal Energy. It is the total amount of energy within the system, excluding the energy in the surroundings. Internal Energy can be divided into two parts: Kinetic Energy and Potential Energy. We say that the change in Internal Energy equals to ∆U, where the final Internal Energy is subtracted from the initial Internal Energy to obtain the change in Internal Energy, ∆U.

Internal Energy is a state function because of the fact that its value depends on the present state of the system, as it is independent of the path. In calculating the change in Internal Energy, by whatever process, as long as the initial and final states are the same, the Internal Energy does not differ. Another way to look at the Internal Energy of a system is to take the perspective of work in relation to Internal Energy. Either mechanical work from the change in pressure or volume can result in the change in Internal Energy. Overall, the internal energy of the system does increases as mass is added into the system, hence making Internal Energy an extensive property as it directly proportional to the amount of material in the system at that time.

Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects. It refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.

Internal energy involves energy on the microscopic scale. For an ideal monoatomic gas, this is just the translational kinetic energy of the linear motion of the "hard sphere" type atoms , and the behavior of the system is well described by kinetic theory. However, for polyatomic gases there is rotational and vibrational kinetic energy as well. Then in liquids and solids, there is potential energy associated with the intermolecular attractive forces. A simplified visualization of the contributions to internal energy can be helpful in understanding phase transitions and other phenomena which involve internal energy.

More generally, while external energy is energy due to macroscopic motion (of the system as a whole) or to external fields, internal energy is all other forms of energy, including random motion (relative motion of molecules within the system) and dipole moments and stress.

Molecular Interpretation

Each molecule has a specific number of degrees of freedom, which include translations, rotation, or vibration. Yet, as the equipartition theorem states, the increase in thermal energy is even spread between degrees of freedom of the molecule in question. According to the equipartition theorem, the average energy of each contribution to the total energy is ½*kT so for a monatomic gas that only contains translation the equation looks like this: U_m (T)=U_m (0)+3/2 RT . With the inclusion of rotational energy for linear molecules the equation then becomes: U_m (T)=U_m (0)+5/2 RT . Now, with a non-linear molecules, including both transitional and rotational energies, the equation then becomes: U_m (T)=U_m (0)+3RT .

Internal Energy for an Ideal Gas

With the assumption that the gas being studied is ideal, the definition of Internal Energy can change. Since there are absolutely no intermolecular interactions in a perfect gas, so long as the distance between the molecules take no effect on energy. Then the assumption can be made that the Internal Energy of a gas to be independent of its volume.

The internal energy of an ideal gas is a function of the temperature only.

An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces. One can visualize it as a collection of perfectly hard spheres which collide but which otherwise do not interact with each other. In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature.

An ideal gas can be characterized by three state variables: absolute pressure (P), volume (V), and absolute temperature (T). The relationship between them may be deduced from kinetic theory and is called the Ideal Gas Law: PV=nRT

The ideal gas law can be viewed as arising from the kinetic pressure of gas molecules colliding with the walls of a container in accordance with Newton's laws. But there is also a statistical element in the determination of the average kinetic energy of those molecules. The temperature is taken to be proportional to this average kinetic energy; this invokes the idea of kinetic temperature.

Reference

Physical Chemistry by Atkins and de Paula (9th edition) Entropy (S) is the thermodynamic measure of randomness throughout a system (also simplified as “disorder”). Entropy can also be described as thermal energy not able to do work since energy becomes more evenly distributed as the system becomes more disordered. Entropy is particularly important when describing how energy is used and transferred within a system. As an exact value of entropy is impossible to measure; however, through relationships derived by both Josiah Willard Gibbs and James Clerk Maxwell the change in energy between one state and another can be calculated based on measurable functions, like temperature and pressure. That value in turn gives insight into how chemical reactions are favored and, most importantly, allows for the calculation of Gibbs Free Energy (ΔG = ΔH-TΔS).

Using statistical mechanics of the gas phase, entropy can be estimated by using Boltzmann’s formula. According to the formula, S = k ln W where k, the Boltzmann's constant, equals 1.381 x 10⁻²³ (in J/K). The Boltzmann's constant was calculated by relating to the gas constant R = kNA. W stands for the number of ways that the atoms or molecules in the sample can be arranged while still containing the same total energy.

It is important to note that the change in entropy, like temperature and volume, is a state function: the value is independent of the path used to get from the original state to the final state. Additionally the overall change in entropy of the universe is positive, meaning that the universe is continuously moving to a state of higher disorder.

A simple example where entropy is increased is when ice melts to water. The structure of ice is a well-ordered, crystalline system. When energy is put into the system in the form of heat, molecules begin to move more rapidly and no longer have the neatly ordered structure of ice. Thus, there distribution throughout space is more “random”. Another example where entropy is increased is when a reaction produces more moles of products than the reactants in the same phase.

The favorability of intramolecular reactions over intermolecular reactions is explained entropically. In an intermolecular coupling, two molecules come together to form one thus increasing the order in the system and decreasing the entropy. In an intramolecular reaction there is one molecule to start and one at the end which does not change the entropy of the system in an unfavorable way as is seen in intermolecular reactions.

Entropy can further be divided into thermal disorder, in which the entropy increases as heat is added to the system, and positional disorder, which related to the increase in entropy as the volume of the system is increased.

Entropy is also of particular interest in biochemistry as one of the unofficial definitions of life is an aggregate of molecules that work to decrease entropy in a certain localized area or volume. Additionally, it helps describe many phenomenons found in biochemical systems, which are described next.

Entropy in Biochemical Interactions

Entropy is a measure of the unavailable energy in a closed thermodynamic system that is also usually considered to be a measure of the system's disorder, that is a property of the system's state. It is varies directly with any reversible change in heat in the system and inverserly with the temperature of the system.

Entropy can be a strong driving force in nature. For example, it plays a very large part in the behavior of hydrophobic substances in water. A very common example of entropy at work would be lipids in solution. The lack of polarity in longer hydrocarbon chains tends to "force" water molecules to align themselves in an orderly pattern around the saturated part of the molecule. This orderly pattern decreases entropy as it prevents water from freely associating itself with other water molecules via hydrogen bonding. An increase in entropy would lead to a more negative Gibbs Free Energy, and a spontaneous reaction. The saturating effect of decreasing the change in entropy serves as the driving force for lipids to associate with one another instead of with water. Lipids coalesce to reduce the amount of water surrounding its molecules, and thereby increasing entropy. This phenomenon results in the formation of essential evolutionary components of life, such as lipid bilayer structures such as the lipid bilayer membranein eukaryotic cells.

Another place that the entropic favoring of hydrophobic molecules to dissociate with water can be found is in the active sites of enzymes. Many enzymes have a high concentration of hydrophobic residues in their active sites. The binding of an enzyme to its substrate alleviates the lack of entropy by driving water molecules out of the active site.

The value of understanding entropy’s role in chemistry can be utilized in the lab. For example, ammonium sulfate can be added in high concentrations to an aqueous solution containing at least one or more proteins. Proteins are much larger than ammonium sulfate ions. Thus the condensed charge of the newly dissolved salt attracts the water molecules in solution to form hydrate shells around them. In order to form these shells around ammonium sulfate, water molecules from the hydration shells around proteins must be used. The hydration shells of proteins are generally more ordered since not all of the protein surface is charged nor are the charges as condensed. Thus, the decrease in water molecules around proteins reaches a limit where the proteins become insoluble in water and are precipitated out for isolation and further study. .

However, entropy can also play a negative role in biochemistry.
For example, denatured protein by heating is an example where entropy plays a role in denaturation. In a folded protein, entropy is high due to its packed structure. As the protein becomes unfolded(denatured), the hydrophobic regions in a protein are surrounded by water. Overall there is no change in entropy but the protein becomes denatured.

Oil spill in sea also follows the same argument.

Entropy during phase changes

An entropy diagram for phase changes of water

As mentioned earlier, entropy is the measure of disorder, and this is also the case when it comes to phase changes.

This could be thought of a more simplified manner. For example, in the solid form of H₂O, they are in a very rigid and ordered crystal structure. As the temperature is raised, the rigid crystal structures begin to loosen up from its tight grasp of one another(via hydrogen bonds), and eventually, melting occurs. The ice turns into liquid. There was an increase in the disorder of this system, it went from a solid rigid crystal structure, to a bunch of freely moving molecules. This amount of disorder for this phase change to occur is entropy, specifically in the case of melting, the entropy of fusion.

History of Development

The enthalpy of reaction at specific values of T and P is defined as the heat exchanged between the system and the surroundings as the reactants are transformed into products at conditions of constant T and P. The heat flowing into the system is given a positive sign. Also, the standard enthalpy of formation refers to one mole of the specified reaction at a pressure of 1 bar. The term "enthalpy" and its accurate definition did not come about until the late 1840s. Before then, scientists and engineers knew from experience that a fluid called heat transfers from a hotter object to a cooler object when they are brought into contact. The result is that the cold object becomes warmer, and the hot object becomes cooler as heat transfers between them. Enthalpy is also the heat energy that absorbed or released as the reactants become the products at a constant pressure. When enthalpy is negative, the heat energy is released. However, when enthalpy is positive, the reactants absorb the heat from their surroundings.

A reasonable view is that there is something transferred from the hot object to the cold one. This “something” was referred as heat. Thus, it was convenient to say that heat always flows from higher temperature to lower one. This conclusion established the concept of temperature gradient, which may be viewed as the driving force for the transfer of energy as heat. Later, through conducting more precise measurements and experiments, scientists concluded that the rate of heat transfer from one object to another is proportional to the temperature difference between the two objects. Thus, the heat transfer is negligible when the temperature differences between the two objects are negligible.

A Thermodynamic View of Heat

Heat, or energy transferred between a hot and a cold object, is never regarded as being stored with an object from the thermodynamic point of view. Form this view point, heat is defined to exist only as energy is in transit from one object to another; or in the thermodynamic terminology, from the system to the surroundings. When heat is added or subtracted from a system or its surroundings, it is thought that energy is not stored in or being pulled away from the point of contact. Rather, heat is thought to act as kinetic and potential energy. The addition and subtraction processes are considered to be the transfer from potential energy to kinetic energy, and vice versa.

Relationship between Heat and Enthalpy

In spite of the transient nature of heat, the definition of heat is always viewed as a unique property of the object that is being measured. As mentioned above, temperature changes were used as the primary units to measure heat until the late 1840s. In fact, the definition of the unit of heat was based on the temperature change of a unit mass of water, as seen in the British thermal unit. The British thermal unit is commonly known as the Btu.

The British thermal unit is defined as 1/180th quantity of heat which when transferred to one kilogram mass of water raised its temperature from 0 to 100 degree Celsius at standard atmospheric pressure. This primitive definition of heat failed to provide an accurate measurement. The main reason accounts for this inaccuracy lie in the reference object, which in this case is water. The measure temperature change is partially determined by the purity of the water. The purer the water is, the more amount of heat is need for the temperature to increase by one unit.

In the twentieth century, more and more scientists realized that heat is just another form of energy that is being transferred by making contact of two objects. Thus it is convenient and practical to measure heat by using an energy unit.

The Modern Concept of Enthalpy

The modern concept of enthalpy or heat was developed largely based on the series of experiments carried out by James P. Joule (1818–1889). Joule’s experiment involves carefully measured amounts of water, oil, and mercury in an insulated container. He placed known amounts of these afore mentioned substances in the container, and agitated the fluids with a rotating stirrer. The mechanical work needed to carry out the rotation was carefully calculated, and the temperature changes of the fluid temperatures were accurately measured. In his result, Joule stated that for each fluid that a fixed amount of work was required per unit mass for every degree of temperature rise caused by the mechanical work provided. Furthermore, the original temperature of the fluids could be restored by the transfer of heat through contact as simple as to cooler objects. The quantitative relationship between heat and work was thus firmly established by Joule’s experiment, which proved definitively that heat is a form of energy.

In order to give heat a more concrete definition and moreover, to recognize a common basis for all energy units, international steam table calorie is defined in relation to joule, the SI unit of energy. Joule is the mechanical work done when a force of one Newton acts through a distance of one meter. Upon the establishment of the SI unit, the international steam table calorie was defined as equivalent to 4.1868 Joule (exact, by definition) and thermochemical calorie is equivalent to 4.184 Joule (exact, by definition). As a reference, one international steam table Btu is equivalent to 1055.056 Joule, and one thermochemical Btu is equivalent to 1054.35 Joule.

Mathematical Expression of Enthalpy

Enthalpy(H) is a measure of the heat energy of a reaction. The Enthalpy H of a thermodynamic system whose internal energy, pressure, and volume are U, P, and V is defined as H =U+PV. Since U, P, and V are state functions, H is a state function. note from dw=-PdV that the product of P and V has the dimensions of work and hence of energy. Therefore it is legitimate to add U and PV. Naturally, H has units of energy.

For example, a reaction that is exothermic will have a negative change in enthalpy. This is because the enthalpy of the products is less that that of the reactants.

The enthalpy of water formation.

In mathematical terms enthalpy of a reaction can be known as the following:

ΔH = (sums of bonds broken) - (sums of bonds formed)

ΔH = ΣnH_products - ΣnH_reactants

When ΔH is negative then the reaction is exothermic and more bonds are formed than broken.

If ΔH is positive then the reaction is endothermic. Chemical bonds tend to form spontaneously that the negative value of ΔH represents exothermic reaction.

The enthalpy for the reverse reaction is equal in magnitude, but opposite in mathematical sign.

Enthalpy is a state function and thus according to Hess's Law, the overall enthalpy of the reaction is equal to the sum of the enthalpies of the individual reactions or steps for which the overall reaction can be divided.

Since enthalpy is a function defined for the sake of calculations, it is difficult to measure the actual enthalpy of a substance. The change in enthalpy however, is easily measured and is an important quantity in many calculations.

When enthalpy changes, it signifies there is a change of state happening in the system. But enthalpies are reversible in the sense that the physical state changes are usually reversible. Therefore, since the enthalpy for formation of everything is always given, then at any given change, the new enthalpy can be calculated.

Reference

Slonczewski, Joan L.. Microbiology "An Evolving Science." Second Edition.

Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

Engel, Thomas. Physical Chemistry. Third Edition.

Introduction

Heat is the energy transfer in body. The smaller the temperature change, the greater its capacity gets. The equation of heat capacity is as follow: $C=dQ/dT$

The heat capacity of an object is the amount of energy needed to raise the temperature of a substance 1 degree. The units are J/oC. Heat capacity is an extensive property. This means that a larger object has a larger heat capacity than a smaller object made from the same material. Heat Capacity = heat supplied / temperature rise The heat capacity of an object depends on both the quantity as well as the types of matter in the object. In order to compare heat capacities of different substances, we must relate heat capacity to the amount of material. One way to do this is to refer to a mole of substance. Then, the heat capacity will become the molar heat capacity. A more useful procedure is to compare heat capacities for one gram of material. This is called the specific heat capacity or simply specific heat. Specific heat is the quantity of heat required to increase the temperature of one gram of material one degree Celsius (or one kelvin). When divide the heat capacity of a material by its mass, we will have a specific heat. Specific heat = heat capacity / mass = C / m

To find the heat q required to raise the temperature of a sample by a certain amount, we multiply the specific heat of the substance, s, by the mass in grams, m, and the change in temperature,t. q = s x m x t

Furthermore, the heat capacity is given by the derivative of the internal energy with respect to temperature for a given energetic degree of freedom. There are many types of heat capacities: translational heat capacity, rotational heat capacity, vibrational heat capacity, and electronic heat capacity. [Physical Chemistry]. In the translational heat capacity, the translational energy-level spacing are extremely small. This makes the high temperature approximation is valid. In the rotational heat capacity, at the lowest temperaturs, there is insufficient thermal energy to provide for population of excited rotational energy levels. In contrast, as the heat capacity increases until the high temperature limit is reached. In the vibrational heat capacity, the high temperature limit is not applied to the vibrational degrees of freedom. The last heat capacity is the electronic heat capacity. In this heat capacity, there is no contribution to that constant volume heat capacity from the degrees of freedom since the partition function for the energetic degree of freedom is equal to the ground-state degeneracy. Also, the average energy is zero as well.

Heat Capacity at Constant Volume

$C_{V}=(dU/dT)_{V}$

It is usually called molar or specific heat capacity. U is the molar or specific internal energy, and T is the temperature. $U,C_{V},$ and $T$ are all state function.

It also can be written:

$dU=C_{V}dT$

During this process, the volume should always be constant. If the volume changes during the process, even the initial value and final value are the same; it is not a constant volume. However, because $U,C_{V}$, and$T$ are all state function, the equation applies to any process for initial and final values are the same.

Heat Capacity at Constant Pressure

$C_{P}=(dH/dT)_{P}$

$C_{P}$ is molar and specific heat capacities. H is the molar or specific enthalpy, and T is the temperature. This process is a closed system process.

It also can be written:

$dH=C_{P}dT$

The equation applies to any process for initial and values are the same, which means the pressure would not necessary be constant during the whole process because H, $C_{P}$, and T are all state function.

Importance of Heat Capacity

The heat capacity, otherwise known as the specific heat, of various molecules is extremely important for many different reasons. It is because of the specific heat of the water molecule that allows for life to exist on planet earth. For example, because of water's unique specific heat of 1 cal to 1 gram of water, a large body of water , like a lake, can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees due to allocation, diffusion and spreading of heat throughout the water system. This coupled with waters high specific heat only changes the overall temperature by such a small amount. During nights and winter season, the gradually cooling water can warm the air. This is the reason that contributes to coastal areas having milder climates than inland regions. The high specific heat of water also tends to stabilize ocean temperatures, creating a favorable environment for marine life. Thus because of its high specific heat, the water that covers most of Earth keeps temperature fluctuations on land and in water within limits that permit life. Also, because organisms are primarily made of water, they are more able to resist changes in their own temperature than if they were made of a liquid with a lower specific heat.

Reference

Engel, Thomas and Reid, Philip. Physical Chemistry. Pearson Education. Inc. 2006. Third Edition.

Smith, J. M., and Ness H. C. Van. Introduction to Chemical Engineering Thermodynamics. New York: McGraw-Hill, 1987. Print.

General Information

American scientist Josiah Willard Gibbs (1839-1903) created the theory of available energy, known as Gibbs Free Energy, in 1873. The theory relates the energy changes within the chemical reaction and how they depend upon the following quantities: enthalpy, temperature, reagents concentration and entropy of the system. In other words, these quantities will determine whether the reaction is favorable (exergonic) or not (endergonic).

The free energy change of a reaction (delta G) can tell us whether or not a reaction occurs spontaneously. Reactions that occur spontaneously have a negative delta G value, and such reactions are called exergonic. When delta G is positive, the reaction does not occur spontaneously, and the input of free energy is required for the reaction to proceed, thus it is called an endergonic reaction. When a system is at equilibrium where no net change occurs, then delta G is zero. The delta G of a reaction is the free energy of the final state minus the free energy of the initial state, making it is independent of the reaction pathway. However, the value of delta G provides no information on the rate of a reaction.

Gibbs Free Energy Equation

This is a Gibbs free energy graph by Josiah Willard Gibbs. it shows a plane perpendicular to the axis of v (volume) and passing through point A - represents the initial state of the body. MN is the section of the surface of dissipated energy. Qε and Qη are sections of the planes η = 0 and ε = 0, and therefore parallel to the axes of ε (internal energy) and η (entropy), respectively. AD and AE are the energy and entropy of the body in its initial state, AB and AC its Gibbs free energy and its capacity for entropy (the amount by which the entropy of the body can be increased without changing the energy of the body or increasing its volume) respectively.

We often focus on the use of the Gibbs free energy equation instead of its derivation. The most commonly-used equations for calculations are:

$\Delta G=\Delta H-T\Delta S\,$ (for constant temperature) - equation(1)

$\Delta G=-RT\ln K\,$ (for equilibrium constant that depends on temperature) - equation(2)

Where ΔH is change in enthalpy, T is the temperature of the system (in kelvin (K)), ΔS is change in entropy of the system, R is gas constant, K is equilibrium constant.

Numerical Meaning of ΔG

If ΔG < 0 (negative), then the reaction will proceed spontaneously, meaning the reaction is favorable (exergonic).

If ΔG > 0 (positive), then the reaction will not proceed spontaneously, meaning the reaction is unfavorable (endergonic).

If ΔG = 0 (equal to zero), then the reaction is at equilibrium.

In general, every system wants to achieve a minimum of free energy. Therefore, the more negative the Gibbs free energy, the more favorable the reaction.

Meaning of work to free energy

The sign of entropy (S) can determine whether a reaction is spontaneous or not. However, the sign of work (Delta H) cannot determine the spontaneous process. For instance, the exothermic reaction become spontaneous under certain conditions. And the endothermic reaction can also become spontaneous under different conditions. Silberberg used water as an example to explain such conditions.

"H₂O (l) ---> H₂O (s) Delta H of the reaction = -6.02 KJ (an exothermic reaction; spontaneous when T<0⁰C)

H₂O (s) ---> H₂O (l) Delta H of the reaction = +6.02 KJ (an endothermic reaction; spontaneous when T>0⁰C)"

In both reactions,the sign of enthalpy has no effect on the spontaneous change. Therefore, one cannot use enthalpy as a factor to determine the direction of a spontaneous reaction.

Standard Gibbs Free Energy of Formation

When we have to consider the relationship between Gibbs free energy and the standard-state free energy of a reaction, we use this equation:

$\Delta G=\Delta G^{\circ }+RT\ln Q\,$

to calculate Gibbs free energy at that of time under a specific circumstances. Where ΔG^o is the standard-states - reactants (or components) at 25^oC (degrees Celsius) and 1 atm (atmospheric pressure, 1 atm same as 100 kilopascals), Q is the reaction quotient. The motivation behind it is that these elements, reactants, and substances, are thermodynamically stable at such atmosphere.

Chemical potential

From elementary thermodynamics, Gibbs free energy, G, is defined as,

$dG=-SdT+VdP+\sum _{i=1}^{C}u_{i}dN_{i}$

$u_{i}$ is the partial molar Gibbs free energy of species i.

The chemical potential is not favorable for phase-equilibria calculation when the pressure approaches zero. Then, fugacity is used instead:

$f_{i}=C\exp(u_{i}/RT)$

$f_{i}$ is the partial fugacity of species i. C is a temperature-dependent constant.

Because fugacity has relationship with pressure, then fugacity coefficient of a pure species:

$\phi _{i}=f_{i}/P$

fugacity coefficient of a species in a mixture:

$\phi _{iV}=f_{iV}/(y_{i}P)$

Free Energy of Enzymes

Free energy determines whether a conversion of reactants to products will occur spontaneously. In the case of an enzyme, ΔG determines the rate of a reaction. Enzymes cannot affect thermodynamics of a reaction, and hence do not affect the equilibrium; Additionally, enzymes accelerate the attainment of equilibria but do not shift their positions. The equilibrium position is a function only of the free-energy difference between reactants and products. They are however, able to reach the equilibrium point at a far faster rate than without the presence of an enzyme.

For instance, in the presence of an enzyme, products could form within a second. On the other hand, products could take a as long as days to form without the presence of the catalyst. In both cases, concentration and amount of product formed remains entirely the same- it's equilibrium state. The amount of products it has formed has balanced with the amount of substrate.

Enzymes decrease only the free energy of activation- otherwise known as the activation energy. The Transition state between a substrate and the product is the point between a reaction where the substrates and products "meet in the middle". At this point, the highest free energy exists for the reaction. The activation energy is the energy it takes for a substrate to reach this transition state.

There are many competing theories of how enzymes actually bind their substrates, and each theory has a different graphic representation of the affect of the enzyme on the free energy of the reaction. In the lock and key mechanism theory, an enzyme has the pre-existing conformation to bind to a unique substrate. After binding and catalyzing the reaction, the enzyme will release the final products.

In the induced-fit mechanism theory, a similar approach is hypothesized. The only difference is that the pre-existing, unbound enzyme does not originally assume the exact conformation to bind the substrate; but rather assumes a slightly different structure prior to binding. Then, as the substrate binds to the enzyme, the structure of the active site conforms around the structure of the substrate to fit properly. Both of these mechanisms can be represented similarly in relation to their effect on the free energy of the reaction. Without really changing the pathway of the energy curve, these models serve to decrease the activation energy of a reaction, thereby increasing the rate of the reaction.

Another model has been suggested however, that appears slightly different on the free energy graph. This is the proposed transition-state model. This model suggests that an enzyme is not structurally adept to bind to the substrate itself, but that it is actually optimized to bind to the transition state of the reaction pathway. This produces a small stabilization of the transition state decreasing the overall activation energy as is characteristic of enzymes. The first increase in energy is due to the binding of the enzyme to the original substrate. The return to original free energy state is stabilization of the enzyme-substrate complex before reaction occurs. The next increase in energy comes from achieving the transition state, and the subsequent fall is the creation of the products. This theory is currently accepted as an alternative because the enzyme-substrate complex of the other theories acquires a very low free energy level due to stabilization. To achieve the transition state after this relatively low level of free energy is much more difficult than achieving the transition state from the relatively more energetically free enzyme-substrate complex suggested in this transition-state model. [6]

Formation of Double Helix

Formation of Double Helix

Double-stranded molecules of nucleic acids form the double helix structure such as DNA and RNA. Formation of double helix is one of biological process that the principles of thermodynamics are applied to it. In a solution containing single strands, all stands can easily move around, rotate, and disperse in the solution. In addition, forming conformation is easy in the single strands solution. However, when the double helix forms, it cannot displace as easy as two single strands could before. Moreover, it has less possible conformations. Thus, by forming double helix the randomness and entropy decrease.

Due to the Second Law of thermodynamics, significant heat has to be released to the surroundings for the process to be consistent with increasing the entropy of universe. Measuring the changing of temperature of a solution before and after formation of double helix reveals that approximately 250 kJ/mol (60 kcal/mol)heat is released. This large released energy is sufficient to overcome the effects of formation double helix - increase of order- and make universe more disorder.

Electrostatic freeenergy of the DNA doublehelix in terms of the counterion condensation theory: The polyelectrolyte theory reveals the formation of the double helix. The secondary structure of DNA is similar to the secondary structure of proteins. The number of condensed counterions is the same as for a line charge with charge density equal to the axial charge density of the helix. The logarithmic salt dependence of the electrostatic free energy is equivalent in range of lower salt concentration, thus the limiting laws stays constant. The helical parameters have a large influence on the overall electrostatic free energy and on the internal free energy of the condensed layer of counterions. The free energies of the single and double helix are negative at a higher salt level. Being negative indicates of the stabilization of the helical charge lattices electrostatic, because of mixing entropy of the condensed counterions. On the other hand, when the salt level is low, the free energy of a single helix is higher than the free energy of a double helix. With B-DNA parameters imagined as single helixes, the salt dependence of the free energy of transition from double strand to single is greatest at about 0.2 M salt, which is very similar to the area of the feature of separation of the DNA strand.The electrostatic freeenergy for the transition of the DNA doublehelix from the B to the A conformation can also be calculated. The Bform is the most electrostatically stable over the salt range. The electrostatic freeenergy values are close to the experimental values of the overall (electrostatic plus non-electrostatic) transition freeenergies for A-philic base pair sequences. B-to-A transition for A-philic sequences around concentration of 1 M is watched over by the polyelectrolyte properties of these two orientations of the DNA double helix. On the other hand the effect of ethanol cannot be tied to the lowering of the dielectric constant.

Bond Energies

How is energy being used? Is energy being consumed or absorbed in a reaction?

1) Bonds formed = Energy is released because it forms a more stable state. ΔH < 0 heat is released.

2) Bonds broken = Energy is absorbed because breaking a stable state and moving towards a less stable state. ΔH > 0 heat is absorbed.

Bond Energy products > Bond Energy reactants : spontaneous

Bond Energy products < Bond Energy reactants : non-spontaneous

Bond Energy Diagrams

ΔG = G products - G reactants

Note: You cannot switch the equation to be G reactants - G products.

The key is to understand if energy is being overall released or absorbed in a reaction. This will give you the correct sign for your ΔG.

Example of Gibbs Free Energy

References

Reece, Jane (2011). Biology. Pearson. ISBN 978-0-321-55823-7. {{cite book}}: Text "coauthors+ Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson" ignored (help)

Seader, J. D., and Ernest J. Henley. Separation Process Principles. Hoboken, NJ: Wiley, 2006. Print.

Introduction

The definition of Material Equilibrium is that in each phase of the closed system, the numbers of moles of each substance in that phase remains constant in time. Material equilibrium can be subdivided into Reaction equilibrium and Phase Equilibrium. Reaction Equilibrium is the equilibrium where the conversion of quantity stops between two sets of chemicals. Phase Equilibrium is where the transport of matter reaches a balance point without conversion of one species to another.

Entropy and Equilibrium

In an isolated system which is not at material equilibrium. The spontaneous chemical reactions occur between difference phases in this system are irreversible processes that increase the entropy.Stot>0

Introduction of phase diagrams

This is an example of a phase diagram. In this case, the substance represented on the PT diagram is for water. The dotted region above the critical point is the supercritical fluid.

A phase diagram is a chart that helps define the conditions at which a substance can be in its solid, liquid, and vapor states. The three boundary lines of the chart signify the equilibrium relationships between the phases: fusion curve, vaporization curve, and sublimation curve. There are also two distinct points on the graph: the triple point and the critical point. To better define these terms the picture diagram can be examined. The red line is the transition between solid to gas and is also called the sublimation curve. The green line is from solid to liquid and is also called the fusion curve. The blue is liquid to gas and is called the vaporization curve. The point where all three lines intersect is the triple point. At this temperature and pressure the three phases: solid, liquid, and vapor can coexist. The critical point is the condition of the highest pressure and temperature that the substance can be observed in vapor/liquid equilibrium.

Supercritical

Any condition that exceeds that highest pressure and temperature becomes a region that has no definite phase boundaries. A phase is usually considered a liquid when vaporization comes from pressure reduction at constant T, the same goes for a gas if condensation results from temperature reduction at constant T. Since this process cannot be applied to either situation the region beyond the highest P and T can be regarded as supercritical and as the fluid region.

A supercritical fluid is defined as a substance above its critical temperature (Tc) and critical pressure (Pc). The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. Supercritical fluids are highly compressed gases which combine properties of gases and liquids in an intriguing manner. Fluids such as supercritical xenon, ethane and carbon dioxide offer a range of unusual chemical possibilities in both synthetic and analytical chemistry. Supercritical fluids have solvent power similar to a light hydrocarbon for most solutes. However, fluorinated compounds are often more soluble in scCO2 than in hydrocarbons. This increased solubility is important for polymerisation.

Phase Diagram for proteins

This is a phase diagram used in protein crystallization. It displays the necessary protein and precipitant concentrations for crystallization.

Though phase diagrams are generally viewed as a map that shows the state of a substance at certain conditions, this simple tool can be used to analyze other complicated materials. An example is that the information derived from a phase diagram can be used for the discussion of protein crystallography. This diagram is a representation of what protein and precipitant concentrations are needed for protein crystallography. The red line is called the solubility line and it distinctly separates undersaturated conditions from the supersaturated, which is the condition desired for protein crystallization. The supersaturated part of the diagram consists of three parts: metastable, labile, and precipitation.

Supersaturated

Metastable-Crystals here can grow larger from seeds but they cannot nucleate. There is a lower protein and precipitant concentration with less aggregation events. This zone signifies the concentrations that are usually too slow for crystallization to occur.

Labile-Crystals here can also grow larger from seeds but they can nucleate. There is a higher protein and precipitant concentration with more aggregation events. This zone is also known as the nucleation zone or the crystallization zone and is more favorable for the formation of crystals.

Precipitation-Proteins usually surface as an aggregate or precipitate and are not used for crystallography. This section of the diagram displays less favorable conditions because aggregation and precipitation occur much faster than crystallization.

Nucleation: process of creating a nucleus.

Aggregation: the clumping or clustering of proteins.

Use of phase diagrams for protein crystallization

The connection between phase diagrams and protein crystallization is the placement of the solubility curve (the red line). As described, crystals can only form in supersaturated solutions and having information on the location of the solubility curve allows for the growth of crystals for X-ray crystallography. Though crystals are needed to determine the solubility curve, it is through various trials that the suitable concentrations for crystal formation are found. By this accumulation of data the crystals can be used as a guide for refining conditions for protein crystallization.

This phase diagram is used also because it unveils the interactions between the many components of the solutions. Examples of such interactions are the liquid-liquid phase separation (LLPS) that define the attraction between proteins, a necessary characteristic for crystallization of proteins. This attraction is thoroughly defined in LLPS. Other data that can be extracted from the phase diagram include enthalpies (from LLPS) and entropies (from solubility curve) of the protein in liquid and solid states. Also, additives can change the curves of the phase diagram giving data on the effect of the additive and how it interacts with the protein analyte. It can also be utilized for predictions of the conditions that are needed for a variety of protein crystallization which would be ideal for the crystallization process, less trial and error, more crystallizing.

Liquid-liquid phase separation (LLPS)

A result of adding precipitant to a protein solution is the formation of liquid drops. These liquid drops are sometimes referred to as oils or coacervates and can be observed at changing temperatures, pHs, and other solution condition changes. The drops usually contain high protein concentrations and when influenced by a higher force, gravity, the drops separate from the rest of the solution. The result of this is two liquid phases from the solution. This entire process is called liquid-liquid phase separation. LLPS is useful because the high concentrations of proteins help increase saturation to a supersaturated state which, as a result, increases crystallization.

The left picture shows LLPS of bovine crystallin in sodium phosphate. The picture on the right shows LLPS of thaumatin.

Introduction

More than one phases coexist at the same temperature and pressure.

K-Values

K-values is a phase-equilibrium ratio, for vapor-liquid systems,

$K_{i}=y_{i}/x_{i}$

$y_{i}$ is the mole fraction of a species in vapor phase, and $x_{i}$ is the mole fraction of a species in liquid phase.

For ideal solutions,

$K_{i}=P_{i}^{S}/P$

K-values can be used for calculating bubble and dew points.

Reference

Seader, J. D., and Ernest J. Henley. Separation Process Principles. Hoboken, NJ: Wiley, 2006. Print.

General information

endergonic reaction

An endergonic reaction refers to a chemical reaction in which energy is being used in the overall reaction, making the reaction non-spontaneous and thermodynamiacally unfavorable. Energy is being absorbed as the reaction proceeds, and there is a net loss of energy in the surrounding system. Due to this consumption of energy, standard change in Gibbs free energy (ΔG) is a positive value under constant pressure and temperature: ΔG° > 0.

The magnitude of ΔG also represents the quantity of energy required to drive the reaction. If a chemical process is exergonic in one direction, then the reverse process must be endergonic. Plants get the required energy to make sugar from the environment by capturing light and converting it into chemical energy that can be used for other processes.

Some examples of endergonic reactions are muscle contractions and protein synthesis.

The equilibrium constant of endergonic reaction where ΔG° > 0 is less than 1: K < 1.

Endergonic reactions require an input of energy, usually larger than those of non spontaneous exergonic reactions, from an outside source to disturb the chemical equilibrium to cause changes, such as bond formation. This input of energy is called the activation energy. In certain reactions, a catalyst is available to speed up endergonic reactions. A catalyst can lower the activation energy barrier for the reaction. Thus, it speeds up the reaction process. The energy for an endergonic reaction is obtained by coupling the reaction with an exergonic reaction.

A familiar example of coupling exergonic reactions to endergonic reactions to promote spontaneity comes from ATP. ATP powers cellular work by coupling exergonic reactions to endergonic reactions. It is responsible for mediating most energy coupling in cells, and in most cases, acts as the immediate source of energy that powers cellular work. The synthesis of the amino acid glutamine from glutamic acid and ammonia is naturally endergonic and non spontaneous with a ΔG value of +3.4 kcal/mol but coupling this reaction with the exergonic process of ATP hydrolysis, -7.3 kcal/mol, will drive the reaction forward, making it spontaneous. In the same sense, a exergonic reaction must be coupled with the formation of ATP from ADP in order to make the reaction spontaneous and in most cases, cellular respiration provides the energy for the endergonic process of making ATP and plants use light energy, instead, to produce ATP.

An endergonic reaction can simply be understood by studying the following situation. In a chemical reaction, the reactants make products and an equilibrium is reached. An endergonic reaction is one where more products are made from the equilibrium amount by disturbing the equilibrium with a form of energy. For example, heat will be absorbed into the system and the equilibrium will shift to the right (towards product side). Consequently, more products will be formed.

Summary

An endergonic reaction:

• The free energy of initial state < free energy of final state
• Energy needs to be put into the system in order to go from the initial start to the final state
• +ΔG

References

Zumdahl, Chemistry Seventh Edition

Neil A. Campbell, Jan B Reece. Biology Seventh Edition, 2005 Pearson Education, Inc.

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/exergon.htm

General information

exergonic reaction

An exergonic reaction refers to a reaction where energy is released. Because the reactants lose energy (G decreases), Gibbs free energy (ΔG) is negative under constant temperature and pressure. These reactions usually do not require energy to proceed, and therefore occur spontaneously. In a chemical reaction, breaking and forming bonds between atoms is a form of energy. Since chemical reactions mainly consist of forming and/or breaking chemical bonds, exergonic reactions release energy by breaking less stable chemical bonds and forming more stable bonds. The example of exergonic reactions occur in our body is cellular respiration: C6H12O6 (glucose) + 6 O2 -> 6 CO2 + 6 H2O this reaction release energy which is used for cell activities.

However, some exergonic reactions do not occur spontaneously and require a small input of energy to start the reaction. This input of energy is called activation energy. Once the activation energy requirement is fulfilled by an outside source, the reaction proceeds to break bonds and form new bonds and energy is released as the reaction takes place. This results in a net gain in energy in the surrounding system, and a net loss in energy from the reaction system; hence, the change in Gibbs free energy (ΔG)] is negative (ΔG < 0). A negative ΔG denotes that the reaction is spontaneous and thermodynamically favorable.

Here is an endergonic reactions of ATP give energy. First Breaking down the ATP formed ADP and Pi is an exergonic reaction, where Delta G is less than 0. However, by combining the reaction glucose+Pi ---> glucose 6-phosphate, a thermodynamically unfavorable reaction. The cell can drive an endergonic reaction.

Conclusion

File:Exergonic.png

Exergonic reaction is

• Free energy of final state < free energy of initial state
  
• Free energy is free during the reaction
  
• The energy can be used to do biological work
  
• Able to work in spontaneous reaction
  
• doesn’t have to occur in great rate which is where the enzyme comes in
  
• -ΔG (negative number)
  

Reference

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/exergon.htm Different from classical thermodynamics, essentially a deductive science, molecular thermodynamics focuses on the properties of individual chemical species and their mixtures at molecular level.

History of Development

By the end of the nineteenth century, most laws and postulates of classical thermodynamics were well-established. However, the rapid pace of development in scientific fields relating to chemistry, physics, and chemical engineering urges scientists and engineers to have a more holistic view on the subject of thermodynamics, which has given rise to the birth of molecular thermodynamics.

More often than not, the source of property values is experiment. For instance, the ideal gas equation was evolved as a statement of observed physical behavior of gases and their interconnecting relationships between volume, pressure, temperature, and number of moles of gas present.

At the turn of the century, physicists and chemists who worked with principles of classical thermodynamics increasingly realized that experiments at macroscopic level often failed to provide any insight into why substances exhibit their observed properties. By conducting further experiments, they found that the basis for insight should rather be established on a microscopic view of matter.

Intermolecular Forces

Intermolecular forces are relatively weak forces between molecules in random motion. The energy resulted from this random motion is referred as internal energy.

The ideal gas model was the very first model introduced when internal energy between molecules was studied. An ideal gas is characterized by the absence of molecular interactions. However, ideal gas also possesses internal energy. Real gases, on the other hand, are composed of molecules that have not only the energy of individual molecules, but also energy shared between molecules due to their interactions. The intermolecular potential energy is associated with collections of molecules, and the intermolecular forces are reflected by the existence of energy in this form.

Pair-Potential Function

Two molecules attract each other when they are far apart and repel each another when they are close together. This fact was also established through the study of molecular thermodynamics. A sketch of intermolecular potential energy may reveal that the potential energy for an isolated pair of spherically symmetric neutral molecules is solely dependent on the distance which separates them.

If we let U denote intermolecular potential energy, F denote intermolecular force, and r denote the distance separating the two spherically symmetric neutral molecules, the intermolecular force may be expressed as a function of intermolecular potential energy and distance separating the two spherically symmetric neutral molecules as:

F(r) = - dU(r) / dr

The negative sign shown in the above equation signifies an intermolecular attraction, whereas a positive sign indicates an intermolecular repulsion.

The above differential equation is also referred as the pair-potential function. Specific values of U and r in this form may appear as species dependent parameters in a pair-potential function.

Internal Energy from A Microscopic View

Kinetic theory and statistical mechanics are the two theories that relate the behavior of molecules from microscopic level to macroscopic level. Many thermodynamic properties, such as internal energy, enthalpy, and entropy were able to be explained after the development of the two theories mentioned above. Together these two theories represent nearly all the knowledge we possess about molecular thermodynamics.

Before further exploring molecular thermodynamics from a molecular level, it is crucial to understand how the energy associated with each individual molecules of an ideal gas relates to the macroscopic internal energy of a system defined.

Energy, no matter external or internal, is quantized from a quantum mechanics point of view. In other words, the total amount of internal energy of a system may be treated and analyzed as tiny measurable units that carry discrete amount of energy. One of such an energy unit is often referred as quanta. Thus there are enormous numbers of quanta contained in a system, and the sum of the quanta determines the energy level of the system. The set of energy levels allowed to exist to a closed system, as specified by quantum theory, is determined by its volume. Each energy level of a system has a quantum states associated with its energy level, which is also known as the degeneracy of the level.

Statistical Mechanics' Contribution

From a molecular thermodynamic point of view, the state of a system is firmly established if and only if the temperature and volume of the system are defined. Nevertheless, a fixed temperature and a fixed volume do not guarantee an equilibrium is reached within the system. In the case of ideal gas, the random motion and collision of the gas molecules with each other and the wall of the container result in exchanges of energy with the surroundings. Momentary fluctuations caused by these random motions and collisions shift the energy level back and forth within the system. Therefore, it makes sense to define an average value over the discrete set of energy levels of the allowed quantum states. Moreover, statistical mechanics is sufficient in providing the means for arriving at the proper average value.

One of the fundamental postulates of statistical mechanics for a system with defined volume and temperature is that the probability of a quantum state depends only on its energy. The importance of this postulate is that it relates the energy level of a system with its probability. In other words, all quantum states with the same energy have the same probability. Following the same logic as stated above, a value for the thermodynamic internal energy may be obtained as the average of the energies of the quantum states, which is equivalent to its probability.

Reference

Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help) Equilibrium may be defined as lack of change, or a static condition. In thermodynamics, equilibrium not only implies the lack of change, it also refers to the absence of any tendency toward change on a macroscopic scale.

==Relationship between Equilibrium, Driving Force, and Resistance in a System==

A system of interest at equilibrium exists if and only if there is no net change in the surrounding conditions. In this case, the driving force can be viewed as negligible also since the absence of any tendency toward change also indicates the absence of any driving force. Thus, all forces are said to be in balance when equilibrium exists in a system. One thing that needs to be pointed out here is that whether a change actually occurs in a system not at equilibrium depends on two factors: the resistance as well as the driving force of the system.

Different Driving Forces Produce Different Kinds of Changes

Many kinds of changes might be encountered in a system; these different changes are caused by different driving forces. Driving forces may be classified by their physical properties, which include pressure, temperature, concentration, gradients in chemical potential, and so on.

For example, imbalance of mechanical forces, such as the pressure gradient on a piston, will tend to cause energy transfer in the form of work. Gradients in chemical potentials tend to cause substances to be transferred from one phase to another; temperature differences tend to cause the flow of heat in or out of the system. Nevertheless, the final results of the above listed tendencies are the same--equilibrium will be achieved in a system.

Equilibrium, Free Energy, and Reaction Direction

Thermodynamic equilibrium refes to a condition in which equilibrium exists with respect to P, T, and concentration. Equilibrium is established with respect to a given variable only if that variable does not change with time, and if it has the same value in all parts of the system and surroundings. The equilibrium with respect to concentration exists only if transport of all species across the boundary in both directions is possible. If the boundary is a movable wall that is not permeable to all species. The sign of free Energy (ΔG) can determine the direction of a spontaneous reaction. In addition, one can determine whether the reaction will proceed to the right or the left based on the reaction quotient (Q) and the equilibrium constant (K). As a result, Silberberg states that

If Q < K or Q/K <1 , then the reaction will head to the right

If Q > K or Q/K >1 , then the reaction will head to the left

If Q = K or Q/K =1 , then the reaction will be at equilibrium.

As a result, the equation that combines the equilibrium constant, free energy, and reaction quotient is ΔG= RT ln (Q/K)

Different Driving Forces Produce Different Kinds of Changes

Many kinds of changes might be encountered in a system; these different changes are caused by different driving forces. Driving forces may be classified by their physical properties, which include pressure, temperature, concentration, gradients in chemical potential, and so on.

For example, imbalance of mechanical forces, such as the pressure gradient on a piston, will tend to cause energy transfer in the form of work. Gradients in chemical potentials tend to cause substances to be transferred from one phase to another; temperature differences tend to cause the flow of heat in or out of the system. Nevertheless, the final results of the above listed tendencies are the same--equilibrium will be achieved in a system.

Effects of Chemical Reactions on Thermodynamic Equilibrium

In most of the applications of thermodynamics, the effects of chemical reactions are usually ignored. Since a system may stay in a long-term equilibrium if a chemical reaction is not initiated, it is convenient to not consider the effects that a chemical reaction might bring about to the system. Furthermore, equilibrium is mostly likely to be reached after a chemical reaction is carried out in most thermodynamic cases. Therefore, a purely physical process may be analyzed without regard to possible chemical reactions.

References

Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

Silberberg, Martin S.(2010). Principles of General Chemistry (2nd Edition).McGraw Hill Publishing Company. ISBN978-0-07-351108-05

Equation of State

PV = nRT

Ideal gas: A hypothetical gas that exhibits linear relationships among volume, pressure, temperature, and amount (mol) at all conditions; approximately by simple gases at ordinary conditions. Although no ideal gas actually exists, most simple gases, such as N2, O2, H2, and the noble gases, show nearly ideal behavior at ordinary temperatures and pressures. Ideal gas law: An equation that expresses the relationships among volume, pressure, temperature, and amount (mol) of an ideal gas: PV=nRT.

Types

There are three basic classes of ideal gas:

• the classical or Maxwell-Boltzmann ideal gas,
• the ideal quantum Bose gas, composed of bosons, and
• the ideal quantum Fermi gas, composed of fermions.

The classical ideal gas can be separated into two types: The classical thermodynamic ideal gas and the ideal quantum Boltzmann gas. Both are essentially the same, except that the classical thermodynamic ideal gas is based on classical statistical mechanics, and certain thermodynamic parameters such as the entropy are only specified to within an undetermined additive constant. The ideal quantum Boltzmann gas overcomes this limitation by taking the limit of the quantum Bose gas and quantum Fermi gas in the limit of high temperature to specify these additive constants. The behavior of a quantum Boltzmann gas is the same as that of a classical ideal gas except for the specification of these constants. The results of the quantum Boltzmann gas are used in a number of cases including the Sackur-Tetrode equation for the entropy of an ideal gas and the Saha ionization equation for a weakly ionized plasma.

Internal Energy

a function of temperature only
U = U(T)

Internal energy: is the total of kinetic and potential energies of all the particles in a system.

Implied Property Relations for an Ideal Gas

C_v is a function of temperature only

C_(v )≡(∂U/∂T)_v= dU(T)/dT= C_v (T)
H is a function of temperature only

H ≡U+PV=U(T)+RT=H(T)
C_p is a function of temperature only

C_p= dH/dT= dU/dT+R=C_v+R
Any change of state of an ideal gas
dU = C_(v ) dT ∆U= ∫▒C_(v ) dT
dH = C_(p dT) ∆H= ∫▒C_(p ) dT

Some facts about Ideal Gas Law: The ideal gas law can be rearranged to calculate the density and molar mass of a gas. Also, in a mixture of gases, each component contributes its own partial pressure to the total pressure (this is also called the Dalton's law of partial pressures). The mole fraction of each component is the ratio of its partial pressure to the total pressure. The most important idea is that the total pressure is the sum of the gas pressure and the vapor pressure of water at the given temperature when a gas is in contact with the water. [The Molecular Nature of Matter and Change].

Reference

Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change. 5th ed. 2009

Equation for Process Calculations for Ideal Gases

For reversible, closed-system, work is given by
dW=-PdV
For ideal gases, the first law can be written by
dQ+dW=C_v dT
From two equations above, we get
dQ=C_v dT+PdV
These three equations can be applied to four types of processes: isothermal, isobaric, isochoric and adiabatic.

Isothermal Process

File:Isothermal.png

From left to right the lines signify: isochoric, adiabatic, isothermal, and isobaric.

Isothermal process deals with closed-system that has constant temperature. So ΔT=0:
ΔU=ΔH=0
Q=RTln V_2/V_1 =-RTln P_2/P_1
W=-RTln V_2/V_1 =RTln P_2/P_1
Q=-W (constant T)

Therefore,
Q=-W=RT ln V_2/V_1 = -RT ln P_2/P_1 (constant T)

Isobaric Process

File:Isobaric.png

From left to right the lines signify: isochoric, adiabatic, isothermal, and isobaric.

Isobaric process deals with closed-system that has constant pressure. So ΔP=0.
ΔU=∫▒〖C_v dT〗 and ΔH=∫▒〖C_p dT〗
Q=∫▒〖C_p dT〗 and W=-R(T_2-T_1)
Therefore,
Q=ΔH=∫▒〖C_p dT〗 (constant P)

Isochoric Process

File:Isochoric.png

From left to right the lines signify: isochoric, adiabatic, isothermal, and isobaric.

Isochoric process deals with closed-system that has constant volume. So ΔV=0.
ΔU=∫▒〖C_v dT〗 and ΔH=∫▒〖C_p dT〗
Q=∫▒〖C_v dT〗 and W=-∫▒PdV=0
Therefore,
Q=ΔU=∫▒〖C_v dT〗 (constant V)

Adiabatic Process

File:Adiabatic1.png

From left to right the lines signify: isochoric, adiabatic, isothermal, and isobaric.

Adiabatic process deals with closed-system that has no heat transfer between the system and the surroundings. So ΔQ=0.

dT/T= -R/C_v dV/V

T_2/T_1 =(V_1/V_2 )^(R⁄C_v )

T_2/T_1 =(P_2/P_1 )^(R⁄C_p ) and P_2/P_1 =(V_1/V_2 )^(C_p⁄C_v )

The following equations apply to ideal gases with constant heat capacities that undergo mechanically reversible adiabatic expansion or compression.

〖TV〗^(γ-1)=constant 〖TP〗^(((1-γ))⁄γ)=constant 〖PV〗^γ=constant

γ≡ C_p/C_v

For any adiabatic closed-system,

dW=dU= C_v dT

W= △U= C_v△T

γ ≡ C_p/C_v = (C_v+R)/C_v =1+ R/C_v or C_v= R/(γ-1)

W= C_v△T= (R△T)/(γ-1)

W= (〖RT〗_2-〖RT〗_1)/(γ-1)= (P_2 V_2-P_1 V_1)/(γ-1)

For mechanically reversible process,

W= (P_1 V_1)/(γ-1) [(P_2/P_1 )^(γ-1)-1]= (RT_1)/(γ-1) [(P_2/P_1 )^(((γ-1))⁄γ)-1]

Diabatic Process

Opposite of adiabatic process
There is heat transfer

Polytropic Process

Polytropic process deals with a model of some versatility. So δ=constant.
〖PV〗^δ=constant

〖TV〗^(δ-1)=constant 〖TP〗^(((1-δ))⁄δ)=constant

W= (RT_1)/(δ-1) [(P_2/P_1 )^(((δ-1))⁄δ)-1]

Q= ((δ-γ)RT_1)/((δ-1)(γ-1)) [(P_2/P_1 )^(((δ-1))⁄δ)-1]

Reference

Smith, J. M., and Ness H. C. Van. Introduction to Chemical Engineering Thermodynamics. New York: McGraw-Hill, 1987. Print.

Irreversible Process

For mechanically reversible, closed-system processes,

• every equation is derived.
  
• the equations have property changes- dU,dH,△U,△H.
  

This is applied to reversible and irreversible processes in both closed and open systems.
An irreversible process is a two-step process.

1. W is for mechanically reversible process.
   
2. Through efficiency, the results are multiplied or divided to get the actual work.
   

If work is produced, then the absolute value for the reversible process will be too big and has to be multiplied by the efficiency.
If work is required, then the reversible process value will be too small and has to be divided by the efficiency.

Reference

Smith, J. M., and Ness H. C. Van. Introduction to Chemical Engineering Thermodynamics. New York: McGraw-Hill, 1987. Print.

Application of the Virial Equations

2 forms of virial expansion are infinite series.

1. Z=1+B^' P+C^' P^2+D^('P^3 )+⋯
   
2. Z=1+B/V+C/V^2 +D/V^3 +…
   

• B^'= B/RT
  
• C^'=(C-B^2)/〖(RT)〗^2
  
• D^'= (D-3BC+2B^3)/〖(RT)〗^3
  

When we derive the relations, set Z = PV/RT and solve for P.

Reference

Smith, J. M., and Ness H. C. Van. Introduction to Chemical Engineering Thermodynamics. New York: McGraw-Hill, 1987. Print. Bioenergetics refers to the transformation of energy that occurs within living organisms. In order to fuel the chemical mechanisms within cells, organisms require an input of energy. This energy is used to drive chemical reactions and help store and process information, which is essential in propagating life. Energy may be obtained from sunlight, in which case the organisms are referred to as phototrophs, or it may be extracted from chemicals, in which case the organisms are referred to as chemotrophs. Because energy may not be available at all times to fuel these life processes, organisms have adapted mechanisms to couple chemical reactions so that exergonic reactions can provide energy for those that are endorgernic.

The chemical reactions performed by an organism make up its metabolism. Catabolic reactions involve the break down of chemical molecules, while anabolic reactions involve the synthesis of compounds.

The Laws of Thermodynamics

The energy processes in living organisms are defined by the basic laws of thermodynamics. The first law dictates that the total energy present in the universe always remains constant (Note: though the energy total is static, it often changes forms such as when an animal converts the chemical energy of food to mechanical energy as it moves). Meanwhile, the second law asserts that the total entropy present in the universe is ever increasing.

The Energy Process

For phototrophs, sunlight takes the form of potential energy, while complex molecules serve as potential energy for chemotrophs. In living organisms, energy is commonly used in the form of work. This work energy is acquired by breaking weak bonds and forming stronger bonds. Work may take the form of synthetic chemical reactions, maintaining chemical and ionic gradients (homeostasis), and transferring genetic information. This most important form of work is the polymerization of information-containing macromolecules, such as protein, DNA, and RNA. These macromolecules form the basis of life and are what ultimately drive life’s processes. Though mechanisms have been adapted to make the most efficient use of the acquired energy, some of it is inevitably released in the form of heat or metabolic waste products. These waste products can be eliminated from the body of an organism and consumed by bacteria or other organisms that are able to extract energy from them. Often, materials and compounds useless to one organism are energy sources for another and by this exchange energy is constantly recycled.

Energy Coupling

In order to increase energetic efficiency, cells often couple reactions together. Endergonic reactions are those that require an input of energy. Exergonic reactions are those that release energy. By coupling these two reactions together, the overall chemical process is made exergonic so it can occur spontaneously due to a negative free-energy change. Using this process, unfavorable chemical reactions can be made to proceed.

The basis of reaction coupling is a shared chemical intermediate. After one reaction produces one product, another can use it as a reactant to drive the production of an essential compound.

Gibbs Free Energy

Named after Josiah Willard Gibbs who developed the concept in 1878, the Gibbs free energy describes the overall favorability of a reaction to proceed. It is characterized by the following equation: ΔG = ΔH - TΔS where ΔG is the Gibbs free energy change, ΔH is the change in enthalpy (heat), T is temperature (measured in Kelvins), and ΔS is the change in entropy. A reaction, or process, will occur spontaneously if and only if ΔG < 0 for that reaction. If ΔG > 0 then that reaction must be coupled (see Energy Coupling) with a reaction which has ΔG < 0 such that the overall ΔG of the 2 reactions is <0. ATP hydrolysis is a commonly used reaction for such situations (see ATP). As the equation shows, a reaction is more favorable, the more ΔH < 0 (i.e. the more heat is given off by the reaction) and the more ΔS > 0 (i.e. the more disorder is increased) and the effect of entropy gained or lost is magnified by the temperature surrounding the reaction.

ATP

Adenosine Triphosphate (ATP)

Most coupling reactions use the break down of adenosine triphosphate (ATP) as the intermediate process to drive chemical synthesis. ATP is used as an energy-storing compound. The phosphoanhydride bond between phosphate groups found in ATP stores a significant amount of energy due to the negative charges carried by the phosphate groups. This bond stored energy that is not currently use, but available later for running reactions is called Potential energy. This energy is required to keep the negatively charged groups close to each other in the ATP molecule because they, as do all like-charged groups, repel each other. This energy can be released (Exothermic reaction) for use in the cell to do work, move things and build things by hydrolysis and breakage of the bond.

ATP -> ADP + P + Energy

When carbohydrates and other foods are consumed, they are broken down by enzymes to release the energy within them. The exothermic energy released is used to reattach a phosphate to ADP through Endothermic reactions which will regenerate ATP formation.

ADP + P + Energy -> ATP

Then, the process of bond breaking and bond forming will repeat over and over within human cells to provide energy for all the chemical reactions.

GTP

Guanosine Triphosphate (GTP)

GTP (guanosine-5’-triphosphate) can be used as a source of energy, just like ATP but not in any type of organisms. However GTP is only used in specific areas of the cell, namely protein synthesis. ATP and GTP are similar in structure; both have a purine base, and 3 phosphate groups, but ATP has adenine attached to the purine, whereas GTP has a guanine. The energy stored in GTP is released in the same way as ATP.

Feedback

In order for the body to maintain homeostasis, feedback loops are often used. Negative feedback maintains homeostasis by slowing down or stopping a mechanism once it approaches the appropriate range. For instance, the hydrolysis of ATP is an exergonic reaction, meaning it gives off energy in the form of heat. If the temperature in a cell becomes too high, the cell will die, so negative feedback will stop the reaction to maintain homeostasis in terms of temperature. In positive feedback, a process will speed up once a receptor detects the occurrence of a certain reaction. A positive feedback mechanism is used in the stomach during digestion: for example, HCl secreted by parietal cells in the stomach convert pepsinogen to pepsin, and this reaction causes the pepsin to convert all the pepsinogen to pepsin to aid in the enzymatic breakdown of proteins. Another example of a positive feedback mechanism is childbirth, once the contractions start, they begin to occur with increasing frequency and pressure.

Energy Coupling

The total ∆G is negative because of the coupling of reactions. In this diagram, ∆G(1) stands for the change in G resulting from the reaction of glucose + Pi --> Glucose 6-phosphate, ∆G(2) is the free energy resulting from the reaction of ATP --> ADP + Pi, and ∆G3 is the total change in free energy by coupling these two reactions together.

Many chemicals' reactions are not spontaneous and require energy to occur. The spontaneity of a chemical reaction is determined by its Gibbs free energy value. If negative, the reaction will proceed spontaneously; if positive, the reaction will not be spontaneous. Note that this is not equivalent to kinetics, or the speed of a reaction; Gibbs free energy only determines the spontaneity of a given reaction. How quickly the reaction proceeds is determined by other factors, such as the presence of enzymes, the amount of heat (energy) that the system has available, and the physical properties of the reacting molecules.

Spontaneous reactions occur without the need for extra energy, but they may happen slowly. In order to catalyze non-spontaneous reactions, such as the synthesis of macromolecules, enzymes and coupling are used by the cell. Almost all reactions that take place in a cell are catalyzed by enzymes that decrease the activation energy of the reaction. Essentially, this means that the enzyme opens up a more favorable "pathway" for the reaction, allowing it to initiate more easily than before, and with less energy. In addition, an unfavorable reaction can be coupled together with a favorable one to make the overall reaction favorable. For example, Glucose + P_i -> Glucose-6-phosphate has a positive $delta$ G and is therefore unfavorable. But it can be coupled with ATP -> ADP + P_i (which has a negative $delta$ G) to make the reaction favorable. The overall reaction thus becomes ATP + Glucose -> ADP + Glucose-6-phosphate and has a negative $delta$ G. Therefore, ATP is considered the energy currency of the cell. However, it should be noted that other energy carrying molecules, such as GTP, do exist and are used for certain processes.

Anabolic and catabolic processes are examples of how the cell couples reactions together to create efficient energy exchange cycles. These processes are explained in more detail in their respective sections; however, it is appropriate to mention them here as they provide a relevant example of reaction coupling. Basically, catabolic reactions are those that convert chemical fuels to molecules that the cell can use for energy, such as ATP and other high-energy compounds. Anabolic reactions are those that require some amount of energy to occur. Thus, the cell can conveniently couple anabolic reactions with catabolic ones - the products of catabolic reactions can be used to drive anabolic reactions to completion. This allows the cell to link different types of reactions together efficiently; it's almost a type of "cellular recycling", as the products of one reaction (i.e. a catabolic one) can be re-used to help another reaction reach completion (i.e. an anabolic one).

Organisms are Energy Transducers

Organisms are transducers of energy, since in the transfer of energy they are less than 100% efficient. Organisms employ the energy harnessed to grow, repair, and maintain their bodies. The energy is also use to compete with other organisms, and to produce new organisms (offspring). In the process of doing these things, organisms generate waste, chemicals and heat. Organisms create local regions of order at the expense of using up some fraction of the total supply of useful energy found in the universe.

General Information

Metabolism

Metabolism regulates life through a set of chemical reactions. Chemical reactions are often coordinated with each other and occur in sequence called metabolic pathways, each step of which is catalyzed by a specific enzyme. These pathways are categorized according to whether the reactions lead to the breakdown or synthesis of substances. Catabolic reactions result in the breakdown of molecules into smaller molecules. Such reactions are often exergonic. By comparison, anabolic reactions promote the synthesis of larger molecules from smaller molecules. This process usually is endergonic and, in living cells, must be coupled to an exergonic reaction. These processes are responsible for the growth and reproduction of organisms, maintaining their structures, and responding to changes in the environment. The involvement of enzymes is essential for metabolism because they couple the organisms, which are thermodynamically unfavorable, to other organisms which are thermodynamically favorable and drive the metabolism towards desirable reactions. Not only do enzymes drive organisms toward desirable reactions, but they also regulate metabolic pathways to respond to changes within the cell's environment as well as signals from other cells. The metabolism of an organism also establishes which substances which enter the organism are nutritious and beneficial and which are harmful. Additionally, the speed of metabolism, or the metabolic rate, of an organism affects how much food the organism consumes. Prokaryote's metabolism is diverse. In other words, prokaryotes run all the major nutrient cycles. They play a major role in the Sulfur cycle and biological process that affects oxidation states of minerals in the earths crust. Also, the cyanobacteria both invented photosynthesis and still dominate the carbon fixation on this planet. We can also harness energy from redox couples by releasing all at once or releasing gradually (chemical bonds).

In Metabolism, there are something that we need for life. For example, we need a) the energy/reducing equivalents (ATP, NADH, NADPH). Also, we need b) Carbon Skeletons (glucose, glycine, etc...). We also need c) Other minor stuff (NPK, metals, etc...).

About the various Trophs: 1) Chemotroph: is the energy from redox 2) Phototroph: is the energy from light 3) Heterotroph: is the carbon from organic molecule 4) Autotroph: is the carbon fixed from CO2

In metabolism, glycolysis and the TCA cycle can be used just to produce molecules for growth. About Anaerobic Respiration: It is the use of a terminal electron acceptor other than O2. Moreover, the process produce generally less energy than using O2. Also, it can be considered the Redox Tower.

Assimilative Vs. Dissimilative Metabolism: 1) Assimilative Reduction: Compound gets reduced and incorporated into the organism. For example: NO3- becomes amino group -NH2. 2) Dissimilative Reduction: Compound gets reduced as an electron acceptor and is discarded. For example: NO3- becomes amino group -NH2.

Nitrate Reduction and Denitrification: a) Nitrite is one of the most common alternative electron acceptors. b) Denitrification removes nitrogen from systems as gas. Agriculture is bad and sewage is good.

- Nitrate Reduction: Nitrate Reductase is the critical protein for NO3- reduction. Nitrate reductase production is repressed by O2 and activated by NO3-. There are fewer protons are pumped. - Denitrification: Utilizes 4 reductive enzymes. It can start with nitrite from nitrate reduction. Also, the process can produce more energy than nitrate reduction. - Reduction of Various Metals: a) Geobacter metallireducens is a gram-proteobacterium. It can reduce a number of metals by oxidation of organic compounds. For examples, for iron: Fe3+ to Fe2+; for manganese: Mn4+ to Mn2+; for uranium: U6+ to U4+. Fe3+ compounds are much less soluble than Fe2+ iron compounds. U4+ compounds are much less soluble than U6+ compounds.

Chemoautotrophs: - Generate energy (ATP) from oxidation of inorganic compounds. - Carbon fixation of CO2 is by Calvin Cycle (same as plants/cyanobacteria). - Using the Electron Tower, we can predict metabolisms.

Hydrogen Oxidation: - Many bacteria produce H2 as a metabolic byproduct. - The critical enzymes are hydrogenases.

The Phototrophs: a) Photoautotrophs: Use light for energy and CO2 as carbon source. 1) Oxygenic: Oxidize H2O for electrons generating O2. 2) Anoxygenic: Oxidize other compounds for electrons (ie: H2S). b) Photoheterotrophs: Use light for energy and organic carbon as carbon source.

Photosynthetic Pigments: - All phototrophs contain chlorophyll or bacteriochlorophyll. - Each chlorophyll variant has different absorption spectra. - Different pigments allow bacteria to co-exist in one environment. - All chlorophylls have a porphyrin ring with different substitutions.

Accessory Pigments: a) Function: expand light spectrum and photoprotection. b) Main Pigments: Carotenoids and Phycobilins.

Photosynthetic Structures: - The purpose: a) increase surface area for light absorption. b) cluster proteins/pigments for electron transfer. - Bacteria do not have chloroplasts...they are the chloroplast. - Membrane Folds: Highly folded membrane invaginations. - Chlorosomes: Found in green sulfur and non-sulfur bacteria. - Phycobilisomes: Specific to cyanobacteria.

Reference: Slonczewski, Joan L. Microbiology. 2nd ed. New York, 2009.

Hydrothermal Vents: - Chemoautotrophs are the producers of these communities. - Do not require light for survival. - Can require no input from outside organisms.

Acid Mine Drainage (Fe2+ Oxidation): - Fe2+ is stable in acidic water (won't spontaneously oxidize). - Fe2+ gives e- for pumping out cytoplasmic H+. - e- is also used in reverse electron flow to produce NADH.

Anoxygenic photosynthesis: a) Represented Phyla: - Proteobacteria (purple bacteria) - Green sulfur bacteria - Green non-sulfur bacteria - Heliobacteria - Acidobacteria b) Only a single light reaction occurs. c) Electrons are derived from non H2O.

Purple Sulfur Bacteria: - They are gram negative and they are proteobacteria. They use bacteriochlorophyll a and b. They produce visible sulfur granules. They are generally found in anoxic environments. They thrive in H2S rich environments (hot spring).

Oxygenic Photosynthesis: a) Represented Phyla: - Cyanobacteria - Plants b) Electrons flow through two light reactions. c) Electrons are derived from H2O.

Cyanobacteria: a) Require oxygen for growth b) Primarily use phycobili-proteins and chlorophyll-a c) Grow in a wide range of environments (some extreme) d) Many species can fix nitrogen

Nitrogen Fixation: a) Groups: - Cyanobacteria - Rhizobia - Green sulfur - Azotobacteria b) Very energy intensive process (16-24 ATP) c) Nitrogenase enzyme is O2 sensitive d) Heterocyst Specific Properties: - No active photosynthesis - Transfer N via Glutamate - Specialized extracellular matrix of glycolipids and polysaccharides - Contain polar bodies to inhibit gas exchange

Storage of Nutrients: a) Bacteria Store Nutrients as Polymers - Low solute concentration - Inert b) Nitrogen Storage - Cyanophycin c) Carbon Storage - Glycogen (starch) - PHB - Oils

Glycogen Metabolism in Cyanobacteria: a) Glycogen functions as a major carbon storage polymer in cyanobacteria. b) The genes glgA and glgP show evidence of transcriptional regulation while GlgC is allosterically regulated. c) Glycogen synthesis and degradation is diurnal in cyanobacteria.

Cyanobacteria as Biofuel Producers: - Nitrogen Fixation - Hydrogen Evolution - Flotation/Motility - Dense Mat Growth - Extremophiles (Temp, ph) - Diverse Secondary Metabolites - Naturally Transformable

Fatty Acid Secretion: - Cyanobacteria do not have thioesterases, they come from plants. - Glycogen synthesis competes with pathways for lipid production.

Reference: Slonczewski, Joan L. Microbiology. 2nd ed. New York, 2009.

• ATP

Structure of ATP

During catabolism, useful energy is temporarily conserved as ATP - adenosine triphosphate. ATP is the universal standard of energy exchange in biological systems as the energy is always transformed and conserved as ATP .

• NAD

Structure of NAD

NAD (Nicotinamide Adenine Dinucleotide)also involves in metabolic pathways. The chemical nature of metabolic pathways usually involves oxidation/reduction reactions. In order for a biochemical to be oxidized, its electrons must be removed by an oxidizing agent. The oxidizing agent is an electron acceptor that gets reduced in the reaction. The molecule that usually functions as the electron carrier in the biochemical oxidation-reduction reactions is NAD and its phosphorylated derivative, NADP. NAD or NADP can become alternately oxidized or reduced by the loss or gain of two electrons. The oxidized form of NAD is symbolized NAD; the reduced form is symbolized as NADH₂.

Metabolic pathways are regulated in three general ways:

1.Gene Regulation Because enzymes in every metabolic pathway are encoded by genes, cells can control chemical reactions via gene regulations. For example, if a bacterial cell is not exposed to a particular sugar in its environment, it will turn off the genes that encode the enzymes that are needed to break down the sugar. Alternatively, if the sugar becomes available, the genes are switched on.

2.Cellular Regulation Metabolism is also coordinated at the cellular level. Cells integrate signals from their environment and adjust their chemical reactions to adapt to those signals. Cell-signaling pathways often lead to the activation of protein kinases that covalently attach phosphate groups to target proteins. For example, when people are frightened, they secrete a hormone called epinephrine into their bloodstream. This hormone binds to the surface of muscle cells and stimulates an intracellular pathway that leads to the phosphorylation of intracellular proteins, including the enzymes involved in carbohydrate metabolism. These activated enzymes promote the supply of energy to the frightened individual. Epinephrine is sometimes called the “fight or flight” hormone because the added energy prepares an individual to either stay and fight or run away. After a person is no longer frightened, hormone levels drop and other enzymes called phosphates remove the phosphate groups from enzymes, thereby restoring the original level of carbohydrate metabolism.

3.Biochemical regulation Metabolic reactions can also be controlled by reactions at the biochemical level. In this case, the binding of a molecule to an enzyme directly regulates its function. Biochemical regulation is typically categorized according to the site where the regulatory molecule binds.

Reference: Biology. Brooker. Widmaier. Graham. Stiling. Chapter Seven, Enzymes and cellular respiration.

Example(s) of Metabolic Pathways

Gluconeogenesis

This metabolic pathway involves the generation of glucose from non-carbohydrate carbon substrates (i.e. lactate, glycerol, and gucogenic amino acids). This is one of two main mechanisms (the other being glycolysis) that the human body uses to keep blood glucose levels from dropping to a dangerously low level, a condition called hypoglycemia. This mechanism isn’t exclusive to humans, and is also present in plants, animals, fungi and other microorganisms, with some variation in the locations in which glucogenesis takes place. This mechanism kicks in during periods of fasting, starvation, or intense exercise and is endergonic. It is also associated with ketosis and has been a target of therapy for Type II Diabetes to inhibit glucose formation and stimulate glucose uptake by cells.

The pathway itself consists of eleven enzyme-catalyzed reactions, which can begin in the mitochondria or cytoplasm (depends on the substrate being used). Many of these steps are the reversible reactions of those found in glycolysis.

•It begins in the mitochondria with the formation of oxaloacetate through carboxylation of pyruvate. This part requires ATP and catalytic help by pyruvate carboxylase, which is stimulated by high levels of acetyl-CoA and inhibited by high levels of ADP.

•Oxaloacetate is then reduced to malate using NADH, which will prepare it for mitochondrial exit. Afterwards, it is oxidised in the cytoplasm again to oxaloacetate using NAD+, in which the remaining steps of glucogenesis will take place.

•The next step is the decarboxylation and phosphorylation of oxaloacetate to produce phosphoenolpyruvate, which is catalyzed by phosphoenolpyruvate carboxykinase (PEP carboxykinase). This step also hydrolyses one molecule of GTP to GDP.

•The next steps of the reaction are essentially the same as those involved in reversed glycolysis, with the only difference being that fructose-1, 6-bisphosphatase converts fructose-1,60bisphosphate to fructose-6-phosphate. Note that this conversion is the rate limiting step of the whole process of glucogenesis.

•Next, glucose-6-phosphate is formed from fructose 6-phosphate with the help of phophoglucoisomerase. This product can be used in other metabolic pathways or can be further dephosphorylated to make free glucose. Cell control of intracellular glucose levels is attained by the fact that free glucose can diffuse in and out of the cell, whereas the phosphorylated form is locked in the cell. Glucose formation happens in the lumen of the endoplasmic reticulum. Here, glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to produce glucose, which is then shuttled into the cytosol by glucose transporters located in the membrane of the endoplasmic reticulum.

Relation to Obesity

As the availability of energy-rich food increased, people started to gain more weight by converting excessive energy into body fat. While this kind of environmental factors play a significant role in increasing rate of obesity, lipid based metabolism in the body is also partly in charge of phenomenon.

Diabetes is one of the common metabolic diseases in relation to obesity. There are two types of diabetes:

Type 1 diabetes: Type 1 diabetes is an autoimmune disease which usually starts before 20 years of age. It is caused by destroying insulin-secreting beta cells in the pancreas. Thus, the person with Type 1 needs insulin to stay alive.

Type 2 diabetes: Most people have type 2 diabetes, in which they have a higher level of insulin in their blood (unlike Type 1 diabetics); however, they are unresponsive to a hormone, insulin resistance. Type 2 diabetes is the most common metabolic disease currently. Also, obesity is one of the main factor for developing type 2 diabetes.

Metabolic syndrome

Obesity is one of the main factors to the development of insulin resistance, which leads to type 2 diabetes. The clustering of insulin resistance, hyperglycemia, dyslipidemia is called metabolic syndrome and is presumed to be a precursor of type 2 diabetes.

One of the reasons for obesity is the amount of triacylglycerides one consumes will exceed the adipose tissue's capacity. Thus, other tissues will begin to store the excess fat (usually the liver and muscle)/

The extra fatty acids in the muscles alter metabolism

The mitochondria is not able to process all of the fatty acids by Beta oxidation. Thus, the extra fatty acids accumulate into the mitochondria and eventually go into the cytoplasm. The inability of the mitochondria to process these fatty acids leads to the fatty acids forming into triacylglyverols and then amount of fat increases in the cytoplasm.

Metabolic Linkage Between Diabetes and Cancer

The HBP binds various metabolic inputs to optimally deliver the synthesis of UDPCLcNAc, which is the donor substrate for OGT. Glucose is funneled into the HBP where it will get phosphorylated by the hexokinase to produce glucose 6-phosphate (Glc-6-P) which is then transformed to fructose 6-phosphate (Fruc-6-P) through the presence of phosphoglucose isomerase. During the rate-limiting procedure of the pathway, glutamine:fructose-6-phosphate (GFAT) converts Fruc-6-P into glucosamine 6-phosphate (GlcN-6-O). Enzymatic steps will guide the production of UDP-GLcNAc, which is termed as a negative feedback inhibitor of GFAT. Flux through the HBP and the generation of UDP-GlcNAc and O-GlcNAcylation are greatly affected by the disease states, which include diabetes (indicated by the purple arrows) and cancer (indicated by the teal arrows) through metabolism (glucose= green, amino acid =red, fatty acid = orange, and nucleotide = blue). Resistance of insulin leads to an increase in glucose levels that marks a specific diabetic condition. As a consequence, the increase in glucose will cause HBP to increase HBP flux through the rise of O-GlcNAcylation. Cancer cells require energy, and metabolites are often in excess which can be funneled into the HBP stimulating flux also, precisely glucose, glutamine, and UTP.

O-linked β-N-acetylgluosamine (O-GlcNAc) is a metabolic signaling sugar molecule. More specifically, O-GlcNAc is a post-translational protein modification that is made up of a single N-acetylglucosamine piece, which is attached to an O-β-glycosidic linkage to serine and threonine hydroxyl moieties on nuclear and cytoplasmic proteins. Proteins that contain O-GlcNAc take part in cellular processes such as transcription, translation, signal transduction, and cytoskeletal assembly, along with other functions. Studies have shown that diseases like diabetes and cancer are strongly affiliated with the major alterations in metabolism, which affect the alterations of O-GlcNAcyclation. Such alterations to O-GlcNAcylation interfere with cellular signaling forces and worsen the disease state.

O-GlcNAc signaling is closely associated with cellular metabolism, and ties very closely to phosphorylation due to post-translational modifications that process swiftly in response to internal and external signs. Furthermore, the similarity between O-GlcNAc and phosphorylation is founded in the sugar’s capability to be dynamically connected or detached based on the changed in the cellular environment activated by stress, hormone, or nutrients. Since O-GlcNAc is linked to the serine and threonine residues, the sugar is strictly competing with phosphorylation. Steric hindrance can be encountered when o-GlcNAcylated and phosphorylated residues are in close proximity to each other. O-GlcNAcylation is catalyzed by an enzyme known as uridine diphospho-N-acetylglucosamine (UDP-GlcNAc): polypeptide β-N-acetylglusaminyltransferase (OGT). In most cells, the enzyme OGT dynamically creates many specific holoenzyme proteins complexes that monitor specific activity approaching the myriad of target protein substrates. On the contrary, phosphorylation includes many individual unique kinases. Just like how there are protein phosphatases that detach phosphorylation, there exist a single cytosolic or nuclear β-N-acetylglucosaminidase (OGA) that aim substrates by shaping transient holoenzyme complexes to remove the sugar component.

An important glucosamine, Hexosamine biosynthetic pathway (HBP), which is a prominent precursor in the synthesis of glycosylated proteins, accommodates a collection of metabolic components that are relative to the formation of Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). To do this, the HBP integrates various metabolic inputs that will essentially deliver the synthesis of UDP-GlcNAc, which is implicitly the donor substrate for OGT. Seemingly, a rise in cellular glucose and flux to a particular level of concentration via HBP will increase UDP-GlcNAc levels to a certain degree. Slight increases in UDP-GlcNAc concentration will suffice O-GlcNAc to function as a nutrient sensor. This is due to the unique responsive property of O-GlcNAc transferase to UDP-GlcNAc concentrations. Rising levels of flux via the HBP causes a resistance for insulin. Flux through the HBP, which leads to the production of UDP-GLcNAc and O-GlcNAcylation are influenced by diseases like diabetes and cancer through metabolic effects. Studies have proven that O-GlcNAcylation play a strict role in insulin signaling. When there is O-GlcNAc and OGT that disrupt insulin signaling, the antipodal of decreased O-GlcNAcylation and O-GlcNAcase stimulate proper insulin signaling. Under specific diabetic conditions, such low levels of O-GlcNAcylation are advantageous in the sense it relieves hepatic insulin resistance and saves the diabetic cardiomyocytes functionality.

Much clarity is needed to gain a better understanding of O-GlcNAc signaling. For instance, uncertainty in the approach of how OGT and OGA target their substrates remains a struggle in fully grasping the mechanistic abilities of this specific metabolic signaling molecule. Development in such research can unravel metabolic diseases like diabetes and cancer.

Biochemicals

Amino acids, carbohydrates, and lipids are essential for life; therefore, metabolism focuses on the production of these molecules during the creation of cells and tissues, and the digestion and use of them when they are broken down and used as a provider of energy.

Amino Acids/Proteins

When amino acids arrange themselves as a linear chain joined together by peptide bonds, proteins are formed. Many proteins are enzymes that catalyze the chemical reactions involved metabolism.

The name protein came from the Greek word proteios, meaning "first place." In bacterial cells, almost 50% of the dry mass is made up of proteins. Almost all organisms contain proteins. All functions of living organisms are related to proteins and each of their specific functions.

Proteins can be classified based on their functions in the cell:

 1. Enzymatic proteins
      a. Specifically speed up reactions that are endogenic.
      b. This is the largest group of proteins.
      c. Enzymatic proteins are responsible for metabolic related reactions in cells.
      d. Examples:
          1) Digestive enzymes catalyze the hydrolysis of foods. 
          2)  DNA- and RNA-polymerases
          3)  Dehydrogenases
 2. Structual proteins
      a. Support the shape of the cell.
      b. Maintain the structure in tissues.
      c. Examples:
          1) Collagen is a type of fibrous framework that makes up the connective tissues in animals.
          2) Keratin is a type of fibrous proteins that supplements hair, horns, feathers, and skin.
 3. Storage proteins
      a. Store amino acids.
      b. Contain energy that can be released in metabolic reactions
      c. Examples:
          1) Ovalbumin is a protein used as an amino acid source for the developing embryo in egg whites.
          2) The casein protein is a major source of amino acid for baby mammals in milk.
 4. Transport proteins
      a. Transport substances.
      b. Examples:
          1) Hemoglobin is a protein of verberate blood that carries oxygen from the lungs to other parts of the body.
          2) Membrane protein attaches to the membrane of the cell, transporting substances that are unable to cross the membrane themselves.
 5. Hormonal proteins
      a. Regulate an organism's activities.
      b. Can be classified as peptides because they are usually small
      b. Examples: 
          1) Insulin is a hormone secreted by the pancreas that sends signals to the cells to regulate the concentration of sugar in the blood steam for vertebrates.
 6. Receptor proteins
      a. Response of cell to stimuli from chemicals, neighboring cells, etc.
      b. Examples:
          1) Receptors that are attached to cell membranes detect signals from hormonal proteins.
 7. Contractile and motor proteins
      a. Involved in the movement of organelles
      b. Examples:
          1) Actin regulates the contraction of muscles
          2) Cilia is responsible for the movement of organelles
 8. Defensive proteins
      a. Protect against foreign substances in the body.
      b. Examples:
          1) Antibodies.
 9. Motor Proteins
      a. Convert chemical energy to mechanical energy to facilitate movement
      b. Examples:
           1) Actin and myosin are the proteins within muscles that help in movement.
           2) Microtubules help move organelles within the cell, and chromosomes 

Amino acids are organic molecules that contain a carboxyl and amino groups attaching an alpha carbon as the center. The alpha carbon also contains other various groups symbolized by R and a hydrogen. The R groups are usually called the side chain and they differ for each amino acid.

Amnio acids includes the following:

 1.  Glycine (Gly or G)
      - Nonpolar
      - Smallest R group with only a hydrogen atom
      - evolutionary conserves because most other R group cannot fit into the small space
      - alpha carbon is achiral
 2.  Alanine (Ala or A)
      - Nonpolar/aliphatic
      - R group is a methyl
      - alpha carbon is chiral
 3.  Valine (Val or V)
      - Nonpolar/aliphatic
      - alpha carbon is chiral
 4.  Leucine (Leu or L)
      - Nonpolar/aliphatic
      - alpha carbon is chiral
 5.  Isoleucine (Ile or I)
      - Nonpolar/aliphatic
      - alpha carbon is chiral
 6.  Methionine (Met or M)
      - Nonpolar
      - Sulfur containing
      - alpha carbon is chiral
      - First amino acid of proteins
 7.  Phenylalanine (PHe or F)
      - Nonpolar
      - Aromatic
      - alpha carbon is chiral
 8.  Tryptophan (Trp or W)
      - Nonpolar
      - Aromatic
      - alpha carbon is chiral
 9.  Proline (Pro or P)
      - Nonpolar
      - Cyclic
      - alpha carbon is achiral
 10. Serine (Ser or S)
      - Polar
      - Hydroxy containing
      - alpha carbon is chiral
 11. Threonine (Thr or T)
      - Polar    
      - Hydroxy containing
      - alpha carbon is chiral
 12. Cysteine (Cys or C)
      - Polar
      - Thiol containing
      - alpha carbon is chiral
 13. Tyrosine (Tyr or Y)
      - Polar
      - Aromatic
      - Hydroxy containing
      - alpha carbon is chiral
 14. Asparagine (Asn or N)
      - Polar
      - Amide
      - alpha carbon is chiral
 15. Glutamine (Gln or Q)      
      - Polar
      - Amide
      - alpha carbon is chiral
 16. Aspartic acid (Asp or D)
      - Electrically charged (Acidic)
      - alpha carbon is chiral
 17. Glutamic acid (Glu or E)
      - Electrically charged (Acidic)
      - alpha carbon is chiral
 18. Lysine (Lys or K)
      - Electrically charged (Basic)
      - alpha carbon is chiral
 19. Arginine (Arg or R)
      - Electrically charged (Basic)
      - alpha carbon is chiral
 20. Histidine (His or H)
      - Electrically charged (Basic)
      - alpha carbon is chiral

A proteins consist of one or more polypeptides. Polypeptides consist of different level of structures.

 1) Primary structure
     - the unique sequence of an amino acid
 2) Secondary structure
     - hydrogen bonds interact between polypeptide backbones.
     - polypeptides can fold into structures such as alpha helix, beta pleated sheet, etc.
 3) Tertiary structure
     - hydrophobic interaction and disulfide bridges formed between side chains of polypeptides
 4) Quaternary structure
     - overall protein stuctures consist of two or more polypeptide chains that combine into one functional macromolecule.

Carbohydrates

Carbohydrates are the most abundant biological molecules found in organisms and are responsible for the storage and transport of energy, such as starch and glycogen, and structural components, like cellulose in plants or chitin in animals.

Carbohydrates are classes of macromolecules - large polymers built from monomers. Carbohydrates include both sugars and polymers of sugars. The simplest carbohydrates are monosaccharides, also known as simple sugars. Another group of carbohydrates are the disaccharides, which are created by joining together two monosaccharides by a covalent bond. Polysaccharides consist of many monosaccharides used as building blocks.

 1. Monosaccharides
     a. simple sugar with general formulas, (CH2O)n
     b. the most common monosaccharide is glucose
     c. the molecule contains a carbonyl group and many hydroxyl groups attached to the carbon atom
     d. a monosaccharide can either be an aldose (sugar with an aldehyde) or a ketose (sugar with a ketone) depending on the location of the carbonyl group
     e. monosaccharide can be categorized by the number of carbon on the chain starting with carbon 3
 2. Disaccharide
     a. two monosaccharides join together through a glycosidic linkage - a covalent bond that can be formed by a dehydration reaction
     b. examples:
          1) maltose - two molecules of glucose joined together
          2) sucrose - a molecule of glucose joined with a molecule of fructose
 3. Polysaccharides
     a. polysaccharides are polymers of multiple monosaccharides joined by glycosidic linkages
     b. some polysaccharides are storage materials and some are structural

Storage polysaccharides

 1. storage polysaccharides store sugar for later use
 2. usually joined by 1-4 linkages of alpha glucose monomers
 3. usually form a helical shape
 4. starch
     a. a storage polysaccharide found  in plants that is joined by glucose monomers
     b. the simplest form of starch is unbranched amylose
     c. a more complex form of starch is branched amylopectin
 5. glycogen
     a. a storage polysaccharide in animals that is joined by glucose monomers
     b. an amylopectin-like polymer but contains more branches
     c. mainly stored in the liver and muscle cells

Structural polysaccharides

 1. structural polysaccharides usually give protection to the cell as a form of membrane
 2. usually joined by 1-4 linkages of beta glucose monomers
 3. usually form beta sheets
 4. cellulose
     a. major components of cell walls in plants
     b. cellulose is unbranched
     c. hydroxyl groups on the glucose are able to interact with other hydroxyl groups on other molecules to form hydrogen bonds
     d. cellulose molecules are grouped into units called microfibrils. 
     e. humans are unable to digest cellulose

Lipids

Lipids are the most diverse group of biochemicals. Structurally, the lipids' primary function is to be part of biological membranes, such as the cell membrane, or as a source of energy in organisms. Another major class of lipids that is produced in cells are steroids, such as cholesterol.

Lipids are grouped together by their hydrophobic behavior - meaning they mix poorly with water. Lipid consist mostly hydrocarbons.

Different kinds of lipid include the following:

 1. Fats
     a. fats consist of a glycerol and three fatty acids, which are constructed from hydrocarbons and the carbon at one end containing a carboxyl group.
     b. the fatty acids are joined to the glycerol by an ester linkage.
     c. saturated fats are fats that do not contain any double bond on the hydrocarbon chain
     d. unsaturated fats are fats that contain double bonds on different positions of the hydrocarbon chain
          1) most double bonds in the hydrocarbon chain are cis- double bonds
          2) trans fats contain trans-double bonds
 2. Phospholipids
     a. phospholipids consist of a glycerol, two fatty acids and a phosphate group attached to an alcohol group
     b. phospholipids consist of a hydrophilic head and a hydrophobic tail
     c. phospholipids make up the lipid bilayer in the cell membranes
          1) hydrophobic tails dislike water and try to get away from water by getting as close as possible to other hydrophobic tails
          2) hydrophilic heads like contact with water and act as protection for the hydrophobic tails in the bilayer
 3. Steroids
     a. steroids consist of four fused ring skeletons
     b. different steroids consist of the steroid skeleton and other chemical groups attached to it
     c. example
          1) cholesterol
               a) common component of animal cell membranes
 4. Glycolipid
     a. glycolipids consist of a fatty acid unit and a sugar unit
     b. derive from sphingosine
     c. simplest glycolipid is cerebroside
     d. a more complex glycolipid is ganglioside

References

Slonczewski, Joan L. Microbiology. 2nd ed. New York, 2009.

General Information

Anabolic reactions are those that require energy to occur. Cells can couple anabolic reactions together with catabolic ones to form an efficient energy cycle; the catabolic reactions transform chemical fuels into cellular energy, which is then used to initiate the energy-requiring anabolic reactions.

Anabolism can be thought of as a set of metabolic processes, in which the synthesis of complex molecules is initiated by energy released through catabolism. These complex molecules are produced through a systematic process from small and simple precursors. For example, an anabolic reaction can begin with relatively simple precursor molecules (created previously by catabolic reactions) and end with fairly complex products, such as sugar, certain lipids, or even DNA, which has an extremely complex physical structure. The increased complexity of the products of anabolic reactions also means that they are more energy-rich than their simple precursors.

Anabolic reactions are divergent processes. That is, relatively few types of raw materials are used to synthesize a wide variety of end products. This results in an increase in cellular size or complexity—or both. Anabolic processes are responsible for cell differentiation and increases in body size. Bone mineralization and muscle mass are attributed to these processes. Anabolic processes produce peptides, proteins, polysaccharides, lipids, and nucleic acids. These molecules comprise all the materials of living cells, such as membranesand chromosomes, as well as the specialized products of specific types of cells, such as enzymes, antibodies, hormones, and neurotransmitters.

ATP provides the energy needed for anabolism to take place. ATP is a high energy molecule that couples anabolism by the release of free energy. This energy does not come through the breakage of phosphate bonds; instead it is released from the hydration of the phosphate group.The chemical reaction where ATP changes to ADP supplies energy for this metabolic process. Anabolism is the opposite of catabolism, for example, synthesizing glucose is an anabolic process whereas the breaking down of glucose is a catabolic process. The Gibbs free energy for the synthesis of glucose is positive, meaning that the reaction is not spontaneous and will not go to completion in any time frame. However, when coupled with ATP, this reaction becomes more thermodynamically favorable, as is the case with many other endothermic reactions in the body. Anabolism and catabolism must be regulated in a way that does not allow the two processes to occur simultaneously. Each process has its own set of hormones that switch these processes on and off. Anabolic hormones include growth hormone, testosterone, and estrogen. Catabolic hormones include adrenaline, cortisol, and glucagon.

There is a need for cells to separate the metabolism process into anabolic and catabolic pathways. Anabolism requires the input of energy, which can be described as an "uphill" (energy intake) process. Catabolism is a "downhill" process which energy is released as the organism had used up energy. At certain points in the anabolic pathway, the cell must put more energy into a reaction than is released during catabolism. Such anabolic steps require a different series of reactions than are used at this point during catabolism.

Stages of anabolism

• There are three basic stages of anabolism.

Stage 1 production of precursors such as amino acids, monosaccharides and nucleotides.

Stage 2 use energy from ATP to turn the precursors into reactive form.

Stage 3 the assembly of these activated precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids.

Examples of Biochemical Anabolic Reactions

1) Polysaccharides: Polysaccharides serve as an example of anabolism because polysacchrides are derived from their subunits of simple monosaccharides. A simple example is the formation of glycogen. Glygocen is a polysaccharide that is composed of subunit glucose monosacchrides connected by glycosidic bonds.

2) Polypeptides: Polypeptides serve as an example of anabolism because polypeptides are derived from their subunits of simple peptides. Polypeptides such as hemoglobin are composed of four different proteins (essentially peptides) that come together and form a completely different protein. Peptides them selves are a result of anabolic reactions themselves due to the condensation reactions that simple amino acids go through to combine and thus form peptide chains.

3) Carbon Fixation: Carbon fixation is another example of anabolism because in photosynthetic organisms such as plants, cyanobacteria, algae and other photoautotrophic organisms fixate carbon dioxide into glycerate 3-phosphate which is then further converted into glucose. Beginning with photosynthesis, synthesis of carbohydrates from sunlight and carbon dioxide. This process uses ATP and NADPH produced by the photosynthetic reactions to convert CO2 into glycerate 3-phosphate. This shows anabolism because from a smaller simpler subunit of carbon dioxide, glycerate 3-phosphate is produced, a much larger and complex biochemical compound.

Examples of Anabolic hormones

Growth hormone:

Growth hormone is a protein-based peptide hormone that stimulates growth, cell reproduction, and regeneration in humans and other animals. Growth hormone is often used to treat children with growth disorders as well as adult growth hormone deficiency.

Insulin:

Insulin is a hormone that is essential for regulating fat and steroids metabolism in the body. It causes the uptake of glucose from the blood by cells in the liver, muscle, and fat tissue. Glucose is then stored as glycogen in the liver and muscle. Insulin also contributes to other body functions such as vascular compliance and cognition.

Testosterone:

Testosterone is a steroid hormone found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is mainly secreted in the testes of males and the ovaries of females. However, small amounts of testosterone are also secreted by the adrenal glands. Testosterone is the principal male sex hormone.

Estradiol:

Estradiol is the predominant sex hormone present in females. However, it is also found in males and acts as an active metabolic product of testosterone. Estradiol has major impacts on the reproductive and sexual functions as well as other organs. [7] [Berg]

Side effects may occur when anabolic hormones (or steroids) are used in excessive amount. If used too much by men, it can cause a decline in testosterone secretion, testicular atrophy (wasting away of the testes), and even breast enlargement. For women, excess use of anabolic hormones may cause a decrease in estrogen secretion and the ability to ovulate, the growth of facial hair, as well as regression of the breasts.

References

http://science.jrank.org/pages/319/Anabolism.html#ixzz0UM1KSRXO

http://science.jrank.org/pages/319/Anabolism.html

Berg, Jeremy; Tymoczko, John L.; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. New York. 2007

Overview

Catabolism is the release of energy from a set of metabolic pathways which break down molecules into smaller units, including the breaking down and oxidizing of food molecules. An example would be proteins, nucleic acids, lipids and polysaccharides being broken down into smaller molecules like amino acids, nucleotides, fatty acids, and mono saccharides. By glycolysis, the glucose is broken down into two pyruvates which can be used for later mechanism (Krebs cycle) to produce energy. The oxidation of long-chain fatty acid to acetyl-CoA is a central energy-yielding pathway in many organisms. Its opposite process is anabolism, which combines small molecules into larger molecules. Energy that is released from catabolism will store as ATP within the cell. The cell will then use this source of energy for synthesizing cell components from simple precursors, for the mechanical work of contraction and motion, and for transport of substances across its membrane. Catabolism maintains the chemical energy needed in order to help the cell grow and develop. Some waste products caused by catabolism are carbon dioxide, urea, and lactic acid. Heat is also sometimes released as a by product because these are oxidation processes. Examples of catabolism are the citric acid cycle.

The energy cells contain is liberated through two distinct processes: glycolysis and cellular respiration

GLYCOLYSIS Glycolysis is a series of reactions that break down glucose into two smaller organic molecules.

Glycolytic pathway Glycolysis occurs in the cytoplasm, in the presence or absence of oxygen. The pathway has several steps to convert six-carbon glucose into two molecules of three-carbon pyruvates. The direct generation of ATP from ADP and P_i is known as substrate-level phosphorylation. NAD⁺ is reduced to NADH twice, so we start with two molecules of NAD⁺ and finish with two molecule of NADH.

The net reaction for glycolysis is: Glucose + 2 ADP + 2P_i + 2 NAD⁺ → 2 Pyruvate + 2 ATP + 2 NADH + 2H⁺ + 2H₂O

Most of the chemical energy extracted from the sum is still stored in pyruvate’s bonds. Pyruvate has two potential fates based on the character of the cell’s environment. In aerobic organisms, pyruvate undergoes further oxidation through the mitochondrial electron transport chain. In anaerobic organisms, pyruvate undergoes an oxygen-free process called fermentation. Some cells are obligate aerobes or anaerobes, meaning that they require that designated environment. Others are facultative; they refer one environment over the other but can survive in either.

Fermentation In the glycolysis reaction equation, NAD⁺ is a necessary reagent and must be present for glycolysis to occur. At the end of glycolysis, the coenzyme is present only in its reduced form, NADH. One way to regenerate NAD⁺ is through oxidation in the electron transport chain, but anaerobic organisms would not participate in that. So another method is used: fermentation. Fermentation reduces pyruvate to either ethanol or lactic acid.

Alcohol Fermentation This process occurs in yeast and some bacteria. Pyruvate is first decarboxylated to acetaldehyde, which is then reduced by NADH to ethanol, thereby regenerating the NAD⁺ Pyruvate (3C) → CO2 + Acetaldehyde (2C) Acetaldehyde + NADH + H⁺ → Ethanol (2C) + NAD⁺

Lactic acid Fermentation This process occurs in some fungi and bacteria, as well as in mammalian muscles when oxygen demand exceeds supply. Basically, many glucose molecules are put through glycolysis, yielding twice as many molecules of cellular respiration, so it builds up. Concurrently, NADH builds up, depleting cells’ supply of NAD⁺. To keep muscles working, pyruvate is reduced to lactic acid, and NADH is oxidized back to NAD⁺. Lactic acid decreases the local pH, which we fell as the burn and fatigue effects of strenuous exercise. Once oxygen supply catches up to demand, the lactic acid may be converted back to pyruvate in the process known as the Cori cycle. The amount of oxygen necessary to do this is known as the oxygen debt. Pyruvate (3C) + NADH + H⁺ → Lactic Acid + NAD⁺

CELLULAR RESPIRATION It is the most efficient means of glucose catabolism, generating approximately 36 to 38 ATP per molecule of glucose. Respiration is aptly named; it is an aerobic process using an electron transport chain, with oxygen being the final electron acceptor. There are three key phases: pyruvate decarboxylation, the citric acid cycle, and the electron transport chain. A productive way to keep track of these reactions will be the follow the carbon.

Pyruvate decarboxylation The first step in aerobic respiration is pyruvate decarboxylation. This step itself does not require oxygen, but it only occurs once the cell commits to aerobic respiration—and the commitment is made only in the presence of oxygen. Pyruvate is transported from the cytoplasm into the mitochondrial matrix, where it is decarboxylated. The remaining acetyl (2C) group is bound to a coenzyme A molecule to form acetyl-CoA. Once NAD⁺ is reduced to NADH per pyruvate; in other words, two NAD⁺ molecules are reduced per molecule of glucose. Acetyl-CoA is a key intermediate in the utilization of fat, protein, and other carbohydrate energy reserves.

2 Pryuvate (3C) + 2 CoA + 2 NAD⁺ → 2 NADH + 2 Acetyl-CoA (2C) + 2 CO₂ (1C)

Citric Acid Cycle The citric acid cycle starts with the combination of acetyl-CoA (2C) and oxaloactate (4C) to generate citrate (6C). Through a series of eight reactions, two CO2 molecules are released, and oxaloacetate is regenerated. The citric acid cycle does not directly generate much energy. Each turn of the cycle generates one ATP via substrate-level phosphorylation and a GTP intermediate, for a total of two pyruvates per glucose molecule. The value of the citric acid cycle is its ability to generate high-energy electrons that are carried by NADH and FADH2. For each molecule of acetyl-CoA that enters the cycle, three NADH and one FADH2 are produced by two to account for the fact that the cycle turns twice per molecule of glucose. These enzymes then transport the electrons to the electron transport chain on the inner mitochondrial membrane, where more ATP is produced via oxidative phosphorylation. At the end of the citric acid cycle, oxaloacetate is regenerated in anticipation of the next round.

The overall reaction is: 2 Acetyl-CoA + 6 NAD⁺ + 2 FAD + 2 GDP + 2 P_i + 4 H₂O → 4 CO2 + 6 NADH + 2 FADH₂ + 2 ATP + 4 H⁺ + 2 CoA Structural Biochemistry/Lipids/Fatty Acids Single-molecule techniques have recently become popular in the biophysical department in helping to discover or clarify and better understand some important biochemical properties such as protein-DNA interactions, protein folding, and the functions and capabilities of membrane proteins. Many of the single-molecule techniques were first revealed in the physics and biophysics department, and later were found to be of great assistance to research biological and biochemical molecules. In 1976, the technique called single ion-channel recordings was first discovered and that later became the gateway to recent techniques such as atomic force microscopy (AFM), optical and magnetic tweezers and single-molecule fluorescence spectroscopy. Many of these recent techniques have helped in fields such as protein folding, transcription, replication, translation, molecular motors, membrane proteins, and viral biology. These single-molecule methods have been able to provide information on problems that could not be solved before while also giving a more detailed view into subjects that have already been researched. Not only that, but single-molecule techniques have also been able to steer the biochemists away from the usual averaging of ensembles that results in the use of moles to a more detailed and unitary concept that involves the single molecules and particles.

Static vs. Dynamic Heterogeneity

A useful aspect of single-molecule techniques is that they give the distribution of values for a property instead of an average of the property which is averaged over a large molecule ensemble. This kind of feature allows for more complete and specific data that can speak more about the certain biomolecule that is being assessed than an average which gives a rather unspecific and broad view. The specificity of single-molecule methods also provides for information on the molecular heterogeneity, which is a fundamental aspect of complex biomolecules and their functions. Molecular heterogeneity can be classified as either static or dynamic. Static heterogeneity is when a collection of molecules that have several subpopulations are extremely stable and do not interconvert over the period of time that is being observed. An example of this is inactive molecules. When an ensemble method is being used, one is trying to determine the fraction of active molecules compared to the whole population. However, when using a single-molecule technique, the inactive molecule can be ignored or disregarded because they do not give an experimental signal. Therefore, static heterogeneity allows for the sole research of the species of interest (active molecule) because the inactive ones can be ignored and removed. On the other hand, dynamic heterogeneity is when a sample with subpopulations of molecules interconvert over the time period observed. An example of dynamic heterogeneity is an enzyme that interconverts between two catalytically active states, with each conformation yielding a different affinity for a substrate. When the interconversion of the enzyme is fast compared to the temporal resolution of the single-molecule technique that is being used, the value that comes out will show a weighted time-average mean of the affinity for each of the two states of the enzyme. On the other hand, if the interconversion is slower than the temporal resolution of the single-molecule method, then the observation of the interconversion between the two states can be directly seen. In addition, dynamic heterogeneity can also be observed in enzymes that operate on substrates in multiple steps, with each step having a different rate. With single-molecule analysis, the observation of each single step in the process of the enzyme can be carefully monitored as real time "movies" that show the kinetics of each step and the intermediate structures of the enzyme and substrate.

Kinetics of Single-Molecule Techniques

The type of information that is extracted from single-molecule techniques are also something that can be compared to the ensemble assays that are used frequently. One example is the kinetic information that is provided by the single-molecule methods. Kinetic observation of a reaction by single-molecule methods provide something called dwell times. These dwell times are of a single molecule at each of its certain states along the reaction pathway. Think of two structural states of a molecule called A and B that is being observed by a single-molecule observable. The single-molecule method with enough temporal resolution to look at the two states would be able to determine the dwell time for states A and B. Afterward, a frequency histogram can be plotted to provide information on how long each dwell time lasts. Dwell time distributions can be used to study enzyme-substrate interactions. This example can also be used to reveal the significant aspect of single-molecule technique in its method of identifying the structural states of a biological macromolecule.

Single-Molecule Methods

There is a wide variety of single-molecule techniques that are present right now. The commonly used methods are separated into two different attributes: force and fluorescence. These two groups were separated based upon the different time resolutions, span of observations, and different spatial resolutions. In the force-based detection methods, there are specific techniques called atomic force microscopy, optical tweezers, tethered partical motion, and magnetic tweezers. On the other hand, the fluorescence imaging includes confocal microscopy and total-internal-reflection fluorescence (TIRF).

Force-based Dectection

Atomic force microscope block diagram v2

Atomic Force Microscopy (AFM)

Atomic force microscopy was first used for topographical imaging of molecules on a flat atomic surface. This form of imaging is done by scanning an extremely sharp tip along the sample surface and then measuring the deflection from the tip by using a laser and a quadrant photodetector. Dry and wet solutions can both be observed under this imaging, although the wet solution's temporal resolution is far worse than the dry solution's. The setup of AFM usually has the a small lever (AFM cantilever) that has a sharp nanometer scale tip which is attached to one of the ends of a biomolecule. By using piezoelectric positioning, the surface which the biomolecule is adsorbed can then be scanned in the x,y,z coordinate system (3D spatial) to an angstrom scale resolution. Then, the position of the tip could be measured by deflecting a laser beam off the surface of the tip and onto the photodetector, which will be position sensitive. The lever will act as a linear spring and when the biomolecule is moved relative to the tip, the lever will flex and apply a force on the molecule. Because the lever is very rigid, the force applied can be great and is therefore very useful in measuring the structural properties of folded proteins and chemical bonds.

Optical Tweezers

Generic Optical Tweezers ru

For the method of optical tweezers, the setup involves a high-power infrared laser that is focused tightly because of a high-end microscope lens. The focused infrared laser beam would trap a micron-sized bead at its focal spot, which allows for the control of the position of the bead. Then, the trap acts like a spring, which would exert a higher force on the bead when it moves farther away from the laser axis. And by measuring the position of the bead in relation to the focal spot, the trap's applied force can be determined (this can be done with the quadrant photodetector, like that in AFM). There are two main modes of optical tweezers: constant force and constant position. When it is in the constant force mode, a constant trap force can be maintained by a feedback loop that will displace the optical trap or the sample coverslip surface, which will keep the position of the bead constant within the zone that it is trapped in. In the constant position mode, the trap position's center is held and the bead will experience a progressively growing force as it is being pulled out of the trap.

Magnetic Tweezers

The setup for magnetic tweezers has the small bead that is used in optical tweezers replaced by a small magnetic bead. The small magnetic bead will be controlled by a pair of magnets that is close to the sample. This setup allows for the tethered biomolecule to have a force applied on it and a rotation that will be easily imposed. Magnetic tweezers function in the constant force mode because the force is an important control parameter for the interaction of interest. In addition, compared to the optical tweezers, the magnetic tweezers cannot obtain the exquisite spatial sensitivity. However, it is easier to use and can be compatible with long term analysis of single biomolecules. Magnetic tweezers is usually used for studying the structural properties of DNA and protein-DNA transactions.

Tethered Particle Motion

This method is used for studying the interation of proteins and polymers. The main idea is one bead is bound to the surface while the other bead is attached to the end of the polymer. These are then put into an aqueous solution and it can be seen that the bead has restricted movement in which it moves in a Brownian motion. The position of this bead is then recorded by optical microscopes; the recordings can tell us information about the DNA during transcription. Different bead types have also improved ways of analyzing polymers. Firstly, metal beads are sometimes used due to their high intensity of gold light. Secondly, polystyrene beads are also used in conjunction with the optical tweezers because these beads are less intense than the metal ones.

Fluorescence-based Methods

Fluorescent biomolecules contain several properties that can be used by fluorescent microscopy and spectroscopy to look at their location, structure and dynamics. There are two main methods for single-molecule fluorescence: confocal microscopy for point detection and wide-field imaging for area detection. Confocal microscopy for point detection tries to collect fluorescence that is emitted by a diffraction-limited volume of ~1 femtoliter. On the other hand, for wide-field imaging, there is total-internal-reflection fluorescence (TIRF) microscopy. TIRF is capable of observing hundreds of molecules that are immobile on the surface for extended periods of time through using evanescent-wave excitation within a thin layer just above the surface and using a ultrasensitive camera above surface for imaging.

FIONA or fluorescence imaging with one nanometer accuracy, has enabled motion tracking with precision of 1 nanometer. This can be done with any single fluorophore or light-scattered particle (example: small gold particle) that is attached to the molecule of interest. With that set up, one can identify the presence of a molecule and track its movement on molecular tracks in vitro or its diffusion in vivo. This remarkable tracking ability has been able to reveal how molecular motors like kinesin and myosin move on their tracks. The combination of this precision in tracking and the founding of complicated methods of switching on and off the fluorescence of some probes allows for techniques such as PALM (photoactivated localization microscopy) and STORM (stochastic optical reconstruction microscopy). PALM and STORM have been characterized as "super-resolution microscopies" because of their ability to achieve spatial resolutions that are better than 50 nanometers.

With two probes attached to a molecule, there are several new capabilities that come forth. FRET (fluorescence resonance energy transfer) is distance dependent and can be exploited to measure nanometer distances and the change in distances within single molecules. For FRET, the first probe would act as the FRET donor while the second probe would be the FRET acceptor. The first probe would be fluorescent, and the second probe would quench the donor in a distance-dependent manner (it can also be fluorescent). Because of this setup, the movements that change the donor-acceptor's distance of separation would also change their fluorescence. These changes are used to observe the kinetics of conformational changes or molecular association/disassociation. However, single-molecule FRET is also affected by relative orientation and rotational freedom of the fluorophores, which can be mistaken for conformational changes. This difficulty can be examined by using MFD (multi-parameter fluorescence), which is a method that can tell the fluorescence properties of single molecules such as fluorescence intensity, anisotropy, and lifetime at different wavelengths. Another method that can help the complication is ALEX (alternating laser excitation), which uses two lasers to measure FRET and relative probe stoichiometries.

Combination of Force and Fluorescence Approaches

The combination of force and fluorescence techniques is being pursued right now by single-molecule groups because of it will allow for simultaneous manipulation and visualization of single molecules as they interact and react. This combination works because the force and fluorescence techniques are highly complementary due to the fact that force-based methods can achieve timescales of 50-100ms while fluorescence-based can be many times faster and are not restricted by an applied force. In addition, whereas force-based methods provide information more on the global structural and mechanical rearrangements in biomolecules, fluorescence-based methods reveals the local conformational changes. This complementary ability of the two types of single-molecule method can lead to great discoveries in the future.

References

Kapanidis, A.N. and Strick, T., 2009, Biology, one molecule at a time, Trends in Biochemical Sciences, v. 34, p. 234-243.

Chemistry is the study of composition, structure and properties of matter. This includes studying changes that are observed when undergoing various chemical reactions or understanding the basic structure of complex macromolecules. It provides an understanding for atoms, molecules, crystals which make up everything in this world, while incorporating the concepts of energy and entropy.

Chemical Foundations of Biochemistry

3d Lewis representation of hydrogen, oxygen, nitrogen and carbon to use as organic building blocks

Chemistry is always interconnected with the other sciences, such as astronomy, physics, material science, and biology. In terms of biochemistry, it strives to find chemical explanations to biological form and function. Chemistry shows that all forms of life share common origins through the similarity of various chemical pathways between different organisms. For example, all living cells have the common molecules of amino acids, carbohydrates, lipids, and nucleic acids which all perform the same functions in every cell.

Nature is predominantly made of four elements: Hydrogen (H), Oxygen (O), Nitrogen (N), and Carbon (C). Together they make up over 99% of all living cells. These elements are the lightest atoms that are capable of making up to four stable and strong bonds. There are other elements that are also essential to living organisms such as Sodium (Na), Potassium (K), Calcium (Ca), and Sulfur (S). These are called trace elements and usually are needed to help specific proteins to function.

Organic Chemistry as the Backbone of Biochemistry

Organic chemistry is the study of the structures, properties, synthesis, and reactions of chemical compounds consisting of mainly carbon and hydrogen atoms. It is the backbone science that explains many of the interactions that occur in a cell at the most basic level. The majority of biochemistry, from a chemical point of view, involves the interactions between organic molecules that exist in organisms and how they are utilized in a versatile fashion at a cellular level. The arrangement of bonds within a molecule determines its overall shape, conformation, and the molecules ability to perform specific functions within a cell. Organic compounds that are of particular interest in biochemistry are carbohydrates, nucleic acids, and proteins.

Carbon is unique from the other essential elements for it has the capability to form four stable, covalent bonds and stable double and triple bonds. Carbon can bond with other carbons to create long chains with or without branches or cyclic structures allowing the formation of an endless array of molecules with different shapes, sizes, and compositions. Replacing hydrogen with other elements or functional groups gives the organic molecule different properties such as polarity.

Importance of Chemical Structure

Biochemistry chemistry is stereospecific meaning that the 3-D orientation of a molecules matters and only specific arrangements will interact properly. Stereoisomers are molecules that have the same chemical bonds but have a different spatial arrangement. They may look very similar but they cannot be the same molecule with out breaking any of the covalent bonds.

Human hands are mirror images of each other just like enantiomers.

There are many places where there can be varying spatial configuration. A molecule can have a chiral center which means that an atom (usually carbon) that is attached to four different substituents. These four can arrange themselves any way they want to around the carbon but if the arrangements are not exactly the same then they are stereoisomers. One example is if the molecule is a mirror image of itself. No matter how you rotate the molecule it will never superimpose on to its original form. This specific type of stereospecificity is called enantiomers. If they are not mirror images of each other they are called diastereomers.

Another way is to have a double bond. Double bonds, unlike single bonds, are stiff and do not rotate freely. The four substituents that surround the bond can form either trans, where the two highest ranked substituents are opposite sides of the bond, or cis, where the highest ranked are on the same side of the bond, relationships. This is called either geometric isomers or cis-trans isomers.

While enantiomers have almost the same chemical properties only the specific orientation will have the desired effect in a biological system. In nature, chiral compounds are usually found in only one orientation. For example, in carbohydrates the second to last carbon always has a "D" orientation and amino acid's chiral carbon is always in an "L" orientation. Enzymes can discriminate between enantiomers therefore is essential to have the right configuration.

Water and its Effects on Structure

The field of biochemistry also encompasses the special properties of water. Because all living organisms have cells that interact with aqueous solutions, the properties of water are fundamental to the environment in which biochemical reactions occur. The structure of a molecule determines whether it is hydrophilic or hydrophobic. Its interaction in water determines how these molecules function in a cell, one example being the lipid bi-layer in the cell membrane and how its chemical properties determines its function in the cell (in this case acting as a barrier in a cell). Water also has unique features such as high boiling and melting points. Its liquid form is also more dense than its solid form which allows marine life. An important feature of water in regards to biochemistry is its ability to form hydrogen bonds.

Introduction

Atoms form bonds by gaining, losing, or sharing electrons. Typically, they seek to achieve the electron configuration of a noble gas. Bonding occurs because it lowers the energy of a system and makes the atom more stable. Atoms also a unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons and an equal in number to the number of nuclear protons. The entire structure having an approximate diameter of 10-8 centimeter and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

An atom cannot be broken down further without changing the chemical nature of the substance. For example, if you have 1 ton, 1 gram or 1 atom of oxygen, all of these units have the same properties. We can break down the atom of oxygen into smaller particles, however, when we do the atom looses its chemical properties. For example, if you have 100 watches, or one watch, they all behave like watches and tell time. You can dismantle one of the watches: take the back off, take the batteries out, peer inside and pull things out. However, now the watch is no longer behaves like a watch.

Atoms are made up of 3 types of particles: electrons, protons and neutrons. Each of these particle has different properties. Electrons are tiny, very light particles that have a negative electrical charge (-). Protons are much larger and heavier than electrons and have the opposite charge. So, protons have a positive charge (+). Neutrons are large and heavy like protons, however, neutrons have no electrical charge. Anyway, each atom is made up of a combination of these particles. The proton and electron stay together just like the two magnets (the opposite electrical charges attract each other).

The electron that is constantly spinning around the center of the atom (called the nucleus). The centrigugal force of the spinning electron keeps the two particles from coming into contact with each other much as the earth's rotation keeps it from plunging into the sun. In an electrically neutral atom, the positively charged protons are always balanced by an equal number of negatively charged electrons. As we have seen, hydrogen is the simplest atom with only one proton and one electron. Helium is the 2nd simplest atom. It has two protons in its nucleus and two electrons spinning around the nucleus. With helium though, we have to introduce another particle. Because the two protons in the nucleus have the same charge on them, they would tend to repel each other, and the nucleus would fall apart. To keep the nucleus from pushing apart, helium has two neutrons in its nucleus. Neutrons have no electrical charge on them and they act as a sort of nuclear glue, holding the protons, and thus the nucleus together.

As you add more electrons, protons and neutrons, then the size of the atom increases. We can measure an atom's size in two ways: 1) By using the atomic number (Z) or using the atomic mass (A, also known as the mass number). The atomic number describes the number of protons in an atom. For hydrogen, the atomic number (Z) is equal to 1. For helium Z = 2. Since the number of protons equals the number of electrons in the neutral atom, Z also tells you the number of electrons in the atom. The atomic mass tells you the number of protons plus neutrons in an atom. Therefore, the atomic mass, A, of hydrogen is 1. For helium A = 4.

Electrically neutral atoms are the atoms that have no positive or negative charge on them. Atoms, however, can have electrical charges. Some atoms can either gain or lose electrons (the number of protons never changes in an atom). If an atom gains electrons, then the atom becomes negatively charged. If the atom loses electrons, then the atom becomes positively charged (because the number of positively charged protons will exceed the number of electrons). An atom that carries an electrical charge is called an ion.

While the number of protons for a given atom never changes, the number of neutrons can change. Two atoms with different numbers of neutrons are called isotopes. For example, an isotope of hydrogen exists in which the atom contains 1 neutron (commonly called deuterium). Since the atomic mass is the number of protons plus neutrons, two isotopes of an element will have different atomic masses (however the atomic number, Z, will remain the same).

The quantum mechanical basis for the formation of chemical bonds is an overlap in the probability densities of two or more wavefunctions; this means that electrons have a non-zero chance of being found in a region of space that is forbidden by classical physics.

Ionic Bonds

An ionic bond is the transfer of electrons between a metal and a nonmetal. An example of an ionic bond is that between sodium and chlorine atoms. The sodium atom transfers its lone electron in the 3s state to the chlorine atom. After the electron transfer, the sodium atom bears a +1 charge while the chlorine atom now bears a -1 charge. With this transfer of electrons, the sodium now has the electron configuration of the noble gas neon, while chlorine now has that of the noble gas argon.

Ionic bonding will occur only if the overall energy change for the reaction is favorable – when the bonded atoms have a lower energy than the free ones. The larger the resulting energy change the stronger the bond. The low electronegativity of metals and high electronegativity of non-metals means that the energy change of the reaction is most favorable when metals lose electrons and non-metals gain electrons.

Pure ionic bonding is not known to exist. All ionic compounds have a degree of covalent bonding, which means, ionic bond could be consider as a special type of covalent bond. The larger the difference in electronegativity between two atoms, the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.

Covalent Bonds

Covalent bonds are another type of chemical bond used to achieve a noble gas configuration, or an octet of electrons. Covalent bonds are formed between nonmetals, usually from the Boron, Carbon, Nitrogen, Oxygen, and Halogen families. Metals are rarely involved in covalent bonds. Each covalent bond consists of two electrons, one usually from each atom involved in the bond. The atoms form enough covalent bonds that when the electrons in the bonds are added with the valence electrons, they will have an octet. The key difference between ionic and covalent bonds lies in how the electrons are distributed between the two atoms. In ionic bonds, the electrons are transferred from one atom to the other, giving the atoms effective +1 and -1 charges. However, in covalent bonds, the valence electrons from both of the two atoms are shared between two atoms. Thus, neither atom is given a full positive or negative charge. Instead, the electrons shared between the two atoms - whether it be 2, 4, or 6 electrons - varies from molecule to molecule.

There are two types of covalent bonds: pure covalent bonds and polar covalent bonds. Pure covalent bonds exist when there is no difference between the two atoms sharing the electrons. The electronegativity of the two atoms is identical. Because the electronegativity values do not differ, they pull the electrons that are being shared between them with the same force. Thus, the electrons are shared equally and none of the atoms bears a partial positive or negative charge. An example of a pure covalent bond is a Cl-Cl or a Br-Br bond. Pure covalent bonds rarely exist for bonds that are not between identical atoms. Another example would be the covalent bonds between the carbons in long alkane chains.

Polar covalent bonds are those that exist between atoms of different electronegativities. The electrons in the bond are still being shared, but not equally between the two atoms. Though the exact ratio of the electron density that each atom bears cannot be determined easily, it is very easy to determine which atom pulls more electron density towards itself. The more electronegative atom will pull the shared electrons more, causing it to now bear a slightly negative charge. Because charge has to be conserved, the less electronegative atom must now bear a slight positive charge, equal in magnitude to the negative charge. As an example, consider a bond between carbon and chlorine. Chlorine is much more electronegative than carbon, thus it pulls more of the electrons towards itself. This gives the chlorine a slightly negative charge and the carbon a slightly positive charge. If the difference between the two atoms is so great causing one of the two atoms to posess a lot of the electron density, the bond becomes increasingly ionic and less covalent. For this reason, though H-Cl is considered a covalent bond, it is classified as a very strong acid, meaning it dissociates completely. Because the electronegativity difference is so vast, the chlorine molecule pulls all the electron density towards itself, thereby dissociating into H+ and Cl- ions in the presence in water.

However, it is important to note that a molecule that contains polar bonds can be nonpolar. For example, take the molecule carbon tetrachloride. This molecule has four polar C-Cl bonds. However, due to the orientation of the polar bonds, they cancel out and the molecule as a whole is nonpolar.

Hydrogen Bonds

A hydrogen bond is a bond created by the dipole-dipole interaction of a hydrogen atom and an electronegative atom such as an oxygen or nitrogen atom due to dipole dipole interactions. A common example of this is water where the electronegativity of the oxygen allows it to have a slight negative charge while the two hydrogen atoms have a slight positive charge. The negative charge on the oxygen forms a weak bond with the slight positive charge of another water molecule's hydrogen. This type of bonding is also present in organic fluorine compounds between C and F groups. This force is weaker than covalent bond and ionic bonds, but stronger than Van der Waals interactions.

Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C), or most of the solutions that use water as the solvent. This is because of the strong hydrogen bond, as opposed to other group 16 hydrides. Intramolecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids.

Role of Noncovalent Interactions in Macromolecules

In macromolecules such as proteins, DNA, and RNA, noncovalent interactions are essential. Noncovalent interactions include hydrogen, ionic, hydrophobic, and Van Der Waals bonding. These interactions are described in more specificity in the list that follows this group. When compared to covalent bonds, noncovalent bonds are weak and continuously form and break bonds. However, when several noncovalent bonds are formed, there is a net increase in bond strength. Their combined participation in a macromolecule makes a difference (i.e. substrate binding to enzyme and the lipid bilayer's role in transport). With several hydrogen bonds, ionic, and hydrophobic interactions existent at the same time, it is unlikely that these several weak interactions will break the substrate and enzyme without external energy. This property is the reason why enzymes have specific catalytic power. Protein folding and the unique properties and structures of proteins also depend on these noncovalent interactions.

Metallic Bonding

Metallic bonding is the bonding between metal and metal. The bonding involves electron pooling and metallic bonding. Since metal atoms are larger and can easily lose their valence electrons, the electrons on their outer shell can be pooled and be distributed evenly with other metal atoms. Differently from the covalent bonding, electrons in the metallic bonding are delocalized which means that the electrons can move freely through the metal.

Metallic Bonding

1) Metallic bonding in sodium: Metals tend to have high melting points and boiling points suggesting strong bonds between the atoms. Even a metal like sodium (melting point 97.8°C) melts at a considerably higher temperature than the element (neon) which precedes it in the periodic table. Sodium has the electronic structure 1s22s22p63s1. When sodium atoms come together, the electron in the 3s atomic orbital of one sodium atom shares space with the corresponding electron on a neighbouring atom to form a molecular orbital. This is the same sort of way that a covalent bond is formed. The difference, however, is that each sodium atom is being touched by eight other sodium atoms, the sharing occurs between the central atom and the 3s orbitals on all of the eight other atoms. And each of these eight is in turn being touched by eight sodium atoms, which in turn are touched by eight atoms and so on, until you have taken in all the atoms in that lump of sodium. All of the 3s orbitals on all of the atoms overlap to give a vast number of molecular orbitals, which extend over the whole piece of metal. There have to be huge numbers of molecular orbitals, because each orbital can only hold two electrons. The electrons can move freely within these molecular orbitals. Each electron becomes detached from its parent atom. The electrons are said to be delocalized. The metal is held together by the strong forces of attraction between the positive nuclei and the delocalized electrons.

2) Metallic bonding in magnesium: Magnesium has the outer electronic structure 3s2. Both of these electrons become delocalized, so the "sea" has twice the electron density as it does in sodium. The remaining ions also have twice the charge and so there will be more attraction between ions and sea. Each magnesium atom has 12 protons in the nucleus compared with sodium's 11. In both cases, the nucleus is screened from the delocalized electrons by the same number of inner electrons,the 10 electrons in the 1s2 2s2 2p6 orbitals. That means that there will be a net pull from the magnesium nucleus of 2+, but only 1+ from the sodium nucleus. So not only will there be a greater number of delocalized electrons in magnesium, but there will also be a greater attraction for them from the magnesium nuclei. Magnesium atoms also have a slightly smaller radius than sodium atoms, and so the delocalized electrons are closer to the nuclei. Each magnesium atom also has twelve near neighbors rather than sodium's eight. Both of these factors increase the strength of the bond.

3)Metallic bonding in transition elements: Transition metals tend to have particularly high melting points and boiling points. The reason is because they can involve in the 3d electrons in the delocalization as well as the 4s. The more electrons can involve, the stronger the attraction.

4) The metallic bond in molten metals: In a molten metal, the metallic bond is still present, although the ordered structure has been broken down. The metallic bond is not fully broken until the metal boils. That means that boiling point is actually a better guide for the strength of the metallic bond than melting point. In melting, the bond is loosened and is not broken.

Relative Strength of Chemical Bonds and Intermolecular Forces

All the bonds known to chemistry can be described the various dissociation energies needed to break the bonds. It is these dissociation energies that rank the strength of the various bonds found in chemistry. Ionic bonds, being the strongest bonds, have a dissociation energy of >400 kcal/mol. Covalent bonds where are the second strongest bonds have a dissociation energy of about 400 kcal/mol. Hydrogen bonds, dipole-dipole, and london (van der waals) dispersion bonds are in a sub category of bonds labeled intermolecular forces. These forces are significantly weaker than ionic and covalent bonds because of their nature of being interactive forces between compounds rather than physical bonds. Hydrogen bonds are the strongest of said forces and overall third strongest in all bond interactions with a dissociation energy of 12-16 kcal/mol. Dipole Dipole interactions are the second strongest intermolecular force but the fourth strongest bond interaction with a dissociation energy of 0.5-2 kcal/mol. Lastly comes the london vander waal forces that the weakest interaction in chemical bonding with a dissociation energy of <1 kcal/mol.

References

Berg, Jeremy M. Biochemistry. 6th ed. W.H. Freeman, 2007.

Introduction

This is a common form of a covalent bond where the hydrogens both share one electron each

Covalent bonds are chemical bonds that are formed by sharing valence electrons between adjacent atoms. This type of bonding is mostly seen in interactions of non-metals. Covalent bonds allow elements the ability to form multiple bonds with other molecules and atoms - a fundamental necessity for the creation of macromolecules. In the covalent bond, as the distance between the nuclei decreases, each nucleus starts to attract the other atom's electron, which lowers the potential energy of the system. Anyway, when the attraction increases, the repulsions between the nuclei and between the electrons increase as well. In covalent bonding, each atom achieves a full outer (valence) level of electrons. Each atom in a covalent bond counts the shared electrons as belonging entirely to itself. Most covalent substances have low electrical conductivity because electrons are localized and ions are absent. Overall, the atoms in a covalent bond vibrate, and the energy of these vibrations can be studied with the IR spectroscopy.

Octet Rule

A general rule to follow when looking at covalent bonding is the octet rule, also known as the noble gas configuration. An atom participating in covalent bonding must (with few exceptions) follow the octet rule, which states that an atom must have eight electrons around it. These electrons can be shared or unshared. The two atoms do not need to share their electrons equally; an electron pair can be donated from one atom instead of each atom donating one electron. A periodic table can be used to determine the number of valence electrons an atom. The general rule is that all atoms will be stable if they can have eight electrons around them. Therefore different atoms can share their unpaired electrons with other atoms with unpaired electrons to gain an octet.

There are quite a few exceptions to this rule. Two very important ones are Hydrogen (H) and Helium (He). These atoms do not have octets and only need a total of two electrons to be stable. This is because hydrogen and helium only contain a 1s electron shell, which can only hold two electrons. Other exceptions occur when the total number of electrons in a molecule or between two molecules is an odd number. These molecules tend to be very reactive. Also, atoms past the second row on the periodic table can have more than eight electrons surrounding them. For example, in phosphorus pentafluoride (PF₅) the phosphorus is bonded to 10 electrons, and in sulfur hexafluoride (SF₆) the sulfur atom is bonded to 12 electrons. Molecules can also be electron deficient, meaning there are not enough available electrons to complete full octets around all the atoms in the molecule. An example of an electron deficient molecule is boron trichloride (BCl₃). In this molecule, the boron atom is only bonded to three electron pairs, while the chlorides are surrounded by full octets.

Types of Covalent Bonds

Multiple covalent bonds can be formed between atoms, which are stronger than single bonds and have higher bond energy and shorter bond lengths. The bond order is used to determine the number of pairs of electrons in a covalent bond. When a molecule has double and single covalent bonds, it can have different chemical forms of equal energy as resonance structures, which has more stability and the bond is the average of the double and single covalent bond. The characteristics of a covalent bond can also be effected by the two atoms it joins.

Single Bonds

An electron dot diagram of a covalent bond between chlorine and hydrogen

Single bonds are one of the weaker types of covalent bonds. Single covalent bonds are also called sigma bonds. These are made when only two electrons are shared. This leads to an overlap of the orbitals and a merging of the electron density clouds. Single bonds tend to be very flexible allowing atoms to rotate around the bond. An example of a single bond is a carbon-carbon (C-C) covalent bond has a bond length of 1.54 A and bond energy of 356 kJ/mol.

Note that the properties of a single bond depends not only on the two atoms that is bonds but also on the atoms surrounding those atoms. Sigma bonds have no nodal planes.

Some covalent single bonds will also have double bonds properties, which are shorter, rigid and non-rotated. One example is peptide bond in proteins which connect each amino acid together to form polypeptide. The peptide-bond is 1.32 A which is shorter than 1.54 A (C-C). The energy that needs to break the peptide bond is much higher than the single bond and this non-rotated single bond contributes the planar property in the polypeptide chain, which also makes the peptide bond more stable than the normal single bond. The double bond properties are contributed by the resonance structure of the pepetide bond.

Double bonds

Formation of a Pi-Bond from two p-orbitals.

Double bonds occur when a covalent bond consists of four shared electrons. A double covalent bond contains a sigma bond and a pi bond. Pi-bonds apply to the overlapping p-orbitals. The orbitals can only overlap in a side-by-side arrangement leading to one nodal plane on the internuclear axis. A single covalent bond only contains a sigma bond. Double bonds tend to be shorter than their single bond equivalents and stronger. Double bonds also create electron density around the bond. Unlike single bonds, double bonds are not flexible and the two adjoining atoms cannot rotate about the bond.

Triple Bonds

C₂H₂.

A triple covalent bond contains one sigma bond and two pi bonds where six electrons are being shared. These bonds are stronger than double bonds and shorter. They are more rigid than double bonds and have a larger electron density. The most common triple bonds are on carbons like C2H2. The skeletal form to draw a triple bond is three straight lines connecting the two atoms.

Polar Covalent Bonds

Covalent bonds can be polar or non-polar depending on the electro-negativity value of the atoms bonded together. If there is a very large difference between the two atoms' electro-negativity values, a polar covalent bond is formed. The atoms do not need to possess the same electro-negativity values, or be of the same element, but they need to be relatively close in their values. If the electro-negativity values are closer, the co-valency between the atoms will be stronger. An exception to this rule is when a molecule possesses symmetry. When the overall dipole moment is zero, such as linear molecule of CO2, the molecule is considered non-polar. The more electro-negative atom will attract the electrons, making itself have a partial negative charge and giving the other atom a partial positive charge. These partial negative and positive charges are what account for the dipole-dipole, dipole-induce dipole, and induced dipole-induced dipole interaction. This attraction-to-repulsion stability is what gives the covalent bonds stability. In addition to the electro-negativity differences between atoms, covalent bonding depends on the angles of adjacent atoms relative to each other.

Typical accepted values for determination of type of bonds:

Difference in electronegativity - X < 0.5 - Non-polar covalent bond

Difference in electronegativity - 0.5 ≤ X ≤ 1.9 - Polar covalent bond

Difference in electronegativity - 1.9 < X - Ionic bond

Specific Types Of Covalent Bonds

Disulfide Bonds In Chemical interactions, certain compounds can react to create a disulfide bond, which is a type of covalent bond that is usually derived by the coupling of two thiols (-S-H). These interactions can also be called SS-bonds or disulfide bridges, with the connectivity of these interactions mainly being R-S-S-R .

Role in Protein Folding Disulfide bonds can play a vital role in the tertiary structure of proteins in the effect they have on protein folding and stability. These disulfide bonds between proteins usually are formed between the thiol groups of cysteine residues. The other amino acid group in which sulfur appears is methionine, which cannot form disulfide bonds.

Formation of a Disulfide Bond.

Disulfide bonds help to stabilize the tertiary structure of a protein molecule in several ways, for example, The disulfide bonds destabilize the unfolded form of a protein by lowering its overall entropy, or state or chaos. Also, when the disulfide bonds link two segments of the protein chain, this increases the effective local concentration of protein residues and lowers the effects of water in a that specific region. Since water molecules are known to attack amide-amide bonds, lowering the effects of water in these disulfide bond- regions helps to stabilize a protein.

Covalent Bond: Bond Length and Bond Energy

The bond energy (BE) is the energy required for the attraction or breakage between the atoms. Since it is the energy needed to break the attraction between the atoms, the bond energy is endothermic and positive. However, the energy required for the formation of the bond is exothermic and a negative value. The bond length is the distance between the nuclei of two covalent bonded atoms. It can be calculated based on the total radii of the bonded atoms. As a result, the bond length increases when the covalent radius increases. And the shorter the bond length, the higher bond energy will be needed to break the attraction between the atoms because shorter distance between the atoms means the bond will be stronger and harder to break. On the other hand, the longer the bond length is, the lower bond energy is needed to break a weaker bond. One can use bond energy to determine the ΔH_rxn. In a reaction, when two atoms react with each other to form the product of different atoms, there are two types of bond energy. One is the energy required for the reactant to be broken and the other one is the energy required for products to be formed. As a result, the difference between the two bond energy is the enthalpy or the work of the reaction. ΔH⁰_rxn= ΔH_{reactant bonds broken} + ΔH_{product bonds formed}

References

Silberberg, Martin S. Chemistry "The Molecular Nature of Matter and Change." Fifth Edition.

Background Information

A noncovalent bond is a type of chemical bond that typically bond between macromolecules. They do not involve sharing a pair of electrons. Noncovalent bonds are used to bond large molecules such as proteins and nucleic acids. Noncovalent bonds are weaker than covalent bonds but they are crucial for biochemical processes such as the formation of double helix. There are four commonly mentioned fundamental noncovalent bond types. They include electrostatic interactions, hydrogen bonds, van der Waals interactions, and hydrophobic interactions. Each type differs in geometry, strength, and specificity.

Types of Noncovalent Bonds

Electrostatic interactions

The energy of an electrostatic interaction is given by Coulomb’s Law: E = kq1q2 / Dr2 where E is the energy, q1 and q2 are the charges of two atoms, r is the distance between the two atoms, D is the dielectric constant, and k is a proportionality constant. k = 1389 for energy that’s measured in kilojoules/mol or k = 332 for energy that’s measured in kcal/mol. A charged group on a molecule can attract an oppositely charged group from another molecule. By contrast, an attractive interaction has a negative energy. The dielectric constant is important for the medium.

Hydrogen bonds

A hydrogen bond is the interaction of a hydrogen atom with an electronegative atom. The electronegative atom can be nitrogen, oxygen, fluorine that comes from another chemical group. Hydrogen bonds are responsible for specific base-pair formation in the DNA double helix. The hydrogen atom in a hydrogen bond is shared by two electronegative atoms such as nitrogen or oxygen. A hydrogen-bond donor is the group that includes an electronegative atom where the hydrogen atom is more tightly bound to and a hydrogen-bond acceptor is when an electronegative atom is less tightly bound to the hydrogen atom. The electronegative atom where the hydrogen atom covalently bonds can pull electron density away from the hydrogen atom creating a positive electronegativity charge. The hydrogen atom can also interact with an atom that has negative electronegativity charge.

van der Waals interactions

Van der Waals interaction is the distribution of electronic charge around an atom that fluctuates with time. It is the sum of the attractive or repulsive forces between molecules. The charge distribution is not perfectly symmetric. The attraction increases as two atoms come closer to each other, until they are separated by the van der Waals contact distance. When the distance of the energy is shorter than the van der Waals contact distance, a very strong repulsive force becomes dominant.

Hydrophobic interactions

Hydrophobic interaction is the physical property of a molecule that is repelled from a mass of water. They are also called hydrophobic exclusions. It is the tendency of hydrocarbons to form intermolecular aggregates in an aqueous medium, and analogous intramolecular interactions.

Ion Induced Dipole

An ion induced dipole is a noncovalent bond interaction that results when the approach of an ion induced a dipole in an atom or in a nonpolar molecule by distributing the arrangement of electrons in the nonpolar species.

Dipole Induced Dipole

A dipole-induced dipole interactions results a nonpolar molecule has it's electron density shifted by a charged molecule. If a negatively charged species described by its electron density (for example, in water electron density is centralized on the oxygen atom) is brought near a species with an evenly distributed charged molecule, the electrons go towards the side where the positive species is, and away from the opposite side, creating an artificial dipole.

http://upload.wikimedia.org/wikipedia/commons/thumb/a/a6/Dipole_interactions.png/320px-Dipole_interactions.png

References

Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman and, 2006. Print. A hydrogen bond is formed by a dipole-dipole force between an electronegative atom (the hydrogen acceptor) and a hydrogen atom that attaches covalently with another electronegative atom (the hydrogen donor) of the same molecule or of a different molecule. Only nitrogen, oxygen, and fluorine atoms can interact with hydrogen to form a hydrogen bond donor; this is different than a hydrogen covalent bond. The hydrogen bond acceptor however can be any atom which is in a polar bond, is electronegative, and has a lone pair. In a hydrogen bond, the lone pair electrons on oxygen, nitrogen, or fluorine interact with the partial positive hydrogen that is covalently bonded to one of those atoms. The hydrogen atom in a hydrogen bond is shared by two electronegative atoms such as oxygen or nitrogen.) Hydrogen bonds are responsible for specific base-pair formation in the DNA double helix and a major factor to the stability of the DNA double helix structure. A hydrogen-bond donor includes the hydrogen atom and the atom to which it is most tightly linked with. The hydrogen-bond also play a very important roles in proteins' structure because it stabalizes the secondary, tertiary and quaternary structure of proteins which formed by alpha helix, beta sheets, turns and loops. The hydrogen-bond connected the amino acides between different polypeptide chains in proteins structure. The hydrogen-bond acceptor is the atom that is less tightly linked to the hydrogen atom.

Hydrogen bonds are fundamentally electrostatic interactions and are much weaker than covalent bonds. They are, however, the strongest kind of dipole-dipole interaction. The electronegative atom to which the hydrogen atom is bonded with pulls electron density away from the hydrogen atom, developing a partial positive charge. Therefore, the hydrogen atom can then interact with a partial negatively charged atom through an electrostatic interaction.

Hydrogen bond with ammonia

Components

Hydrogen bonding is a form of electrostatic interaction between a hydrogen atom bonded to two electronegative atoms; one of which is the hydrogen-bond donor that has a stronger bond between itself and the hydrogen. These electronegative atoms are nitrogen, oxygen, and fluorine; this electronegative atom pulls electron density away from the hydrogen atom, giving it a partially positive charge. This partial positive charge is attracted to the partial negative charge of the hydrogen bond acceptor (an electron density rich atom). The chemical bond formed between the hydrogen-bond donor, hydrogen atom, and hydrogen-bond acceptor has a straight, linear structure.

H-bonding

Hydrogen bonding (H-bond) is a non-covalent type of bonding between molecules or within them, intermolecularly or intramolecularly. This type of bonding is much weaker and much longer than the covalent bond and ionic bonds, but it is stronger than a van der waals interaction. It also carries some features of covalent bonding: direct and straight. In other words, H-bond donor and H-bond acceptor lie along the straight line. In order to form an H-bond, an H-bond donor and H-bond acceptor are required. The H-bond donor is the molecule that has a hydrogen atom bonded to a highly electronegative, small atom with available valence (N, F, and O follow the above description the best because they are very electronegative, making H, which is covalently attached to them, very positive). The H-O, H-N, and H-F bonds are extremely polar; as a result, the electron density is easily withdrawn from the hydrogen atom towards the electronegative atom. The partially positive hydrogen in one molecule attracts to partially negative lone pair of the electronegative atom on the other molecule and H-bond forms as a result of such an interaction. All the hydrogen bonds vary in strength

Other important facts about hydrogen bonding are as follows. The small sizes of nitrogen, oxygen, and fluorine are essential to H bonding for two reasons. One is that it makes those atoms electronegative that their covalently bonded H is highly positive. Other reason is that it allows the lone pair on the other oxygen, nitrogen, or fluorine to come close to the H. Also, hydrogen bonding has a profound impact in many systems. Hydrogen bonding is also involves in the action of many enzymes [The Molecular Nature of Matter and Change].

Properties of Water Due to Hydrogen Bonding

Ammonia, water, and hydrogen fluoride all have higher boiling points than other similar molecules, which is due to hydrogen bonds. Bonds between hydrogen and these strongly electronegative atoms are very polar, with a partial positive charge on hydrogen. This partially positive hydrogen is strongly attracted to the partially negative oxygen on the adjacent molecule. In general, boiling points rise with the increase molecular weight, both because the additional mass requires higher temperature for rapid movement of the molecules and because heavier molecules have a greater London forces. Water's freezing point is also much higher than other similar molecules. An unusual feature is that it decreases in density when it freezes. The tetrahedral structure around each oxygen atom, with two regular bonds to hydrogen and two to other molecules. This requires a great amount of space between the ice molecules. Clathrates are molecules trapped in holes of solid, like ice, that is theorized to be able to be used as anesthesia.

Hydrogen bond and physical properties

Hydrogen bonding has a significant influence on a molecule's boiling points. The boiling point usually increases with the increase of the molar mass. However, molecules that are involved in intermolecular H-bonding bonding have much higher boiling points in comparison with the molecules of the same molar mass that are not involved in H-bonding. This is because the unusually strong H-bonding forces allow for stronger interaction between water molecules and therefore creating a stronger bond and higher boiling point. In addition, H-bonding is responsible for many unusual proprieties of water, such as its high boiling point, melting point, heat of vaporization, high dielectric constant, surface tension, capillary action etc.

Hydrogen bonding can occur between hydrogen and four other elements. Oxygen(most common), Fluorine, Nitrogen and Carbon. Carbon is the special case in that it only really interacts in hydrogen bonding when it is bound to very electronegative elements such as Fluorine and Chlorine.

Hydrogen bonding is an important component of the three major macromolecules in biochemistry such as proteins, nucleic acids, and carbohydrates. The H-bonding is responsible for the structure and properties of proteins(enzymes). Hydrogen bonding is applicable in these biomolecules because of functional groups present. Some such are the carboxylic acid, alcohol or even amine groups. These provide either an hydrogen, oxygen or nitrogen for possible hydrogen bonds.

Hydrogen bond in proteins

Hydrogen bonding within the "green fluorescent protein" 1RRX.

As previously mentioned, hydrogen bond can be intermolecular (ex. the bonding of water molecules) as well as intramolecular (ex. the bonding of protein and DNA). The secondary structure of protein forms as a result of H-bonding between amino acids. For example, an α-helix is a rod-like secondary structure that forms as a result of H-bonding between the carboxyl group of (i) amino acid to the amino group of (i+4) amino acid. The turn (loop family) is a secondary structure which forms as a result of H-bond between carboxyl group of (i) amino acid and amino group of (i+3) amino acid. The β-sheet is a secondary structure which forms as a result of H-bonding between two or more β-strands. An anti-parallel β-strands forms hydrogen bonds that are straight due to the carbonyl group and the amino group being directly aligned, while a parallel β-strand forms hydrogen bonds that are slightly weaker in comparison to the anti-parallel because the carbonyl group and the amino group don't align perfectly, which forms a longer and weaker hydrogen bond.

The solubility of proteins in water is dependent on the ability to form hydrogen bonds with the protein surface. Proteins that have a greater hydrophilic surface content are generally more capable of forming hydrogen bonds with the surrounding water. The alteration of salt concentration of the solution, as is performed in salting out/in, creates a shielding effect that reduces the ability to form an H-bond with the hydrogens in water. The protein precipitation method of salting out utilizes this concept in protein fractionation.

Hydrogen bonding in water

Hydrogen bonding in water

The simplest example of a hydrogen bond can be found in water molecules. A water molecule consists of one oxygen atom attached to two hydrogen atoms. A hydrogen bond can be formed between two molecules of water. In the case of liquid water where there are many water molecules present, each water molecule could potentially hydrogen bond with up to 4 other molecules (2 through its 2 hydrogen atoms with each hydrogen bonding to another oxygen and another 2 through its 2 lone pairs on the oxygen that can hydrogen bond to 2 other hydrogen atoms).

Although water has a low molecular mass, it has an unusually high boiling point. This property can be attributed to the large number of hydrogen bonds that exist within the water. Since these bonds are difficult to break, water’s melting point, viscosity, and boiling point are relatively high in comparison to other liquids that are similar but lack the hydrogen bonding. Water contains substantially more hydrogen bonds (up to 4) relative to certain other liquids that also have hydrogen bonding. An example would be ammonia in which the nitrogen only has one lone pair but 3 hydrogen atoms and thus only capable of forming up to 2 hydrogen bonds.

Hydrogen bonding can also explain why the density of ice is less than the density of liquid water. In water's liquid form, the hydrogen bonding that keeps the molecules close together is constantly being broken and remade repeatedly at room temperature. But as the water turns into ice, the hydrogen bonding causes the water molecules to form a rigid, lattice structure, which causes large gaps between the molecules, resulting in it's smaller density yet larger volume.

Hydrogen bonding also accounts for water's high surface tension. The large availability of hydrogen bonding between water molecules (4 hydrogen bonds to one water molecule) proves how well they can stick to each other, forming a strong and stretchy surface. Common examples from which this characteristic can be observed include a cup filled slightly over the top without spilling over, or small organisms that are able to stay on top of the water without breaking its surface.

Water has a different number of hydrogen bonds depending upon the temperature. It is estimated that at 0oC each water molecule has an average of 3.69 hydrogen bonds, while at 25oC it has an average of 3.59 hydrogen bonds, and at 100oC it has an average of 3.24 bonds. The decreasing hydrogen bonds with an increase in temperature can be attributed due to the increase of molecular motion.

Hydrogen bonding in DNA

DNA contains four bases: Guanine, Cytosine, Adenine, and Thymine. The complementary base pairs of guanine with cytosine and adenine with thymine connect to one another using hydrogen bonds. These hydrogen bonds between complementary nucleotides are what keeps the two strands of a DNA helix together. Each base can also form hydrogen bonds with the external environment such as with water. Although these internal and external hydrogen bonds are fairly weak, the consolidated power of all the millions of hydrogen bonds in DNA make it a stable molecule. Also, the hydrogen bonds on the phosphate groups on each nucleotide interact inducing two strands of DNA to conform to a helical structure.

The base pairing in the DNA (one purine and one pyrimidine base) can be explained in more details. In addition to holding the DNA strands together, the hydrogen bonding between the complementary bases also sequester the bases in the interior of the double helix. Therefore, the hydrogen bonding between the bases reinforces the hydrophobic effects that stabilize the DNA. The hydrophobic bases are again kept in the inside of the helix, whereas the polar exterior is touching the solvent water. The hydrogen bonding is a weak molecular force, but it is an additive effect that stabilizes the DNA molecule. The bases are precisely held by hydrogen bonding with the energy of 1 to 5 kcal/ mol (4 to 21 kJ/mol).

The hydrogen bonding in the DNA bases of one purine (guanine and adenine) and one pyrimidine (cytosine and thymine) creates a similar shape. The pairing of guanine and cytosine shape and structure is very similar to that of the pairing of adenine and thymine. Cytosine and Guanine are held together by three hydrogen bonds. The pairing of adenine and thymine share two hydrogen bonds, thus the bond is slightly weaker and slightly longer.

G-C hydrogen bonding

A-T hydrogen bonding

References

Silberberg, Martin S. Chemistry "The Molecular Nature of Matter and Change." Fifth Edition. 2009.

Berg, Jeremy; Tymoczko, John; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. 2007. (8) http://en.wikibooks.org/wiki/Structural_Biochemistry/Chemical_Bonding/Dipole_interaction http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/DNA http://en.wikibooks.org/wiki/Structural_Biochemistry/Chemical_Bonding/Covalent_bonds http://en.wikibooks.org/wiki/Structural_Biochemistry/Proteins http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid http://en.wikibooks.org/wiki/Structural_Biochemistry/Carbohydrates http://en.wikibooks.org/wiki/Structural_Biochemistry/Proteins/Structures http://en.wikibooks.org/wiki/Structural_Biochemistry/Proteins/Amino_Acids http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/Nitrogenous_Bases/Purines/Guanine http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/Nitrogenous_Bases/Purines/Cytosine http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/Nitrogenous_Bases/Purines/Adenine http://en.wikibooks.org/wiki/Structural_Biochemistry/Nucleic_Acid/Nitrogenous_Bases/Purines/Thymine http://en.wikibooks.org/wiki/Structural_Biochemistry/Water

Definition

The tendency of nonpolar molecules in a polar solvent (usually water) to interact with one another is called the hydrophobic effect. The interactions between the nonpolar molecules are called hydrophobic interactions. The relative hydrophobicity of amino acid residues is defined by a system known as hydrophobicity scales.

General Information

Hydrophobic oil immiscible in water

The interactions between nonpolar molecules and water molecules are not as favorable as interactions amongst just the water molecules, due to the inability of nonpolar molecules to form hydrogen bonding or electrostatic interactions. When nonpolar molecules are introduced to the water molecules, the water molecules will initially surround the nonpolar molecules, forming a "cages" around the molecules. However, the tendency of nonpolar molecules to associate with one another will draw the nonpolar molecules together, forming a nonpolar aggregate.

Based on the second law of thermodynamics, the total entropy of the system plus its surrounding must always be increasing. Therefore, it is favorable for the nonpolar molecules to associate without the interference of water. The water molecules that initially "caged" the nonpolar molecules are released from the nonpolar molecules' surfaces, creating an increase in entropy in the surrounding. The favorable release of water molecules from nonpolar surfaces is responsible for phenomenon of the hydrophobic effect.

Problem

Hydrophobic interactions can also be seen in the clustering of amphipathic/amphiphillic molecules such as phospholipids into bilayers and micelles. The hydrophobic areas of amphipathic molecules cluster together to avoid the ordered "cage" of water molecules that would surround them and orient the hydrophillic ends as a shield-like outer structure that interacts amicably with the polar water molecules. Micelles occur when a spherical fatty acids structure is formed with a hydrophobic core and hydrophillic outer shell. Bilayers can be commonly seen in cell membranes with hydrophillic outer (outside the cell) and inner (inside the cell) linings has hydrophobic (inside the membrane) center. The Lipid bilayer is a more favored formation in nature due to the micelle formation may contain bulky fatty acids causing hindrance in its formation.

Lipid Bilayer(1) and Micelle (2)

Electric Properties of Plasma Membrane

Most cell membranes are electrically polarized, such that the inside is negative [typically -60 millivolts (mV)]. Membrane potential plays a key role in transport, energy conversion, and excitability. For example, membrane transport. Some molecules can pass through cell membranes because they dissolve in the lipid bilayer. Additionally, most animal cells contain a high concentration of K1 and a low concentration of Na1 relative to the external medium. These ionic gradients are generated by a specific transport system, an enzyme that is called the Na1–K1 pump or the Na1–K1 ATPase. The hydrolysis of ATP by the pump provides the energy needed for the active transport of Na1 out of the cell and K1 into the cell, generating the gradients. The pump is called the Na1–K1 ATPase because the hydrolysis of ATP takes place only when Na+ and K+ are present. This ATPase, like all such enzymes, requires Mg2+

Solution

When two nonpolar molecules come together, structured water molecules are released allowing them to interact freely with bulky water. The release of water from such cages is favorable. The result is that non-polar molecules show an increased tendency to associate with one another in water compared with others - less polar and less self-associating solvents. This tendency is called the hydrophobic effect and the associated interactions are called hydrophobic interaction.

The release from the cage-like clathrates is more favorable because it increases the entropy of the system.

References

http://en.wikibooks.org/wiki/Structural_Biochemistry/Second_law http://en.wikibooks.org/wiki/Structural_Biochemistry/Water http://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Biological_Membranes Van Der Waals interactions (also known as London Dispersion forces) are weak attractions that occur between molecules in close proximity to each other. The basis of these interactions is that the distribution of electronic charge around an atom fluctuates with time. As two atoms come closer to each other, this attraction increases until they are separated by the van der Waals contact distance. When two molecules are too close to each other, the potential energy due to repulsion is very high, which means that it is unstable. There is repulsion even though the molecules are neutral, because there is an electron cloud surrounding each molecule. When these molecules get too close to each other, repulsion between the molecules occur. As the molecules move farther apart from each other, the potential energy due to repulsion decreases. This force was named after Johannes Diderik van der Waals, a Dutch physicist who studied them extensively.

An example of interaction energy-distance relation graph obtained from argon dimer. Because of London dispersion forces, the interaction energy greatly increases when the distance between two atoms is smaller than 3.8A

Van der Waals radius

Vander Waals attraction

Two molecules can interact by Van der Waals forces when they are at a certain distance apart. The molecules are stabilized by Van der Waals interaction at the Van der Waals contact distance because the potential energy of the system at this distance is at its lowest. In the potential energy diagram shown to the right, the minimum potential energy point corresponds to the Van der Waals contact distance.

As the distance between the molecules increases, the weakly bonded molecules lose their stability and are no longer affected by the Van der Waals forces due to their large distance apart. When the distance between the two molecules decreases, however, the stabilization is decreased as well due to the electrostatic repulsion between the molecules. This level of repulsion is felt more drastically and more intensely in Van der Waals interactions than in ionic interactions, where the level of repulsion is felt more gradually.

Energy in Van der Waals interaction

Energies associated with van der Waals interactions are quite small. Usually, they are about 2 to 4 kJ/mol per atom pair. When the surfaces of two large molecules come together, a large number of atoms are in Van der Waals contact, and the net effect, summed over many atoms pairs can be substantial. Macromolecules such as proteins and DNA contain numerous sites of potential van der Waals interactions that the cumulative effect of these small binding forces can be enormous; hence the most stable structure for macromolecules is that where weak interactions are maximized.

Van der waals interaction in proteins structure

In addition to hydrogen-bonds and disulfide bonds, protein structure can also be stabilized by Van der waals interactions. In the coiled-coil protein, there are Heptad repeat which form by the side chain interaction between each alpha helix; hepad-repeat is repeated in every 7th residues. If these repeating residues are hydrophobic, such as leucine, van der waals interaction will be formed to stabilize this protein structure.

References

Berg, Jeremy; Tymoczko, John; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. 2007. (8)

Nature of Dipole Bonding

The nature of dipole bonding begins when atoms differ in their electronegativity, which quantifies an individual atom's ability to attract electrons to itself. A classic example of an electronegative atom is fluorine.

When a covalent bond forms between two atoms, the electrons will be distributed between the two atoms unequally; the more electronegative atoms will have the larger electron density. This unequal sharing of electrons creates a charge separation, and the molecule under inspection will develop partial charges where the electronegative atom will develop a partial negative charge and its adjacent atom will develop a partial positive charge. The molecule is then said to be polarized due to this charge separation. When molecules exhibit this charge separation, there is a pseudo-electrostatic force between the partial charges of molecules.

The key to dipole bonding is charge separation within a molecule.

Permanent dipole-dipole interactions

Dipole moment

When the covalent bonds in a molecule are polarized so that one portion of the molecule experiences a positive charge and the other portion of the molecule experiences a negative charge. This separation of opposite charges creates an electric dipole. Depending on the orientation of two dipoles, molecules can be attracted to each other as the partial negative charge is attracted to the partial positive charge.

• Ex. Carbon Monoxide (CO)

The permanent and induced dipole interaction of nonpolar molecules bonded to nonpolar molecules in polar molecules, which forms more stable structure.

Besides, when the polar molecules lie near one another, as in liquids and solids, their partial charges act as tiny electric fields that orient them and give rise to dipole-dipole forces. So, the dipole-dipole forces is the positive pole of one molecule that attracts the negative pole of another. The greater the dipole moment, the greater the dipole-dipole forces between the molecules are.

Induced dipole interactions

Induced dipoles occur due to the fact that electrons are in constant motion within a molecule. For nonpolar molecules, the averaged charge distribution within a molecule would suggest that there is no permanent dipole. However, instantaneous charge distributions indicate the presence of a transient dipole. Imagine that if you take a photograph of a molecule, you can see an electron and a nucleus in a fixed arrangement. This fixed arrangement implies that there is a dipole moment, if only for a brief moment. We call this type of instantaneous dipole an induced dipole. Brief partial negative charges will be attracted to brief partial positive charges.

Induced dipoles are typical of nonpolar molecules such as fatty acid chains, aliphatic chains, and aromatic hydrocarbons.

Hydrophobic interactions

Hydrophobic effect

When nonpolar molecules clump together when surrounded by polar molecules.

• The nonpolar molecules are not held together by bonds which is why it is called an instantaneous dipole-induced dipole interaction. This phenomenon is due to the reduction of the number of water molecules surrounding the hydrophobic portion of molecules.

clathrate

The structure that is formed when hydrophobic interactions take place (when the hydrophilic molecules line the outside of the hydrophobic molecules). This forces the hydrophobic molecules to become highly ordered

• Example: When water molecules surround lipids

NaCl forming an Ionic Bond.png

An Ionic Bond is a specific type of chemical bond formed between a "metal" and a "nonmetal." "Metals" involved in ionic bonds are usually the Alkali and Alkaline-Earth metals - also known as the first two columns on the period table - as well as several transition state metals. The non-metals usually involved in the ionic bonds are the halogens.

The goal of forming chemical bonds is to have an octet. "Octet" means that an element has eight electrons. Another way of putting this phenomenon is to state that each element wants to have the electron configuration of a noble gas. For the non-metals, the goal is to achieve the electron configuration of the noble gas in the same row. For the metals, the goal is to have the electron configuration of the noble gas in the row directly preceding it. For example, chlorine wants the electron configuration of Argon while sodium wants that of Neon. Ionic bonds help achieve the octet because the metal effectively transfers its valence electrons to the nonmetal. In this way, both metal and nonmetal achieve noble-gas electron configurations.

However, pure ionic bonds do not exist. There is a level of covalent-bond character in each ionic bond. As a general rule of thumb, the larger the electronegativity difference between the metal and nonmetal, the more ionic the bond, and therefore, the less covalent the bond. Electronegativity is the ability to draw electron density while in the presence of another atom. The more electronegative the atom, the greater its ability to pull electron density towards itself.

Electrostatic interactions are also known as Charge-charge interactions and Ionic interactions. An electrostatic attraction exists when there are closely packed ions of opposite charges. An electrostatic repulsion is present between different ions that have the same charge.

Coulomb's Law

The force between two point charges can be calculated by Coulomb’s law,

F_1on2 = F_2on1 = kq₁q₂/r²

In other words, the bond energy is directly proportional to the charges of the two atoms and inversely proportional to the square of the distance between the two atoms. F has the unit N (Newtons), r is the distance between the center of the two-point charges in meter, q₁ and q₂ are the charges (in C, Coulombs) of each atom respectively. k is the constant, approximately equal $9.00x10^{9}$ N•m²•C⁻². If the force is negative (F < 0), it represents the existence of attraction since the only possible way to have F smaller than zero is if the sign of q₁ and q₂ are different, or opposite, meaning the sign of charges for these two atoms are opposite. In other words, if the force is positive (F > 0), it means the two charged atoms repel each other, due to the same sign of charges (both positive or negative charges.)

Ionic bond energy

The ionic bond energy between two-points charges can be calculated using the formula of electrical potential energy,

U_ele= Kq₁q₂/r²

In other words, the bond energy is directly proportional to the charges of the two atoms and inversely proportional to the distance (bond length in microscopic level) between the two charged atoms. U_ele has the unit J (Joules), r is the distance between the ion centers in nanometers, and q₁ and q₂ are the numerical ion charges. K is the constant, approximately equal $2.31x10$⁻¹⁹ J•nm. It is also equal to kQ² x 10⁻⁹ (Conversion to nanomemter, nm) or Q²/4$\pi \epsilon _{0}$ x 10⁻⁹ where k is approximately equal to $9.00x10^{9}$ Nm²C⁻² and Q is equal to the charge of a proton, $1.60x10$⁻¹⁹C; $\epsilon _{0}$ approximately equal to $8.85x10$⁻¹² C²N⁻¹•m⁻² or F•m⁻¹, representing the permittivity of free space.

Lattice Energy

Lattice energy (ΔH⁰_lattice) is the released energy from the interaction between ions. The positive sign of this energy indicates that the energy is needed for ions to form a solid. On the other hand, the negative sign of the energy indicates that the energy is needed for ionic solids to be separated into its gaseous ions. In addition, ionic interactions can be explained based on the lattice energy because the qualitative number of the energy indicates the ions' hardness, solubility, and melting point. Lattice energy can be predicted based on the effect of ionic size and ionic change. As one goes down the periodic table, the ionic radius increases. And as the radius increases, there would be a decrease in the electrostatic energy between the positively charged atom and negatively charged atom. As a result, there would be a decrease in the lattice energy. In addition, ionic charge can also determine the lattice energy. A greater amount of energy will be needed to bring a larger charged ions together than to form the smaller charged ions.

Introduction

A disulfide bond, also called an S-S bond, or disulfide bridge, is a covalent bond derived from two thiol groups. In biochemistry, the terminology R-S-S-R connectivity is commonly used to describe the overall linkages. The most common way of creating this bond is by the oxidation of sulfhydryl groups. (2 RSH → RS-SR + 2 H+ + 2 e-) This process of oxidation can produce stable protein dimers, polymers, or complexes, in which the sulfide bonds can help in protein folding. The process mostly occurs with the thiol groups in cysteine.

Formal depiction of disulfide bond formation as an oxidation.

Disulfide bonds can occur in two ways: intramolecularly and intermolecularly. Intermolecular disulfide bonds occur between polypeptide chains while intramolecular disulfide bonds occur within a polypeptide chain and are usually responsible for stabilizing tertiary structures of proteins. On the other hand, intermolecular disulfide bonds are attributed to stabilizing quaternary protein structures.

Disulfide Bonds in Proteins

Two cysteine residues can be linked by a disulfide bond to form cystine.

Disulfide bonds in protein membranes are found in both bacteria and eukaryotes. Extracellular proteins often have several disulfide bonds, whereas intracellular proteins usually lack them. In proteins, these bonds form between the thiol groups of two cysteine amino acids. Cross-linkage between multiple linear polypeptide chains is not uncommon in proteins. Most of the cross-linkages are from disulfide bonds formed by the oxidation of two cysteine amino acids. The result is a disulfide bond called cystine connecting the polypeptide chains. The cysteine amino acid group is the only amino acid capable of forming disulfide bonds, and thus can only do so with other cysteine groups. These bonds are responsible for the stabilizing the globular structure and are the strongest type of bond that a protein can possess and are one of the major forces responsible for holding proteins in their respective conformations, and therefore have an important role in protein folding and stability.

The typical bond dissociation energy of a disulfide bond ranks at 60 kcal/mole and has a bond length of 2.05 Å. Fairly low energy is required to produce rotations about the S-S bonds, thus these rotations are common. At dihedral angles near 90°, the bonds tend to be more stable. However, the bonds become significantly better oxidants at angles approaching 0° and 180°. Disulfide bonds have been identified in the protein folding in E. Coli. They are used in many processes, including DNA replication.

Disulfide Bonds in cyclic peptides

Most cyclic peptide bonds are formed between disulfide bonds. As a result, the denaturation of cyclic peptides can often be attributed to the stability of disulfide bonds. In the study with the peptide 1 (cyclo(1,4)-Cys-Gly-Phe-Cys-Gly-OH), where it was conducted in buffer solutions between pH 1-11 at 70 degrees C. It was found that the most stability came from pH ~ 3 and a Vshape between pH ~1-~5. As the pH goes from neutral to basic, degradation was found between Gly2-Phe3, which is due to the breaking of disulfide bonds.

Disulfide bonds hold proteins, such as this one, in its conformation

Making disulfides

Multiple ways to make disulfides

In the journal article “Multiple ways to make disulfides” by Neil J. Bulleid and Lars Ellgaard, they discuss how disulfides can be formed in the endoplasmic reticulum (ER), and the different enzymes that catalyze the pathways of formation. Disulfides increase the stability of the protein and also “regulate redox-dependent functions,” and over the years, our ideas of how disulfides form in proteins have drastically changed. [2]

Disulfides are created in the presence of enzymes in the protein disulfide isomerase (PDI) family. They act as a oxidizing agent, oxidizing the thiol group on a protein. If the protein's amino acid residues, specifically cysteines, are close to one another they will form a disulfide bond even if it is not properly folded. If a disulfide bond forms when the protein is not properly folded, they call this a non-native disulfide. This could be a misfolded protein, or it could be one of the intermediates before the protein folds into its native state. PDIs help non-native disulfides become native disulfides by acting as a catalyst to the isomerization process (they have to help brake the non-native disulfide bond so that the protein can finish folding properly before they can form the native disulfide bond) (Figure 1). [2]

Figure 1: http://imgur.com/sEvem

Ero1

Bulleid and Ellgaard studied an enzyme in yeast to understand how disulfides were formed de novo (Latin for “in the beginning”). The enzyme they studied was ER oxidoreduct in (Ero1p). Experiments showed that “Ero1p and the mammalian homologues ERO1(alpha) and ERO1(beta) are able to catalyze oxidation by coupling de novo disulfide formation to the reduction of oxygen to hydrogen peroxide (H2O2).” Ero1p was shown to oxidize PDI, which allowed PDI to exchange disulfides on the protein. Using knockout experiments they were able to show that while in yeast, knockout of Erop1 interrupted disulfide formation, in higher eukaryotes (for example, mice and humans) knockout of ERO1(beta) only caused misfolding in proinsulin and the double knockout of ERO1(alpha) and ERO1(beta) did not do much worse than just the knockout of ERO1(beta). In fact, after some time, “double knockout cells re-established normal ER redox conditions after a strong reductive challenge, albeit at a slower rate than in wild-type cells.” This tells us that ERO1 is not as necessary in higher eukaryotes as it is in yeast and implies that there are other pathways to forming disulfide bonds. [2]

PRDX4

Because hydrogen peroxide is produced when disulfides are formed via ERO1 catalysis, and H2O2 can cause damage biomolecules, Bulleid and Ellgaard believed there had to be other proteins in order to remove the H2O¬2¬. This is where peroxiredoxin (PRDX4) comes in. PRDX4 is a group of enzymes located in the ER that both removes H2O2 and also forms disulfides. In this process the peroxidatic cysteine in PRDX4 takes an oxygen from H2O2 to make water and a –SOH group, this then reacts with the adjacent –SH group to form a disulfide bond (Figure 2). This can now be exchanged with the –SH groups on some PDI proteins so it can then exchange with substrate proteins (Figure 1). [2]

Figure 2: http://imgur.com/oyzmc

References

1. He, HT. "Synthesis and chemical stability of a disulfide bond in a model cyclic pentapeptide: cyclo(1,4)-Cys-Gly-Phe-Cys-Gly-OH." The University of Kansas, Lawrence, Kansas 66047, USA.

2. Neil J. Bulleid, Lars Ellgaard, Multiple ways to make disulfides, Trends in Biochemical Sciences, Volume 36, Issue 9, September 2011, Pages 485-492, ISSN 0968-0004, 10.1016/j.tibs.2011.05.004.(http://www.sciencedirect.com/science/article/pii/S096800041100082X)

Overview

Researchers initially thought that enzyme ERO1, endoplasmic reticulum oxidoreductin 1, couples oxygen reduction to de novo formation of disulfides, but it was recently discovered that mammals that are deficient in this enzyme still survive and form disulfides. This suggests that there exist alternative pathways to forming disulfides. Discoveries have found that peroxiredoxin 4 is involved in peroxide removal and disulfide formation. Many other different pathways for disulfide formation in the mammalian ER include quiescin sulfhydryl oxidase, ER-localized protein disulfide isomerase peroxidases and vitamin K epoxide reductase. These various pathways for forming disulfides are regulated by glutathione.

Disulfide-bond

Disulfide formation in the endoplasmic reticulum

Many important secretory and cell-surface proteins like antibodies, plasma membrane receptors and channels, extracellular matrix proteins, and blood clotting factors contain disulfide bonds because disulfides heighten protein stability and control redox-dependent functions. Disulfides in the endoplasmic reticulum undergo a procedure catalyzed by membranes of the PDI, protein disulfide isomerase, family and then undergo co-translational translocation to the ER. As the polypeptide starts to fold, cysteine residues that come into close contact form disulfides, even if they do not form in the end product. These disulfides that do not end up folding into the final product are sources of problems for misfolded proteins but may also serve as intermediates in normal folding processes. In order to obtain the final correctly folded disulfide, incorrectly folded disulfides must be disintegrated in a reaction catalyzed by the PDI family. The PDI family thus plays a vital role in the course for the correct formation and reduction of disulfides in order for proper folding of proteins that enter the endoplasmic reticulum

The correspondence of a disulfide between the enzyme and substrate is needed to cause a catalytic reaction to form a disulfide. Since PDI family members each accommodate at least one thioredoxin domain, a CXXC motif alternates between dithiol and disulfide states at their active site. When a disulfide is transferred to the substrate protein, a reduction of the active site occurs. It is necessary that the active site be deoxidized in order for the enzyme to carry out further oxidation. It was initially thought that GSSG, glutathione disulfide, was relevant in how the active site is reoxidized. In Vitro experiments were conducted and showed that a GSH/GSSG ratio similar to that found in the endoplasmic reticulum would efficiently oxidize active site cysteines in PDI, thus transferring disulfides onto substrate proteins. These experiments didn’t show how disulfides were introduced de novo. With the revelation of ERO1p, an enzyme in yeast called endoplasmic oxidoreduction, it was found that this enzyme was necessary for the formation of disulfide. ERO1p plays an important role in oxidizing PDI instead of secreting proteins or low-molecular-weight molecules like GSH and catalyzing oxidation by coupling de novo disulfide formation to the reduction of oxygen to hydrogen peroxide. While securely regulated in order to prevent overproduction of reactive oxygen species, ERO1 showed how disulfides could be formed de novo and also identified the ultimate electron acceptor for the pathway.

Ero 1 is essential in yeast but not in higher eukaryotes

Knockout of the gene encoding ERO1p in various organisms showed different conclusions of its importance. When the gene encoding ERO1p was knocked out in yeast, it demonstrated its importance as an essential protein in yeast. Knockouts in higher eukaryotes showed different results. When knocked out in D. mlanogasterI, this led to a relatively mild phenotype with a certain problem in its folding of the cell-surface receptor Notch. Two ERO1 paralogs, pair of genes ERO1α and ERO1β, exist in mice and humans. When knocked out of ERO1 β, a defect in the folding of proinsulin occurred. ERO1α is thought to drive disulfide development in other tissues. It was found that a double knockout of ERO1α and ERO1β did not result in a more severe reaction than when ERO1 β is knocked out by itself. This showed that there existed an ERO1-independent pathway for the formation of disulfides in mammalian cells and double knockout cells helped re-establish normal endoplasmic redox conditions.

Potential pathways to generate disulfides de novo in the ER

PRDX4

Since H2O2 is also generated when a reaction is catalyzed by ERO1, this indicates that additional proteins might be present in the endoplasmic reticulum to remove this reactive oxygen species. Enzymes called peroxiredoxins metabolize H2O2 ensuing disulfide formation. PRDx4 is active in both H2O2 removal and disulfide formation. Studies show a new responsibility for PRDx4 in de novo disulfide formation. Since PRDX4 and several PDI family members within the ER and equivalent, this shows that the enzyme is an abundant endoplasmic resident protein. Under circumstances where PRDX4 was efficiently reduced by some PDI family members during incubation of the two proteins at equal concentrations, some PDI protein was oxidized. This showed that even though GSH was a reductant on its own, when a PDI is included, the efficiency of PRDx4 reduction is enhanced. This proposes that disulfide exchanged between PRDx4 and GSH to form GSSP depends on the presence of PDI. If PRDX4 were to be reduced by PDI family members, a rapid disulfide formation in secretory proteins could occur. These results are confirmed by in vitro evidence. Since PRDX4 enhances a temperature-sensitive mutant of ERO1, viability and disulfide formation in yeast occurs at non-permissive temperatures. However, overexpression of PDI family members can also lead to decrease or increase in the ability of PRDX4 to be reduced in the endoplasmic reticulum. In conclusion, by combining the ERO1 and PRDX4 pathways, every oxygen molecule that is reduced causes two disulfides to be introduced, therefore making the whole process more effective than using ERO1 alone.

GPX7 and GPX8

GPX7 and GPX8, homologous enzymes, belong to the family of thioredoxin GPX-like peroxidases that can also reduce H2O2. They are PDI peroxidases that couple the reduction of H2O2 to oxidation of certain PDI family members. In the presence of these GPXs, certain PDI family members are easily oxidized. With the presence of both GPX and PDI, oxidative refolding of a reduced model protein mediated by H2O2 proceeds faster. Physical associations between ERO1α and both GPXs in cells are shown by bimolecular fluorescence complementation. With the addition of GPX7, the in vitro rate of oxygen consumed by ERO1α increased, indicating a more efficient process in its presence. These biochemical results show an important role for GPX7 and GPX8 in disulfide formation.

QSOX

Erx2p, a sulfhydrl oxidase, when overexpressed could suppress the ero1-1 mutation. This different pathway shows that alternative proteins could potentially fulfill the essential function of ERO1 in yeast and other organisms. QSOX, a flavoprotein known to introduce disulfides into proteins in vitro, is similar to Erv2p. QSOX plays a part in catalyzing de novo disulfide formation by coupling disulfide oxidation to the reduction of oxygen in order to form H2O2. Unlike ERO1, which specially oxidizes only PDI family members, QSOX has a much broader substrate specificity that can introduce disulfides into protein substrates. However, PDI has the ability to greatly enhance native disulfide formation since QSOX cannot isomerize non-native disulfides. QSOX has the ability to complement Δero1 yeast strain when overexpressed. This indicates that QSOX is involved in disulfide formation when in vivo. When ERO1 and QSOX are both knocked down, a more severe phenotype occurs than when ERO1 is knocked down alone indicating that QSOX might provide some function when ERO1 is absent. QSOX currently remains a candidate for de novo disulfide formation independent of ERO1 due to its promiscuous substrate specificity and location in the secretory pathway.

VKOR

The VKOR enzyme exhibits another potential ERO1-independent pathway for disulfide formation . VKOR is a four transmembrane helix protein in the endoplasmic reticulum. VKOR functions by catalyzing the two steps in the reduction of vitamin K epoxide so as to generate vitamin K hydroquinone. When vitamin K epoxide is reduced, a CXXC motif in VKOR is then oxidized to form a disulfide bond. Members in VKOR family exchange this disulfide with thiredoxin-like oxidoreductases in order to oxidize substrate proteins. Since human VKOR does not contain a thioredoxin domain, PDI family members instead serve as VKOR substrates. WHen overexpression occurs with active-site CXXA mutants, VKOR can be trapped in a mixed-disulfide complex with PDI family members, mainly the transmembrane-bound TMX and TMX4. Results of these experiments show that VKOR sustaining disulfide formation is still unclear but proves a potential pathway.

Relative contribution and interplay between oxidative pathways

With all these potential pathways for disulfide formation, now their relative contribution has to be figured out. ERO1 pathway is an important pathway for disulfide formation under normal physiological conditions. TRDX4 hyperoxidation in cells expresses a deregulated form of ERO1 and shows that the enzyme is active in the ER and has the ability to produce hydrogen peroxide. Even though mammals do no necessarily need this pathway, if it didn’t exist, it would compromise disulfide formation. ERO1 is important in determining the redox status of PDI in cells and also forms mixed disulfides with PDI. This shows that ERO has the ability to oxidize PDI. If animals did not have ERO1 to survive and still formed disulfide-bonded proteins, this shows that other pathways exist. Their relative contributions still have to be explored.

PRDX4-dependent pathway doesn’t provide a significant contribution to disulfide formation. If it did, it would be predicted that the Prdx4 knockout mouse should have a severe phenotype and even though the Prdx4 knockout mouse is viable, it is sterile due to oxidative stress in the testis. PRDX4 pathway is then determined to be crucial for correct function of specific tissues though not essential for survival. If there were a higher presence of PRDX4 in higher eukaryotes, it would show mild phenotypes of ERO1 knockouts as an alternative source of hydrogen peroxide that is available in order to make the PRDX4 pathway work. More research is needed to determine which sources provide the H2O2 necessary to drive disulfide formation. The importance of the PRDX4 pathway needs to be determined, even if PRDX4 provides an alternative pathway to ERO1 for disulfide formation.

An endogenous level, enzymes GPx7 and GPx8 cannot substitute for PrdxIV in ERO1 knockout cells. These enzymes could have a similar function to PRDX4. Research shows that proteins of the peroxiredoxin family have a higher rate of reactivity towards H2O2than GPXs. Evidence shows that interaction between Ero1α with the GPXs in vivo might compensate for their relatively slower rates of reactivity. Further understanding of their characterization is needed of the enzymes in order to explain their roles in de novo disulfide formation. Current research shows that the cellular function of VKOR in disulfide formation remains essentially uncharacterized. In VKOR, the mixed disulfide trapped with PDI family members does suggest a role in the process though. The pathway of oxidation of PDIs by VKOR being coupled strictly to γ-carboxyglutamate formation is not important due to its low flux. This would not be the case if the hydroquinone could be reoxidized through an alternative electron acceptor. This could bring about a more important role in the pathway.

As shown in vitro experiments, direct oxidation by H2O2 of cysteines in PDI and substrate proteins to form disulfides could potentially occur. Although, these pathways are shown to not be important due to faster kinetics of substrate refolding when the reaction mixture contains H2O2 and PDI family members together with either of the PDI peroxidases or PRDX4. Dehydroascorbate is an addition source of oxidizing equivalents in the ER. DHa can be moved into the ER from the cytosol, generated in the ER, and like H2O2, DHA directly oxidizes PDI and unfolds reduced proteins in vitro . Due to its slow rate, the former pathway is shown to not be a main route for the reduction of DHA. A faster pathway occurs with a faster PDI-independent oxidation of protein substrates.

References

Bulleid NJ, Ellgaard L. Trends Biochem Sci. 2011 Sep;36(9):485-92. Epub 2011 Jul 19. Review. PMID: 21778060 [PubMed - indexed for MEDLINE]a http://www.ncbi.nlm.nih.gov/pubmed/21778060

Overview

Valence bond theory – this is used to describe hybrid orbitals and electron pairs. It is an extension of the electron dot and bybrid orbital representations. Crystal field theory – this is used to describe the split in metal d-orbitals, which approximates the energy levels from the ultraviolet and visible spectra, but it does not describe bonding Ligand field theory – this is an expansion of the crystal field theory that can be used to describe bonding between the metal ion and the ligands by focusing on the orbital interactions. Angular overlap method – this is used to estimate the orbital energies in a molecular orbital calculation.

Valence Bond Theory

Valence bond theory was originally proposed by Pauling as a way of hybridization between atomic orbitals. This was one of the major theory used to describing the bonding of coordination compounds, but it's rarely used today. The valence bond theory is filled by the Aufbau principle, which states that electrons are most stable when they filled each orbital with one electron before further filling it. This leads to the Madelung rule, which states that orbitals filled by n+l are filled first before orbitals filled by n+l of higher energy orbitals.

Period 4

Period 5

Period 6

Period 7

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Lanthanum

57

[Xe] 6s2 5d1

Actinium

89

[Rn] 7s2 6d1

Cerium

58

[Xe] 6s2 4f1 5d1

Thorium

90

[Rn] 7s2 6d2

Praseodymium

59

[Xe] 6s2 4f3

Protactinium

91

[Rn] 7s2 5f2 6d1

Neodymium

60

[Xe] 6s2 4f4

Uranium

92

[Rn] 7s2 5f3 6d1

Promethium

61

[Xe] 6s2 4f5

Neptunium

93

[Rn] 7s2 5f4 6d1

Samarium

62

[Xe] 6s2 4f6

Plutonium

94

[Rn] 7s2 5f6

Europium

63

[Xe] 6s2 4f7

Americium

95

[Rn] 7s2 5f7

Gadolinium

64

[Xe] 6s2 4f7 5d1

Curium

96

[Rn] 7s2 5f7 6d1

Terbium

65

[Xe] 6s2 4f9

Berkelium

97

[Rn] 7s2 5f9

Scandium

21

[Ar] 4s2 3d1

Yttrium

39

[Kr] 5s2 4d1

Lutetium

71

[Xe] 6s2 4f14 5d1

Lawrencium

103

[Rn] 7s2 5f14 7p1

Titanium

22

[Ar] 4s2 3d2

Zirconium

40

[Kr] 5s2 4d2

Hafnium

72

[Xe] 6s2 4f14 5d2

Rutherfordium

104

[Rn] 7s2 5f14 6d2

Vanadium

23

[Ar] 4s2 3d3

Niobium

41

[Kr] 5s1 4d4

Tantalum

73

[Xe] 6s2 4f14 5d3

Chromium

24

[Ar] 4s1 3d5

Molybdenum

42

[Kr] 5s1 4d5

Tungsten

74

[Xe] 6s2 4f14 5d4

Manganese

25

[Ar] 4s2 3d5

Technetium

43

[Kr] 5s2 4d5

Rhenium

75

[Xe] 6s2 4f14 5d5

Iron

26

[Ar] 4s2 3d6

Ruthenium

44

[Kr] 5s1 4d7

Osmium

76

[Xe] 6s2 4f14 5d6

Cobalt

27

[Ar] 4s2 3d7

Rhodium

45

[Kr] 5s1 4d8

Iridium

77

[Xe] 6s2 4f14 5d7

Nickel

28

[Ar] 4s2 3d8 or [Ar] 4s1 3d9 (disputed)

Palladium

46

[Kr] 4d10

Platinum

78

[Xe] 6s1 4f14 5d9

Copper

29

[Ar] 4s1 3d10

Silver

47

[Kr] 5s1 4d10

Gold

79

[Xe] 6s1 4f14 5d10

Zinc

30

[Ar] 4s2 3d10

Cadmium

48

[Kr] 5s2 4d10

Mercury

80

[Xe] 6s2 4f14 5d10

Crystal Field Theory

Crystal field theory was originally developed to describe the structure of metal ions in crystals. The energies of the d orbitals are split by electrostatic field. This was developed in the 1930s, which ignored the covalent bonding since ionic crystals didn't describe it. When the ligands come close to the metal, a big destabilization occurs. The dx^2-y^2 and dz^2 have eg symmetry whereas the dxy, dxz, dyz have t2g symmetry, which implies that they're triply degenerate. eg accounts for 40% of the energy difference while t2g accounts for 60% of the energy. While eg's energy rises, t2g's energy falls to accounts for the difference in rising so that their energies cancel out. This energy difference is called crystal field stabilization energy. This also leads to the idea of high spin and low spin. High spin occurs when Δo > the pairing energy with a weak ligand field and low spin occurs when Δo < pairing energy with a strong ligand field.

Factors that influence Δo: 1) charge : the greater the charge on the central ion, the greater the pull from the ion, which results in an increase in oxidation state.

2) identity of the metal : the greater #d orbitals will have a greater Δo. For example, the 5d orbital can interact more efficiently than 3d, which results in a greater Δo.

3) identity of the ligand : stronger Lewis bases have greater Δo

Ligand Field Theory

Ligand field theory is used to describe ligand-metal orbital interactions. The metal's d-orbitals match the irreducible representations Eg and T2g. The metal's s and p orbitals have the symmetry A1g and T1u. This leads to a total of 3 bonding orbitals.

Coordination Numbers

Factors involved in determining the overall shape of a coordination compound

1) the number of bonds – since most bonds are exothermic, the creation of bonds will lead to a greater stabilization of the structure

2) VSEPR considerations

3) Occupancy of d-orbitals – the number of d electrons affect the geometry of the coordination compound

4) Steric interference – occurs when large ligands surround the central metal

5) Crystal packing effects – the regular shape is distorted when packed into a crystal. This would hinder the understanding of the original structure since it'd be unclear as if the distortion came from the crystallization process or naturally.

Coordination number 4 – this is usually obtained from square planer and tetrahedral structures with four ligands. Some examples are: CrO4 2-, Ni(CO)4, [Cu{py)4]+. Coordination number 5 – this is usually obtained from trigonal bipyramid, square pyramid, and pentagonal planes. Some examples are: Fe(CO)5 and PF5. Coordination number 6 – this is the most common coordination number since it is usually obtained from the structure of an octahedral. Some examples are: [Co(en)3]3+ and [Co(NO2)6]3-. Coordination number 7 – this is usually obtained from pentagonal bipyramid, capped trigonal prism, and capped octahedron. Some examples of molecules in this structure are: [NiF7]2- and [NbF7]2-. Coordination number 8 – this is usually only obtained in simple ionic lattice structures like CsCl.

References

Miessler, Gary. Inorganic Chemistry. 4th Edition.

Essentiality of Aluminum and Silicon

Darwin, natural selection and the biological essentiality of aluminum and silicon

By definition, natural selection is a competition in which winners and losers are defined by selection pressures which act upon competitors that are constrained within specific boundaries or arenas. Through experimentation and discovery, it has been assessed that natural selection can be viewed as a force of nature which is as important in biochemical evolution as it is in speciation. Exley takes a closer look at the biological essentiality of both aluminum and silicon. Aluminum in particular is deemed critical and also the most widely abundant metal on earth. In fact, it is the third most abundant element in the Earth’s crust. However, an element’s wide abundance does not correlate with its biological importance. Instead, aluminum is severely hindered by its biological unavailability. Selection of essential metals for biological use is mainly based on several factors including reaction kinetics and reaction thermodynamics. For example, kinetic constraints involve how a biochemical reaction comes to equilibrium which dictate reaction kinetics and ultimately which biochemical pathways are most efficient or favorable and thus predominate. Equilibrium constants dictate properties of reaction products by ways of solubility equilibrium and complex stability.Living systems however concentrate on the importance of kinetics rather than thermodynamics. Specifically kinetics rely on concentration of reactants, products, competitors, and interferences which all aid living systems in its attempt to avoid chemical equilibrium. Thus, kinetics affect the natural selection of which metals become essential to living systems.

Natural Selection of Aluminum

This takes us to a more enhanced view at the metal of aluminum. Despite its abundance, it has no essential role in any biological system in an organism. Silicon, on the other hand, is second most abundant, but is indeed viewed as an essential element. It was later found evident that aluminum’s lack of an indispensable biological role could be attributed to its non-participation in addition to the possibility of simply being selected out of systems altogether. The absence of aluminum is actually quite unfortunate, due to its versatility as a biologically reactive element. With further investigation, it was deemed that the lack of aluminum was not because it was selected out of biochemical systems but because the lack of available biologically-reactive aluminum present for selection. The unavailability of aluminum can be explained by the lack of aluminum in biotic cycles. Less than .001% of aluminum which is cycled through abiotic processes such as rain-fuelled dissolution of mountains, is actually cycled through biotic processes. Even more puzzling is that there are no known biological mechanisms that specifically keep aluminum out from biota, nor are there "biological footprints" left behind in evolutionary encounters with biologically reactive aluminum.

Aluminum has the ability to bind to oxygen-based functional groups, participate in critical redox reactions, and serves the role as an excellent immunogen as an antigen to enable widespread use of particular vaccines. However, aluminum was not selected --due to slow ligand exchange rates. While it is able to bind to oxygen-based functional groups it does not do so quickly enough to efficaciously serve as a metal co-factor for enzymes. Also, the prevalence of the biologically reactive form of Silicon, Silicic acid, reacted with biologically reactive Aluminum Al3+ and thus reduced the amount of Al3+. Exley asserts due to this, other less abundant metals were able to outcompete aluminum.

Exley went on to study salmon, which unveiled an interesting fact about the emergence of silicic acid, which sought out to protect against the toxicity of aluminum. In more familiar terms, it can be said that silicic acid took geochemical control over the availability of aluminum. Silicic acid is the only available biological form of silicon because silicon’s bonds are extremely tough to break and is selected against when it comes to participation in reactions. Silicon is essential, however, but does not possess any biochemical importance. Silicic acid is a weak acid that participates in an interaction with aluminum hydroxide to produce HAS. In doing so, it was successful in significantly reducing the biological availability of aluminum and has further promoted less selection for metals as well. The metals, however, that were selected have been deemed essential. Metals such as Magnesium, Iron, Calcium, Zinc, and Copper have created a cloud over Aluminum’s head, metaphorically, of course. The aluminum environment has lately been progressing through human activities and a sort of biochemical evolution that must account for a biologically reactive aluminum.

Non-Selection of Silicon

While aluminum was selected against due to its lack of availability of its biologically reactive form Al3+, Silicon's biologically reactive form Si(OH)4 has always been available for selection. Silicon has been seen in biological systems such as in the form of silicic acid which can pass through permeable biological membranes mimicking water and protecting against the toxicity of Aluminum. However today, there is little evidence in biotic systems to prove the essentiality of silicon to living systems.

While there is no silicon biochemistry, silicon still asserts evolutionary pressures being an essential element to life. However the only form of biologically reactive Silicon exist in the form of Silicic acid. Silicic acid is a polyprotic weak acid which loses its first proton at relatively high pH 10. The majority of biochemical reactions occur around neutral pH which thus limits the bioorganic and bioinorganic chemistry of silicon which only exists as a small neutral molecule.

Sequence, Structure and Biophysical Properties of Proteins

Environmental pressures can shape the sequence, structure and other properties of proteins. Proteins which occupy extreme environments such as proteins which are present in high temperature environments need to be more stable. This stability is mostly determined by the thermodynamic stability of the protein which is dictated by the energy gap between the native state and the unfolded and/or the misfolded states.

Changes in sequence and structure that affect both native and unfolded states are both observed by thermophilic proteins. Characteristics such as higher compactness, tightly packed secondary structure are observed within these proteins. These features are naturally selected for higher tolerances to thermal environmental pressures.

Adaptation of Viruses

Viruses are another example to adaptation to extreme environments such as presence outside host cells. However once within a host, they must avoid countermeasures of the host and survive. Virus genomes are usually packed tightly rather than loosely packed. Viruses are also prone to high rates of mutation exemplified by overlapping reading frames in RNA viruses which enable a single mutation to affect more than one protein. Natural selection must select how the protein is packed and thus its sequence and thus the mutational tolerance of the virus. A virus while having a compact protein structure, it is not as compact as thermophilic proteins. While thermophilic proteins are resistant to mutation due to its tightly packed proteins, a virus with looser packed proteins can thus mutate more frequently observed in the high rates of viral mutations.

Reference

Tokuriki N, Oldfield CJ, Uversky VN, Berezovsky IN, Tawfik DS. Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100,Israel.

Darwin, natural selection and the biological essentiality of aluminium and silicon. Exley C. The Birchall Centre, Lennard-Jones Laboratories, Keele University, Staffordshire, ST5 5BG, UK.

Overview

Valence bond theory – this is used to describe hybrid orbitals and electron pairs. It is an extension of the electron dot and bybrid orbital representations. Crystal field theory – this is used to describe the split in metal d-orbitals, which approximates the energy levels from the ultraviolet and visible spectra, but it does not describe bonding Ligand field theory – this is an expansion of the crystal field theory that can be used to describe bonding between the metal ion and the ligands by focusing on the orbital interactions. Angular overlap method – this is used to estimate the orbital energies in a molecular orbital calculation.

Valence Bond Theory

Valence bond theory was originally proposed by Pauling as a way of hybridization between atomic orbitals. This was one of the major theory used to describing the bonding of coordination compounds, but it's rarely used today. The valence bond theory is filled by the Aufbau principle, which states that electrons are most stable when they filled each orbital with one electron before further filling it. This leads to the Madelung rule, which states that orbitals filled by n+l are filled first before orbitals filled by n+l of higher energy orbitals.

Period 4

Period 5

Period 6

Period 7

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Element

Z

Electron Configuration

Lanthanum

57

[Xe] 6s2 5d1

Actinium

89

[Rn] 7s2 6d1

Cerium

58

[Xe] 6s2 4f1 5d1

Thorium

90

[Rn] 7s2 6d2

Praseodymium

59

[Xe] 6s2 4f3

Protactinium

91

[Rn] 7s2 5f2 6d1

Neodymium

60

[Xe] 6s2 4f4

Uranium

92

[Rn] 7s2 5f3 6d1

Promethium

61

[Xe] 6s2 4f5

Neptunium

93

[Rn] 7s2 5f4 6d1

Samarium

62

[Xe] 6s2 4f6

Plutonium

94

[Rn] 7s2 5f6

Europium

63

[Xe] 6s2 4f7

Americium

95

[Rn] 7s2 5f7

Gadolinium

64

[Xe] 6s2 4f7 5d1

Curium

96

[Rn] 7s2 5f7 6d1

Terbium

65

[Xe] 6s2 4f9

Berkelium

97

[Rn] 7s2 5f9

Scandium

21

[Ar] 4s2 3d1

Yttrium

39

[Kr] 5s2 4d1

Lutetium

71

[Xe] 6s2 4f14 5d1

Lawrencium

103

[Rn] 7s2 5f14 7p1

Titanium

22

[Ar] 4s2 3d2

Zirconium

40

[Kr] 5s2 4d2

Hafnium

72

[Xe] 6s2 4f14 5d2

Rutherfordium

104

[Rn] 7s2 5f14 6d2

Vanadium

23

[Ar] 4s2 3d3

Niobium

41

[Kr] 5s1 4d4

Tantalum

73

[Xe] 6s2 4f14 5d3

Chromium

24

[Ar] 4s1 3d5

Molybdenum

42

[Kr] 5s1 4d5

Tungsten

74

[Xe] 6s2 4f14 5d4

Manganese

25

[Ar] 4s2 3d5

Technetium

43

[Kr] 5s2 4d5

Rhenium

75

[Xe] 6s2 4f14 5d5

Iron

26

[Ar] 4s2 3d6

Ruthenium

44

[Kr] 5s1 4d7

Osmium

76

[Xe] 6s2 4f14 5d6

Cobalt

27

[Ar] 4s2 3d7

Rhodium

45

[Kr] 5s1 4d8

Iridium

77

[Xe] 6s2 4f14 5d7

Nickel

28

[Ar] 4s2 3d8 or [Ar] 4s1 3d9 (disputed)

Palladium

46

[Kr] 4d10

Platinum

78

[Xe] 6s1 4f14 5d9

Copper

29

[Ar] 4s1 3d10

Silver

47

[Kr] 5s1 4d10

Gold

79

[Xe] 6s1 4f14 5d10

Zinc

30

[Ar] 4s2 3d10

Cadmium

48

[Kr] 5s2 4d10

Mercury

80

[Xe] 6s2 4f14 5d10

Crystal Field Theory

Crystal field theory was originally developed to describe the structure of metal ions in crystals. The energies of the d orbitals are split by electrostatic field. This was developed in the 1930s, which ignored the covalent bonding since ionic crystals didn't describe it. When the ligands come close to the metal, a big destabilization occurs. The dx^2-y^2 and dz^2 have eg symmetry whereas the dxy, dxz, dyz have t2g symmetry, which implies that they're triply degenerate. eg accounts for 40% of the energy difference while t2g accounts for 60% of the energy. While eg's energy rises, t2g's energy falls to accounts for the difference in rising so that their energies cancel out. This energy difference is called crystal field stabilization energy. This also leads to the idea of high spin and low spin. High spin occurs when Δo > the pairing energy with a weak ligand field and low spin occurs when Δo < pairing energy with a strong ligand field.

Factors that influence Δo: 1) charge : the greater the charge on the central ion, the greater the pull from the ion, which results in an increase in oxidation state.

2) identity of the metal : the greater #d orbitals will have a greater Δo. For example, the 5d orbital can interact more efficiently than 3d, which results in a greater Δo.

3) identity of the ligand : stronger Lewis bases have greater Δo

Ligand Field Theory

Ligand field theory is used to describe ligand-metal orbital interactions. The metal's d-orbitals match the irreducible representations Eg and T2g. The metal's s and p orbitals have the symmetry A1g and T1u. This leads to a total of 3 bonding orbitals.

Coordination Numbers

Factors involved in determining the overall shape of a coordination compound

1) the number of bonds – since most bonds are exothermic, the creation of bonds will lead to a greater stabilization of the structure

2) VSEPR considerations

3) Occupancy of d-orbitals – the number of d electrons affect the geometry of the coordination compound

4) Steric interference – occurs when large ligands surround the central metal

5) Crystal packing effects – the regular shape is distorted when packed into a crystal. This would hinder the understanding of the original structure since it'd be unclear as if the distortion came from the crystallization process or naturally.

Coordination number 4 – this is usually obtained from square planer and tetrahedral structures with four ligands. Some examples are: CrO4 2-, Ni(CO)4, [Cu{py)4]+. Coordination number 5 – this is usually obtained from trigonal bipyramid, square pyramid, and pentagonal planes. Some examples are: Fe(CO)5 and PF5. Coordination number 6 – this is the most common coordination number since it is usually obtained from the structure of an octahedral. Some examples are: [Co(en)3]3+ and [Co(NO2)6]3-. Coordination number 7 – this is usually obtained from pentagonal bipyramid, capped trigonal prism, and capped octahedron. Some examples of molecules in this structure are: [NiF7]2- and [NbF7]2-. Coordination number 8 – this is usually only obtained in simple ionic lattice structures like CsCl.

References

Miessler, Gary. Inorganic Chemistry. 4th Edition.

The science of studying carbon-containing molecules is known as organic chemistry. One of the properties of the carbon atom that makes life possible is its ability to form four covalent bonds with other atoms, including other carbon atoms. This binding ability with comes from having four electrons in the carbon’s outer shell, causing it to need four additional electrons for its outer shell to be full.

Role of Carbon in organic chemistry

In living organisms, carbon atoms most commonly form covalent bonds with other carbons and with hydrogen, oxygen, nitrogen and sulfur atoms. Bonds between two carbon atoms, between carbon and oxygen, or between carbon and nitrogen can be single or double in organic compounds. Bonds of a higher order between these atoms can be found in inorganic compounds however. The combination of carbon with itself and with different elements and different types of bonds allows a vast number of organic compounds to be formed from only a few chemical elements. This is made all the more impressive because carbon bonds may occur in configurations that are linear, ring like, or highly branched. Such molecular shapes can produce molecules with a variety of functions. One last feature of carbon that is important in biochemistry is that carbon bonds are stable at the different temperatures associated with life. This property arises in part because the carbon atom is very small compared to most other atoms, and therefore the distance between carbon atoms forming carbon – carbon bonds is quite short. Shorter bonds tend to be stronger and more stable than longer bonds between two large atoms. Thus, carbon atoms are compatible with what we observe about life today, namely that living organisms can inhibit environments ranging from the earth’s icy poles to deep-sea vents. Aside from the simplest hydrocarbons, most organic molecules and macromolecules contain functional groups – group of atoms with special chemical features that are functionally important. Each type of functional group exhibits the same properties in all molecules in which it occurs. For example, the amino group (NH₂) acts like a base. At the pH found in living organisms, amino groups readily bind H⁺ to become NH₃⁺, thereby removing H⁺from an aqueous solution and raising the pH.

Synthesis of Carbon-Carbon Bond

The synthesis of new carbon-carbon bonds in organic reactions is an important synthetic organic technique that leads to the production of artificial chemicals such as new drugs and plastics. In the carbonyl chemistry many synthetic techniques are based on natural processes for the formation of carbon-carbon bonds in biological systems. Some examples of organic reactions forming new carbon-carbon bonds include Aldol reactions, Claisen condensation, Diels–Alder reaction, and Michael reaction.

Claisen Condensation

An Aldol reaction is a powerful technique forming new carbon-carbon bonds in organic chemistry, since it unites two simple molecules into a complex one. The reaction combines two carbonyl compounds to form a new β-hydroxy carbonyl compound. Producsts of such reactions are called aldols, known as the product of aldehyde + alcohol. A typical Aldol reaction involve the nucleophilic addition of a ketone enolate to an aldehyde. Aldol condensation takes place when the aldol product lose a water molecule to form an α,β-unsaturated carbonyl compound. Nucleophiles that can be employed in the aldol reaction include the enols, enolates, and enol ethers of ketones, aldehydes, and other compounds carrying the carbonyl function, whereas the electrophilic reagent is usually an aldehyde or ketone. When different nucleophile and electrophile are used, the reaction is called a crossed aldol reaction. On the other hand, a reaction in which the same nucleophile and electrophile are employed is called aldol dimerization.

Typical Aldol Reaction Mechanism

A Claisen condensation occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, which results in a β-keto ester or a β-diketone. In Claisen condensation, attack of an ester enolate on a carbonyl group generates a new carbon-carbon bond. The reaction mechanism involves ester enolate formation by reacting ethyl acetate with a stoichiometric amount of reagents with ester function, nucleophilic addition of another ester molecule that furnishes a ketoester, elimination of the alkoxide group, and finally the deprotonation of ketoester followed by protonation upon aqueous work-up. The overall process is endothermic and all steps before the deprotonation of ketoester are reversible. The deprotonation of ketoester drives equilibrium, since it removes the base needed to catalyze the previous steps. To prevent transesterification, both the alkoxide and ester are usually derived from the same alcohol.

Claisen Condensation Mechanism

In a Diels-Alder reaction, a conjugated diene adds to a substituted alkene to yield substituted cyclohexane derivatives. Such reaction is a special case of cycloaddition reactions between pi systems; four conjugated atoms containing four pi electrons reacts with a double bond containing two pi electrons. The four-carbon component is called diene and the alkene added is called dienophile. The reaction is also called a [4+2]cycloaddition. This type of reaction can still be carried out in the absence of carbon in the newly formed ring. Diels-Alder reactions that are reversible are called the retro-Diels-Alder; for example, the decomposition reaction of the cyclic system.

Diels-Alder Cycloaddition Mechanism

The Michael reaction is the nucleophilic addition of a carbanion or another nucleophile to an alpha, beta unsaturated carbonyl compound. The stabilized anions derived from β-dicarbony compounds with α,β-unsaturated carbonyl compound leads to 1,4-additions. It is one of the most useful methods for the mild formation of new carbon-carbon bonds. A Michael addition is base-catalyzed and works with α,β-unsaturated ketones, aldehydes, and other carboxylic acid derivatives; they are known as Michael acceptors. A Michael donor is an electron-withdrawing group on the nucleophile such acyl and cyano. [Vollhardt] [8]

Michael Reaction Mechanism

Isomers in organic chemistry

Organic molecules also have isomers. Isomers are molecules that contain the same number of atoms and also the same kind of atoms. However, they have different bonding arrangements. Types of isomers include constitutional (or structural) isomers, and stereoisomers.

Constitutional isomers (structural isomers) are the compounds that have the same molecular formula but differ in how the atom are arranged and connected. Chain isomers, positional isomers, and functional group isomers are constitutional isomers.

Chain isomers

Example: Pentane and 2-Methylbutane

vs.

Positional isomers

Example: 3-Hexanone and 2-Hexanone

vs.

Functional isomers

Example: Ethanol and Dimethylether

Stereoisomers include conformational isomers and configurational isomers. Conformational isomers are compounds that posses the same molecular formula and atomic connectivity but differ in a rotation about a bond. In other words, conformational isomers can be interconverted by rotation about single bonds. They are not separable at room temperature. There are different kinds of conformational isomers. They are eclipsed, staggered, anti, and gauche conformations. Configurational isomers are those isomers which can only be interconverted by breaking bonds. There are two different types of configuration isomers. They are enantiomers and diastereomers. Enantiomers are non-superimposable mirror images. Diastereomers are non-superimposable non-mirror images.

Glucose and fructose, which both have the same chemical formula of C₆H₁₂O₆ but a different arrangement in atoms, would be a good example of constitutional/structural isomers. Enantiomers would be a good example of a type of stereoisomer. Stereoisomers are isomers that have exact same bonding between atoms, but differ in their specific spatial arrangements. For example, enantiomers are isomers that are mirror images of each other. They can be superimposed on each other (achiral) or not (chiral), and may exhibit either R (clockwise) or S (counter-clockwise) configurations. Each mirror image of a chiral molecule expresses different properties than its counterpart. A way to determine whether a molecule is chiral or not is by looking for and identifying chiral centers. A chiral center for carbon, will have four different groups bonded to it. It must be sp3 hybridized and be tetrahedral in shape. A molecule's chiral center can also be referred to as a stereocenter.

Fullerenes are organic molecules that consist only of carbon atoms.

The role of organic synthesis research in Biochemistry

Organic synthesis is the science of constructing molecules. There are two major areas of research in organic synthesis: exploratory and target oriented. Research in both fields requires innovation, imagination, and artistic creativity.

Exploratory research involves the development of new organic reactions. Many researchers in this field focus on the optimization of previously known reactions. There are many factors when developing new organic reactions such as reactant, solvent, temperature, pH, etc. The main goals for any researcher in this field are to maximize yield of desired product, minimize side reactions/products, and be reliable for a broad spectrum of starting material. The advancement of the methodology aspect of organic synthesis expands the tools and techniques used by target oriented research.

Target oriented research involves the development of organic molecules through a series of organic reactions. Researchers in this field use preexisting reactions and commercially available materials to synthesis desired products. “Target” molecules are either natural products or designed molecules. A linear synthesis (a linear series of reactions conducted one after another) will suffice with simple molecules. Other approaches such as convergent synthesis (independent synthesis of key intermediates) are used for complex molecular structures. Methods such as solid phase synthesis are exceptionally useful in the synthesis of proteins.

“Target” molecules are biologically, medicinally, and/or theoretically interesting products. The field of bioorganic synthesis began with the synthesis of urea by Friedrich Wöhler in 1828. Natural product synthesis has been recognized with Nobel Prize in Chemistry on several occasions. Target oriented natural products are immensely useful for medical research. Cancer inhibiting molecules have been found in several natural products including marine natural products.

New reactions are constantly being developed and optimized. Many of these reactions are particularly useful in the synthesis of drugs and biomarkers. Reactions such as the palladium cross-coupling reaction, winner of the 2010 Nobel Prize in Chemistry, have been immensely useful in the synthesis of drugs, biomarkers, and other useful molecules.

The simplest Suzuki coupling reaction involves a palladium cross-coupling reaction of phenylboronic acid and bromobenzene to yield phenylbenzene.

Straying away from traditional synthetic methods has brought monumental advancements in alternate molecule building techniques in the last few decades. An example of an innovative technique is solid phase synthesis, a method in which molecular building blocks are attached to a bead and the “target” molecule is obtained with a linear series of reactant solutions. There are several advantages of solid phase synthesis compared to traditional solvent-based synthesis. Some advantages are that functional groups can be easily protected and also it is easier to extract unwanted byproducts or reactants from the desired “target” molecule. Solid phase synthesis is exceptionally useful with the synthesis of peptides, deoxyribonucleic acid (DNA), and other sequence-based molecules.

“There is excitement, adventure and challenge, and there can be great art in organic synthesis” – R.B. Woodward (Nobel Prize, 1965)

Synthetic Approach to Activated Amino Acids

The use of coupling agents in peptide synthesis is way of activating the carbonyl carbon of one amino acid, thus rendering it more reactive to the adjacent amino acid's amine group. Dicyclohexylcarbodiimide and various uronium salts are predominantly used as coupling agents in the field [1]. Although the use of coupling agents isn't a daunting task, constructing an intrinsically activated amino acid would circumvent the use of these reagents, saving time, money, and reaction yield.

The general synthetic approach at obtaining an activated amino acid will be described herein. Reaction of a specific isocyanide with an arbitrary aldehyde yields and alpha-hydroxy indole species [2].

Indole

Converting the hydroxy function on this species to an amino group would yield an activated amino acid. Notice that the indole function is aromatically stabilized, acts as a great leaving group, and thus activates the carbonyl carbon. It is imperative that converted amine function is protected, so that cross reaction between monomers doesn't occur. Carrying out this task isn't easy, because many chemical syntheses require extreme conditions which aren't suitable for peptide synthesis. High temperature or pH fluctuations can easily break peptide bonds, rendering the targeted peptide destroyed. Therefore, mild conditions in carrying out this displacement are sought after. [9] [Gianneschi]

Click Chemistry

The ability of generating molecular modification with high selectivity is invaluable for studies of chemical and biological systems. Click chemistry is the chemical philosophy of synthesizing molecules from a core group of highly effective reactions developed by Sharpless, Finn, and Kolb in 2001. The inspiration for the development of Click chemistry came from the idea that nature tends to produce substances from smaller subunits. The logic behind click chemistry is to bind small molecular units together to produce products with reactions that proceed rapidly in high yields under ambient conditions. These reactions are required to have a high thermodynamic driving force that is orthogonal to other functional groups that may be present in biomolecules. Click chemistry reactions are effective for labeling biomolecules. They also proceed in biological conditions with high yield. An important aspect of the reactions is that they are bioorthogonal, meaning that they don’t react with functional groups in the biological systems. Some examples of Click chemistry reactions are (a) Azide-Alkyne Cycloaddition, (b) Copper-Free Azide Alkyne Cycloaddition, and (c) Staudinger Ligation shown in the schemes below.

The major goals of Click Synthesis were to simplify the methods of how molecules are synthesized and, consequently, improve the process of identifying and synthesizing molecules with biological importance. These methods have proven to be beneficial in modern drug development; particularly useful for in situ fragment-based drug design. In situ drug development via Click chemistry has been extended to the selective generation of potent inhibitors of carbonic anhydrase and HIV-1 protease. A schematic representation of the process of inhibitor development with aid of Click chemistry is shown below.

These techniques have been extremely useful in combinatorial drug development. The product of the azide-alkyne cycloaddition, triazole, is favorable in drug design because it possesses a variety of useful properties. The highly tuned properties of these reactions possessing high yields, selectivity, and ability to undergo transformation in mild (biological) environments allow the products to be directly analyzed for activity without purification (which is a significant shortcut!). Example of direct screening of click chemistry products with alpha-1,3-fucosyltransferase (fuc-T) show below.

Click chemistry designs include reactions that are opposed to the reaction with biological molecules. This is a very useful property for selectively labeling molecules to detect in biological systems. The Staudinger ligation and azide-alkyne cycloadditions have proven to be very helpful for tasks that were very challenging prior.

There are is an extensive range of applications, a brief description and example schematic depictions are presented below:

(1) Introduction of Unnatural Amino Acids Bearing Reactive Tags into Proteins.

(2) Labeling of Viral Surfaces.

(3) Incorporation of Labeled Probes onto Proteins via Post-Translational Modification.

(4) Labeling of Nucleotides for Imaging DNA and RNA.

(5) Derivatization of Lipid Probes.

(6) Activity- Based Protein Profiling.

The benefit of these methods is clear from the broad spectrum of applications in chemical biology. The scope of these reactions is quite broad and takes innovation of the researchers using these methods to maximize its potential. These methods expand to pure organic chemistry lab as well. Without constraints of biological environments, these reactions have obvious beneficial aspects with precise control of target structures. The progression of bioorthogonal chemistry (has and) continues to produce new and effect tools for the future of research. [Best]

Wöhler's Synthesis of Urea

Wöhler's synthesis is one of the examples of synthesis in which molecules are made. Carbon compounds, organic products, in this reaction are made from inorganic salt.

Pb(OCN)₂ + 2 H₂O + 2 NH₃ -----> 2 H₂N(C=O)NH₂ + Pb(OH)₂

Lead cyanate Water Ammonia Urea Lead hydroxide

The effect of acid on food digestion and stomach acid

The normal human stomach has about 0.02 M of HCl per day. If there's an increase of HCl in the stomach, the pH of stomach juice would falls from 2.5 to 1.0 . HCl would mess up the normal folded shapes of protein molecules in food. As a result, the acid would destroy many digestive enzymes in the stomach. Therefore, to protect itself from the increase of such acid, the cells in the stomach must work together to prevent it from happening. First, the stomach tissues are made of protein molecules with the interior covered with layers of gastric mucosa. When stimuli such as smelling and tasting activate the cells in the gastric mucosa, the signal molecule, histamine, would make the parietal cells to hide the acid juices in the stomach. As a result, there would be an increase in the acid juices production in the stomach. In order to prevent it from happening, active ingredients such as cimetidine, famotidine, and ranitidine help to reduce the acid by obstructing the histamine from helping the parietal cells.

References

http://en.wikipedia.org/wiki/Claisen_condensation

http://en.wikipedia.org/wiki/Aldol_reaction

http://en.wikipedia.org/wiki/Claisen_condensation

http://en.wikipedia.org/wiki/Diels%E2%80%93Alder_reaction

http://en.wikipedia.org/wiki/Michael_reaction

Vollhardt and Schore. Organic Chemistry. 6th edition. New York: W.H. Freeman and Company

http://en.wikipedia.org/wiki/Carbon%E2%80%93carbon_bond

http://en.wikipedia.org/wiki/Peptide_synthesis

Gianneschi, C. N., Rubinshtein, M., James, R.C., Kobayashi, Y., Yang, J., Young, J., Yanyan, J.M. Org. Lett., 2010, 12 (15), pp 3560–3563

Best, M.D. Biochemistry, 2009, 48 (6571), pp 6571–6584

General information

The term “functional group” is one that is used almost exclusively in organic chemistry. In organic chemistry, functional groups are a set of specific atoms within a molecule that determine the molecule’s overall reactivity and properties. Small differences in functional groups, like the difference between an aldehyde and a ketone, can result in drastic differences in the properties and reactivity of the molecules. Compounds that have the same functional groups will have the same type of reactivity, though small deviances will result if other substituents or functional groups are present on the molecule.

An example of a Grignard reaction

There are 7 important functional groups in the chemistry of life: Hydroxyl, Carbonyl, Carboxyl, Amino, Thiol, Phosphate, and aldehyde groups.

1) Hydroxyl group: consists of a hydrogen atom covalently bonded to an oxygen atom. The hydroxyl group is denoted by -OH in chemical structures and has a valence charge of -1 when in the hydroxyl ion form. It is present in Alcohols and Carboxylic Acid molecules.

2) Carbonyl group: is written as a covalent C=O double bond. It is a very polar molecule and the carbonyl carbon can serve as a reaction site for many reactions. It is present in Aldehydes, Ketones, Esters, Anhydrides, and Carboxylic Acids.

3) Carboxyl group: is the monovalent group -COOH, consisting of a carbonyl group bound to a hydroxyl group. It is the main functional group in organic acids (carboxylic acids), in which a proton can dissociate and lead to a strong stabilizing resonance that forms between the two oxygens.

4) Amino group: The amino group (–NH2) consists of one atom of nitrogen attached by covalent bonds two atoms of hydrogen, and a spot where it is attached to an alkyl group, or another hetero atom. This leaves a lone valence electron pair on the nitrogen which is available for bonding to another atom.

5) Thiol group: a functional group containing a sulfur atom bonded to a hydrogen atom. General formula: -SH. The amino acid cysteine contains a thiol group. It is the thiol derivative of a hydroxyl group, and can undergo similar reactions.

6) Phosphate group: A functional group or radical comprised of phosphorus attached to four oxygen (a diester), and with a net negative charge, thus represented as PO4-. The phosphate group is important in living things in varying ways. Firstly, it is an important structural backbone component of nucleotides, which is the basic structural unit of DNA and RNA. Secondly, it is used as an electron transfer component of energy-rich molecules, such as ATP. Thirdly, it is also bound to coenzymes like NADP / NADPH involved in anabolic reactions (such as photosynthesis in plants and lipid synthesis in animals). It is also a part of the hydrophilic head of phospholipids in biological membranes.

7) Aldehyde group: Aldehydes are the second compound containing the carbonyl group (C=O). In aldehydes, one of the two groups attached to the carbonyl carbon is an alkyl group, while the other is a hydrogen atom. This means that aldehyde carbonyl occur at the end of a chain of carbon atoms (i.e. in sugars, R-CHO is at the end of a chain, while a carbonyl inside the chain attached to two carbons is a ketone). They can form acetals and hemiacetals when exposed to water, or in the presence of hydroxy groups. When an aldehyde group is the highest priority functional group present in the molecule, it is names as an alkanal (note, the -e is dropped). The numbering scheme used will be the one that gives the carbonyl carbon atom the lowest possible number (i.e. number 1). Other functional groups are located by this numbering scheme. Since the carbonyl group is always at carbon number 1, there is no need to indicate the location. It is possible to have the -CHO group directly attached to a ring. If it is the highest priority group in the molecule, the ring is a substituent to the aldehyde and it is named as a cycloalkyl carbaldehyde or cycloakanecarbaldehyde (an older way is as a carboxaldehyde). There are a number of compounds which were named before IUPAC developed the standardized nomenclature rules. Many of these compounds are still referred to by these common names.

Other common functional groups include:

• Haloalkanes: can also be named as alkyl halides despite the fact that the halogens are higher priority than alkanes. The alkyl halide nomenclature is most common when the alkyl group is simple.

Example of Haloalkane

• Ethers: If both groups are simple alkyl groups, then the ether is usually named as alkyl alkyl ether. If the two alkyl groups are the same, then it's a dialkyl ether.
• Thiols: A sulfur-containing organic compound having the general formula RSH, where R is another element or radical.
• Alkenes: The root name is based on the longest chain containing both ends of the alkene unit, the C=C. The chain is numbered so as to give the alkene unit the lowest possible numbers. The locant for the first carbon of the alkene is used in the name.
• Alkynes: The alkynes are the third homologous series of organic compounds of hydrogen and carbon, where there is at least one triple-bond between the atoms in the molecules.
• Aromatic Compounds: a hydrocarbon containing one or more benzene rings that are characteristic of the benzene series of compounds.

Aromatic Compound With Alcohol Substituent

• Anhydrides: are used in the preparation of esters. Ethyl acetate and butyl acetate (from butyl alcohol and acetic anhydride) are excellent solvents for cellulose nitrate lacquers. Acetates of high-molecular-weight alcohols are used as plasticizers for plastics and resins. Cellulose and acetic anhydride give cellulose acetate, used in acetate rayon and photographic film. The reaction of anhydrides with sodium peroxide forms peroxides (acetyl peroxide is violently explosive), used as catalysts for polymerization reactions and for addition of alkyl halides to alkenes. In Friedel-Crafts reactions, anhydrides react with aromatic compounds, forming ketones such as acetophenone.

Anhydrides react with water to form the parent acid, with alcohols to give esters, and with ammonia to yield amides; and with primary or secondary amines, they furnish N-substituted and N,N-disubstituted amides, respectively.

Read more: http://www.answers.com/topic/acid-anhydride#ixzz2DkMTM1Db

Read more: http://www.answers.com/topic/acid-anhydride#ixzz2DkMNN84B

• Esters: is any of a class of organic compounds corresponding to the inorganic salts and formed from an organic acid and an alcohol.
• Nitriles: are typically undergo nucleophilic addition to give products that often undergo a further reaction. The chemistry of the nitrile functional group, C=N, is very similar to that of the carbonyl, C=O of aldehydes and ketones.

Basic Information

The hydroxy-group is one of many functional groups studied in organic chemistry. The presence of a hydroxy group indicates that the molecule is either an alcohol or a carboxylic acid. The chemical representation of a hydroxy group is ^–OH, indicating an oxygen atom covalently bonded to a hydrogen atom.

Within the general functional group of alcohol, there are three subgroups: primary, secondary, and tertiary alcohols. An alcohol is classified as a primary, secondary, or tertiary depending on the carbon it is attached to. An alcohol is primary if it is attached to a carbon with only one carbon-carbon bond. By the same token, an alcohol is secondary if the carbon to which it is attached is bound to two other carbons, and tertiary if the carbon’s remaining three bonds are bonded to other carbons. The following examples of primary, secondary, and tertiary alcohols are ethanol, isopropanol (IUPAC nomenclature: propan-2-ol), and tert-butyl alcohol (IUPAC nomenclature: 2-methylpropan-2-ol), respectively.

Ethanol, a primary alcohol

Isopropanol, a secondary alcohol

t-Butyl Alcohol, a tertiary alcohol

Physical Properties of Alcohols

The bond existing between oxygen and hydrogen is significantly shorter than the bonds formed between carbon and hydrogen. Hydroxy groups are inherently polar. The oxygen in the covalent bond is highly electronegative; thereby, pulling the majority of the electron density shared in the covalent bond towards itself. From this information it becomes clear that the oxygen bears a significant partial negative charge, while the hydrogen bears a partial positive charge of the same magnitude.

The presence of a highly electronegative group allows alcohols to hydrogen bond with other molecules. For example, in ethanol, the negatively charged oxygen on one ethanol molecule is attracted to a positively charged hydrogen molecule on another ethanol molecule. The ability of alcohols to hydrogen bond amongst themselves explains their high melting points. Alcohols also have higher boiling points than the hydrocarbons and ethers which contain a comparable number of carbon atoms.

The solubility of alcohols in water is interesting because alcohols have hydrophobic and hydrophilic centers, making them amphipathic or amphiphilic. The hydrocarbon portion of the alcohol is nonpolar, and thus resists dissolving in water; the polar alcohol group is hydrophilic and thereby promotes dissolution in water. In a small molecule such as propanol, the alcohol dissolves because the polar alcohol group outweighs the very short hydrogen chain. However, as the hydrocarbon chain increases beyond butanol to pentanol and hexanol, alcohols become virtually immiscible in water.

Alcohols are amphoteric species that may exist as both acids and bases. At low pH, or when surrounded by strong acids, they may be protonated to alkyloxonium ions. In contrast, at high pH and when surrounded by strong bases, they may exist as alkoxide ions.

Phenols

Structure of Phenol

Phenols are hydroxy-arenes. They have some similarities to alcohols in chemical reactivity at oxygen, but they are much more acidic and much less basic. They appear to be enols (hydroxy attached to an alkene) and should be unstable relative to the keto form, but the gain in aromaticity in being in the enol favors the enol form relative to the keto. Examination of the resonance forms indicates that the hydroxyl donates electron density to the ring, hence the hydroxyl group is activating and ortho-para directing.

Nomenclature of Phenols

Substituted phenols are named as derivatives of the parent compound phenol with the hydroxyl-bearing carbon being designated as C-1. Some examples are:

2-Methyl-4,6-dinitrophenol	Benzene-1,3-diol

Acidity of Phenols

The pka of phenols is around 8-10. Phenols are substantially more acidic than alcohols. This is because phenoxide (the corresponding conjugate base) is resonance stabilized.

Synthesis of Phenols: Nucleophilic Aromatic Substitution

Direct concerted displacement of leaving groups on aromatic rings is not possible. In appropriately substituted rings, leaving groups may be replaced by hydroxyl involving nucleophilic processes via a 2-step mechanism: nucleophilic aromatic substitution.

Alcohol-like Reactivity of Phenols

Phenols undergo many of the same reactions as alcohols, especially where the nucleophicity of the corresponding phenoxide (alkoxide) is concerned.

Phenol alkylation via Williamson-Type Reaction

Synthesis of Alcohols

In industry, methanol is made on a multibillion-pound scale from a pressurized mixture of CO and H₂ called synthesis gas.The reaction involves a catalyst consisting of copper, zinc oxide and chromium(III) oxide.

Changing the catalyst to rhodium or ruthenium leads to 1,2-ethanediol (ethylene glycol), an important industrial chemical that is the principal component of automobile antifreeze. Ethanol is prepared in large quantities by fermentation of sugars or by the phosphoric acid-catalyzed hydration of ethene (ethylene).

On a smaller than industrial scale, we can prepare alcohols from a wide variety of starting materials. For example, conversions of haloalkanes into alcohols by Sn2 and Sn1 processes featuring hydroxide and water. These methods are not as widely used as one might think, however, because the required halides are often accessible only from the corresponding alcohols. They also suffer from the usual drawbacks of nucleophilic substitution: Bimolecular elimination can be a major side reaction of hindered systems, and tertiary halides from carbocations that may undergo E1 reactions. Some of these drawbacks are overcome by the use of polar, aprotic solvent.

Conceptually, the easiest way to reduce a carbonyl group would be to add hydrogen, H-H, across the carbon-oxygen double bond directly. Although this can be done, it requires high pressures and special catalysts. A more convenient way is a polar process, in which hydride ion, and a proton, are delivered to the double bond, either simultaneously or sequentially.

Organometallic Reagents

Organometallic reagents of magnesium and lithium are used in the synthesis of alcohols. Since the metal in organometallic compounds are extremely electropositive, the carbon can react as if it were carrying a negative charge (giving it nucleophilic behavior). These organometallic compounds can attack the carbonyl group of aldehydes and ketones. An important feature of this reaction is that a new carbon-carbon bond is formed in the process.

Synthesis of Tertiary Alcohol From Ketone

The first step of this reaction involves the nucleophilic carbon attacking the carbonyl carbon of a ketone or aldehyde to form the carbon-carbon bond. Simultaneously, two electrons from the carbon-oxygen double bond (carbonyl) are pushed by the electrons from the attacking carbon to form a metal alkoxide with the metal from the organometallic compound. Upon aqueous work-up, an alcohol is formed by hydrolyzing the metal alkoxide.

Different types of alcohols can be formed by changing the compound reacting with the organometallic compound. For example, a reaction between an organometallic compound and formaldehyde will result in a primary alcohol. Reaction with any other type of aldehyde will produce secondary alcohols; reaction with ketones will yield tertiary alcohols.

Hydroxylation

An important function of metabolic degradation is the introduction of hydroxy groups into unfunctionalized parts of molecules in our body (hydroxylation). A set of proteins, known as the cytochrome proteins, are vital biomolecules that are present in almost all living cells, which assist with the process of hydroxylation. An example of a cytochrome protein is cytochrome P-450. Cytochrome P-450 uses O₂ to add hydroxy groups to alkyl substituents. The importance of hydroxylation can be signified by the excretion of drugs to prevent toxic accumulation. Hydroxylation helps increase the water solubility of compounds through addition of hydroxy groups to specific regions of the compounds. Thus in the liver, hydroxylation helps to accelerate the excretion of medicine that we ingest by making it more soluble. If it weren't for this process, ingested drugs would stay in our bodies much longer and have a much greater chance of toxic accumulation.

Hydroxylation can also be selective in the addition of hydroxy groups to a molecule. Protein's undergoing hydroxylation can control the order in which hydroxy groups are added, as well as, the positions to which they are added. This is a key function of the process of hydroxylation, as it allows molecules to undergo oxidation at specific regions of the molecule. Also it allows molecules to bind or react with other compounds in specific orientations.

Nomenclature

The system used to name organic molecules – IUPAC – is relatively simple to use for alcohols, though it can get more complicated if the molecule containing the alcohol group contains substituents containing other functional groups, such as alkenes, alkynes or halogens. The simplest case is when the molecule contains only hydrocarbons and the hydroxy group is the sole functional group, making it purely an alcohol.

In general, the presence of functional groups is indicated by a characteristic suffix that is distinct for each functional group and a number designating the location of the functional group on the molecule. For alcohols, the characteristic suffix used is –ol.

Case 1: Hydrocarbons and alcohols In this case, the longest chain of the hydrocarbons is counted – this becomes the primary hydrocarbon chain of the molecule. Then the longest carbon chain is numbered with the goal of putting as small a number as possible on the hydroxyl group. The longest chain is named as it is for alkanes, but the suffix for alcohols is –ol. Thus, for a propane chain which contains an alcohol group on the second carbon is named propan-2-ol or 2-propanol. Because the alcohol is the only functional group, it does not matter which of the two names is used. It is assumed that the “2” corresponds with the location of the hydroxy group. However, this rule does not always hold true when other functional groups are present.

Case 2: This is more complex because there is a hierarchy of functional groups in organic chemistry. Each functional group when isolated wants to be labeled as the smallest number. However, some functional groups are of “higher importance” and thus are numbered first. Also, if the alcohol is no longer the most “important” functional group, the molecule does not the suffix for the alcohol. This may present a slight problem initially, because the –ol was the only hint that the molecule possessed an –OH group. However, the presence of the –OH group is designated by using the word “hydroxy” after the number of the carbon on which the –OH group is bound to. An example is: 1-chloro-2-hydroxybenzene.

If there are multiple –OH groups present, the nomenclature is modified only slightly. The suffix used in the name is the number of alcohol groups followed by –ol. For example, a compound containing two –OH groups has a suffix of “diol”; for a compound with three –OH groups, the suffix is a “triol.” The numbers of the carbons containg the alcohol groups are stated in the same manner as if there were only one; giving the hydroxy group the lowest number possible. For example, a diol would be named as 3,3 pentandiol. This name indicates that the third carbon in the five-carbon chain is bonded to two –OH groups.

Unique Infrared Spectroscopy of Alcohols and Phenols

Infrared Spectroscopy is a technique used by chemists to elucidate structural information about an organic compound. Organic molecules absorb electromagnetic radiation in the infrared region and promote specific vibrational states. These vibrations are called fundamental absorptions, and they arise from the excitation from the ground state to the lowest-energy excited state. These modes of vibration are then picked up by a machine which interprets the radiation in the form of a spectrum. Since every type of covalent bond has a different natural frequency of vibration, two different molecules will never share the same infrared spectrum. The absorption of certain bonds have very distinct vibrational frequencies. A small range of absorption can be allocated to a specific type of bond. For instance, an absorption around 1715 is normally due to the carbonyl group.

Alcohols and Phenols have unique Infrared spectrum and usually show strong and broad streching bands between 3400 and 3300 cm⁻¹. In solution, it is sometimes possible to observe a free stretching band at around 3600⁻¹ with a sharp but weaker peak.

The broad peak that is well known as the alcohol or phenol peak is created by the stretching vibrations between the O-H. Intermolecular hydrogen bonding usually weakens the O-H bond, and shifts the band to a lower frequency. Phenols are known to have a broader O-H band than alcohols. However, it is hard to determine the difference by looking at the O-H band due to their similar shapes. The bending vibrations between the C-O-H usually yield weak and broad peaks around 1440–1220 cm⁻¹. However, these peaks are difficult to see due to interference by similar bands from strong CH₃ bending peaks. Strong C-O bond stretches are observed from 1260 – 1000 cm⁻¹. Since the C-O bond is adjacent to the C-C bond, the shape and size of the peak can be used to determine whether an alcohol is primary, secondary, or tertiary. It can also be used to distinguish between phenols and alcohols. In addition, alcohols that are present in amides, ethers, and aldehydes can also be distinguished in IR spectrum. Infared spectrum is a useful analytical tool that is used to identify which function groups are present or which ones are not.

NMR of alcohol groups

Nuclear magnetic resonance (NMR) is a technique used to identify organic compounds. Since the atomic nucleus has a spinning charge due to the positively charged protons, it can generate a magnetic field. Because of this, the protons will have two orientations, a β and α spin state that have different energies in the presence of an external magnetic field. When the compound is exposed to electromagnetic radiation, resonance (when the nuclei is flipped back and forth between orientations) will occur and when the amount of radiation matched the difference in energy of the two spin states, an NMR signal is recorded and the NMR detector records peaks.

One of the most common forms of NMR is the proton NMR (H⁺ NMR). The NMR spectrum is read by the chemical shift of the different groups. In a proton NMR a compound with an -OH group will have a chemical shift (peak) at around 0.5-5.0 ppm. For example, ethanol will have a peak at around 4.0 ppm indicating the hydroxide groups while its two other peaks at around 1.0 ppm and around 3.0 ppm will indicate the position of the C-H protons in accordance to their position in the compound.

References

1. Schore, Neil E. (2011). Organic Chemistry Structure and Function 6th edition. W. H. Freeman.

2. Hammond, Christina N., Mohrig, Jerry R., Schatz, Paul F.. (2010). Techniques in Organic Chemistry 3rd Edition. W.H. Freeman.

3. Atkins, Peter, Jones, Loretta. (2008). Chemical Principles: The Quest for Insight 4th Edition. W.H. Freeman.

General Information

A ketone. Carbonyl is in between two hydrocarbons.

An aldehyde. R can be a anything but a hydroxyl group or alkoxy group attached to a carbonyl.

A carbonyl group is a functional group that is comprised of a carbon atom doubly bonded to an oxygen atom. The carbon attached to the oxygen can have single bonds to different atoms. The atoms that the carbon is bound to distinguish it as a ketone, aldehyde, carboxylic acid, ester, or amide.

==The Nomenclature of Carbonyl Compounds==

1, Naming Carboxylic Acids:

The functional carboxylic acid is called a carboxyl group. In systematic nomenclature, a carboxylic acid is named by replacing the last alphabet "e" with "oic acid." For instance, a two carbon alkane is called ethane and two carboxylic acid is ethanoic acid. Notice that a carboxylic acid with 6 or less carbons are usually known by their common names. ( methanoic acid = formic acid, ethanoic acid = acetic acid, propanoic acid= propionic acid, butanoic acid= butyric acid, pentanoic acid= valeric acid, hexanoic acid= caproic acid)

2, Naming acyl halides

Acyl halides have a halide in place of the -OH group in a carboxylic acid. Acyl halides are named by replacing "-ic acid" in the acid name with "-yl halide" (eg; -yl chloride, -yl bromide, -yl flouride etc). If acid has a name with "-carboxylic acid," then replace "carboxylic acid with "-carbonyl halide" (e.g. -carbonyl chloride, -carbonyl bromide, -carbonyl fluoride)

3, Acid anhydrides

An acid anhydride is formed by two molecules of carboxylic acid reacting with each other to lose one water molecule. An anhydride is a symmetrical anhydride if the two reacting acid molecules are the same. If two reacting acid molecules are different then they are going to from a mixed anhydride. Symmetrical anhydrides are named by replacing "acid" in acid name with "anhydride." Mixed anhydrides are named by stating the names of both acids in alphabetical order then followed by "anhydride."

4, Naming Esters

If there is an -OR group in place of the -OH group of a carboxylic acid, the name of the group attached to the carboxyl oxygen should be

stated first, then state the name of acid by replacing "-ic acid" with "-ate."

Salts of carboxylic acids are named in the same way. The cation is named first then name the acid with replacing "-ic acid" by "-ate"
Notice that cyclic esters are called lactones. Their common names are derived from the common name of the carboxylic acid, which designates the length of the carbon chain, and a Greek letter to indicate the carbon to which the carboxyl oxygen is attached.

5, Naming Amides

An amide has an -NH2, -NHR, or -NR2 group in place of -OH group of a carboxylic acid. They are named by replacing "-oic acid", "-ic acid", or "-ylic acid" of the acid name with "-amide." If a substituent is bonded to the nitrogen, the name of the substituent should be stated first (if there is more than one substituent bonded to the nitrogen, they should be stated alphabetically), then state the name of the amide. The name of each substituent is preceded by capital N to indicate that the substituent is bonded to a nitrogen. Cyclic amides are called lactams. Their nomenclature is similar to that of lactones.

Carbonyl Compounds

Compound	Aldehyde	Ketone	Carboxylic Acid	Ester	Amide
Structure

Each of the characteristic specific to each compound is detailed on their specific pages.

General Reactivity

Oxygen is far more electronegative than carbon, so the electron density is higher near the oxygen and lower near the carbon. This creates a dipole moment, where the oxygen bears a negative charge and the carbon bears a positive charge of the same magnitude. This distribution of charges makes carbon an electrophile and oxygen, the resulting nucleophile.

The reactivity of each carbonyl compound is dependent on the group attached directly onto the alpha carbon. This is because of the resonance stabilized structures which form due to the donation of an electron by this group. The stronger the contribution of this resonance structure, the stronger the stability of the carbonyl. An example of this can be seen in carboxylic acids which upon deprotonation create a degenerate structure and thus increases the acidity of the proton. Peptide bonds are unusually strong and carry more of an $sp^2$, planar hybridization due to the resonance contribution of the amide electron onto the alpha carbon. This decreases the reactivity of peptide bonds considerably and thus requires much energy or a protease to catalyze the bond. Carbonyls by themselves are very stable bonds and the energy of their formation are usually very high. This makes the formation of carbonyls in organic synthesis to be highly thermodynamically favorable and usually the creation of a carbonyl bond as the end product will drive a reaction to formation.

The carbonyl group has a short, strong, and very polar double bond. Its reactivity of its double bond is very different from the double bond of the alkenes because of their oxygen's electronegativity along with the lone pair of electrons. The carbonyl carbon is also electron withdrawing since it is so close to the highly electronegative oxygen. The polarization of aldehydes and ketones also alters the physical constants. The polarization of the carbonyl group is the reason why their boiling points are higher than those of the hydrocarbons of similar molecular weight. the carbonyl group's polarity causes the smaller molecules to be completely miscible in water. Carbonyl compounds with more than six carbons are considered large, which is insoluble in solution. The larger the compound, the larger its hydrocarbon chain, the more hydrophobic the molecule is, so its solubility would then decrease.

Inductive Effect Take important notice that the electrophilicity of the carbon is highly dependent on the nearby atoms and the atoms it is directly bounded to. For example, the carbonyl carbon of a carboxylic acid will not be as electrophilic as a carbonyl carbon of a ketone because of resonance stabilization. On the other hand, acetyl chloride (the carbon is bound to an R-group, doubly bonded to the oxygen, and then bound to the fluorine), will have a more electrophilic carbonyl carbon because the electronegative chlorine will increase the induced positive dipole of the carbon. Electrophilicity plays a key role in chemical reactions, and less electrophilic carbonyl carbons are not as readily reactive.

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Biochemical Synthesis of Carbonyl Groups

Nicotinamide Adenine Dinucleotide

Within biological systems, carbonyl compounds can be formed by the oxidation of alcohols. An oxidation reaction in organic chemistry is one that is characterized by a process which either adds electronegative atoms or removes hydrogen from a molecule. An example of this oxidation includes the oxidation of ethanol by the oxidizing agent nicotinamide adenine dinucleotide (NAD⁺). NAD⁺ is composed of a pyridine ring, two ribose molecules, and the heterocycle adenine. When the two enantiometers of 1-Deuterioethanol are reacted with NAD⁺, in the presence of enzyme alcohol dehydrogenase, the biochemical oxidation is found to be stereospecific (NAD⁺ and only removes the hydrogen attached to the C₁ atom in 1-Deuterioethanol. Similar to the oxidation of 1-Deuterioethanol described above, other alcohols can be oxidized biochemically to form carbonyl groups. Another example includes the oxidation of methanol to formaldehyde.

'HNMR

In 'HNMR spectroscopy, aldehyde compounds have a very unique chemical shift which appears to be between 9-10 ppm. Which means that the aldehyde formyl hydrogen is also very strongly deshielded. Aldehyde C₂ hydrogens are also slightly deshielded because of the electron withdrawing from the oxygen from the carbonyl group. In ketones, this is also similar. The alpha-hydrogens also experience this deshielding; which has a chemical shift between 2-2.8 ppm.

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¹³CNMR

The carbon-13 NMR spectra for aldehydes and ketones are the same since it is of the chemical shift of the carbon participating in carbonyl group. Since that carbon is bound to an oxygen, it will appear at a lower field approximately 200 ppm. The carbons adjacent to the carbonyl carbon are also deshielded (just like the hydrogens in 'HNMR). The carbons further away from the carbonyl group are less deshielded.

IR Spectroscopy

The IR absorbtion bands for all carbonyl compounds absorb in the 1760-1665 cm^-1 region, which is due to the streching vibration of the C=O bond. Generally, carbonyl groups have a high intensity and narrow regions, making them useful for diagnostic purposes.

Range	Type of Compound	Such as:
1750-1735 cm-1	saturated aliphatic esters	CH3-CH2-COOR
1740-1720 cm-1	saturated aliphatic aldehydes	CH3-CH2-COH
1730-1715 cm-1	α, β-unsaturated esters	R'(R")C-CH-COOR
1715 cm-1	saturated aliphatic ketones	CH3-CH2-CO-CH2-CH3
1710-1665 cm-1	α, β-unsaturated aldehydes and ketones	R'(R")C-CH-COH

Introduction

A carboxyl group consists of a carbon double-bonded to an oxygen and also bonded to a -OH group. Compounds with carboxyl groups are called carboxylic acids or organic acids. The carboxyl group can act as an acid when by donating a proton (H⁺) to a solution and becoming ionized. Under biological conditions at pH~7, carboxyl groups are usually deprotonated, meaning they lose a H⁺, and become negatively charged. An example of a carboxyl group in the body would be carbonic acid, formed from the hydration of a carbon dioxide. Under biological conditions, carbonic acid usually dissociates into bicarbonate ion.

An ester functional group

Properties

Carboxyl groups have an electronegative oxygen atom double bonded to a carbon atom. This carbon-oxygen bond is very polar and the fact that its a double bond increases the polarity of the bond. As a result of the polarity, compounds containing carboxyl groups usually have higher melting points, boiling points and have hydrophilic centers. Moreover, the higher melting point and higher boiling point can be attributed to the fact that they can form hydrogen bonds both in the liquid and solid state. Fatty acids are examples of compounds that have hydrophilic centers due to their carboxyl groups. Also, carboxyl groups, especially when present in molecules with a low molecular weight tend to be highly volatile and therefore tend to have strong odors. The pKa of carboxyl groups usually range from 4-5.

Nomenclature

In naming organic molecules with multiple functional groups, the carboxyl group takes precedence in naming over any other functional group. Therefore when naming a molecule such as an alkane that contains a carboxyl group, the -e on the alkane is replaced by -oic acid. Also, when numbering the chain of the organic molecule that contains a carboxyl group, the carboxy carbon is labeled as the number 1 carbon. Molecules with two carboxyl groups would use instead the -dioic suffix.

Bonding

The polarity of the carbon-oxygen bond makes the carbon very susceptible to nucleophilic attack. Upon attack, the electrons of the double bond will migrate to the oxygen atom in order to maintain the octet for the carbon atom. The oxygen will now be negatively charged and a tetrahedral intermediate has been formed. The double bond will reform when the migrated electrons on the oxygen atom move back into the double bond to oxygen while the carbonyl carbon attacked expels the -OH group as a leaving group. While the expulsion of an -OH group is energetic unfavorable, the formation of the energetically favorable carbon-oxygen double bond helps overcomes this obstacle. Other ways to overcome this obstacle is to convert the -OH group into a better leaving group. The polarity giving the oxygen a partially negative charge also makes the carboxylic acid susceptible to electrophilic attack. An example of this is the hydrolysis of a carboxylic acid under acidic conditions where a proton acts as an electrophile and attacks at the oxygen which is doubly bonded to the carbon.

Amino Acids

The carboxyl group is a major component of amino acids. The carboxyl group, along with the amino group, allows amino acids to be zwitterions where both the amino group and the carboxyl group are charged. Since the carboxyl group can be deprotonated, it can impart a negative charge onto the amino acid. The carboxyl group is also key in the formation of peptide bonds. The carboxyl group of an amino acid can be attacked by the amino group of another amino acid. The nitrogen group of the amino group acts as the nucleophile and attacks the carbon of the carboxyl group. Carboxyl groups are also present on the side chains of two amino acids, Aspartate and Glutamate. These amino acids allow for hydrogen bonding and the formation of salt bridges, which help stabilize the structure of proteins.

From Nature

• Formic acid (HCOOH), the simplest carboxylic acid with only one carboxyl group, is primarily responsible for the pain caused by insects' bites(mostly Hymenoptera, like bees and ants).
• Acetic acid (CH₃COOH) can be biologically synthesized by either aerobic or anaerobic fermentation, a process used to make vinegar. The aerobic process requires warm ethanol (CH₃CH₂OH) and oxygen with Acetobacter. The anaerobic process requires only sugar(C₆H₁₂O₆) as input chemical, and acetogen can then give carboxylic acids as output. It should be noted that aerobic process is still dominantly applied, because acetogens used for anaerobic processes show less tolerance to acidic environments. In other words, acetogens will be killed if too much acid is produced.
• Propanoic acid (CH₃CH₂COOH,or C₂H₅COOH). It can be formed by breakdown of fatty acids with odd numbers of carbon atoms. During such metabolic process, propanoic acid undergoes condensation reaction with the thiol end of CoA (coenzyme A) to form propionyl-CoA. Propanoic acid can also be synthesized by anaerobic respiration of bacteria called propionybacterium.
• Butanoic Acid (C₃H₇COOH) can be found in some naturally occurring esters, such as hexyl butanoate from oil of Heracleum and octyl butanoate in parsnip. The fermentation (biological) method of butanoic acid production was discovered by Louis Pasteur in 1861. The overall process takes one mole of glucose and the products are one mole of butanoic acid, two moles of carbon dioxide, two moles of hygrogen gas and three moles of ATP. During such process, glucose is cleaved into two pyruvate molecules first. The pyruvate is then oxidized to acetyl CoA releasing carbon dioxide and hydrogen as by-products. The ATP is released after the acetyl CoA undergoes various enzyme reactions.
• Benzoic acid (C₆H₅COOH) is the main component of benzoin resin. However, it will be fairly pricey to extract benzoic acid directly from benzoin resins. Therefore most benzoic acid in the market is manufactured industrially.
• Citric acid (HOOCCH₂-COH(COOH)-CH₂COOH).The simple structure is similar to glycerol, which is also a biologically abundant molecule in fat. Citric acid is a well known organic acid in variety of citrus fruit such as lemon and lime. It is the essential material for citric acid cycle, a very important metabolic process. It is also very useful for modern pharmaceutical, cosmetics and other industries that include chemical processes. The industrial production of citric acid experienced a transformation from juice extraction to biosynthesis. Industrial citric acid production began in 1890 using extractions from Italian citrus exports, following the first crystallization of citric acid by Swedish chemist Carl Wilhelm Scheele in 1784. The biological methods of production was discovered in 1893 using Penicillium mold and sugar, but such process was not popular until World War I cut the Italian citrus exports. A more efficient biological production was then discovered by American food chemist James Currie by using cheap sugary mixture and mold called A. niger. Such industrial process has been used by major pharmaceutical companies like Pfizer. In addition, the extracted or biologically synthesized citric acid is precipitated by calcium hydroxide for isolation and acid is converted back from precipitate at the end.
• Oxalic acid (HOOC-COOH) is found in kidney stones as calcium oxalate, and it leads to kidney failure. It is therefore risky to eat carambola (commonly known as starfruit) and monstera due to their high oxalate content. Oxalate is also included in citric acid cycle.

Introduction

The mechanism of peptide bond formation.

Amino groups are composed of a N atom bonded to two H atoms. Amino groups can act as a base because they can pick up an H⁺ from a solution. Amino groups can be ionized with a 1⁺ charge under basic condition. Organic compounds with an amino group are called "amines"; organic compound containing an amino group and carboxyl group are called amino acids which are the building blocks of proteins.

Amines can readily form hydrogen bonds. The amines in the bases of DNA form hydrogen bonds with nearby nitrogen or oxygen atoms and keep the two strands together.

The amino group of lysine[[|]], for example, has be proven useful in the regulation of genes. In particular when the amino group of Lysine in the histones is acetylated, it can no longer function as it regularly does. This is a regulating step that is involved with gene expression and replication.

Another role amino groups play in biochemistry is in enzymes. In the case of a protease, an enzyme which cleaves amino acids, a tetrahedral transition state is formed when the hydroxy group of serine attacks the carbonyl carbon of the amino acid. Because the tetrahedral transition state has a negative charge, the positive NH₃⁺ charges help to stabilize the transition state, forming what is called an oxyanion ring.

Amines can also act as nucleophiles because of its lone pair of electrons. This is the basis by which peptide bonds are formed, with the carbonyl carbon acting as the electrophile in a dehydration reaction.

Structure and Physical Properties

Inversion of amine

The amine nitrogen is sp3-hybridized and tetrahedral shape. The nitrogen non-bonded pair acts like a substituent—the geometry is tetrahedral around nitrogen and the bond angles are all around 109°. Unlike carbon, sp3 hybridized nitrogen is not rigid and undergoes rapid inversion at room temperature. It is similar to an umbrella flipping inside-out and similar to the inversion of configuration which occurs in an Sn2 reaction.

Nomenclature

All the usual IUPAC rules are followed. The suffix –amine is added to the name at the end and the position of the amino group must be specified. For 2° and 3° amines, the largest alkyl substituent is chosen as the parent and the other alkyl groups on nitrogen are named as substituents with the prefix N- to denote that they are attached to nitrogen. Because the amine functional group has the lowest priority in naming, it is often named as a substituent on more highly functionalized molecules.

Acidity of Amines

Amines are much less acidic than alcohols with a pKa ~ 35 and Keq = 10-35. A very strong base like alkyllithium must be used to completely deprotonate an amine.

Basicity of Amines

Amines are the most basic of the common organic functional groups, but are still fairly weak bases. Protonation occurs on the non-bonded electron pair exclusively. The basicity of amines is directly dependent on the “electron density” at the nitrogen atom. Both inductive and resonance effects can alter the basicity of a nitrogen atom.

Hybridization on the N also affects basicity. An increase in s character on an atom increases the electronegativity of that atom which favors acidity and therefore disfavors basicity. Hence sp³-hybridized nitrogen is more basic than either sp² or sp hybridized nitrogen.

Synthesis of Amines

Gabriel synthesis to 1° amines

1. Cyanide displacement of alkyl halides followed by reduction

2. LiAlH4 reduction of amides

3. Displacements with azide followed by reduction

4. Gabriel synthesis to 1° amines

5. Reductive amination

IR Spectroscopy for Amines

Primary Amines will give two short, sharp equal peaks at around 3200 cm-1 – 3500 cm-1

Secondary Amines will give one short, sharp peak at around 3320 cm-1

Tertiary amines will not give a peak in any region.

Reference

1. http://orgchem.colorado.edu/Spectroscopy/irtutor/aminesir.html

General Simple Amide

Amide with -R Groups on Nitrogen

An amide functional group consists of a carbonyl group bonded to a nitrogen. In simple amides, two hydrogen atoms are bonded to the nitrogen (-CONH2) while in more complex amides, the nitrogen is bonded to one or two aliphatic or aromatic groups (-CONR).

Nomenclature

Naming amides is very similar to naming carboxylic acids. For IUPAC nomenclature, first name the carboxylic acid, and then drop the -oic acid and add amide. For example, propanoic acid would become propanamide and ethanoic acid would become ethanamide. The first part of the name depends on the carbon chain the amide is attached to.

Properties

Basicity

Differing from similar amines, amides show no measurable basicity due to the delocalised lone pairs on the nitrogen. In normal amines or -NH2 functional groups, the lone pair on the nitrogen can accept hydrogen atoms acting as a base. However, the pi bond formed between the double bonded carbon and oxygen in an amide contains p orbitals that are positioned almost parallel to the lone pair on the nitrogen. This causes the electron pair to be delocalised and shared throughout the carbonyl part of the molecule. Delocalization reduces an amide's basicity because the electron pair is not associated with a single atom reducing the intensity and focus of its proton drawing ability. Delocalization also helps to stabilize the overall structure of the amide and as a result more energy would be required to break the shared electron structure.

Melting Point

Methanamide is a liquid at room temperature while other amides remain solid. Relative to their size, amides have relatively high melting points due to the hydrogen bonding between the partially positive hydrogen atoms the -NH2 group and another electronegative oxygen. Each simple amide has two partially positive hydrogen atoms and two pairs of electrons on the oxygen allowing for multiple possible sites of hydrogen bonding. A lot of energy is required to break these hydrogen bonds increasing the melting point of amides.

Solubility

Small amides are soluble in water because they may have hydrogens bond with water molecules. Larger amides have trouble dissolving because of their long hydrophobic carbon chains. Amides are typically less soluble than amines and carboxylic acids because they can both donate and accept hydrogen bonds.

Preparation

Carboxylic acids can be used to prepare amides by reaction with solid ammonium carbonate in acid to form a ammonium salt. Upon heating, this salt dehydrates to produce and amide and water. Acyl chlorides (acid chlorides) will react violently ammonia to create ammonium chloride and an amide of the acyl chloride. Acid anhydrides will also react with ammonia to produce amides such as the reaction of ethanoic anhydride with ammonia producing ethanamide and ammonium ethanoate.

Synthesis of Amides

Amides are derived from a reaction between an amine and a carboxylic acid. Between these two molecules we have two competing nucleophiles, the oxygen of the alcohol group in the carboxylic acid molecule and the nitrogen of the amine. A nucleophile is a chemical species that donates a pair of electrons to an electrophile to create a chemical bond in a reaction. With nitrogen lying to the left of the oxygen on the periodic table, nitrogen serves as a better base and better nucleophile than the alcohol. The reaction between an amine and a carboxylic acid is based on an addition and elimination reaction. Although this is a simple and easy reaction, it is not the most effective and efficient way of producing amides. The reaction between the two also contains a competing acid-base reaction, which produces a salt. Therefore, with competing products, the addition-elimination reaction between the two is not the most effective way to isolate an amide. A better procedure would be the reaction between an acyl halide, an activated carboxylic acid derivative, and an amine. The replacement of the hydroxyl group in the carboxylic acid with a halide produces a reactive molecule called an acyl halide. With halogens being the most electronegative atoms, the presence of it in the molecule pulls the electrons away from the carbon of the carbonyl atom, creating an electrophilic site. With an electrophilic site present, the nucleophilic nitrogen of the amine will easily react with the acyl halide to form an amide.

938 × 212px

Step 1: The nucleophilic nitrogen attacks the carbon of the carbonyl, pushing the electrons of the double bond of the carbonyl to the oxygen. Formation of a zwitterion occurs (negative charge on the oxygen, positive on the nitrogen).

Step 2: The favored and more stable carbonyl is reformed, kicking out the halide.

Step 3: The positive charge is quenched as the halide comes back and removes a hydrogen from the nitrogen, forming an amide and a hydrogen halide.

Reactions

Hydrolysis

Hydrolysis of amides can occur under both acidic and basic conditions. Under acidic conditions, amides catalyzed by dilute acid react with water to form carboxylic acids and ammonium chlorides. An example would be heating ethanamide in dilute hydrochloric acid to form ethanoic acid and ammonium chloride. If heated under basic conditions such as sodium hydroxide solution, ethanamide will form ammonium gas and sodium ethanoate salt.

Dehydration and Hofmann Degradation

Amides can be dehydrated by reaction with phosphorous (V) oxide such as heating ethanamide with phosphorous oxide to produce ethanenitrile with the loss of water. The Hofmann Degradation reaction involves reaction of an amide with a mixture of bromine and sodium hydroxide resulting in the loss of the carbonyl group such as degradation of ethanamide into methylamine.

Practical use of polyamides

Chemical Structure of Kevlar

Polyamides are in general polymers held together by amide links. Nylon consists of repeating chains of carbons held together by amide chains while kevlar is made up of chains of benzene instead of carbon. Nylon is formed from the loss of water between a reaction of hexanedioic acid and diaminohexane while kevlar is formed from the reaction of benzene dicarboxylic acid and diaminobenzene. Nylon is used commercially for clothing, carpets, ropes, and tires while the high strength to weigh ratio of kevlar makes it practical for use in bullet proof vests and other lightweight sturdy needs.

General Overview

A sulfhydryl is a functional group consisting of a sulfur bonded to a hydrogen atom. The sulfhydryl group, also called a thiol, is indicated in chemistry nomenclature by "-thiol" as a suffix and "mercapto-" or "sulfanyl" as a prefix. Thiols have great affinity for soft metals. Sulfhydryls play an important role in biochemistry, as disulfide bonds connect necessary amino acids together for functional purpose in secondary, tertiary, or quaternary proteins structures.

Several Important Roles in Biochemistry

Sulfhydryl groups can be found in the amino acid cysteine. When two cysteine residues are in close proximity to each other, they can form a disulfide bridge also called cystine. The formation of a disulfide bond is an example of a post translational modification. It can be helpful to the structure of proteins, but it can make it difficult to accurately determine the sequence of a protein through the technique of Edman Sequencing. Disulfide bridges often play important structural and functional roles in proteins. Their hold are crucial in the formation of many tertiary structures of proteins. Extracellular proteins usually contain disulfide bonds whereas intracellular proteins usually lack disulfide bonds. These disulfide bonds can be broken with the addition of beta mercaptoethanol. Beta mercaptoethanol reduces the disulfide bonds back into their sulfhydryl form. It was found by Christian Anfinsen that proteins denatured by beta mercaptoethanol in urea will spontaneously reform its disulfide bonds if trace amounts of beta mercaptoethanol are present. This reformed protein was found to be fully functional. If the protein was reformed in the absence of urea, it was found that the function was greatly reduced. This is due to the fact that the wrong disulfide bonds were formed and the amino acid became "scrambled."

Structure

The C-S bond in thiols are 180 picometers in length, and the C-S-H bonds are at a 90° angle, significantly more acute than alcohols. Their main cohesive forces are van der Waals interactions, and sulfur is less electronegative than the oxygen in hydroxyl groups, making them less polar.

Reactions

Thiols can be easily oxidized, and thiolates act as potent neucleophiles.

S-Alkylation

The conjugate bases of thiols can be alkylated to give thiolethers.

RSH + R'Br + base → RSR' + [Hbase]Br

Redox

Thiols, in the presence of a base, are oxidized to give an organic disulfide.

2 R–SH + Br₂ → R–S–S–R + 2 HBr

Thiols can be oxidized by more powerful reagents such as sodium hypochlorite to yield sulfonic acids.

R–SH + 3H₂O₂ → RSO₃H + 3H₂O

Thiols can also undergo thiol-disulfide exchange.

RS–SR + 2 R'SH → 2 RSH + R'S–SR'

Organolithium compounds can react with sulfur to make thiols.

RLi + S → RSLi RSLi + HCl → RSH + LiCl

References

1. Berg, Jeremy M. (2007). Biochemistry, 6th Ed., Sara Tenney. ISBN0-7167-8724-5. 2. Campbell, Neil A. Biology. 7th ed. San Francisco, 2005. 3. E. Jones and I. M. Moodie (1990), "2-Thiophenethiol", Org. Synth.; Coll. Vol. 6: 979

Introduction

A phosphate group consists of a phosphorus atom bonded to four oxygen atoms. It is usually ionized and attached to the carbon skeleton by one of its oxygen atoms. Compounds with phosphate groups are called organic phosphates and they are frequently involved in energy transfer reactions. Phosphates can also be found in the backbone of DNA forming phosphodiester bonds (two esters with a common posphonyl). The negative charges on phosphates are part of the reason why the backbone of DNA is on the outside. A good example of transferring energy is the three phosphate group which is found in the energy compound ATP(adenosine triphosphate).

Phosphorylation of proteins is important for several reasons. Phosphate has a charge of negative two, which is important for disrupting or forming electrostatic interactions that alter the structure and function of proteins. Phosphate is also capable of forming three hydrogen bonds. The free energy of phosphorylation is large allowing it to change the equilibrium between conformational states. Phosphorylation can also take place very quickly or very slowly making it very flexible and versatile in meeting the needs of the body. Phosphorylation also results in highly amplified effects where hundreds of target proteins are phosphorylated by one activated kinase in a short time span. Phosphorylation is prevalently seen with the application of ATP, which aids in the regulation of metabolism.

Properties

A phosphate group is a hypervalent molecule containing five bonds which makes ten electrons. Phosphate salts can form through ionic bonds between a cation and one of the oxygen anions. Molecules with a phosphate group are usually not very soluble in water. Phosphate can exist in four different forms. In acidic conditions, it can take the form of phosphoric acid while in the more basic conditions it can be fully deprotonated to the phosphate ion. The forms phosphoric acid, dihydrogen phosphate ion, hydrogen phosphate ion, and phosphate ion all act and behave as individual weak acids. Each pK value differs greater than 4. This creates three pH regions of very acidic, moderately acidic to moderately basic, and very basic, depending on which deprotonation stage. These characteristics give the phosphate group the functionality of being a good buffer. Polymeric ions such as pyrophosphate and metaphosphates can be formed. When a pyrophosphate binds to a calcium ion, calcium phosphate is formed. Phosphate is and important component to the strength and solidity of animal teeth and bones. The exoskeleton of crustaceans and insects contain calcium phosphate as well.

Phosphate Applications

Phosphate can be used chemically as polish metal alloys. Phosphoric acid polishers work by reacting with various metal ions and generating hydrogen gas thus releasing some of the metal ions. By doing so, the metal is "phosphatized" with a phosphate coating that prevents corrosion and increases paint adhesion. This also plays a role in producing and maintaining a system of drinkable water because water usually runs through pipes and other water distribution sources that can easily erode. Because phosphate acts as a good buffer, it ensures that the pH of drinking water is stable and resists dramatic changes.

Introduction

Organic Synthesis is the part of Organic Chemistry that deals with the creation of compounds from other available compounds. There are lots of different ways to make substances and synthesis studies this. There are four very basic reaction schemes that can help in seeing what synthesis consists of: S_N1 (Unimolecular Nucleophilic Substitution), S_N2 (Bimolecular Nucleophilic Substitution), E1 (Unimolecular Elimination), and E2 (Bimolecular Elimination).

S_N2

As the name implies, S_N2 reactions deal with the substitution of some group for another. There are certain things the group coming in and the group leaving need to have in order for the reaction to happen because, for example if the leaving group cannot leave or is a group that does not want to leave, the reaction will not happen. The group coming in is referred to as the nucleophile. The nucleophile has electrons that can be donated to some atom lacking electron density, the electrophile. in an substitution reaction, the electrophile has the leaving group attached. To be considered a good nucleophile, the group needs to have lots of electron density usually being charged, and are not able to balance these extra electrons. There are several ways a molecule can balance its electron density. Three of these ways are: resonance, atomic radius, inductance. Of these, resonance is the biggest factor although atomic radius also plays a big role. inductance could be important but it is a balancing force that loses its power rather quickly. Resonance refers to the phenomenon where a molecule oscillates between two different states, usually by moving around double bonds. This makes it so that charges move around in a molecule. This moving of the charge is more of an oversimplified way of seeing this. In reality, the molecule exist in a hybrid state of all the different resonance forms. This hybrid state actually spreads the electron density all around the molecule.

The resonance of benzene and the hybrid state it actually exists in

The resonance of acetate. The charge is spread between the two oxygens and the connecting carbon and the charge is better balanced

Atomic radius plays a role in stabilizing charge. For example, all the halogens have a -1 ionic form. The charge is the same for all of them. What changes is the atomic radius. a larger radius means the electron density can be dispersed more, thus balancing the charge better.

atomic (gray) and ionic (red and blue) radii of some elements

Inductance is the ability of electronegative atoms to pull electron density towards themselves and can stabilize charge in that way. This force dies out rather quickly though as the electronegative moves farther away from the charge.

A molecule that can donate electrons is called a Lewis base and indeed most nucleophiles are basic, although a good base is not necessarily a good nucleophile. A molecule that can withstand a negative charge (by using what was mentioned above) could be a good leaving group.

In the molecule of bromomethane, the leaving group is the bromo group and the electrophile is the carbon. this bromo group is very electron withdrawing and because it has a big atomic radius it can dissipate the charge, a term called polarizability. A typical S_N2 reaction involves this bromomethane and NaOH

After the reaction has occurred the stereochemistry of the new product will be the exact opposite than before. For example if it was an (R)-iodopropane reacting with bromine it would because (S)-bromopropane. The reason for this inversion of the stereocenter is because of the backside attack the nucleophile does in Sn2 reactions.

Bromomethane molecule. Bromine serves as the leaving group

Bimolecular Nucleophilic Substitution of Bromomethane

Ways to increase the rate of the reaction
1. Increasing the concentration of either the nucleophile or substrate will increase the rate of the reaction. This is because it is a bimolecular reaction, where rate = k[nucleophile][substrate]: the rate is dependent on the concentrations of nucleophile and substrate. Increasing the concentration of nucleophile will increase the rate, since rate is proportional to the concentration of nucleophile and concentration of substrate. The same applies to the substrate.

2. Using a good leaving group.

3. Increasing the temperature of the system.

4. Anything that will not sterically hinder the nucleophile will increase the rate of the reaction.

5. Using a polar aprotic solvent, meaning the solvent does not contain an acidic proton.

S_N1

The reason for the 2 in and S_N2 reaction is the fact that the leaving group and the nucleophile are both in the rate law of the reaction. This means that the concentration of both bromomethane and NaOH affect the rate of the reaction. This is because the reaction happens in one step with one intermediate where the leaving group-electrophile bond is breaking and the nucleophile-electrophile bond is forming. In an S_N1 reaction there are two steps and two intermediates. The first is the formation of a carbocation where the leaving group leaves and the carbon is left with a positive charge. The second is the formation of the nucleophile-electrophile interaction. Of these two The first is the slowest and called the rate determining step. This means that no matter how much nucleophile there is the rate of reaction will not changed until the concentration of the substrate changes. The first step is very slow because forming a carbocation is not very favorable. The only way for this to happen is if the leaving group is a really good one and for there to be a way to stabilize the carbocation. The first one is simple, find a molecule with a very good leaving group. By making the carbocation a secondary or tertiary carbocation, the charge is stabilized better. This is done by resonance or hyperconjugation. The electrons in the methyl group(s) interact with the carbocation and give it some stabilization.

The nucleophile can attack the carbocation from both faces, the alpha or beta face of the substrate. As a result one will generally see a racemic mixture, or 50:50 mixture of a R and S conformation of the resulting compound.

Formation of carbocation stabilized by the ethyl groups

In order for the slow first step to happen, the second faster step needs to be slowed down. The way to do this is to have not have a very good nucleophile. instead of the hydroxide ion, water its conjugate acid can be used.

The first step in the unimolecular substitution reaction is the departure of the leaving group. the second is the attack of a weak nucleophile

Ways to increase the rate of an Sn1 reaction
1. Another way to increase the rate of the reaction for Sn1 reactions is to use a polar protic solvent. The polar protic solvent will increase the rate of the reaction because it will help stabilize the leaving group. It will stabilize the leaving group because it will solvate the charge. Anything that can help stabilize the leaving group will make it more likely to leave because it will be going to a lower energy state.

2. Another way to increase the rate of the reaction for Sn1 reactions is to add a salt. The salt will help stabilize the leaving group through ionic interactions, and because the leaving group would be more stable it is more likely to leave and form the carbocation that much faster.

3. Another way to increase the rate of the reaction for Sn1 reaction is to increase the polarity of the solvent. The polarity of the solvent will help stabilize the carbocation by hydrogen bonding. Therefore if one wants to increase the rate of reaction of an Sn1 reaction he or she simply needs to increase the polarity of the solvent used.

4. Another way to increase the rate of the reaction is to increase the temperature of the system.

5. Using a polar, protic solvent, meaning the solvent has acidic hydrogens.

Beta branching effect on the rate of the reaction
Beta branching means that there is a carbon chain on the carbon next to the carbon that is bonded to the leaving group. . For SN2 reactions beta branching will slow down the reaction because of steric hindrance, thus the less beta branching the alkyl halide has the faster the reaction will be for SN2. Based on the experiment results in SN1 reactions more beta branching will slow down the Sn1 reactions because of steric hindrance that hinders the nucleophilic attack on the carbocation.

Ring effect
Experiments has shown that the bromocyclopentane reacted instaneously while the bromocyclohexane required some time for the reaction to proceed showing that 5 membered ring reacts faster than a 6 member ring. The reason behind this is because of transannular strain, the unfavorable interactions between ring substituents on non-adjacent carbon, the less transannular strain there are the slower the reaction will be for SN1 and SN2. A 5-member ring is the only exception to this rule, but in general 3, 4, 6 member ring will react slowly because it is more stable and low in energy.

Aromatic Ring Effect
Aromatic rings will enable primary alkyl halide to proceed SN1 reactions because the aromatic ring can stabilize the carbocation through resonance. The position of the aromatic ring has to be one carbon away from the leaving group so the electrons can resonate and stabilize the carbocation. Electron-donating groups will contribute to the electron density so it will “activate” the ring and make it more susceptible to electrophilic attack, increasing the rate of reactions, while electron-withdrawing group will decrease the rate of reactions because it removes electron density from the aromatic ring, as shown with experiments the methoxybenzyl chloride, an electron donating group, reacted, while the nitro, an electron withdrawing group, did not react at all.

Reference

1. Vollhardt, K. Peter C., and Neil Eric Schore. Organic Chemistry: Structure and Function. New York: W.H. Freeman, 2011. Print.

Macromolecules

They are polymers made of preexisting monomers. They can form functional units by connecting together. Forming these functional unites requires energy. Examples of macromolecules include proteins, nucleic acid, lipids and polysaccharides.

References

1 ^ "http://en.wikipedia.org/wiki/Macromolecule," 2009.

2 ^ Nelson, D.L., "Lehninger Principles of Biochemistry," 2008.

   Macromolecules consist of proteins, nucleic acids, carbohydrates and lipids. 

1) Proteins are made of smaller building blocks called amino acids. There are 20 different amino acids. The chains of amino acids fold up in complex ways, giving each protein a unique 3D shape. 2) Nucleic acids allow organisms to transfer genetic information from one generation to the next. 3) Carbohydrates have several roles in living organisms. Carbohydrate derivates are actively involved in fertilization, immune systems, the development of disease, blood clotting and development. 4) Lipids include fats, phospholipids and steroid. Lipids also have function in the body.

Background Information

Carbohydrates consist of numerous functions that are important to living organisms. They are also known as saccharides, or sugar if they exist in small quantities; these names are used interchangeably to describe the same thing. The simplest carbohydrates are the monosaccharides, also known as simple sugars. Disaccharides are double sugars, consisting of two monosaccharides joined by a covalent bond. Carbohydrates also include polysaccharides, which are polymers composed of many sugar building blocks. The name "carbohydrate" is derived from 'hydrates of carbon', and they arise from photosynthesis, where they exist as products.

Carbohydrates are the most abundant aldehyde compounds found in living organisms. They provide storage, transport starch and glycogen that provide energy to bodies, and contain structural components such as cellulose in plants and chitin in animals. Additionally, they contribute to the immune system, fertilization, pathogenesis, blood clotting, and development.

Structure of Carbohydrates

Examples of Carbohydrates.

The common chemical formula for carbohydrates is C_n(H₂O)_n, where the ratios are usually 1 Carbon: 2 Hydrogens: 1 Oxygen. Trioses, pentoses, and hexoses are found most commonly among monosaccharides.Their structure is composed of the functional groups, aldehyde and ketone, which are attached with various amount of hydroxylgroups. The hydroxyl groups are usually attached to the carbons not a part of the aldehyde or ketone functional groups, to form aldoses and ketosesStructural Biochemistry/Carbohydrates/Ketoses, respectively. The most elementary carbohydrates comprise polyhydroxyaldehydes (an aldehyde moiety) or polyhydroxyketones (a ketone moiety). There are four general classes of carbohydrates: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Classification and Nomenclature

The most important carbohydrate is glucose. In general, monosaccharides have one carbonyl group (aldehyde, ketone, or acid), and the remaining carbons each bear one hydroxyl group. Monosaccharides can be linked together via ether and/or acetal bonds to form very large polymers called polysaccharides. A disaccharide consists of 2 linked monosaccharides and so on. Almost all saccharides in nature have at least one chiral carbon and they occur in nature as a single enantiomer. Glucose has 4 chiral carbons and has 15 other stereoisomers for a total of 16 possible stereoisomers of this gross structural formula.

The suffix –ose is often used in describing and naming carbohydrates. For example:

• A carbohydrate with 6 carbons is called a hexose
• A carbohydrate with 5 carbons is called a pentose
• A carbohydrate with an aldehyde as its carbonyl unit is called an aldose
• A carbohydrate with a ketone as its carbonyl unit is called a ketose

Looking at glyceraldehyde:

Enantiomers behave identically whether its a D or L conformation they both have the same boiling point, melting point and solubility. A different conformation seen in carbohydrates are diastereomers. Diasteriomers have the same chemical formula but different connectivity. A monosaccharide that has diastereomer conformation have different chemical and physical property. the D/l configuration applies to the highest numbered stereocenter (in most cases the highest stereocenter is the second to last hydroxyl group in a sugar molecule). The D/L configuration, like chiral molecules, means that the molecules will gradually rotate under polarization direction of linearly polarized light as it passes through, even under solution. D/L configuration also can be used in nomenclature to distinguish carbohydrate stereoisomers (meaning they have the same physical and chemical properties as well as the same formula, but rotate differently in polarized light) with the same formula. Naturally-occurring glyceralde has an R-configured chiral carbon. This is called D-glyceraldehyde. The opposite enantiomer is called L-glyceraldehyde. The D and the L refer to the configuration of the highest numbered chiral carbon when viewed in Fischer configuration. D-has the hydroxyl on the right side and the L has the hydroxyl on the left side. Almost all naturally occurring carbohydrates are of D-configuration.

Monosaccharides

The most basic carbohydrate is the monosaccharides (e.g. glucose, fructose, and galactose with the structural formula C₆H₁₂O₆), and it consists of 3 carbons or more within the molecule. Monosaccharides may appear to be linear molecules, but when they come in contact with aqueous solutions, they tend to form 5-carbon ring structures, which makes the molecules more stable. Monosaccharides are classified by their placement of the carbonyl group and its chirality. If the carbonyl group is a ketone, the monosaccharide is referred to as a ketose. However, if the carbonyl is an aldehyde, the monosaccharide is called a aldose. There are more possible configurations of the aldose form than ketose due to the presence of more chiral carbons found in aldoses. Carbons that have a hydroxyl group (-OH), disregarding both the first and last carbons are asymmetric. The asymmetric carbons lead to the two possible forms (R and S)which corresponds to the D- and L- configurations. Combining two monosaccharides together would result in a disaccharide, linked via a glycosidic bond; and condensation reaction is the process that fuses two monosaccharides together. Such reactions form a disaccharide by removing a hydroxyl group from one monosaccharideMonosaccharides and a proton from the other.

Ring Structure of Monosaccharides After hemiacetals and hemiketals form, the carbohydrate will form a ring structure. For example, in Glucose the Hydroxyl group from the Carbon 6 will attack the carbonyl Carbon from Carbon 1, with a hemiacetal intermediate. The Carbon that will then change from a carbonyl carbon to a carbon with a hydroxy will then be called an anomeric carbon. An anomeric carbon is the hemiacetal or hemiketal that is bounded by an alcohol group and an ester bond. Anomers are another form of isomers that differ in the hemiacetals or hemiketals.The most common structure of a glucose ring is the alpha conformation in contrast with the sterically hindered Beta conformation. The alpha conformation is structure in which the hydroxyl in Carbon 1 is facing the opposite of the plane as carbon 6, while the beta conformation is facing in the same plane as the Carbon 6 thus causing steric hinderance. For this reason of steric hinderance the alpha conformation is much more stable than beta.

Monosaccharide Streoisomer

Monosaccharide stereoisomers

Modified Monsaccharide

Modified Monosaccharide example

Disaccharides and Polysaccharides

Monosaccharides not only form disaccharides, but polysaccharides as well. It is called an oligosaccharides if monosaccharides are linked by O-glycosidc bonds. The only difference between disaccharides and polysaccharides is that there are more monosaccharides combined together into a long chain, whereas disaccharides only consist of 2 combined monosaccharides. Polysaccharides are frequently long chains of glucose monomers bonded together. There are two types of polysaccharides: Homo-polysaccharides and Hetero-polysaccharides. Homo-polysaccharides are chains of one type of monosaccharides, while Hetero-polysaccharides consist of multiple types of monosaccharides. Both types of polysaccharides can exist in either branched or unbranched forms. Some crucial homo-polysaccharides, like starch, glycogen and dextrans, play a role in energy storage, while other homo-polysaccharides like cellulose and chitin have more structure-based roles. The diversity of these branched and unbranched carbohydrates is due to the number of hydroxyl groups present in the sugar. Any one of these hydroxyl groups can act as the alcohol in the formation of the glycosidic linkage. One sugar acts as the alcohol while the other has the anomeric carbon that ca for the O-glycosidic bond. This reaction can occur 1,4 or 1,6 depending on the alpha (pointing down) and beta (pointing up) orientations of the hydroxyl groups. These orientations organize the sugars into different structures. Alpha and Beta sheets form with sugars arranged with the least amount of steric hindrance.

Monosaccharides and disaccharides seem to be stable compounds, but that is not necessarily true, since they are hemiacetals with reactive carbonyls in their structure. Hemiacetals, with the general formula R1R'1C(OH)OR2 where R2 is not a hydrogen, and is formed by the reaction of carbonyl compounds with alcohols, and the carbonyl group is fairly reactive. Because the carbonyl groups are very reactive, they can oxidize to products in a short period of time. However, some carbohydrates are acetals and, as such, prevent carbohydrates from oxidizing. This occurs because the anomeric carbon is fixed in a glycosidic linkage. Because of this resistance against oxidation, acetals are known as the non-reducing sugars, like sucrose, while the hemiacetals are known as the reducing sugars, such as glucose, maltose, and lactose.

Two or more monosaccharides are linked by a glycosidic bond. The links can be alpha or beta depending on the position of the bond formed. Monosaccharides can also form bonds with amino acids to form glycoproteins. A sugar linked to an asparagine is N-linked, or linked from the sugar to the nitrogen of ASN. A sugar attached to Serine or Threonine is O-linked, or linked from the carbon on the sugar to the oxygen in those two amino acids. An Asparagine residue can only accept an oligosaccharides if the residue is a part of an Asn-X-Ser or Asn-x-Thr sequence, where x can be anytype of amino acid. Therefore, potential site can be detected within amino acid sequences.

Glucogen Metabolism Glucose metabolism and various forms of it in the process is described by the process below. Glucose-containing compounds are digested and taken up by the body in the intestines, including starch, glycogen, disaccharides and as monosaccharide. Glucose is stored in mainly the liver and muscles as glycogen. It is distributed and utilized in tissues as free glucose.

Examples of glucogen metabolism.

Hemiacetal and Hemiketal

An aldehyde or a ketone can react with an alcohol to yield a hemiacetal or a hemiketal.Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol to the carbonyl group. When the alcohol group is replaced by a second alkoxy group, an acetal or a ketal, respectively, is formed. For example, the acetal formation ends with having two ethers on the target carbon, whereas the hemiacetal and hemiketal has both an alcohol and ether group.

Example of Hemiacetal and Hemiketal Formation

1- Hemiacetal Formation.

2- Hemiketal formation.

In Carbohydrates

The same way aldehydes and ketones react with alcohols to form hemiacetals and hemiketals, respectively, carbohydrates react intermolecularly to form rings. When forming a ring 5 or 6 membered ring is most favorable and will only be formed. The Carbon 1 will be attacked by either the Carbon 5 or Carbon 6 hydroxyl group to form a 5 or 6 membered (respectively)carbohydrate ring.

The carbohydrates are a major source of metabolic energy, both for plants and for animals that depend on plants for food. Aside from the sugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material (cellulose), a component of the energy transport compound ATP, recognition sites on cell surfaces, and one of three essential components of DNA and RNA. Carbohydrates are called saccharides or, if they are relatively small, sugars.

References

Berg, Biochemistry, 6th Edition

http://themedicalbiochemistrypage.org/carbohydrates.html

http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/biochemistry/biochemi.htm

Viadiu, Hector. "Carbohydrates." Chem 114A. UCSD, La Jolla. 19 Nov. 2012. Lecture.

Structure and Terminology of Nucleic Acids

Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are the two types of nucleic acids. The structure of DNA differs from RNA only because the DNA molecule does not have a hydroxyl group on the 2' carbon atom of the sugar ring. The missing hydroxyl group keeps DNA from being hydrolyzed, making it more stable than the ribose sugar of RNA.

The sugars in nucleic acids are bonded together by a phosphodiester bond between the 3' carbon on one sugar and the 5' carbon on another sugar.

Deoxyribose sugar

Ribose Sugar

The phosphodiester bond is negatively charged to repel nucleophilic attack by hydroxide ions and is less susceptible to hydrolytic attack. This negative charge, as well as the lack of a hydroxide group on the 2' carbon of DNA, makes the DNA molecule more stable than the RNA molecule, which could explain why DNA is the hereditary information carrier of all cells.

The chain of sugars made by the phosphodiester bonds form the backbone of the nucleic acids. The backbone of every nucleic acid is the same, but each sugar can contain one of four different bases. Two of the bases, (A)adenine and guanine (G), are derivatives of purine. The other two bases, cytosine (C) and thymine (T) are derivatives of pyrimidine.

Ribose, unlike deoxyribose, contains a base called uracil (U) instead of the base thymine (T). Thymine has an extra methyl group on the 5-carbon atom, making it slightly different than Uracil.

A nucleoside is a combination of a sugar and a base.

The nucleosides in DNA are called deoxyadenosine, deoxyguanosine, deoxycytidine, and thymidine, and the nucleosides in RNA are called adenosine, guanosine, cytidine, and uridine. If the base is a purine, then the N-9 (nitrogen) is bonded to the C-1' (carbon) of the sugar. If the base is a pyrimidine, then the N-1 is bonded to the C-1' of the sugar.

A nucleotide is a nucleoside joined to one or more phosphate groups by ester linkages. Therefore, it consists of the sugar, base, and phosphate(s). Nucleotides are the monomers that form DNA and RNA strands.

Most nucleotides are formed by the attachment of a phosphate group to the C-5' of the sugar. However, some nucleotides are also formed by the attachment of the phosphate group to the C-3' of the sugar. The four nucleotides that compose DNA are called deoxyadenylate, deoxyguanylate, deoxycytidylate, and thymidylate.

A DNA chain has polarity. One end of the chain will have a free 5' OH group. The other end will have a free 3' OH group. By convention, the base sequence of a DNA strand is written from the 5' to the 3' end.

The three-dimensional structure of DNA is a double helix. The two helices are coiled around a common axis and they run in opposite directions. The hydrophilic backbone of the helices make the outside of the helix, while the hydrophobic bases make up the inside of the helix. The helical structure of DNA is stabilized by the hydrogen bonding between complementary base pairs. Purines are larger than pyrimidines, so in order to have a regular structure, base pairs are formed between one purine and one pyrimidine. The base pairs of DNA are adenine (A) with thymine (T) and guanine (G) with cytosine (C). In RNA the thymine is replaced with uracil (U). The DNA structure is further stabilized by van der Waals forces between stacks of base pairs and the rigid five-membered ring structure of the backbone sugars.

RNA

RNA is nucleic acid, and its single-stranded, helical structure is constructed by nucleotides of nitrogenous bases, a ribose sugar, and phosphate group(s); the bases that make up RNA are adenine, guanine, cytosine, and uracil, for which, 1’ nitrogen of pyrimidine base and 9’ nitrogen of purines base are bonded to 1’carbon of pentose sugar by glycosidic bond; base pairs of adenine and uracil are hydrogen bonded to cytosine and guanine, respectively; the ribose is a pentose sugar of carbon numbered from 1’ to 5’ and has a hydroxyl group on the 2’ carbon; the 3’ and 5’ carbons of ribose sugar are bonded to phosphate group by phosphodiester bond; more importantly, the structure is of A-form geometry, which is constructed as of vast and thin major groove and of flat and broad minor groove, the structure can fold on itself to form secondary structure, such as tRNA and rRNA, and the secondary structure that are stabilized by hydrogen bonds, domains of loops, and metal ions, such as Mg 2+, form specific tertiary form.

List of Nitrogenous bases found in DNA and RNA

Name	Abbreviation	Structure	Classification	Found in
Cytosine	C		Pyrimidine	DNA, RNA
Thymine	T		Pyrimidine	DNA
Uracil	U		Pyrimidine	RNA
Adenine	A		Purine	DNA, RNA
Guanine	G		Purine	DNA, RNA

References

Berg, Jeremy; Tymoczko, John; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. 2007.

Lipids Overview

Lipids are naturally occurring (organic) compounds that are insoluble in polar solvents such as water . Their insolubility can be attributed solely to their long hydrophobic hydrocarbon chains. These hydrophobic chains may be saturated or unsaturated. Unsaturated chains contain double or triple covalent bonds between adjacent carbons while saturated chains consist of all single bonds. Lipids are composed of a glycerol molecule bonded to long hydrocarbon chain(s) (can be single or multiple) and, depending on the lipid, to other molecules—such as a phosphate group (phospholipids).

Some examples of the types of lipids are: neutral, saturated, (poly/mono) unsaturated fats and oils (monoglycerides, diglycerides, triglycerides), phospholipids, sterols (steroid alcohols), zoosterols (cholesterol), waxes, and fat-soluble vitamins (vitamins A, D, E, and K). Lipids have many different biological functions such as fuel molecules, structural building blocks for phospholipids and glycolipids, covalent attachments to guide molecules to specific membrane locations, and intracellular messengers.

"Chemical structure of the saccharolipid lipid A as found in E. Coli."

There are three common types of Membrane Lipids. They are phospholipids, glycolipids, and cholesterol. [Structural Biochemistry].

Fats Fats consists of glycerol and 3 fatty acids. Fats are created via 3 condensation reactions creating ester linkages that link the fatty acid carboxyl groups to the hydroxyl groups in glycerol. There are two different types of fatty acids, saturated and unsaturated. In a saturated fatty acid, it has the maximum number of hydrogen atoms possible, thus there are no double bonds. There are only single bonds. Since saturated fatty acids are only single bonds, it can pack more tightly together at room temperature and this makes it a solid at room temperature. An example of a saturated fatty acid is butter. An unsaturated fatty acid has one more double bonds. These double bonds create a kink in the hydrocarbon tail, which in return results in looser packing. At room temperature, it is a liquid. An example of this is oil.

Phospholipids They are found in biological membranes. The components of phospholipids include a hydrophobic tail and hydrophilic head. The hydrophobic tail consists of two hydrocarbon chains. The hydrophilic head consists of choline, phosphate, and glycerol. The fatty acids give a hydrophobic barrier, whereas the remainder of the molecule has hydrophilic properties. Phospholipids spontaneously form lipid bilayers due to amphipathic nature of lipid molecules. Phospholipids are found in all cell membranes.

Cholesterol Cholesterol is a steroid and they are built from 4 fused hydrocarbon rings. The hydrocarbon tail is connected to the steroid at one end, and a hydroxyl group is connected to the other end. Cholesterol is a steroid important in cell membranes and acts as a precursor to some sex hormones. However, prokaryotes do not have cholesterol.

Triglycerides

Neutral fats (triglycerides) are composed of fatty acid hydrocarbon chains bonded to a single glycerol molecule. Fatty acids consist of long hydrocarbon chains with a carboxyl group while glycerol consists of 3 carbons and 3 hydroxyl groups. Fatty acids are the building blocks of fat molecules. The method by which the three fatty acid chains in a triglyceride attach to a single glycerol molecule is called dehydration synthesis. Dehydration synthesis is also used in various other reactions, including the joining of two monosaccharides to form a disaccharide. Triglycerides function primarily in energy storage, as a form of insulation, and to protect and cushion cells and organs.

There is an image of a triglyceride molecule with three neutral fatty acid chains and a glycerol group

Saturated fatty acids contain single bonds between the carbons of the hydrophobic chain. Saturated fatty acids originate from animals and are found as component chains in a triglyceride molecule. Saturated fatty acids exist in the solid state at room temperature. Unsaturated fatty acids however contain one (monounsaturated) or more (polyunsaturated) double bond(s) between the carbons of the hydrocarbon chain, which causes the molecule to bend. Triglycerides with too many bends cannot be packed as closely together as neutral fatty acids and therefore are less dense. Below is an example of a saturated fatty acid

A Saturated Fatty Acid

Triglycerides composed of many fatty acids that melt at lower temperatures than those triglycerides with saturated fatty acids. These unsaturated fatty acids do not bind at their maximum number of hydrogen’s because of double bonding between the carbons of the chain. Unsaturated fatty acids originate from plants and are found as component chains to triglyceride molecules. Unsaturated fatty acids exist in the liquid state at room temperature.

(E)-4-oxohexadec-2-enoic acid, An Unsaturated Fatty Acid

These images depict a saturated fatty acid chain (contain single carbon bonds) and an unsaturated fatty acid chain (contain double carbon bonds).

Phospholipids

Phospholipid structure.

Phospholipids are modified triglycerides with one of the fatty acid chains replaced with a phosphate group. They are made by four distinguished groups: fatty acid chains, a platform, a phosphate group, and an alcohol attached to the phosphate. The fatty acid chains are hydrocarbon chains that are typically 14-24 carbons in length. The platform is either glycerol or sphingosine, which is an amino alcohol with a hydrocarbon chain. Phospholipids have a very characteristic non-polar fatty acid chain portion and a polar phosphate portion.The amphipathic character of phospholipids contribute in its' crucial role in phospholipid bilayers. The polar phosphate group is capable of interacting with water molecules and spontaneously forms a bilayer in an aquatic environment. Phospholipids orientate themselves so that the polar heads are facing the water molecules and the hydrophobic fatty acids are oriented toward the inside of the bi-layer. The bi-layer environment enables the non-polar fatty acid chains to stay together, avoiding the water while the hydrophilic phosphate group is oriented toward the water. Phospholipids participate in the formation of the cell membrane by the coming together of two layers of phospholipids. The phospholipids are responsible for the membrane's semi-permeability and fluidity.

The structure describe a phospholipid:

1-Acylglycerin-3-phosphoethanolamin

This image illustrates the components and orientation of the hydrophilic phosphate group and the hydrophobic fatty acid chains that form the lipid bi-layer.

Phospholipid is the most common group of lipids. In fact, cell membranes as well as organized cellular compartments are all made up of these phospholipids. They can form structures called micelles, in which when phosopholipids congregate, the hydrophobic fatty acid tails join together in the center of the sphere away from the aqueous environment and the polar heads are exposed to the outside. Structures such as liposomes can also be artificially formed from these lipids: using high frequency sound waves to sonicate the sample containing phospholipids and molecules of interest to create phospholipid vesicles that contain the molecules of interest. This is often used to deliver drugs to the cells and study how drugs pass through the membrane.

The plasma membrane is made up of the phospholipid bilayer. The membrane is an amphipathic sheet-like structure that is fluid and electrically polarized. The membrane itself has little functions, but the proteins that are integrally and peripherally integrated to the membrane help mediate many of the functions that we contribute to membrane. The membrane is asymmetric in that the proteins are randomly distributed across the membrane, some are attached inside the cell, some outside, and others integrated within membrane. Also, rapid lateral diffusion and slow transverse diffusion contribute both to the membrane's asymmetric characteristic and fluidity. In transverse diffusion, phospholipids are flipped inside-out or outside-in, and this flipping is regulated by flipases. However, the longer the fatty acid chains are, the less likely for transverse diffusion to occur. Longer chains also decreases the fluidity of the plasma membrane. There are other factors that may affect plasma membrane's fluidity. For example, the better arranged the fatty acids chains are, the less fluid the membrane is. On top of that, the more unsaturated the fatty acids are, the more fluid the membrane is. This is because the double bonds bend the chains that allows sloppy arrangements. The interruption of cholesterol within the membrane also causes more fluidity since the polar hydroxy group in cholesterol disrupts the hydrophobic environment within the phospholipid bilayer.

Glycolipids

Glycolipids are sugar(glyco-)containing lipids. They are derived from sphingosine instead of a form of phospholipids that derives from glycerol (phospholipids exist in both derivatives from glycerol and sphingomyelin platform). Another difference from phospholipids is that glycolipids contain a sugar unit (can be glucose or galactose) instead of a phosphate group.

Examples: Glycolipid molecules exist from the most basic molecule, cerebroside which contains 1 fatty acid unit, a sphingosine backbone, and 1 sugar unit (glucose or galactose), to the most complex molecules containing branched chains of multiple sugar residues (up to seven residues in gangliosides).

Properties: When glycolipids exist in membranes, their sugar residue terminal always face the extracellular side.

Chemical structure of Glycolipids

Structure of Glycolipids'

Cholesterol

Cholesterol

Cholesterol is a form of lipids that differs from the rest of its relatives. It is relatively medium molecule that contains 4 adjacent cyclic hydrocarbon molecules with three six-member rings and one five-member ring that has a hydroxyl and a saturated hydrocarbon chain terminals.

The molecule functions as a bufferor a temperature stabilizer for the membrane in which it can make up of 25% of the membrane. When exist in membranes, the 4 cyclic molecules in the cholesterol molecule lay parallel to the fatty acid chains of the phospholipids, meanwhile the hydroxyl terminal points in the direction with the polar phospholipid heads in which it interact with.

Cholesterol molecules exist primarily in nerve cells. The molecule binds to the myelin sheath membrane which provides an outer coating that protects the nerve cell from its surroundings.

It is an essential predecessor to sex hormones that exists in males (testosterone) and females (oestradiol). Also an essential component in vitamin D that enables the body to utilize calcium to form bones.

Animals acquire very little cholesterol from the food they eat; they make cholesterol within the body. Although cholesterol is essential for many processes and structural function, it can be detrimental to have excess cholesterol. Too much cholesterol in the blood will cause blockages in the arteries which can result in heart disease, high blood pressure, and stroke. Only 0.25% of human beings retain High Cholesterol disease from heredity, however people are gaining high cholesterol in their blood from the food they eat (especially people in America).

Membrane Properties

The cell membrane has a set of properties that are contributed to the presences of the lipids as well as proteins. 1. Structures are lik sheets. 2. They are formed by lipids, proteins, and carbohydrates. 3. The membrane is amphipathic (contain hydrophobic and hydrophilic regions). 4. Each protein allows for the function of the membrane. 5. Membranes are held on by weak, non-covalent bonds. 6. The structure is asymmetric. 7. It is high in fluidity. 8. The membrane is polarized.

Fluidity

Lipid unsaturation effect

The presence of the lipids in the membrane structure of a cell is vital for the cell especially affecting its fluidity. As addressed in the section below, this is necessary to allow things to flow in and out of the cell. One factor that plays a big role in this is cholesterol as shown below. Another one is the presence of double bonds. The more the double bonds, the greater the amount of kinks or curve in the lipid and therefore more free space. The length of the lipids also plays a role. Lipids move in two distinct ways. Most commonly they interact in lateral diffusion where they switch places with the lipid to the left or right of them. Other times, they go through transverse diffusion where they flip flop with the ones whose tails they are facing. This is due to the weak, Van der Waals interaction of the lipid molecules. The longer the lipid is, the stronger this interaction is, therefore decreasing the mobility of the lipids. Decrease lipid mobility yields in decreasing the fluidity of the membrane.

Cholesterol and fluidity

This shows the cholesterol molecules submerged in the lipid bilayer

Cholesterol is an important factor in membrane permeability, that is, how much can flow through the cell. Cholesterol acts as a 'buffer' to prevent against any extremes. Obviously, the membranes permeability cannot be too fluid as to allow anything inside the cell(i.e. harmful agents),but at the same time, the membranes permeability must be fluid enough as to let out/let in important agents that need to enter the cell.

Cholesterol is a hydrocarbon steroid with one single alcohol group, which leads to its amphiphatic nature.

Generally,

1.AT LOW TEMPERATURES:Cholesterol in a membrane leads to a more fluid membrane.

2.AT HIGH TEMPERATURES:Cholesterol in a membrane leads to a less fluid membrane.

Ether Lipids with Branched Chains

An ether lipid.

Two major factor that separates Archea(bacteria) from Bacteria is the Archea's cell membranes phospholipid consists of ether linkages and the fatty acid hydrocarbon chains are completely saturated and branched with a methyl group every 5 carbons. These simple structural differences provides Archeabacteria drastic difference from bacteria in terms of their habitat's harsh environments. These 2 factors contribute to the chemical properties that their membrane is more resistant to hydrolysis (ether versus ester linkages) and resistant to oxidation (branched saturated hydrocarbon chains).

Phospholipids and Glycolipids Readily Form Bimolecular Sheets in Aqueous Media

Phospholipids and glycolipids have amphipathic characteristics which enables them to form a micelle or a "lipid bilayer". Due to the hydrophobic hydrocarbon tail and the hydrophillic polar head group, the lipids arrange in a form where the polar groups face water while the tail is away from water. One formation is the micelle where the lipids arrange themselves in a circle with the head groups making the circumference while the tails are inside. A more favorable formation is the lipid bilayer or the bimolecular sheet. This arrangement has the lipids form a barrier where the polar head groups face the aqueous media and the hydrophobic tail face inside away from water. This type of formation is favorable to cell membranes for it forms a barrier from the extracellular fluid and protects the cytoplasm within the cell. Integral and peripheral proteins may be present in the lipid bilayer to allow certain functions to occur such as transportation of ions or acting as pumps.

References

Berg, Jeremy; Tymoczko, John; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. 2007. Berg, Jeremy M., Tymoczko, John, L., Stryer, Lubert. Biochemistry. Seventh Edition.

The Organics of Biochemistry:[12]

"Lipid bilayer." Wikipedia. 7 December 2012. 7 December 2012 <http://en.wikipedia.org/wiki/Lipid_bilayer>.

Viadiu, Hector. "Lipids and Cell Membranes." UCSD, 19 November 2012. Protein molecules contain polypeptide chains made from sequences of the 20 amino acids. These amino acids are linked together by a peptide bond that is formed by condensation of two amino acids with the elimination of the elements of water. Protein function is dependent on its tertiary structure. Proteins tend to fold into three- dimensional structures because of the sequence of amino acids. Proteins also contain functional groups from each amino acid. These groups are reactive and also contribute to protein function. Proteins also interact with one another and with other macromolecules. Proteins can be rigid or flexible. This allows certain proteins to be found in different parts of the cell such as the cytoskeleton or in soft tissue.

Importance of Proteins

Enzymes are proteins that catalyze chemical reactions. Enzymes speed up the reactions in biological systems by lowering the activation barrier needed to start that reaction.

Hormones are proteins that are chemical messengers in the body. These proteins are sent to different parts of the body to send or receive messages. Hormones are very important in regulating the human body and keeping the body in a state of homeostasis. Some protein hormones include insulin, growth hormone, Luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). These proteins are part of the glycoprotein hormones.

Transport Proteins are also used to transportation. For example, hemoglobin is a metalloprotein(protein that contain a metal as cofactor) that transports an oxygen in the red blood cells with the help of iron.

Motor Proteins help convert chemical energy to mechanical energy which relates to muscular motion in an organism. Examples are actin and myosin.

Protective Proteins protect cells by releasing, making antibodies, fighting and destroying foreign objects. Antibodies are gamma globulin proteins.

Structural Proteins help maintain the structure of a variety of biological components like cells and tissues in an organism. Collagen, elastin, α-keratin, sklerotin, and fibroin are all examples of proteins that contribute to the formation of an organism's body.

Storage Proteins that contain energy and can be digested during metabolism of the organism. Examples are egg ovalbumin and milk casein.

Membrane Proteins include receptors and membrane transport. The receptors in the membrane of cells allow ions to pass through. These prevent unwanted objects from coming into the cell. These receptors also determine if the cell is excited to create an action potential or not. Membrane transport is important because it allows ions, proteins, and other macromolecules to pass through the cell membrane.

Classification of proteins by location

External proteins-proteins outside of cells and are found in multicell organisms.

Internal proteins-proteins that are inside cells and perform functions for intercellular needs.

Membrane proteins-proteins that are embedded in the bilayer of the membrane of on the edges of the membrane helping with intracellular interactions.

Virus proteins-usually the coat for viruses

Enzymes

Enzymes are proteins that speed up the reaction rate. Many reactions cannot occur without the use of an enzyme.

This graph shows how the presence of an enzyme lowers the activation energy and therefore speeds up the reaction. Reactions with enzymes can speed up to 10 billion times faster than those without an enzyme. The rate at which the enzyme works is affected by the substrate and enzyme concentration, temperature, and pH.

Amino Acids

There are 20 amino acids that make up proteins. The main chain has an N terminal which is an amino group (NH₂) and a C terminal which is a carboxyl group (COOH). The side chains make each amino acid unique.

The 20 different amino acids can be classified into six different classes based on their side chain(R group).

1. Aliphatic - carbon side chain. The longer the aliphatic chain, the more hydrophobic.

Glycine, Alanine, Valine, Leucine, Isoleucine

2. Hydroxyl or Sulfur containing- The OH is reactive, hydrophilic (water loving), polar and uncharged. Sulfur is very reactive.

Serine, Threonine, Cysteine, Methionine

3. Cyclic- Proline

4. Aromatic-

Phenylalanine - purely hydrophobic

Tyrosine - OH reactive

Tryptophan - less hydrophobic due to its NH groups

5. Basic- hydrophilic and positively charged.

Lysine, Arginine, Histidine

6. Acidic and their amide- negatively charged.

Aspartate, Glutamate, Asparagine, Glutamine

This is the general structure of an amino acid in its unionized form.

This table shows all the amino acids and the their side chains. From left to right they are: glycine(Gly), alanine(Ala), valine(Val), Leucine(Leu), isoleucine(ILe), methionine(Met), phenylalanine(Phe), proline(Pro), aspartic acid(Asp), glutamic acid(Glu),serine(Ser), threonine(Thr), cystine(Cys), tyrosine(Tyr), asparagine(Asn), glutamine(Gln), tryptophan(Trp), lysine(Lys), arginine(Arg), and histidine (His).

Peptide Bonds

Proteins are made from many amino acids. They are connected together by peptide bond. Peptide bonds are formed by condensation, the loss of awater molecule and between the carboxyl group with the aminogroup.

The reaction above shows how two alanine are linked together by a peptide bond. The bond is formed between the n terminal amino group and the c terminal carboxyl group. Two hydrogens and an oxygen come out in this reaction which produces water. The peptide bond acts almost like a double bond due to the resonance of the carbonyl. Because of this, there is no rotation about this bond so therefore conformation is limited. This limits stereochemistry in that almost all peptide bonds in proteins are trans isomers to limit steric hindrance between the R groups. Only proline can be either cis or trans because the energy levels these two isomers show are about the same(the side chain of proline has similar distance to the adjacent R group in either isomer). The reason for that is because proline's side chain form a ring with the alpha-amino group. Proline is the only amino acid whose side chain form a ring with the alpha-amino group.

Structures

Proteins can fold into four different structures. These structures determine protein function and characteristics.

Primary Structure- The primary structure of a polypeptide is its amino acid sequence, from beginning to end. The primary structures of polypeptides are determined by genes. Genes carry the information to make polypeptides with a defined amino acid sequence. For the protein to function correctly, each amino acid needs to be in order as the genes assigned. Even a little change in the amino acid sequence would affect the shape of the protein and its ability to function. An average polypeptide is about 300 amino acids in length, and some genes encode polypeptides that are a few thousand amino acids long.

Secondary Structures- The amino acid sequence of a polypeptide, together with the laws of chemistry and physics, cause a polypeptide to fold into a more compact structure. Amino acids can rotate around bonds within a protein. This is the reason proteins are flexible and can fold into a member of shapes. Folding can be irregular or certain regions can gave a repeating folding pattern. Such repeating patterns are called secondary structures. The two types are the α-helix and β-sheet. In an α-helix, the polypeptide backbone forms a repeating helical structure that is stabilized by hydrogen bonds. These hydrogen bonds occur at regular intervals and cause the polypeptide backbone to form a helix. In a β-sheet, regions of the polypeptide backbone come to lie parallel to each other. When these regions form hydrogen bonds, the polypeptide backbone form a repeating zigzag shape called a β-sheet.

Tertiary Structure- As the secondary structure becomes established due to the primary structure, a polypeptide folds and refolds upon itself to assume a complex three-dimensional shape called the protein tertiary structure. The tertiary structure is the three-dimensional shape of a single polypeptide. It's usually a result of interactions among the R groups of the amino acids that make up the polypeptide. For some proteins, such as ribonuclease, the tertiary structure is the final structure of a functional protein. Other proteins are composed of two or more polypeptides and adopt a quaternary structure. Tertiary structure is important in regards to enzymatic activity.

Quaternary Structure- Most functional proteins are composed of two or polypeptide that each adopt a tertiary structure and then assemble with each other. The individual polypeptides are called protein subunits. Subunits may be identical polypeptides or they may be different. Each subunit has a non-protein component that is necessary for the protein to function correctly. These components are called heme. When proteins consist of more than one polypeptide chain, they are said to have quaternary structure and are also known as multimeric proteins, meaning many parts.

Factors that influence protein structure

Several factors determine the way that polypeptides adopt their secondary, tertiary and quaternary structures. The amino acid sequences of polypeptides are the defining features that distinguish the structure of one protein from another. As polypeptides are synthesized in a cell, they fold into secondary and tertiary structures, which assemble into quaternary structures for most proteins. As mentioned, the laws of Chemistry and physics, together with amino acid sequence, govern this process. Five factors are critical for protein folding and stability:

1. Hydrogen bonds

2. Ionic bonds and other polar interactions

3. Hydrophobic effect

4. Van der waals forces

5. Disulfide bridges

The image shows the two subunits with alpha units in red and beta units in yellow.

Recently, the nature of protein structure space has been widely discussed in the literature. The traditional discrete view of protein universe as a set of separate folds has been criticized in the light of growing evidence that almost any arrangement of secondary structures is possible and the whole protein space can be traversed through a path of similar structures. Here we argue that the discrete and continuous descriptions are not mutually exclusive, but complementary: the space is largely discrete in evolutionary sense, but continuous geometrically when purely structural similarities are quantified. Evolutionary connections are mainly confined to separate structural prototypes corresponding to folds as islands of structural stability, with few remaining traceable links between the islands. However, for a geometric similarity measure, it is usually possible to find a reasonable cutoff that yields paths connecting any two structures through intermediates

There has recently been much discussion on the origins of protein structure space. Researchers have been debating whether proteins are made up of discrete structure groups or a continuum. The traditional view of distinct structural folds has been questioned and many researchers are supporting the continuous view. The discrete view sees proteins as separate folds whereas continuous view supports the idea that any arrangement of secondary structures can be possible. Instead of debating on which of the two is correct, researchers have started to assert that both continuous and discrete views represent a duality in the sense that each view is necessary and present in protein structure space. Discrete and continuous views are actually complementary, protein structure space is discrete on an evolutionary level but continuous geometrically. Evolutionary connections are made by looking at certain folds as island of structural stability. To view protein structures for their geometric similarities, we see paths that connect any two structures through intermediates.

Discrete view:

The traditional discrete view was developed under the idea that there are many structural similarities present in protein structures. This idea was developed by utilizing X-ray crystallography to study the earliest protein structures, myoglobin and hemoglobin. Scientists have found that both myoglobin and hemoglobin have similar structures despite having different sequences. Other examples of structural similarities in protein structures include chymotrypsin and trypsin, several TIM beta/alpha barrels, Rossmann folds, and immunoglobin-like beta sandwiches. All of these structures are unique and recognizable..

The concept of ‘folds’ was developed to describe this discreteness in protein structure. Therefore, newly determined structures can be recognized as being one of these clear and recognizable structures or could be used to establish a ‘new’ fold so other protein structures could identify with it in the future. This idea was supported by the fact that most structures matched these commonly seen prototypes of protein structure.

The TIM-barrel fold is an example of a prototype. A great number of metabolic enzymes belong in the TIM-barrel fold prototype. Although most of these families did not show much similarity in terms of sequence, X-ray crystallographers found TIM-barrels in may enzyme families. These TIM-barrel families are known to have been created from gene duplications and build-up of metabolic pathways.

The fold discovered by Michael Rossmann is another example of a prominent structural prototype. This doubly wound fold is recognized by two right-handed βαβ units that are placed in a centrosymmetric form to make hydrogen bonds between the first strands of the units.

There is an evolutionary process that is occurring to evolve into a thermodynamically stable structure. Mutations that occur diverge from the evolutionary process by shying away from thermodynamic stability. Thus, these mutated proteins are eliminated by selection. The stabilizing selection that drives evolution makes it so movements between folds are uncommon. If we look at protein structure space in an evolutionary view, protein discreteness is characteristic of it.

Continuous view:

Many publications suggest that structure space is continuous. Similarity metrics like TM-score have allowed scientists to find a connection between any two structures through no more than seven steps.

Continuous space is not caused by evolution but by rules of hydrogen-bond formation and antiparallel and parallel arrangements of secondary structures. A continuous protein structure space also does not involve transitivity. The continuous view is more involved with geometry, which is also not transitive.

The term ‘fold’ has been questioned in a few publications on the account that categorizing structures into nonoverlapping folds can lead to missing important functional connections in different folds.

Grouping commonly seen structures with very distinct geometry can be very helpful for categorization and visualization. Using these groupings such as ‘TIM-barrel’. ‘Rossmann fold’, and ‘OB-fold’ that were discussed earlier could be helpful to determine structure and function. On the other hand, other types of ‘folds’ may be more ambiguous and only particular to a certain evolutionary group. Take proteins of different alpha-helical folds for instance. They are often characterized by progressive changes in the angles of helix packing in comparison to discrete topological differences. In cases of ambiguity such as these, the continuous view may be more appropriate. To determine protein function, it is necessary to study the lists of structure similarities rather than just fold assignments because placement and conformation of functional sites can be common even for protein structures with different folds. For example, TIM-barrels and Rossmann folds have discretely distinct geometries and their active sites have similar locations in between beta-strands and alpha helices.

‘Fold’ is a term that is essential to the SCOP classification (structural classification of proteins) and is a standard for making evolutionary connections between proteins. Proteins are said to have the same fold if they possess the same major secondary structural elements in the same mutual orientation and connectivity. This type of classification can cause problems because subjectivity is involved in choosing which secondary structure is major.

The definition of ‘fold’ is more of an empirical approximate ‘art’ because classification criteria for proteins are very loose and based on several things: structural data, and evolutionary and functional considerations.

Scientists are now using a revised term called ‘new fold’. Using this case, scientists are easy to miss meaningful connections between evolution and function with protein structure.

Scientists have questioned the immediate and practical value of protein structure space. They are fundamentally important, and especially necessary in structure prediction. The continuous viewpoint says that all structures are predictable if approximately 40% of overlapping structures are present. Another way these viewpoints can be applied is in predicting functional properties. Discrete and continuous views are both helpful for protein prediction. Both views are applicable in determining functionality, therefore one should not be seen as less important than the other, and neither should be criticized.

This idea of both continuous and discrete views playing a part in protein structure space suggests that there is a duality present in the nature of protein structure space. On an evolutionary standpoint, protein structure space is mostly discrete, and certain regions of stability correspond to certain protein folds.  There are visible evolutionary connections between these regions.  Geometrically speaking, the protein structure space is continuous.  Any arrangement of secondary structures is possible, and almost any two structures can be connected by a path of intermediate and locally similar structures.  An important note to remember about both discrete and continuous views is that homology is transitive, and homology is a property of discreteness; structure similarity is not.  The continuous view is not transitive.

Model vs. Structure

Amidst research and study of proteins, there are confusions between 'structure' and 'models'. Often confused with structure, model refers to the arrangement of models based on the interaction of the molecules (which can include the side chains and bonds), homology between the molecules, or general reasoning. It is often part the data retrieved from experiments used to guide towards a model building. Models can be very simplistic or intricate. It can include arrows to represent the α or β structures, or arrows to show a path. They can also show the constrains of a measurement.
The structure of a protein refers to the spatial interaction of atoms with their covalent and non-covalentness. A structure is often obtained through experiments using x-ray crystallography, electron microscopy, NMR, etc. Electron microscopy and crystallography provide accuracy with the spatial resolution, while NMR experiments provide information on the atoms’ arrangements and their coordination. _[1]

Chemical Reactions with Proteins

Proteins are made, modified, and identified in organic chemistry. Organic chemical reactions are necessary for proteins to exist and to function. The previous sections talk about reactions link together proteins. An important part of biochemistry and organic chemistry is to identify proteins and the sequence the amino acid are linked together. Identifying the sequences allows for comparison between other proteins. They are also valuable for making DNA and for encoding genes. The following techniques involve organic chemistry to help identify the sequence of proteins.

Edman Degradation

Edman degradation determines the amino acid at the N terminal. The reagent Phenyl isothiocyanate is used to cleave off the N terminal amino acid while leaving the rest of the chain intact. This is useful in identifying short peptides. However, it will take a long time to determine the sequence if the chain is long since it only identifies one amino acid at a time. Efficiency of this method decreases each time it is repeated. Therefore the best way to identify longer chains are to break them into smaller peptides for analysis.

Chemical cleavage

cyanogen bromide- CNBr cleaves chains at the carboxyl side of methionine

O-lodosobenzoate- cleaves carboxyl side of trypotophan

Hydroxylamine- cleaves asparagine- glycine bonds

2-nitro-5-thiocyanobenzoate- amino side of cysteine residues

Enzymatic cleavage

trypsin- cleaves carboxyl side of lysine and arginine

clostripain- cleaves carboxyl side of arginine residues

staphylococcal protease- carboxyl side of aspartate and glutamate

thrombin- cleaves carboxyl side of arginine

chymotrypsin- cleaves carboxyl side of tyrosine, tryptophan, phenylalanine, leucine, and methionine, mostly aromatic side chains

carboxypeptidase A- cleaves the amino side of C-terminal amino acid except arginine, lysine, or proline

Protein Degradation

Protein degradation is an important process that breaks down proteins into their smaller subunit amino acids. Degradation occurs in order to maintain and provide the body with a steady amount of amino acids. Misfolded or damaged proteins are broken down as they serve no purpose and must be destroyed. Dietary proteins are also degraded. Dietary proteins contain amino acids that cannot be synthesized in the body and therefore must be obtained through the foods we eat. These amino acids, known as essential amino acids, are a vital part in the resynthesis of new proteins.

Proteins are first digested in the stomach where the acidic conditions provide an optimal environment for the denaturing of the protein. The low pH enables proteolytic enzymes to unfold and degrade the protein. Pepsin is the main enzyme found in the stomach. Further break down of proteins occur in the lumen of the intestine. The pancreas provides the main source of the proteolytic enzymes that help in the break down of the protein. The ubiquitin-proteasome degradation of proteins is one pathway the body uses to regulate the supply of amino acids. A smaller protein called ubiquitin first tags the protein that needs to be degraded. This creates a marking on the protein that signals the proteasome to further digest it.

Once the protein is fully digested, the free amino acids counterparts are carried through the blood and delivered to necessary tissues to be absorbed and used for new protein synthesis. The free amino acids can also serve as energy in the process of cellular respiration. The amino group from the amino acid must first be removed so that the carbon skeleton can then be converted into a carbohydrate or fatty acid. The amino group is processed through the urea cycle where the nitrogen is disposed of.

Reference

Berg, Biochemistry, 6th Edition

http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/biochemistry/biochemi.htm
[1] Fandrich, Marcus, Schmidt, Matthias, and Grigorieff, Nikoaus: Trends Biochem Sci. 2011 June; 36 (6) 338-345. “Recent Progress in understanding Alzheimer’s β-amyloid structures” A reagent is an inorganic or small organic molecule that helps the reactant react in a chemical reaction.

List of Reagents, Its Uses and Information

1. AIBN [azobis (isobutyronitrile)] is used for radical initiator. AIBN is a white acicular crystal, which is insolvable in water, solvable in organic solvent such as methyl alcohol, ethanol, acetone, ethyl ether and light petroleum etc. Ice point of pure product is 105 degree Celsius. The product is decomposed flashily and it releases nitrogen gas in the position of melting. It decomposes slowly under ordinary temperature, which should be stored under 20 degree Celsius. AIBN is maily used as polymerization initiator of monomer such as chloroethylene, vinyl acetate,acrylonitrile, etc. Also, it is used as blowing agent for PVC, polyalkene, polyurethane, polyvinyl alcohol, acrylonitrile/butadiene copolymer, chloroethylene copolymer, acrylonitrile/ butadiene/styrene copolymer, polyisocyanate, polyvinyl acetate, polyamide and polyester, etc. Moreover, it is also used in other organic synthesis.
2. AlCl₃ (aluminum trichloride) is used for Lewis acid catalyst. It is a yellowish or grayish-white, crystalline powder with a sharp oodor. It is used as a chemical intermediate for Aluminum compounds, as a catalyst for cracking petroleum, in preserving wood, and in medications, disinfectants, cosmetics, photography and textiles.
3. BF₃ (boron trifluoride) is used for Lewis acid catalyst of chemical reactions. It is a colorless gas with a pungent odor. It reacts readily to form coordination complexes with molecules having at least one pair of unshared electrons.
4. BH₃ (borane) is used for hydroboration. Borane-lewis base complexes are often found in literature. Borane-tetrahydrofuran (BTHF) and borane-dimethyl sulfide (BMS, DMSB) are often used as a borane source. Both reagents are available in solution (e.g. 1 M in THF), and are therefore easier to handle than diborane. Volatility and flammability are always a drawback. BMS is more stable than BTHF but has an unpleasant odor.
5. Br₂ (bromine) is used for radical bromination and dibromination. Bromine compounds are used as pesticides, dyestuffs, water purification compounds, and as a flame-retardants in plastics. 1,2-dibromoethane is used as an anti-knock agent to raise the octane number of gasoline and allow engines to run more smoothly. This application has declined as a result of environmental legislation. Potassium bromide is used as a source of bromide ions for the manufacture of silver bromide for photographic film.
6. CCl₄ (carbon tetrachloride) is used for nonpolar inert solvent. It is a manufactured chemical that does not occur naturally. It is a clear liquid with a sweet smell that can be detected at low levels. It is also called carbon chloride, methane tetrachloride, perchloromethane, tetrachloroethane, or benziform. Carbon tetrachloride is most often found in the air as a colorless gas. It is not flammable and does not dissolve in water very easily. It was used in the production of refrigeration fluid and propellants for aerosol cans, as a pesticide, as a cleaning fluid and degreasing agent, in fire extinguishers, and in spot removers. Because of its harmful effects, these uses are now banned and it is only used in some industrial applications.
7. CHCl₃ (chloroform) is used for polar, nonflammable solvent. It is also a highly volatile, clear, colourless, heavy, and highly refractive.
8. CH₂Cl₂ (dichloromethane) is used for polar, nonflammable solvent. Chloroform has a relatively narrow margin of safety and has been replaced by better inhalation anesthetics. In addition, it is believed to be toxic to the liver and kidneys and may cause liver cancer. Chloroform was once widely used as a solvent, but safety and environmental concerns have reduced this use as well. Nevertheless, chloroform has remained an important industrial chemical.
9. CH₂I₂ (diiodomethane) is used for Simmons-Smith cyclopropanation. It is a colorless liquid. It decomposes upon exposure to light liberating iodine, which colours samples brownish.
10. CH₂N₂ (diazomethane) is used for making methyl esters from acid and cyclopropanation. It is not only toxic but also potentially explosive.
11. DIBAL (diisobutylaluminum) is used for selective reduction of esters, amides, and nitriles to aldehydes.
12. Dicycolhexylborane is used for hydroboration of alkyne derivatives and anti-Markovnikov hydration.
13. Dioxane is used for good solvent for dissolving water and organic substrates. It is a colorless liquid with a faint sweet odor similar to that of diethyl ether. It is classified as an ether.
14. DMD (dimethyldioxirane) is used for epoxidation of alkenes. It is the most commonly used dioxirane in organic synthesis, and can be considered as a monomer of acetone peroxide.
15. DMF (dimethylformamide) is used for polar aprotic solvent. This colourless liquid is miscible with water and the majority of organic liquids. DMF is a common solvent for chemical reactions.
16. DMSO (dimethylsulfoxide) is use for polar aprotic solvent. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water.
17. Et₂O (diethyl ether) is used for medium polarity solvent. It is a colorless, highly volatile flammable liquid with a characteristic odor.
18. FeBr₃ (iron tribromide) is used for Lewis acid catalyst in the halogenation of aromatic compounds.
19. H₂ (hydrogen) is used for hydrogenation and reduction of nitro. Hydrogen is the only element that can exist without neutrons. Hydrogen’s most abundant isotope has no neutrons. Hydrogen forms both positive and negative ions. It does this more readily than any other element. It is the most abundant element in the universe. Hydrogen is the only atom for which the Schrödinger equation has an exact solution. Moreover, it reacts explosively with the elements oxygen, chlorine and fluorine: O2, Cl2, F2.
20. H₂O₂ (hydrogen peroxide) is used for oxidative workup of hydroboration. It is used to help stop infection in cuts or scrapes, we can use it as a mouthwash when diluted with water, and it is also used to bleach hair.
21. Hg(OAc)₂ (mercuric acetate) is used for oxymercuriation. Mercuric Acetate can affect in berating and by passing through skin. Also, mercuric acetate should be handled as a teratogen with extreme caution. Mercury poisoning can cause "shakes", irritability, sore gums. It increased saliva, personality change and permanent brain or kidney damage. Mercury accumulates in the body.
22. HgSO₄ (mercuric sulfate) is used for Markovnikov hydration of alkynes. It is an odorless solid that forms white granules or crystalline powder. In water, it separates into an insoluble sulfate with a yellow color and sulfuric acid.
23. HIO₄ (metaperiodic acid) is used for oxidative cleavage of 1,2-diols. In dilute aqueous solution, periodic acid exists as discrete hydronium and metaperiodate ions.
24. HMPA (hexamethylphosphoramide) is used for preventing aggregation (polar aprotic solvent). It is a phosphoramide having the formula [2N]3PO.
25. K₂Cr₂O₇/H₂SO₄ (potassium dichromate) is used in oxidation of alcohols. (Jones Reagent)
26. LAH (lithium aluminum hydride) is used for very strong hydride source and reduces esters to alcohols. It is an inorganic compound with the chemical formula LiAlH4.
27. LiAl(Ot-Bu)₃H [lithium tri(t-butoxy) aluminum hydride] is used for modified hydride source and reduces acid chlorides to aldehydes.
28. LDA (lithium diisopropylamide) is used for strong, hindered base.
29. Lindlar's catalyst is used for reducing alkynes to cis-alkenes.
30. mCPBA (m-chlroperbenzoic acid) is used for epoxidation of alkenes.
31. MnO₂ (manganese dioxide) is used for selective oxidation of allylic alcohols.
32. MsCl [methanesulfonyl chloride(mesyl chloride)] is used for converting hydroxyl to a good LG.
33. NaBH₄ (sodium borohydride) is used for mild source of hydride. It is an inorganic compound.
34. NaBH₃CN (sodium cyanoborohydride) is used for reductive amination and hydride source stable to mild acid.
35. NaNO₂ (sodium nitrite) is used for diazotization of amines. Sodium nitrite is a salt and an anti-oxidant that is used to cure meats like ham, bacon and hot dogs. Sodium nitrite serves a vital public health function: it blocks the growth of botulism-causing bacteria and prevents spoilage. It is also gives cured meats their characteristic color and flavor. Also, USDA-sponsored research indicates that sodium nitrite can help prevent the growth of Listeria monocytogenes, an environmental bacterium that can cause illness in some at-risk populations.
36. NBS (N-bromosuccinimide) is used for bromine surrogate. It is a brominating and oxidizing agent that is used as source for bromine in radical reactions. For example: allylic brominations and various electrophilic additions. The NBS bromination of substrates such as alcohols and amines, followed by elimination of HBr in the presence of a base, leads to the products of net oxidation in which no bromine has been incorporated.
37. n-BuLi is used for strong base.
38. NCS (N-chlorosuccinimide) is used for chlorine surrogate.
39. O₃ (ozone) is used for oxidative cleavage of alkenes. It is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope, breaking down with a half life.
40. OsO₄ (osmium tetroxide) is used for dihydroxylation of alkenes.
41. PCC (pyridinium chlorochromate) is used for selective oxidation of primary alcohols to aldehydes.
42. PPH ₃ (triphenylphosphine) is used for making Wittig reagents.
43. SOCl₂ (thionyl chloride) is used for converting alcohols to alkyl chlorides.
44. THF (tetrahydrofuran) is used for medium polarity solvent.
45. pTsCl [p-toluenesulfonyl chloride (tosyl chloride)] is used for converting hydroxyl to a good LG.
46. pTsOH [p-toluenesulfonic acid (tosic acid)] is used for oragnic-soluble source of strong acid.
47. Zn(Hg) (zinc amalgam) is used for Clemmensen reduction (with HCl).
48. Jones Reagent (CrO3, H2SO4, H2O) is a solution of chromium trioxide in diluted sulfuric acid that can be used safely for oxidations of organic substrates.
49. SOCL2 - forms alkyl chlorides from alcohols
50. Clemmensen Reduction (Zn(Hg), HCl) - removes a ketone and replaces it with hydrogens.
51. Grinard reagents (R-Mg-X) - an organometalic chemical reaction where an alkyl-magnesium halide is added to a carbonyl group in an aldehyde or ketone.

References

1. http://en.wikipedia.org/wiki/Reagent
2. http://www.reagents.com/
3. http://www.chem.wisc.edu/areas/reich/handouts/namereagents/namedreag.htm

Fischer Esterification

The Fischer esterification mechanism is a reversible reaction between an alcohol and a carboxylic acid. Upon acid catalysis, the carboxylic acid is protonated and transforms into an activated electrophile. The alcohol, being the nucleophile, attacks the carbonyl carbon because of its positive charged electronegativity experienced by the withdrawing oxygen atom. The result of this attack is a tetrahedral intermediate. Once the intermediate is formed, the alcohol group from the original carboxylic acid undergoes deprotonation while the hydroxyl group protonates. With a protonated hydroxyl group, the functional group is removed by delocalized electrons from the carboxyl group resulting in the removal of a water molecule, leaving the final product as an ester. Due to equilibrium mechanics, all the steps mentioned in the Fischer esterification mechanism can be reversed, which is not desired in order to achieve a final ester product. Le Chatelier’s principle can be used to manipulate the reaction in order for the creation of ester to be more favorable than the reverse reaction. In order for this to happen, an excess of carboxylic acid or alcohol can be added along with the removal of water, the side product of the reaction found in the forward reaction.

When a chemical reaction is introduced, oftentimes only the reactants and products are shown. In the example shown below, normally the ester would be shown as the starting material, along with its product, the carboxylic acid. It is our job to describe the overall transformation taking place in a reaction, in the example below—the hydrolysis. In the example shown below, the ester was able to hydrolyze to the carboxylic acid through a series of reaction intermediates. With organic mechanisms it is our job to study how and when bonds break and form, and how different changes in substrates might affect the outcome of a reaction.

Base-Catalyzed Hydrolysis and Transesterification

Upon addition of a base, the old ester bond breaks and a new one forms. This is known as transesterification

Step 1: Catalysis: The base attacks the carbonyl carbon, pushing the electrons from the double bond to the oxygen.

Step 2: Leaving Group: The electrons from the oxygen anion reform the double bond, kicking out the old ester group, completing the hydrolysis.

Step 3: Deprotonation: If an OH- group would have been the nucleophile, then the hydrogen would be deprotonated.

Acid-Catalyzed Hydrolysis and Transesterification

Upon acid catalysis, a old ester bond breaks and a new one is formed.

Step 1: Catalysis: The carbonyl oxygen is protonated by the acid catalyst.

Step 2: Nucleophilic Attack: The nucleophile attacks the carbonyl carbon, pushing the electrons from the double bond to the oxygen. The oxygen from H2O serves as the nucleophile during the attack at the electrophillic carbon of the carbonyl. This creates a tetrahedral intermediate.

Step 3: Intramolecular Protonation: The original -OMe group picks up a proton from the newlyadded water group. This protonation converts -OMe into a good leaving group.

Step 4: Hydrolysis: The electrons from the hydroxl group moves down forming a double bond, thus kicking out the old ester group.

Step 5: Deprotonation: The solvent picks up the proton from the newly formed double bonded oxygen. The deprotonation of the carbonyl oxygen leads to regeneration of the acid catalyst. This leads to the final product.

This shows one example of a mechanism used in organic chemistry. A good understanding of the properties of organic molecules and the nature of their bonding allows us to predict mechanisms.

Hydrolysis without Transesterification

The above images show an ester bond being broken and replaced with another ester group.

Below, you will find the same mechanism, but without the reforming of a new ester bond a.k.a transesterification

Keto- Enol Equilibria

Keto and enol tautomers are in equilibrium in either acidic or basic conditions. An enol is short for an unsaturated alcohol and keto is short for a ketone. In the process of going from a keto to an enol, an enolate is formed which is the resonance structure with a charge delocalized over the oxygen and the carbon. Both the acid and the base catalyzed reactions occur very quickly and that the keto form is more likely to be found in solution.[1]

Acid-Catalyzed Enol-Keto Equilibrium

Step 1: Protonation: The alkene attacks a free acidic hydrogen proton in the acidic solution.

Step 2: Relief of charge: The lone pair from the alcohol moves down to relieve the positive charge on the carbon.

Step 3: Deprotonation: The resulting carbonyl compound with is then deprotonated, most likely from the solvent or water.

Note that it is the same mechanism but in reverse to go from a ketone to alcohol in acid conditions where the carbonyl is protonated and then a proton from a carbon is pulled off leaving the electrons to form an alkene.[1]

Base-Catalyzed Enol-Keto Equilibrium

Step 1: Deprotonation: A base pulls off the hydrogen proton from the alcohol group.

Step 2: Resonance: Resonance structures show that the negative charge can be somewhat stabilized on the carbon.

Step 3: Reprotonation: The negatively charged carbon then pulls in a hydrogen proton, most likely from the base or solvent.

Note that it is the same mechanism but in reverse to go from the ketone to an alcohol in basic conditions where the base would pull of the hydrogen forming the alkoxide intermediate which then takes back the hydrogen from the base.[1]

Aldol Condensation

Aldol Condensation is the attack of an enolate ion on a carbonyl functional group to form a carbon-carbon bond. The name aldol comes from aldehyde alcohol. Aldol condensation can be done with ketones as well as aldehydes, either the same aldehyde/ketone as the starting enolate or a different aldehyde which would be called a crossed aldol condensation. It is a condensation reaction because it releases water. Also note that it regenerates one equivalent of the base used. [1]

Step 1: Enolate Formation: A base (hydroxide in this example) deprotonates a hydrogen and forms an enolate ion with two resonance structures.

Step 2: Enolate Attack: The carbon with the negative charge attacks a second aldehyde (or ketone) at the carbonyl carbon. Note that the enolate alkoxide is more stable with the negative charge than the carbon so the resonance structure is shown to accurately portray the carbon-carbon bond that will form.

Step 3: Protonation: The resulting alkoxide after the enolate attack is then protonated to give the final product. Note that the base is regenerated.

Michael Addition

Similar to other nucleophiles, enolates can do conjugate additions to α,β-unsaturated aldehydes or ketones to form carbon-carbon bonds by the Michael addition reaction. As before, an enolate ion is formed and then the carbon attaches to another carbon of a second reactant that has a conjugated double bond with a carbonyl, called a Michael acceptor. It is a similar mechanism reaction to the aldol condensation.[1]

Step 1: Enolate Formation: A base such as hydroxide deprotonates a hydrogen from the carbon and forms an enolate ion.

Step 2: Enolate Attack: The alkoxide pushes down its extra set lone pair of electrons reforming the carbonyl and the carbon attacks the conjugated carbon of the Michael acceptor. In a chain reaction of electron pushing another alkoxide is formed after the two reactants are fused together by a carbon-carbon bond.

Step 3: Protonation: the newly formed alkoxide is then protonated by either the solvent or water.

References

[1] Vollhardt, Peter, and Neil Schore. Organic Chemistry: Structure and Function. 6th ed. New York: W. H. Freeman and Company, 2011. Stereochemistry: By definition, stereochemistry is the arrangement of different atoms in space. Stereochemistry is a 3d representation of a carbon that is sp3 hybridized. There are many different types of stereoisomers. Let's first discuss a few basic concepts.

Background

Although most people are accustomed to thinking of organic chemistry as a bunch of drawings and structures, this "paper chemistry" is not really how these molecules act in real life. Of course, these molecules are really three-dimensional shapes, and not just 2d drawings. Stereochemistry aims to explain the natural phenomena of spatial arrangements of these organic molecules.

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Stereocenter: Any atom in a molecule that is attached to 4 different atoms. Also known as chiral center. A chiral molecule is special in that it is not identical to its mirror image, or in other words, the only criterion for chirality is that the object and its mirror image must be non-superimposable. For example, methane (CH4) is identical to its mirror image; therefore, this molecule is not chiral. In general, molecules with "n" chiral centers have 2^n stereoisomers. For example, a molecule with 3 stereocenters would give rise to a molecule with 8 stereoisomers. Stereoisomers are isomers of molecules with the same formula and connectivity, but with different arrangements of their atoms in space. Two stereoisomers have atoms linked together in the same order, but the two molecules do not have the same three-dimensional shapes. Wedges indicate bonds coming towards the viewer, while dashed lines indicate bonds going away from the viewer.

A molecule classified as achiral either is not sp3 hybridized or the molecule is sp3 hybridized but has two or more substituents that are the same. These molecules are not chiral. An achiral molecule is super-imposable on itself.

A Brief Introduction to Isomers

This is a flow chart showing the different categories of isomers. Although this chart is in Danish, the spelling is very similar to English. Do not get the spelling confused.

Molecules that are isomers have the same molecular formulas but their structures are different from each other. There are two main categories of isomers: constitutional isomers and stereoisomers. Constitutional isomers, also known as structural isomers, are molecules that have the same molecular formulas but their connectivity is different. Oftentimes constitutional isomers have very different physical and chemical properties. Stereoisomers can be divided into two sub-categories: enantiomers and diastereomers. Enantiomers are molecules that are mirror images of one another and are not super-imposable. These molecules have the same connectivity but differ in their 3D arrangement of carbon substituents. Oftentimes enantiomers have very similar physical properties but they may differ in chemical properties. Diastereomers are molecules that are not related as mirror images but, like enantiomers, have the same connectivity but differ in their 3D arrangement of carbon substituents.

Enantiomers

Enantiomers are structures that are mirror images of one another that are non-superimposable. Non-superimposable means that no matter what way you rotate it, you will not be able to place it directly on top of the other facing the same way. You can easily demonstrate this with your hands because your hands are also non-superimposable, as you cannot place your hands on top of one another with your thumbs facing in the same direction, while your palms face the same way. Enantiomers have identical physical and chemical properties.

This is a depiction of both R and S enantiomers.

Enantiomers are generally classified with either an R or S configuration. To determine whether a stereocenter is labeled R or S, one must first rank its four substituents by molecular weight. The highest priority substituent (rank 1) will be the substituent atom with the largest molecular weight; conversely, the lowest priority substituent (rank 4) will be the substituent atom with the lowest molecular weight. For this reason, a hydrogen substituent is always given the lowest priority. Once the four substituents have been ranked by priority, a series of rules can be followed to determine the stereocenter's R or S configuration.

Rule 1: The lowest priority substituent is always placed as far away as possible, or in other words, placed into the paper or board.

Rule 2: Once the lowest priority substituent (generally a hydrogen) has been placed "into the paper," there are only two possible arrangements of the remaining three substituents (R and S). Looking down the lowest priority substituent bond with the carbon, if the remaining three substituents are increasing in the clockwise direction (1, 2, 3), the stereocenter is classified as R. If the three substituents are increasing in the counter-clockwise direction, the stereocenter is classified as S.

Rule 3: Point of difference rule. This rule helps determine the priority of substituents that have the same rank when looking at the atoms directly attached to the stereocenter. A simple example is the difference between a methyl and an ethyl substituent. Following the substituents beyond the initial carbon atom, a methyl is attached to three hydrogens, while the ethyl's carbon is attached to two hydrogens and another carbon. This is the point of difference in the substituents. Since the carbon atom in the ethyl group is a higher priority than the 3rd hydrogen in the methyl group, the ethyl substituent is ranked higher in priority than the methyl group.

Rule 4: Double and triple bonds can be treated as single bonds when determining priority

Rule 4: Double and triple bonds can be viewed as if they were single bonds, but with the extra bonds added to each end of the multiple bond as single bonds. This can be seen on the figure to the right. The carbon is attached to a thiol group and is double bonded to an oxygen. This double bond can be seen as the carbon attached to two single bonded oxygens, and the second oxygen is bonded a carbon (representing the double bond to the initial carbon).

Enantiomers are optically active, meaning they have the ability to rotate plane polarized light. A dextrorotatory (+) molecule rotates plane polarized light in a clockwise direction, while a levorotatory molecule rotates plane polarized light in a counterclockwise direction. Enantiomers differ in which direction each compound rotates plane polarized light.

Enantiomers: As you can tell from this picture, the methyl is sticking out in one drawing, and sticking back in another. If you rotated one of these, you would NOT be able to superimpose it on the other. An example of Enantiomers.

As you can tell from the drawing, on the left, the methyl group is pointing out at us, while the hydroxyl group is pointing back. On the molecule in the right, this order is reversed. This is a prime example of enantiomers.

Confusion with Enantiomers

One might think that you can simply just rotate the molecule and they would be exactly the same, but this is not the case. If you rotated the molecule above, the hydroxyl and methyl will be imposed on one another, however, the fluorine and hydrogen will now be on opposite sides. This is what it means to be non-superimposable

Oftentimes there are confusions with meso compounds. For example, say there are two molecules: one characterized with 1R, 2S configurations at its stereocenters (chiral center), the other molecule characterized with 1S, 2R configurations at its respective stereocenters. One might wonder why these molecules aren't enantiomers of each other—after all, they do have opposite configurations at all their respective stereocenters. However, these molecules aren't enantiomers because they are meso compounds. In this situation, each compound has a line of symmetry dividing its two stereocenters. Due to the plane of symmetry of each molecule, each compound is considered achiral (not chiral).

Racemization of Enantiomers

Two molecules are considered to be racemic if they consist of the same amount of each enantiomer, a 50:50 ratio of the left-handed, and right handed enantiomer.

A racemic molecule is not shown to rotate plane-polarized light. Individually each enantiomer rotates plane-polarized light, but does so in equal and opposite directions, thus leading to a net rotation of 0.

Optical activity is directly proportional to the ratio of the two enantiomers of a compound. When an equal ratio of the two enantiomers of a compound are present, the sample is considered optically inactive or racemic. If only one of the enantiomers is present, then the sample is considered optically pure. To describe a mixture that falls in-between these two extremes, one can use the enantiomer excess (ee) relation. This is used for when one enantiomer is in excess of the other enantiomer in a mixture. The enantiomer excess equation is:

Enantiomer excess (ee) = % of major enantiomer - % of minor enantiomer

The enantiomer excess equation tells us how much of one enantiomer is in excess of a racemic solution (since in a racemate the ratio of enantiomers is 1:1). The enantiomer excess is also known as optical purity and can be found by the relation:

Enantiomer excess (ee) = optical purity = ([a]_Mixture/[a]_{Pure enantiomer}) X 100%

Example

A solution of (+)-alanine from an artifact has a value of [a]_Mixture = 3.7; [a]_{pure enantiomer} = 8.5. Find optical purity and actual enantiomer composition of the sample.

Solution

Enantiomer excess = optical purity = (3.7/8.5) X 100% = 43.5 % Therefore, 56.5% of the sample is racemic and 43.5% of the sample is the pure (+) isomer. Thus, it can be concluded that the artifact under examination has an actual composition of 71.75% (+) and 28.25% (-) alanine.

Optical Activity

If two enantiomers of a chiral molecule were isolated into two different containers in pure form, it would be extremely difficult to distinguish the two based on just their physical properties such as boiling points, melting points, and densities. Because only certain enantiomers of certain drugs are effective while the other enantiomers are ineffective or even detrimental to the body, it is very important to be able to distinguish enantiomers.

Fortunately chiral molecules possess a unique property that rotates the chiral molecules in a particular direction when a special kind of light called plane-polarized light is passed through a sample containing the enantiomers. This unique reaction that chiral molecules have with plane-polarized light is called optical activity which results in enantiomers often being referred to as optical isomers. The rotation of a particular chiral molecule as seen from the perspective of the viewer facing the light source can either be clockwise, dextrorotatory (“dexter” is Latin for “right”), or counterclockwise, levorotatory (“laevus” is Latin for “left”) [2]. Molecules that rotate clockwise are referred to as (+) enantiomers while molecules that rotate counterclockwise are referred to as (-) enantiomers [2]. The direction of rotation of any particular chiral molecule will always be the same and the enantiomer of that chiral molecule will have an opposite direction of rotation.

This is a simplified depiction of how polarizers filter out a beam of light leaving only the plane-polarized light to pass through.

Plane-polarized light result from passing a beam of ordinary light through a special material called a polarizer. A polarizer serves as a filter for the light waves that pass through it and filters out all but one light wave, the plane-polarized light wave [2]. Once the light has been filtered to just the plane-polarized light, the light wave travels through the chiral molecules. The electric field of the light wave interacts with the electrons around the chiral molecule and causes the molecule to rotate at a particular direction; this rotation is called optical rotation [2]. If there is optical rotation in the sample, then the sample is referred to as optically active. To determine if a sample is optically active, optical rotations have to be measured with a polarimeter.

Oftentimes reactions that yield chiral molecules are not stereospecific which may lead to products that have a 1:1 mixture of enantiomers. This reaction mixture is referred to as a racemic mixture because when a plane-polarized light passes through this sample, half of the chiral molecules would be rotating clockwise while the other half would be rotating counterclockwise. This results in a sample with no net optical activity. However, when the ratio of enantiomers is not 1:1, then there will be optical activity. One of the major goals of pharmaceutical companies is to perform reactions that yield optically pure products because this saves them very much time from the purification process of optically impure samples, not to mention money on the reactants used during production.

Stereochemistry of Amino Acids

This is a depiction of both L and D amino acids.

Amino acids are building blocks of protein. The general structure of an amino acid has an sp3 hybridized carbon atom linked to four other substituents. Three of the four substituents attached to the carbon of all amino acids are an amino group and a carboxylic acid group and a hydrogen atom. The last substituent is often represented with an R if the amino acid isn’t specified. R can represent one of the twenty different side chains the twenty different amino acids have.

The stereochemistry of amino acids are represented with an L,D system. The L,D system is essentially the same as the R,S system for absolute configuration with the exception that the L,D system is for amino acids. For the L,D system, L is the equivalent of the S absolute configuration leaving D to be the equivalent of the R absolute configuration.

Amino acids exist dominantly in the L absolute confirmation. There is no conclusive evidence for the dominance of the L isomer over the D isomer at the moment. Many speculate that the L isomer became the dominant amino acid configuration due to chance rather than any physical or chemical property. There have been experiments done trying to elucidate the reason behind the dominance of the L isomer. In the experiments, a protein would be artificially synthesized with only amino acids with the D absolute configuration. The conclusion was that the proteins made of only D amino acids were equally as active as the proteins made of L amino acids with the only difference being that the proteins made of D amino acids had reactions that were the reverse of the proteins made of L amino acids.

Diastereomers

Diastereomers are any stereoisomers that are not enantiomers. Thus, diastereomers are not mirror images of each other. Because diastereomers are stereoisomers that aren't mirror images of each other, they can be distinguished by different physical and chemical properties. Thus, it is possible to separate diastereomers by standard laboratory techniques. Diastereomers arise when stereoisomers of a molecule have different configurations at at least one of their stereocenters (but not all, for they would then be enantiomers). Each stereocenter possesses two configurations, so the number of stereoiosmers is increased by a factor of two. Cis/trans isomerism is a form of diastereomerism (cis-two substituents pointing in the same direction; trans-two substituents pointing in opposite directions).

This picture shows the cis and trans diastereomers of a butene molecule. For alkenes only, the cis and trans prefix is often replaced with Z or E.

This picture shows the cis and trans diastereomers of a cyclic molecule.

While enantiomers have identical chemical properties, diastereomers do not.

There are 3 stereocenters in the molecule below. The atoms have been switched in the first and third carbon. This is an example of diastereomers. If the second carbons atoms were switched also, then these two molecules would be enantiomers.

Stereochemistry for Alkenes

Cis and trans prefixes are essentially absent when naming alkenes with an E,Z system being used instead. The E,Z system sets priorities for carbon substituents the same way as the R,S system with the difference being that the double-bond having no bearing on the priority, only the two substituents that are single-bonded to the carbon. An E isomer (E comes from the German word “entgegen” which means “opposite”) would be where the highest priority substituents on each carbon on the double bond are opposite of each other [1]. A Z isomer (Z comes from the German word “zusammen” which means “together”) would be where the highest priority substituents on each carbon on the double bond are on the same side of each other [2].

Epimers

Epimers are diastereomers that differ at only ONE stereocenter.

There are 3 stereocenters in the molecule below. The atoms have been switched at the stereocenter in ONLY the third carbon(on right). This is an example of epimers.

Anomers

"Anomers are isomers that differ at a new asymmetric carbon atom formed on ring closure." An example of this is Alpha-D-Glucose and Beta-D-Glucose (shown below).

Alpha-D-Glucose

Beta-D-Glucose

Fischer Projections

D-Glyceraldehyde

In Fischer projections, bonds attaching substituents to the central carbon are shown using horizontal and vertical lines. Similar to the hashed-wedged line structures, horizontal lines are indicative of bonds coming towards the viewer, while vertical lines indicate bonds going away from the viewer. It is a simplified way of showing tetrahedral carbon atoms and their substituents in 2 dimensions. Converting hashed-wedged structures to Fischer projections is a simple process. Here is an example shown below.

Shortcuts for Stereochemistry

A simple method to differentiate between an enantiomer and a diastereomer is to look at the molecules being compared and, assuming they both are in fact stereoisomers, look at each stereocenter. If the carbon substituents of each stereocenter have been switched once, then the molecules are related as enantiomers. If not all of the carbon substituents of each stereocenter have been switched once, then the molecules are related as diastereomers. In this case, the switching of carbon substituents means, for example, that a dashed OH substituent and a wedged H substituent of the same carbon switched to a wedged OH substituent and a dashed H substituent while the other two carbon substituents remain in the same positions. By this logic, if the carbon substituents of the stereocenters have been switched twice, then the molecules are related as enantiomers, not diastereomers.

Chiral Drugs

Today, the majority of chiral medicines are sold as racemic mixtures. In two enantiomers, there would be one that is inactive. According to Vollhardt, the other enantiomer would stop the activity of the other enantiomer by becoming the blocker of the biological receptor site. As a result, the FDA started to produce individual enantiomers of the medicine. Consequently, they began to test the pure enantiomers to help increase the performance of the medicine and it may help to lengthen the patent of medicine. Also, they applied the "chiral switch" method to develop different ways of enantioselective synthesis. "Chiral switch" is the method of switching achiral reactant to a chiral product in a reaction that can be catalyzed by a chiral catalyst. Vollhardt states that such methods have been applied to the production of medicine such as antiarthritic, analgesic naproxen, and antihypertensive propranolol.

References

1. Schore, Neil E. (2011). Organic Chemistry Structure and Function 6th edition. W. H. Freeman.

2. Berg, Jeremy Mark, John L. Tymoczko, and Lubert Stryer.Biochemistry. 6th. New York: W H Freeman & Co, 2012.

3. Schore, Neil E. (2007). Organic Chemistry Structure and Function 5th edition. W. H. Freeman.

3. Berg, Jeremy M. (2002). Biochemistry 5th edition. W. H. Freeman.

4. Vollhardt, Peter and Schore, Neil. (2009). Organic Chemistry 9th Edition. W.H. Freeman and Company. ISBN 978-1-4292-0494-1. Structural Biochemistry/Chemistry of important organic molecules in Biochemistry/

Introduction to Vitamins

Vitamins are critical organic molecules necessary for life. Despite their vital importance, they cannot be synthesized by animals and they must be acquired through diet, whether they are obtained from food or through supplemental vitamins. This odd characteristic may have evolved because of the biological complexity of vitamin synthesis; it is more efficient to obtain vitamins in food in trace amounts than to develop the numerous enzymes necessary to synthesize vitamins. In addition, vitamins must be molecularly modified after ingestion before they can be used in the body. Vitamins can act ascoenzymes, signaling molecules, antioxidants, and hormones, as well as serving various other functions. Vitamin deficiency can lead to conditions such as: scurvy from lack of Vitamin C, and eye damage from lack of Vitamin A. In fact, many activated carriers in metabolism are derived from vitamins, including carriers important for redox reactions, such as electron transport (FADH2, for example), and carbon dioxide transfer (biotin). Because of the critical roles vitamins play in the functioning of the human body, they are an essential topic of study despite the low vitamin requirements of most animals.

Water-Soluble vs. Fat-Soluble

Approximately 14-20 vitamins have been discovered. There are two categories that vitamins fall under: water-soluble as well as fat-soluble.

Defined
Water-soluble: vitamins that easily dissolvable in water and easily excreted out of the body via urine output. As a result of this type of vitamin that can be dissolved in water, individuals cannot overdose on them because all excess will simply be excreted. Moreover, this type need to be replaced more on a regular basis. Some water-soluble vitamins are B vitamins(B₁, B₂, B₃B₆, B₇, B₁₂) and vitamin C.(1)

Fat-soluble: vitamins that are soluble in fat or lipids and are absorbed into the body through the intestinal tract or more specifically the small intestines.(1):
Some fat-soluble vitamins are vitamins A, D, E, and K.(1) Vitamins included in this category include vitamin A, vitamin D, vitamin E, as well as vitamin K. An acronym to help remember these fat-soluble vitamins- ADEK. Main areas in which fat-soluble vitamins are stored are the liver and adipose tissues. Fat-soluble vitamins except for vitamin K, are stored for long periods at a time and then excreted after this long duration of time has passed. For this reason overdosing on fat-soluble vitamins is highly feasible if ingested at high, toxic levels and it could possibly lead to hypervitaminosis.

Structure of Vitamins

Vitamin	Structure
Vitamin A
Vitamin B1 (Thiamin)
Vitamin B2 (Riboflavin)
Vitamin B3 (Niacin)
Vitamin B5 (Pantothenic Acid)
Vitamin B6 (Pyridoxine)
Vitamin B7 (Biotin)
Vitamin B9 (Folic Acid)
Vitamin B12 (Cobalamin)
Vitamin C
Vitamin D3 (Cholecalciferol)
Vitamin E
Vitamin K

Functions of Vitamins

Vitamin A: is used for growing healthy new cells like skin, bones, and hair. Also is used for surface lining upkeep of the eyes, urinary tract, intestinal tract, and respiratory system. Night vision is also assisted by Vitamin A. Vitamin A also performs other major functions in the body. It is required for reproductive functions such as normal growth and development of sperm and ovaries. Vitamin A also helps vision by keeping cells which are used for transduction of light into nerve signals healthy. Vitamin can be found from certain foods such as egg yolk, whole milk, and butter.

Vitamin D: is needed for the body to properly use calcium and phosphorous. It is also used in the formation of some RNA, maintain a normal heart, keep a stable nervous system and blood clotting. Along with absorbing calcium, Vitamin D can also help regulate the amount of calcium and phosphorus that is present in the blood. Vitamin D can be found in dairy products, fish, and oysters. Vitamin D deficiency caused severe growth retardation. The lack of calcium in the bones resulted in deformities of the skeleton, characterized by a widening at the ends of the long bones because of a disorganization in the hypertrophy and maturation of chondrocytes in the epiphyseal plates. Vitamin D deficiency is also associated with a low-normal blood calcium, low or low-normal fasting blood phosphorus, and elevated parathyroid hormone (PTH) levels that cause a mineralization defect in the skeleton.

Vitamin E: is an antioxidant that helps the body get rid of free radicals to keep tissues healthy. It is also used in the creation of red blood cells. The use of vitamin A,C and K are assisted by Vitamin E. Although the role of Vitamin E is not completely understood but it is clear that it presents antioxidant properties in the body. They get rid of the free radicals in the body by preventing the oxidation of lipid-based cell membranes. Free radicals are very reactive and can steal electrons from membranes which could ultimately damage DNA. Good sources of Vitamin E are almonds, spinach, wheat, and asparagus. Of the many such dietary components, vitamin E has commanded most interest because of its availability, strong marketing potential, overall health impact, and central role in preventing oxidation at the cellular level.

Vitamin K: assists in creating proteins in the body like those that create blood clots. It also allows for calcium regulation within the body. Vitamin K’s ability to help the clotting of blood is important for healing. The clotting ability could help in slowing or stopping bleeding in injured patients. During surgery, Vitamin K is often given to patients to reduce bleeding. Sources of Vitamin K include spinach, Brussels sprouts, asparagus, and broccoli.

Vitamin B: are essential for creating dopamine, epinephrine, serotonin, and myelin. They also help the mind focus, help hemoglobin hold oxygen and lower cholesterol. Vitamin B is essential to good health. It is also used for energy production in the human cells. B vitamins help convert food often consumed as carbohydrates into fuel. They also help the nervous system function properly. Good sources of Vitamin B are bananas, potatoes, whole grans, and chili peppers.

Vitamin C: helps regulate the immune system and relieve pain caused by tired muscles. It also is needed in the manufacture of collagen and norepinephrine. Vitamin C is also an antioxidant which can enhance the immune system by stimulating white blood cells in the body. Vitamin C also helps to benefit the skin, teeth, and bones. Vitamin C is often in citrus fruits such as papayas, oranges, and lemons.

The Different B Vitamins(4)

Vitamin B1:

Thiamin is another name for vitamin B1. It helps to convert blood sugar into energy for your body. It also helps the mucous membranes of the muscular, cardiovascular, and nervous systems in good shape. Some good sources of Vitamin B1 is from some whole grain cereals, pork, navy beans, and wheat germs.

Vitamin B2:

Riboflavin is another name for vitamin B2. It works with the other B vitamin complexes to process the carbohydrates, proteins, and fats into calories for energy in body. The body also needs this for healthy skin, good vision, growth, and red blood cell creation. Some good sources of Vitamin B2 is Dairy, red meats, and leafy green vegetables.

Vitamin B3:

Niacin is another name for vitamin B3. It also works with other B vitamin complexes to process the carbohydrates, proteins, and fats into calories for energy in the body. The difference is that it helps the digestive systems functions along with promoting a healthy appetite and healthy nerves. Large doses of niacin could lower LDL cholesterol but large doses is recommended to be taken under physician supervision. Some good sources of Vitamin B3 is yeasts, meat, and peanuts.

Vitamin B5:

Panththenic Acid is another name for vitamin B5. Like B3 and B2 it hlps break down carbohydrates, proteins, and fats for energy. Some good sources of Vitamin B5 is from meats, peas, and whole grain cereals

Vitamin B6:

Pyridoxine is another name for Vitamin B6. Vitamin B6 working along with B12 and B9 helps prevent heart attacks. Just like B2 B3 and B5 this vitamin helps the body process proteins, carbohydrates, and fats into energy. Some good sources of B6 is from meats, eggs, soybeans, whole grains, and nuts.

Vitamin B7:

Vitamin H or Biotin are other names for Vitamin B7. Vitamin B7 helps the formation of fatty acids and glucose to be used as fuel for the body. Some Good sources of B7 is from bananas, yeast, cereal, and liver.

Vitamin B9:

Folic Acid is another name for vitamin B9. It is very important during pregnancy since it is used for making and maintaining new cells. B9 prevents anemia by keeping up the production of red blood cells and prevent low birth weight and prematurity in births. Some good sources of B9 is from mushrooms, leafy greens, peas, and broccoli.

Vitamin B12:

Cobalamin is another name for vitamin B12. It works with B9 in keeping red blood cells healthy and also helps keep the central nervous system healthy. Some good sources of B12 are meat, eggs, and dairy. The two organizations that create guidelines for vitamin intake are by the Food and Nutrition Board of the National Academy of Sciences and the Food and Drug Administration(FDA).

Disruption in vitamin metabolism

Excess consumption of ethanol can lead to biochemical changes in the body, which further interrupts the vitamin A metabolism. Since both ethanol and retinol, the animal form of vitamin A, are alcohols, they are likely to interact and compete with each other for the same pathways; this also means they are likely to disrupt one another’s reaction. Retinol, like ethanol, reacts with dehydrogenases. The reaction forms retinoic acid. In the case of overconsumption of ethanol, retinol’s functions are affected. The retinoic acid signaling pathway is disrupted by the metabolized ethanol, which leads to various cancers and fetal alcohol syndrome.

People who often drinks may likely to develop Wernicke-Korsakoff syndrome, a disorder that affects the brain due to an insufficient intake of vitamin thiamine. Some characteristics of Wernicke-Korsakoff syndrome are confusion and abnormal movement of the limbs and eyes. Because thiamine can be converted to thiamine pyrophosphate, a coenzyme that is part of the pyruvate dehydrogenase complex, the insufficiency of thiamine leads to the formation of an altered pyruvate dehydrogenase complex that causes neurological defects.

Alcoholics are especially susceptible to scurvy, a disease resulting from low levels of vitamin C in the body. People with scurvy experience easy bruising, gum disease, loss of teeth, abnormal changes of the skin, and other symptoms. These symptoms appear because the lack of vitamin C stops the production of collagen, which is a protein fiber that makes up most of the dry skin weight. More specifically, vitamin C is needed for the production of the enzyme called prolyl hydroxylase. Prolyl hydroxylase is in charge of producing an amino acid, 4-hydroxyproline, that is needed in collagen. Hydroxyproline is essential because it forms hydrogen bond within the collagen triple helix, thus making the collagen more stable.

Dietary Supplement Information

The Institute of Medicine has created a few guidelines for people to reference on how much to take, recommended requirements, and what would be considered too much.
This is also known as the Dietary Reference Intake(DRI)(2):
The applications of RDA are: evaluating the adequacy of the national food supply, establishing a nutritional policy for public institutions and organizations, establishing labeling regulations, and setting guidelines for product formulation. The amount of vitamin intake for children, males, and females can be found on this external link: (1)

Recommended Dietary Allowance(RDA): is the average intake per day that satisfies the nutrient requirement for about 97-98% of all people whom are healthy in a gender group or life group.

Adequate Intake: is the recommended amount to intake, based on estimations of nutrient intake by people who are healthy and this amount is assumed to be sufficient. This measurement is used when the Recommended Dietary Allowance is unable to be determined.

Tolerable Upper Intake level: this is the utmost upper limit that is permitted without any detrimental risk to health and well-being for most of the people of a population. Ingesting or taking over this limit, increases the probability of detrimental effects significantly.

Estimated Average Requirements: daily intake number that has been evaluated to suffice 50% of the healthy peoples of a particular age or gender group.

Deficiency or Excess: although vitamins are essential building blocks of the human body, too much or too less of these compounds could be dangerous. An example would be the deficiency of folic acid (one of the vitamin B's) and a derivative of vitamin B2 called FAD. These two molecules help regulate the production of a potentially harmful compound called homocyesteine. Homocyestein is an essential intermediate of the production of an essential non-toxic amino acid methionine. To help lower the risks of this compound, vitamin B helps improving the fit of the enzyme that converts homocyestein to methionine. The deficiency of vitamin B in this case could increase the risk of heart attacks, strokes, and in the case of pregnant women, child defects. (7)

Vitamin Deficiencies and Excess(1)

Vitamin	Deficiency	Excess
Vitamin A	mild - night blindness, diarrhea, impaired vision severe - eye inflammation, causes blindness in children, keratinization of eyes and skin || Headache, vomiting, hair loss, irritability, liver and bone damage, blurred vision	Intracranial pressure, dizziness, nausea, headaches, liver damage.
Vitamin D	severe - children will get rickets, or softening and or weakening of bones; adults will get osteomalacia, which is also softening of the bones	brain, cardiovascular, and kidney damage
Vitamin E	Potential of anemia is babies that are born with a low-weight, deterioration of nervous system	Increase the risk of heart failure; can act as an anticoagulant and excessive intake may increase the risk of over bleeding.
Vitamin K	Results in extreme bleeding.	liver damage, anemia
Vitamin C	dry skin, anemia, and bruising.	Gastrointestinal sickness
Vitamin B	(it can be one of eight types of B vitamins): (3) Cheilosis (Inflammation of the lips where there is scaling and fissures), photophobia (fear of bright light), Glossitis(inflammation of the tongue)	ranges from symptoms such as liver damage (B2) to poor coordination and numb feet (B6)

Amount of Vitamins Needed Per Day(4)

Amount for adult Males/Females

Vitamin A - 1000mcg/800mcg

Vitamin B1 - 1.5 mg/1.1 mg

Vitamin B2 - 1.7 mg/1.1 mg

Vitamin B3 - 15–19 mg/15 mg

Vitamin B5 - 4–7 mg/4–7 mg

Vitamin B6 - 2 mg/1.6 mg

Vitamin B7 - 30-100mcg

Vitamin B9 - 200mcg/180mcg/400mcg for pregnant women/280mcg for breast feeding

Vitamin B12 - 2mcg/2mcg

Vitamin C - 60 mg/60 mg

Vitamin D - 5mcg/5mcg for 19-51 year olds and 10mcg/10mcg for 51 and older

Vitamin E - 10 mg/8 mg

Vitamin K - 80mcg/65mcg

Introduction to Cofactors

Cofactors are small molecules or compounds that bind to enzymes and allow the enzymes to function. Cofactors are not proteins nor amino acids. Enzymes catalyze many type of reactions. However, most of the enzymes require the presence of cofactors in order to be active. The cofactor's role and mechanism varies with different enzymes. Usually, cofactors perform chemical reactions that the twenty amino acids cannot undergo. Proteins can perform acid-base reactions, form certain covalent bonds, and engage in intermolecular interactions. However, proteins are not equipped to facilitate reactions involving oxidation-reduction and group transfers.

Inactive enzymes without a cofactor attached are referred to as apoenzymes. Conversely, enzymes that are complete with cofactors and ready to catalyze reactions are called holoenzymes.

There are two categories of cofactors: metals and coenzymes. Metal cofactors are essentially metal ions, such as Zn²⁺, Mg²⁺, and Ni²⁺. Coenzymes are small organic compounds, commonly generated from vitamins. Furthermore, coenzymes are characterized by how strong they bind to their respective enzymes. Coenzymes that bind strongly to enzymes are called prosthetic groups. The enzyme-coenzyme complexes with prosthetic groups are generally permanent. In others words, the prosthetic group does not dissociate from the enzyme. Coenzymes that loosely bind to enzymes function like cosubstrates. In this case, the enzyme-coenzyme complexes can form and dissociate, which make the enzyme active or inactive.

Some Coenzymes and their Respective Enzymes

Coenzyme	Enzyme
Thiamine pyrophosphate	Pyruvate dehydrogenase
Flavin adenine nucleotide	Monoamine oxidase
Nicotinamide adenine dinucleotide	Lactate dehydrogenase
Pyridoxal phosphate	Glycogen phosphorylase
Coenzyme A (COA)	Acetyl COA carboxylase
Biotin	Pyruvate carboxylase
5'-Deoxyadenosyl cobalamin	Methylmalonyl mutase
Tetrahydrofolate	Thymidylate synthase

Coenzyme Functions:

Thiamine pyrophosphate: helps with tissue respiration.

Coenzyme A transports acetyl groups from one substrate to another subtrate.

For more information on Cofactors, see Cofactor (biochemistry).

References

1. Anderson, J. and L. Young. "Fat-Soluble Vitamins." Food and Nutrition Series. No.9.315. 2008 August. <http://www.ext.colostate.edu/pubs/foodnut/09315.html>.
2. "Dietary Reference Intakes: An Update." International Food Information Council Foundation. 2002 August. <http://www.ific.org/publications/other/driupdateom.cfm?renderforprint=1>.

3. Thiamin pyrophosphate. <http://www.uic.edu/classes/phar/phar332/Clinical_Cases/vitamin%20cases/thiamin/thiamin_pyrophosphate.htm Cofactor functions>

4. Vitamins. <http://www.lifeclinic.com/focus/nutrition/>

5. Campell. "Chapter 41 Animal Nutrition." Biology. 8th ed. Benjamin-Cummings Pub, 2007. 877. Print.

6. Holick, F. Michael, :Vitamin D." Boston University Medical Center, Boston. 1999

7. "The Chemistry of Health", NIH Publications, reprinted August 2006

8. Berg, Jeremy "Biochemistry", Chapter 27 The Integration of Metabolism. pp 812-813. Seventh edition. Freeman and Company, 2010.

9. Leo, MA ; Lieber, CS. Alcohol, vitamin A, and beta-carotene: adverse interactions, including hepatotoxicity and carcinogenicity. AMERICAN JOURNAL OF CLINICAL NUTRITION Volume: 69 Issue: 6 Pages: 1071-1085 JUN 1999

Introduction

Most of the vitamins are known to act as coenzymes. However, Vitamin D, also known as calciferol, do not behave as coenzymes. Instead, Vitamin D are known to regulate calcium and phosphate metabolisms. Through recent research, Vitamin D are known for preventing different cancers and cardiovascular disease, protecting against diabetes, and enhancing the ability of muscle works.

Naturally, the best sources for vitamin D can be found in fatty fish like salmon and tuna and in fish liver oils. Smaller amounts of vitamin D can also be found in beef liver, cheese, and egg yolks. Vitamin D can also be taken in the form of supplements, either in the form D₂ or D₃. Most of people are attain their vitamin D through sunlight exposure. The UV-B radiation of sunlight penetrates the skin and converts 7-dehydrocholesterol to previtamin D3 which then becomes vitamin D₃.

Health effects of supplements

Up until today, the effects of vitamin D supplementation on health are uncertain. A United States Institute of Medicine (IOM) report states that outcomes that are related to disease such as cancer or diabetes could not be linked reliably with calcium or vitamin D intake. However, some researchers claimed that the IOM made a mistake when calculating the blood level of vitamin D associated with bone health. In return, the IOM members said that they used a standard procedure for dietary recommendations and that the result is just based on what they had in their data. Basically, many researchers use large scale of clinical trials to do research on vitamin D supplements.

Bone Health

Osteomalacia also known as rickets (when happen to children) are caused by vitamin D deficiency. Low serum vitamin D levels have been associated with falls, and low bone mineral density. The United States Preventive Services Task Force in the year of 2012 issued a draft statement conveying that there is not enough evidence to indicate that healthy postmenopausal women should use supplemental doses of calcium or vitamin D to prevent any fractures.

However, there are also other studies that show that any supplementation containing vitamin D and calcium may improve bone mineral density. Supplementation containing vitamin D may also decrease the risk of falls and fractures in only certain groups of people and not all. Further studies, even proved that elders in the age of 65 may take these supplements and decrease the risk of fractures. However, there is not much evidence that can prove this actually works. In the end, it appears that without a good amount of calcium, the amount of vitamin D that a person consume is not enough to benefit the bone health.

Mortality

Low blood levels of vitamin D are related with increased mortality. Elderly women that are giving supplementary vitamin D3 seems to decrease the risk of death. However, there are vitamins that are not seen effective. One such vitamin is vitamin D2, and other vitamins are like alfacalcidol and calcitriol. It is important to note that consuming vitamin D is good for the health, but an excess and a deficiency in vitamin D appear to cause abnormal functioning and premature aging.

Cancer

Taking supplements containing vitamin D does not appear to help people with prostate cancer. However, there some cancers that are related to low vitamin D levels and with worse outcomes in other cancers. As of today, there is not enough evidence to support supplementation in those with cancer.

Pregnancy

It is actually good for pregnant women to take a good amount of vitamin D because it will give positive immune effects. However, there are many women who are pregnant that does not take the recommended dose of vitamin D and this will definitely create a negative impact on the mother and her baby in the stomach.

Multiple Sclerosis

Multiple sclerosis is related to low vitamin D. Although there are uncertainties, many supplementation that contains vitamin D may have a protective effect.

The reason why vitamin D deficiency is thought to case multiple sclerosis is due to the following reasons:

• MS often increases with increasing latitude, which is strongly inversely correlated with duration and intensity of UVB from vitamin D concentrations and the sunlight.
• At higher latitudes in populations with high consumption of vitamin D (rich fatty fish), the prevalence of MS are lower than expected.
• With migration from high to low latitudes, MS risk decreases.

Since the year 2011, Vitamin D3 is tested to be as a treatment of Multiple Sclerosis.

Synthesis

Vitamin D is synthesized from 7-dehydrocholesterol to calcitriol (1,25-dihydroxycholecalciferol) in three different steps. The first process in this synthesis is by utilizing ultraviolet light to photolyze 7-dehydrocholesterol to previtamin D₃. The second process occurs immediately when the previtamin D₃ isomerizes to create vitamin D₃, also known as Cholecalciferol. The last process occurs when vitamin D₃ (Cholecalciferol) goes through the process of hydroxylation reaction to form calcitriol (1,25-dihydroxycholecalciferol). The images shown below indicate the process of synthesizing Vitamin D in three different steps:

7-Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conjugated diene unit in its B ring. 7-Dehydrocholesterol is present in the tissues of the skin, where it is transformed to vitamin D3 by a sunlight-induced photochemical reaction. Vitamin D 3 is a key compound in the process by which Ca²⁺ is absorbed from the intestine. Low levels of vitamin D 3 lead to Ca²⁺ concentrations in the body that are insuffi cient to support proper bone growth, resulting in the bone disease called rickets.

7-Dehydrocholesterol is transformed to vitamin D 3 by a sunlight-induced photochemical reaction

Deficiency of Vitamin D

Deficiency of Vitamin D may cause severe damages to both children and adults. Children should be aware of the disease known as rickets which is mainly caused by Vitamin D deficiency. The symptoms of rickets are deformation of skeletal bones which affects in impaired growth. Adults should be aware of the disease known as osteomalacia. Symptoms for osteomalacia is bending of bones due to its weakness. A primary way to escape from Vitamin D deficiency is through fortified foods. Milk is an excellent way to consume an acceptable portion of Vitamin D for our daily lives.

People at risk of not getting enough vitamin D include:

• breastfed infants - their vitamin D intake directly corresponds to their mother's vitamin D levels
• older adults - As they age their skin decreases in its efficiency to synthesize vitamin D. Also can be caused because they are more likely to stay indoors
• people with limited sun exposure - As UV rays from the sun are the main source for vitamin D, limiting the sun exposure can drastically decrease the vitamin D intake.
• dark-skinned people - The greater amounts of melanin reduces the skin's ability to produce vitamin D from sunlight.
• obese people - Those who are obese may need higher amounts of vitamin D than those who are not obese.

References

4. Carey, Francis A. "24." Organic Chemistry. 8th ed. Boston: McGraw-Hill, 2003. 1101. Print.

http://en.wikipedia.org/wiki/Vitamin_D

"Dietary Supplement Fact Sheet:Vitamin D". Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ Vitamin K is a fat-soluble compound that plays an essential role in the physiology of mammals. Its primary function pertains to the blood-clotting process. Vitamin K has an aromatic structure that undergoes oxidation-reduction during the formation of prothrombin, a protein that is crucial in blood clotting. Prothrombin converts fibrinogen to fibrin, which holds blood clots together. Deficiency in vitamin K slows blood clotting, which could ultimately lead to death.

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Vitamin K is a group of fat-soluble vitamins. The letter, “K,” derives from its German name, Koagulationsvitamin, which signifies its ability to help blood clot or coagulate. Not only does it contribute to blood clotting, some studies have also proven its capability of maintaining strong bones in the elderly. All Vitamin K group members are composed of a methylated naphthoquinone ring, which is also known as menadione. Vitamin K1, phylloquinone, contains four isoprenoid residues, in which one is unsaturated, in its side chain. Vitamin K2, menaquinone, can be composed of varying numbers of unsaturated isoprenoid residues.

Shown is Vitamin K1, or phylloquinone, which is composed of a methylated naphthoquinone ring. The naphthoquinone ring has been widely accepted as a functional group that gives rise to the function and distinctive nature of all vitamin K group. In this phyloquinone, a phytyl side chain is also presented

Vitamin K2, also known as menaquinone, comprises of varying numbers of unsaturated of isoprenoid residues. Similar to vitamin K1, a methylated naphthoquinone ring is presented. The common numbers of isoprenoid resides are usually 4 and 7, giving rise to MK-4 and MK7, respectively.

Vitamin K can be categorized into two natural vitamers: vitamin K1 and vitamin K2. Because they vitamin K1 and vitamin K2 are both 2-methyl-1,4-naphthoquinone (3-) derivatives, they have similar metabolic pathways during reactions. Overall, vitamin K contains a functional naphthoquinone ring and an aliphatic side chain. Vitamin K1 has a phytyl side chain attached to the homolog while vitamin K2 are composed of menaquinones that are made up of varying numbers of isoprenoid residues. In 1929, Henrik Dam, a Danish scientist, first discovered vitamin K1 while he performed an experiment to study the role of cholesterol. He gave chickens a cholesterol-deplete diet and found out that these livestock developed hemorrhages. This symptom cannot be reversed even if purified cholesterol was added back to the diet. Later, he found out that it was because coagulation vitamin was missing in the diet. This coagulation vitamin became essential to animal health, and was later identified to be vitamin K1.

Vitamin K1, also known as, phylloquinone, is synthesized by green plants. It is found primarily within photosynthetic tissues of plant such as, lettuce, cabbage, collard greens, broccoli, spinach, and turnip greens. Not only involved in plants, vitamin K1 can also be active within animal tissues although it operates in a completely different way in terms of its biochemical reactions.

Vitamin K2 can be classified into several menaquinone forms, or MK-n, where n signifies the number of 5-carbon units. The common forms of vitamin K2 are MK4 and MK7. While MK4 is generally found in meat or dairy products, MK7 is present is fermented, natural products such as, cheese or natto. Through transformation of vitamin K1 in the testes, pancreas and arterial walls, MK4 can be produced and synthesized. In animals, vitamin K2 can be synthesized in the large intestine by bacteria. This synthesis pathway, however, is commonly carried out in germ-free rats, and is proven to be gut bacteria-independent. Studies have also shown that that the amount of MK4 present is cell tissues has direct influence over the conversion of K1 into MK4. While scientists are still attempting to better understand the production of MK7, some have reported that it can be synthesized from vitamin K1 in the colon with the help of E-coli.

Understanding of how vitamin K can contribute to blood clotting is crucial for severe bleeding can be life threatening. In many proteins, gamma-carboxylation of specific glutamic acid residues allows them to bind to calcium ions (Ca2+). The main cascade of coagulation is formed by Factor II, VII, IX, and X primarily. These factors are proteins specialized in binding to calcium ions. The activated form of Factor II can be enhanced when protein Z is involved in the mechanism since protein Z stimulates a higher interaction between the phospholipids in cell membranes. To counterbalance this blood clotting mechanism, protein C, protein Z, and protein S can be occasionally utilized to control the coagulation cascade for they are both anticoagulant proteins. In face, Vitamin K factor originates from liver. Therefore, a healthy liver is essential in delivering and carrying out the complete function of vitamin K and its coagulation cascade. In addition, this is why liver disorders can give rise to problematic vitamin K-dependent clotting formation, resulting in severe hemorrhage.

Besides its indispensable contribution to blood clotting formation, vitamin K also helps bone development. Studies have shown that higher levels of vitamin K in human bodies can furnish higher bone density, providing better structure and higher level of rigidness. Osteoporosis is a common disorder in which patients whose levels of vitamin K do not meet the standard. Osteoporosis patients, especially postmenopausal women, tend to develop fragile bone structures that can easily contribute to bone fractures.

Other than Vitamin K’s two natural vitamers, vitamin K1 and vitamin K2, synthetic forms of vitamin K are also available in the market. Vitamin K1 is a common U.S. supplement customers can obtain from pharmacies. It can be found in tablet capsule, and liquid forms, depending on the preference of the customer. A healthy, average diet do not generally lack in vitamin K. Vitamin K deficiency is rather rare in health adults. As mentioned earlier, postmenopausal women and newborn infants are at a higher risk of deficiency. Those who suffer from liver disorders can also face higher likelihood of developing vitamin K deficiency. Beside hemorrhage, common symptoms of vitamin K deficiency also includes anemia, bruising, bleeding of the gum of nose, and heavy menstrual bleeding in women.

References

2. "Disclaimer." University of Maryland Medical Center. University of Maryland Medical Center, 2011. Web. 22 Nov. 2012. <http://www.umm.edu/altmed/articles/vitamin-k-000343.htm>.

3. Higdon, Jane. "Linus Pauling Institute Micronutrient Research for Optimum Health." Linus Pauling Institute at Oregon State University. N.p., 19 Dec. 2011. Web. 22 Nov. 2012. <http://lpi.oregonstate.edu/infocenter/vitamins/vitaminK/>.

4. NG, ClueBot. "Vitamin K Deficiency." Wikipedia. Wikimedia Foundation, 22 Nov. 2012. Web. 22 Nov. 2012. <w:en:Vitamin K deficiency>.

5. Nordqvist, Christian. "What Is Vitamin K?" Medical News Today. MediLexicon International, 22 Mar. 2011. Web. 22 Nov. 2012. <http://www.medicalnewstoday.com/articles/219867.php>.

6. Vitamin K." Vitamin K. American Cancer Society, 21 Oct. 2010. Web. 22 Nov. 2012. <http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/herbsvitaminsandminerals/vitamin-k>.

7. Vitamin K." Vitamin K. American Cancer Society, 21 Oct. 2010. Web. 22 Nov. 2012. <http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/herbsvitaminsandminerals/vitamin-k>.

8. Wikipedia. Wikimedia Foundation, 27 Dec. 1997. Web. 22 Nov. 2012. <http://en.wikipedia.org/wiki/File:Phylloquinone_structure.svg>.

Vitamin C

Vitamin C (ascorbic acid), is an essential nutrient for humans and certain animals. It is a cofactor in many enzymatic activities and it prevents the disease scurvy. It can also act as an antioxidant against oxidative stress.

Vitamin C was first named as “ascorbic acid” by Walter Norman Haworth after it was discovered to be an anti-scurvy factor. “Ascorbic” means “against scurvy”. Casimir Funk introduced the term “vitamin C” around 1920.

Biochemical Role

An accumulation of mutations in the gene coding for I-guolonolactone oxidase (enzyme in ascorbic biosynthetic pathway) caused primates and a few other animal species unable to synthesize ascorbic. One theory as to why this happened is that maybe the loss of being able to synthesize ascorbic promoted mutations to occur for more genetic variability and therefore helping to further evolution. Another theory is that the vitamin C in fruits and vegetables was sufficient enough to compensate for not being able to synthesize it. A byproduct of synthesizing ascorbic is hydrogen peroxide so the loss may have been advantageous.

Scurvy is caused by ascorbic deficiency. The symptoms include vision problems, neurological disorders and the alteration of the extracellular matrix in blood vessels, bones, skin, gums, and tendons. Many of the symptoms can be explained by the inactivation of a few ascorbic-dependent enzymes. These enzymes commonly share a catalytic mechanism that requires Fe2+, 2-oxoglutarate, and ascorbic as co-substrates. The enzymes are in the class of 2-oxoglutarate dependent dioxygenases (2-ODDs).

Different 2-ODDs catalyze reactions such as hydroxylation, desaturation, and oxidative ring-closure or expansion. These reactions lead to the addition of oxygen into a substrate. These steps are critical in a large number of biochemical pathways. Ascorbic is thought to merely maintain the Fe2+ state but many electron donors do not seem to be able to replace it in maintaining 2-ODDs in their active form. Studies on the mechanism of peptidyl-prolyl-4-hydroxylase (P4H) show the complexity of the reaction. Without ascorbic, P4H is quickly inactivated by self-oxidation. Inactivation of P4H from lack of ascorbic is the first identified cause of scurvy. The hydroxylation of peptidyl-proline is necessary for collagen folding. In humans, unhydroxylated collagen is more flexible and it is degraded in the endoplasmic reticulum (ER). Different collagen types have specific functions in maintaining skin, tendons, cartilage, bones, teeth, cornea, muscles, blood vessels, other tissues, and organs. Collagen is the most abundant protein in animals.

Vitamin C, also called ascorbic acid, is much more than just an antioxidant that prevents scurvy. It plays a very important role in signaling that occurs in cells as well as gene expression. In addition to 2-ODDs helping incorporate oxygen into organic substrates, ascorbic acid also participates in hypoxia-inducible factor (HIF) hydroxylation signaling. This reaction requires the presence of oxygen because it is catalyzed by dioxygenases [1]. Hypoxic (low oxygen) conditions are obviously not favored for this reaction, so animal cells have developed a fascinating way to detect if oxygen is present and actually have defenses available against hypoxia. HIF1α is a transcription factor, which contains two proline residues that are hydroxylated. This is the basis behind oxygen detection in mammalian cells [3, 4]

The HIF family contains transcriptionally activating genes, HIF1 is specifically involved in angiogenesis, metabolism, nutrient transport, and cell migration [5]. HIF1 is a heterodimer that contains both an α and β subunits. Two proline residues of HIF1α (Pro402 and Pro564) are hydroxylated by prolyl hydroxylases (HIF-P4H) [1]. The HIF-P4H complex is located in the cytosol and its Km value for oxygen are above atmospheric concentration. This shows that the HIF-P4H complexes are able to successfully sense oxygen levels.

Normoxic conditions (when there is oxygen available) allow the proline residues of HIF1α to be hydroxylated. This is a necessary step for the von-Hippel-Lindau (pVHL) protein to bind. Subsequently, HIF1α undergoes degradation by a proteasome-mediated pathway. Hypoxia conditions prevent hydroxylation from taking place, and thus HIF1α is unable to bind to pVHL. So, HIF1α cannot be degraded and it is transported to the nucleus. Here, HIF1α binds with HIF1β, forming a dimer that binds the Hypoxia Responsive Element in the promoter region of a vast variety of hypoxia-induced genes. These genes play important roles in cancer such as angiogenesis [6]. Ascorbic acid may have potential to help in certain cancer treatments. This is a highly debated idea but is centered around the concept that HIF1α has the ability to stimulate angiogenesis, which plays a major role in cancer growth [6]. It has also been shown that cancer patients can have lower amounts of ascorbic acid [7].

HIF1α builds up in hypoxic conditions as well as from nonhypoxic stimulation. Nonhypoxic stimuli, can come from pyruvate and oxaloacetate from the Krebs cycle. The nonhypoxic activation of HIF is partially controlled by ascorbic acid [1].

Different pathways for HIF1 depending on presence of co-substrates needed for its hydroxylation.

Gene Expression

Ascorbic is thought to be involved in gene transcription, RNA stabilization, post-translational modification of proteins, hydroxylation, epoxidation and desaturation of many substrates. This hypothesis suggests that ascorbic acid seems to stabilize certain mRNAs. It was observed that stabilization of collagen transcript is ascorbic dependent. Ascorbic is required for the expression of osteocalcin which is a calcium-binding protein that is made by osteoblasts. The mechanisms of how ascorbic controls gene expression is still unclear. Ascorbic acid has also been observed to induce cell differentiation. It is hoped that ASC can be used for in vitro synthesis of differentiated cells from embryonic stem cells. This can be useful in future clinical treatment.

Antioxidant

Ascorbic acid is a well-known antioxidant. It acts as a reducing agent and reverses oxidation in liquids. Oxidative stress is caused by there being more free radicals in the body than there are antioxidants. This condition affects cardiovascular disease, hypertension, diabetes, and chronic inflammatory diseases.

Non-antioxidant

Ascorbic acid is not only an antioxidant, it also behaves as a “non-antioxidant” (Pro-oxidant). It has been shown to reduce transition metals such as iron from Fe3+ to Fe2+.

Vitamin C sources

Vitamin C can be found in fruits and vegetables. It is also present in certain meats such as liver. Citrus fruits are the most commonly known sources of vitamin C. The Kakadu plum (Australia) and the camu camu fruit (Amazon Rainforest) have the highest concentration of vitamin C.

References

1. "Beyond the Antioxidant: The Double Life of Vitamin C - Springer." Home - Springer. N.p., n.d. Web. 6 Dec. 2012. <http://link.springer.com/chapter/10.1007/978-94-007-2199-9_4/fulltext.html>.
2. "Vitamin C - Wikipedia, the free encyclopedia." Wikipedia, the free encyclopedia. N.p., n.d. Web. 6 Dec. 2012. <http://en.wikipedia.org/wiki/Vitamin_C>.

[3] Ivan M, Kondo K, Yang H, Kim W, Valiando J, Ohh M, Salic A, Asara JM, Lane WS, Kaelin WG Jr (2001) HIF targeted for VHL-mediated destruction by proline hydroxylation: implications for O2 sensing. Science.

[4] Jaakkola P, Mole DR, Tian YM, Wilson MI, Gielbert J, Gaskell SJ, Kriegsheim A, Hebestreit HF, Mukherji M, Schofield CJ, Maxwell PH, Pugh CW, Ratcliffe PJ (2001) Targeting of HIFalpha to the von Hippel-Lindau ubiquitylation complex by O2-regulated prolyl hydroxylation. Science.

[5] Chun YS, Kim MS, Park JW (2002) Oxygen-dependent and -independent regulation of HIF1α.J Kor Med Sci.

[6] Harris AL (2002) Hypoxia – a key regulatory factor in tumour growth. Nat Rev Cancer.

[7] Mayland CR, Bennett MI, Allan K (2005) Vitamin C deficiency in cancer patients. Palliat Med.

Overview

Vitamin A is a fat-soluble vitamin that is derived from two sources: preformed retinoids and provitamin carotenoids. Retinoids, such as retinal and retinoic acid, are found in animal sources such as liver, kidney, eggs, and dairy produce. Carotenoids, such as beta-carotene (which has the highest vitamin A activity), are found in plants such as dark or yellow vegetables and carrots.

Natural retinoids are present in all living organisms, either as preformed vitamin A or as carotenoids, and are required for biological processes such as vision and cellular growth. A major biologic function of vitamin A (as the metabolite retinal) is in the visual cycle. Research also suggests that vitamin A may reduce the mortality rate from measles, prevent some types of cancer, aid in growth and development, and improve immune function.

Recommended dietary allowance (RDA) levels for vitamin A oral intake have been established by the U.S. Institute for Medicine of the National Academy of Sciences to prevent deficiencies in vitamin A. At recommended doses, vitamin A is generally considered nontoxic. Excess dosing may lead to acute or chronic toxicity.

Vitamin A deficiency is rare in industrialized nations but remains a concern in developing countries, particularly in areas where malnutrition is common. Prolonged deficiency can lead to xerophthalmia (dry eye) and ultimately to night blindness or total blindness, as well as to skin disorders, infections (such as measles), diarrhea, and respiratory disorders.

Functions of Vitamin A

Gene Regulation

Vitamin A, while it is in its retinoic acid form, plays a very crucial role when it comes to gene transcription. Retinoic acid regulates gene transcription by binding to nuclear receptors known as retinoic acid receptors, or also known as RARs. These RARs are bound to DNA as heterodimers with the appropriate retinoid receptors. When RAR binds to these specific retinoid receptors found in the DNA, known as RXRs, they form a a heterodimer known as a RAR-RXR complex before completely binding to DNA. It should be noted that the receptors found on DNA does not necessarily need to bind to RAR, but can form homodimers known as RXR-RXRs that can then form larger heterodimers with many other nuclear receptors. However, the RAR-RXR heterodimer recognizes retinoic acid response elements known as RAREs on DNA while RXR-RXR homodimers can recognize retinoid X response elements on the DNA. The heterodimers of RXR can bind to various distance response elements on the DNA to control processees not regulated specifically by Vitamin A. In other words, the alternate forms of vitamin A, specifically retinoic acid, can form these heterodimers with retinoic receptors to form complexes of response systems that can then regulate greater processes that normal Vitramin A could not do alone. For examples, upon initial binding and creation of the RAR-RXR complex, the receptors undergo a conformational change that causes co-repressors to dissociate to receptors. At this point, coactivators can then bind to the receptor complex and help loosen chromatin structures from histones. This response that is regulated by the retionic-retionic receptor complex (RAR-RXR) can regulate expression of target genes by release and attachment of various corepressors and coactivators.

Reference

http://www.mayoclinic.com/health/vitamin-a/NS_patient-vitamina

Overview of Vitamin E

Vitamin E is a name collectively for a group of fat-soluble compounds that contain antioxidant distinctive activities. It can be found in vegetable oils, nuts, seeds, green vegetables like broccoli, or taken as a dietary supplement. Vitamin E exists in 8 chemical forms; alpha, beta, and delta-tocopherol and alpha-, beta-, gamma-, and delta-tocotrienol. In general, Vitamin E is a fat-soluble antioxidant that prevents the production of ROS formed during the process of the oxidation of fat. Scientists are currently investigating whether vitamin E could help prevent or slow the chronic diseases associated with free radicals.

Deficiency in vitamin E can cause nerve and muscle damage to the point of loss of feeling in arms and legs, muscle weakness, and vision problems. It can also result in a weakened immune system. Vitamin E deficiency can be linked to diseases like Crohn's disease, cystic fibrosis as these are diseases where fat is not properly absorbed/digested. It can also be linked to a few genetic diseases like abetalipoproteinemia.

The most common forms of Vitamin E in a natural diet is γ-tocopherol followed by α-Tocopherol.

http://www.uic.edu/classes/phar/phar332/Clinical_Cases/vitamin%20cases/vitamin%20E/vit-E.gif

Function

Vitamin E has many important biological functions which include:

1. Vitamin E is an antioxidant.
2. Enzymatic Activities
3. Gene Expression
4. Neurological Functions
5. Cell Signaling
6. Protects Lipids and Prevents Oxidation of Polyunsaturated Fatty Acids

Forms

Vitamin E has 8 forms split between two types - Tocopherols and Tocotrienols. These forms are then named with prefixes such as alpha, beta, gamma, and delta. Among these forms, α-Tocopherol is the most studied as it is the form which humans consume and require.

α-Tocopherol

Alpha Tocopherol appears to mainly serve as an antioxidant in humans. As alpha-tocopherol is a fat soluble molecule it takes on the role of preventing free radicals from destroying (through oxidation) fats in the body. As fats are essential for cell membranes, this is very important for our bodies hence why it is a Vitamin.

Other functions that alpha-tocopherol has include inhibiting the activity of protein kinase C. Protein kinase C is an important molecule for cell-signaling. Alpha-tocopherol also affects the activity of enzymes in immune and inflammatory cells.

γ-Tocopherol

Gamma Tocopherol is the most common form of Vitamin E in a North American diet. However, its function is not as well known as alpha tocopherol and surprisingly its concentration levels in the bloodstream is around 1/10th of alpha-tocopherol. It would appear that the reason for this is that alpha-tocopherol lowers gamma-tocopherol levels. Through limited studies, it has been suggested that gamma-tocopherol may act as an antioxidant as well. However, gamma-tocopherol is also suspected to increase risk of developing prostate cancer. More research on Gamma Tocopherol needs to be conducted at this point.

References

1. "Vitamin E." — Health Professional Fact Sheet. N.p., n.d. Web. 07 Dec. 2012.

2. "Dietary Supplement Fact Sheet:Vitamin E". NIH Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/VitaminE-QuickFacts/?print=1 Vitamin B is a group of vitamins that are able to be water dissolved. Usually, people thought vitamin B was one vitamin but in actuality vitamin B is a group of vitamins such as vitamin B12 and vitamin B6. These can be just called vitamin B complexes. Vitamin B is found in unprocessed foods because processed foods have lowered vitamin B. It has something to due with the creation. Vitamin B complexes are essential for overall health. They help promote growth and development throughout the body. Vitamin B complexes play a large roll in structural biochemistry in that it is needed for enzymes and proteins to control the chemical reactions in the body. This helps turn food into energy. Low vitamin B count can lead to increased chance to develop cancer. Vitamin deficiency has always been known to cause disease. Low levels of vitamin B also leads to digestion and growth problems.

Introduction

Vitamin B12, also known as cobalamin, is a vitamin that is water soluble and has a key role in ensuring that the brain and the nervous system functions normally. It is also essential for the formation of blood. It is usually involved in the metabolism or all cells within the human body and it affects DNA synthesis and regulation. Vitamin B12 is noted as the largest and most structurally complicated vitamin. It is typically produced industrially through bacterial fermentation-synthesis.

Medical Usage

Vitamin B12 is utilized to treat hereditary deficiency of transcobalamin II, cyanide poisoning, and vitamin B12 deficiency. It is also provided to detect pernicious anemia through the Schilling test. A sufficient amount of vitamin B12 levels can effectively protect elders from brain atrophy or shrinkage, which is linked to Alzheimer’s disease and impairment in cognitive function.

Typically, the recommended intake for vitamin B12 is approximately 2 to 3 micrograms per day. It is usually taken orally in amounts that do not exceed the recommended dietary allowance.

Adverse Effects

• Even after an intake of enormous amounts, vitamin B12 does not necessarily appear harmful to individuals who are healthy due to its low toxicity.
• Treatment of vitamin B12 deficiency can result in the unmasking of polycythemia vera, which can be described as an increase in blood volume and red blood cells. Using vitamin B12 to correct megaloblastic anemia can cause fatal gout and hypokalemia.
• Vitamin B12 is contraindicated in early Leber’s disease. Leber’s disease is a hereditary optic nerve atrophy.

References

1. Lonn E, Yusuf S, Arnold MJ, et al. (2006). "Homocysteine lowering with folic acid and B vitamins in vascular disease".The New England Journal of Medicine 354
2. Yamada K, et al. (2008) "Degradation of vitamin B12 in dietary supplements." Int J Vitam Nutr Res. 78 (4-5): 195-203.

Introduction

B vitamin supplement tablets

Vitamin B6, a naturally found water-soluble vitamin, is one of the most useful and versatile coenzymes in the body, performing a wide variety of functions that involve over 100 enzyme reactions. The focus of Vitamin B6 is protein metabolism. One of the significant roles it plays is cognitive development through synthesis with neurotransmitters. It also participates in glycogenolysis, immune function, and hemoglobin formation. Vitamin B6 is widely found in a large variety of food, most especially in the fish, liver and plenty of other organ meats, starchy vegetables like potato, and even fruit. It is even available as supplement multivitamins to be included daily diet. There are some risks in deficiency and inadequacy but they are all fairly uncommon since it is so involved is our daily diets.

Interactions with Medicine

Vitamin B6 has important factors to take notice when involved with medications. There are several types of medication that directly depend on the level of vitamin B6. For example, Cycloserine, used to take tuberculosis, can combine with Pyridoxine (Vitamin b6) and help prevent or make mild the severe seizures and neurotoxicity that may occur. For "Antiepileptic Medications", they increase catabolism rate of all vitamin B6s, which leads to a lower plasma PLP concentration and hyperhomoscyten. This could lead to increase of seizures and epilepsy. Another example is "Theophyline", which treats or prevents various breathing problems, such as wheezing or asthma.

Pyridoxine structure

=== Vitamin B6 and Health ===

CARDIOVASCULAR DISEASE

Research has shown that certain levels of Vitamin B6 and B12 can have an affect on cardiovascular disease and in fact reduce it by lowering the homocysteine levels. Research is still in process currently, but various clinical trials have shown that there may be a relationship between the two, which can eventually lead to vitamin B6 to help reducing the risk and severity of cardiovascular disease.

CANCER

Other research has proved that low plasma vitamin B6 concentration is associated to an increased risk of various cancers.

COGNITIVE FUNCTION

Several studies have presented that there also may be a correlation between the vitamin B6 level status and cognitive decline, especially in the brain function of the elderly. Memory, mood, and other bodily functions are all effected by the levels of Vitamin B6.

Pyridoxal-phosphate-3D-balls

Citation: Dietary Supplement Fact Sheet: Vitamin B6; Office of Dietary Supplement National Institutes of Health. 2011 15 09. <http://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/>

Organic Reactions in Biochemistry

Biochemistry has a very close relation with organic chemistry, as a good amount of reactions within the body occur through organic reactions. Thus, it is important to look at some of the important Biochemical molecules and the reactions surrounding them through the perspective of organic chemistry. Through the formation of peptide Bonds, long polypeptide chains form which then can fold into proteins.

Phosphodiester Bonds

A phosphodiester bond is a bond between a two pentoses and a phosphate group through esterification. It is formed by strong covalent bonds, in which the phosphate groups that are bonded together are negatively charged, so that they repel nucleophilic species like they hydroxide ion. This protects the phosphodiester bonds from hydrolytic attack, unlike with most other esters. These bonds are an important part of all life and make up the backbone of nucleic acids(including DNA and RNA). The phosphodiester bond links the 3’ end of a strand of DNA to the 5’ end of another strand by bonding the hydroxyl group of the 3’ end of the sugar to the hydroxyl group of the 5’ end of the other sugar. Because of the free 2’ hydroxyl group in the sugar groups, the phosphodiester bonds between two ribonucleotides can be broken through the process of alkaline hydrolysis.

Phosphodiesterases

Phosphodiesterases are enzymes that catalyze the hydrolysis of phosphodiester bonds and repair DNA sequences.

DNA Polymerase

DNA polymerases are enzymes that help to catalyze the formation of phosphodiester bonds through the reaction:

(DNA)n + dNTP <=> (DNA)n+1 + PPi

In this formula, dNTP is deoxyribonucleotide and PPi is pyrophosphate ion.

References

1. Berg, Jeremy M. (2007). Biochemistry, 6th Ed., Sara Tenney. ISBN0-7167-8724-5.

Importance of Peptide Bonding

Peptide bonding (or amide bonding) is one of the most important reactions in biochemistry, as it is the bond used by amino acids to form proteins. Amino acids form peptide bonds with other amino acids when the amino group of the first amino acid bonds with the carboxyl group of the second amino acid. The dipeptide formed is followed by the loss of water. A chain of amino acids connected by peptide bonds is called a polypeptide chain, while each individual amino acid is referred to as a residue. These chains then fold due to various internal and external forces in order to become proteins.

Partial-Double Bond of Peptide Bond

The stability of the peptide bond is due to the resonance of amides. With resonance, the nitrogen is able to donate its lone pair of electrons to the carboxyl carbon and push electrons from the carboxyl double bond towards the oxygen, forming the oxygen anion. This resonance effect is very stabilizing because the electrons can be delocalized over multiple atoms, with one especially stable resonance structure containing the highly electronegative oxygen as an anion. The double bond character of the C-N bond results in a relatively short bond (1.32 angstroms vs. the normal length of a pure C-N bond, 1.47 angstroms). The double bond resonance form of the peptide bond helps to increase stability and decrease rotation about that bond. The partial double bond character is either strengthened or weakened depending upon the environment that it is in. An example would be a hydrophobic environment where the double-bond form would be highly discouraged since the double-bond form has a positive charge on the nitrogen and a negative charge on the oxygen.

The partial double bond results in the amide group being planar thus causing them to take either the cis or trans conformation. In the cis configuration, the two alpha carbon atoms fall on the same side of the peptide bond. In the trans configuration, these groups are on opposite sides of the peptide bond. While the proteins exist in their unfolded state, the peptide groups can isomerize at free will and thus often take the form of both conformations. This however, is not true in the folded state where with very rare exceptions, only one conformation is taken up at each position. In peptide bonds, the vast majority take the trans conformation with a cis:trans ratio of around 1:1000. There is preference for the trans configuration over the cis orientation because with trans there is less steric hindrance between groups attached to the alpha carbon atoms.

Amide Bond Resonance

Unlike the rigid peptide bond, the bond linking the amino group to the alpha carbon atom and the bond linking the alpha carbon atom to the carbonyl carbon are single bonds. These two bonds are free to rotate about the amide bonds, allowing them to take on a variety of orientations. The enhanced freedom of rotation with regards to these two bonds allows proteins to fold in a variety of shapes. The degree of rotation of each of these bonds can be quantified by their torsion angle. A torsion angle is defined somewhere between -180 and +180. Torsion angles are also called dihedral angles. The angle of rotation in regards to the bond between the nitrogen and the alpha carbon is referred to as phi, while the angle of rotation between the alpha carbon and the carbonyl carbon is referred to as psi. Moving clockwise from the nitrogen atom towards the alpha carbon, or from the carbonyl carbon to the alpha carbon yields a positive value on the Ramachandran diagram. The Ramachandran diagram illustrates various orientations of polypeptides and shows specific orientations that are not possible due to steric hindrance based on phi and psi values. At certain phi and psi values some structures may not exist due to clashes between the atoms.

Overall Reaction

The overall reaction involves the reaction of two amino acids which is catalyzed by enzymes enzyme in the body to produce the two amino acids combined and a water molecule.

Reaction Mechanism

This reaction to form peptide bonds involves reacting the amine group of one amino acid (the N-terminal) to the carboxylgroup of another amino acid (the C-terminal). A peptide bond is a dehydration reaction, or condensation reaction, meaning it releases a molecule of water through the course of the reaction. The molecule formed by a peptide bond is called an amide.

In the presence of water, the peptide bond will break spontaneously; this is called amide hydrolysis. This occurs because the peptide reaction possesses an equilibrium that pushes the reaction towards hydrolysis (heading in the reverse direction), which means the reaction is endergonic, and requires energy to proceed. Although this reaction requires an input of energy, peptide bonds are still stable bonds as the rate of hydrolysis is incredibly slow. Enzymes facilitate the hydrolysis reaction of peptides to form proteins in living organisms.

The hydrolysis reaction is very slow because the bond between the amino group and carboxyl group is stable due to resonance. The lone pair from the nitrogen donates electrons to the carbonyl. The resonance decreases the electrophilicity, and stabilizes the carbonyl forming the peptide bond. Even though peptide bonds are stable, they can still react. The reactions of peptide bonds involve attack at the carbonyl carbon and the formation of a tetrahedral intermediate.

To form peptide bonds between specific amino acid residues, the functional groups of the amino acids must be protected. The amino end is usually blocked using a phenylmethoxycarbonyl group (or Cbz). The amino group is then restored by a hydrogenolysis reaction using the reagents H2 and Pd-C. Another amino protecting group used is 1,1-dimethylethoxycarbonyl (or tert-butoxycarbonyl, Boc). This can be removed under acidic conditions (for example, with HCl). On the other end, the carboxy terminus can be protected by the formation of an ester. Deprotection of the carboxy end can be achieved by treatment with base. Polypeptides can be synthesized by coupling an amino protected amino acid with another amino acid in which the carboxy end is protected with the help of dicyclohexylcarbodiimide (DCC).

References

1. Berg, Jeremy M. (2007). Biochemistry, 6th Ed., Sara Tenney. ISBN0-7167-8724-5.

2. Nelson, David. Lehninger Principles of Biochemistry. 5th. New York : W. H. Freeman and Company, 2008.

3. Vollhardt, Peter. Organic Chemistry Structure and Function. 6th Ed., 5th. New York : W. H. Freeman and Company, 2011.

Overview

Techniques in organic chemistry applied to biology can produce many accommodations such as medicines that are useful to everyone. Methods of separating, isolating, purifying, and identifying unknown compounds as well as known compounds in unknown mixtures are helpful to the pharmaceutical industry. By isolating and purifying an unknown compound and then performing other analyses such as NMR or electron microscopy, the structure of the compound can be identified and with further analyses of the structure, its biological implications and effects can be understood using chemistry. Ever since the extraction of morphine from opium, many natural products have been isolated and their structures determined. Once the structure was determined, chemists were able to devise synthetic methods to synthesis these compounds, creating modern pharmaceutical chemistry. One example of this is Caffeine. By isolating, purifying, and identifying its structure, scientists have been able to understand how the consumption of caffeine results in wakefulness and alertness. These biological effects are the results of its structural similarity to adenosine and cyclic adenosine monophosphate (cAMP). This similarity allows caffeine to bind to the active site of receptors or enzymes that normally react with adenosine and its derivatives. Adenosine is important in the regulation of brain activity as build up of adenosine levels in the brain during the day causes the concentration to increase and if it is high enough, adenosine binds to brain receptors that activates mechanisms causing drowsiness and sleep. When caffeine binds to the adenosine receptors, it prevents the binding of adenosine and therefore delays sleepiness and increases alertness. Also, Caffeine inhibits the enzyme cyclic nucleotide phosphodiesterase (cAMP-PDE) that converts cAMP to noncyclic monophospate. cAMP is a secondary messenger responsible for increasing blood pressure and delivering more oxygen to the brain. When caffeine binds to cAMP-PDE, it stops the breakdown of cAMP and consequently prolongs the excitation effects. In conclusion, the use of organic chemistry techniques to isolate, purify, identify, and understand compound structures can lead to a chemical understanding of the biological phenomenon that result for chemical interactions which forms the basis for biochemistry.

References

Mohrig, Jerry R. Techniques in Organic Chemistry. 2006, W.H. Freeman and Company

Organic Chemistry Lab, Chemistry 143A

Liquid-Liquid Extraction

Liquid-liquid Extraction

Extraction is a technique used for separating a compound from a mixture. An example is separating a water-insoluble organic compound from an aqueous mixture by extracting it into a water-insoluble organic solvent. These extractions are often part of the workup procedure for isolating and purifying the product of an organic reaction. Because trace amounts of water are often present at the end of an extraction process, a drying reagent is needed to ensure a dry product. The process of liquid-liquid extraction involves the distribution of a compound between two solvents that are insoluble in each other. By taking advantage of the differing solubilities of a solute in a pair of solvents, compounds can be selectively transported from one liquid phase to the other.

This occurrence is quantified by the partition coefficient (K):

${\text{ K}}={\frac {\text{ Concentration of solute in organic phase}}{\text{ Concentration of solute in aqueousphase}}}.$

The larger the value of K, the solute will be found in greater amounts in the organic solvent. In an extraction procedure, an aqueous phase, usually water, and a immiscible organic solvent known as the organic phase are generally shaken in a container. The solutes are then allowed to distribute themselves between the two layers according to their solubility, the denser layer is always on the bottom of the container. After they have distributed themselves, each layer can be removed and analyzed separately by drying the extractions. The factors that need to be considered in selecting a drying agent are its capacity for removing water, its efficiency, the speed with which it works, and its chemical inertness. Once the extraction has been dried, the solvent must be removed to recover the desired organic product. This can be achieved by heating the container of the mixture and allowing for the solvent to evaporate, leaving a solid or liquid residue or allowing the mixture in to evaporate in a fume hood, depending on the volatility of the solvent.

Liquid-Liquid Extraction FlowChart

Here is an example of a simple liquid-liquid extraction problem.

You want to isolate a mixture of compounds, and you do this by taking advantage of the basicity and acidity of certain compounds. It is also important to note those structures that are rather neutral. Some key concepts are, bases will react with acid and vice versa. Also, the aqueous layer will contain a charged molecule, or ion, most of the time.

Step 1: Upon dissolving the mixture in a good solvent such as methylene chloride, a strong acid is added to the mixture.

The strong acid is added so that it will react with any basic compounds in the mixture, which will lead it into the aqueous layer. Thus, one of the compounds has been separated. Adding base will deprotonate the proton from the nitrogen and give you the isolated product in the organic layer.

Step 2: To the remaining 3 compounds, a weak base is added, such as sodium carbonate.

The weak base will deprotonate the most acidic of the hydrogens, which is in this case, the carboxylic acid. The carboxylic acid containing structure will be separated into the aqueous layer. Keep in mind, that the base is not limited to only deprotonating one structure, but this all just depends on the acidity of the compounds.

Step 3: To the final 2 compounds, a stronger base such as potassium hydroxide which will deprotonate the phenol.

Generally phenols are not very acidic, that is why a strong base must be used. The cyano containing compound is left in the organic layer.

Reaction Complete. All 4 compounds have been isolated into either aqueous or organic, and as mentioned earlier, it is very easy to bring an aqueous compound back into the organic, that is by adding the complementary acid or base.

Column Chromatography

Column Chromatography

Column chromatography is a preparative method for separating and isolating compounds from mixtures which can later be analyzed using thin-layer chromatography (TLC), gas chromatography, or IR. The method is used for obtaining compounds from natural sources or for purifying products from reaction mixtures. This method can be considered an upside-down version of TLC. Instead of having a thin layer of adsorbent attached to a solid support, a column is filled with a larger amount of adsorbent, often silica gel, and the mixture is loaded on top of it. While TLC depends on capillary action, column chromatography depends on a solvent or eluent moving down the column using the force of gravity. As this fluent moves down the column, it carries the soluble compounds with it and compounds having strong interactions with the adsorbent move more slowly than compounds having weaker interactions with the adsorbent. By taking advantage of the material used for the stationary and mobile phase, the compounds will separate in distinctive bands and each band will come out of the column individually. This will allow for the collection of each separate band in different vials which can then be analyzed with TLC and then dried to later be analyzed based on their melting points.

Polarity plays a large role in the process, where the using a non-polar solvent allows for a non-polar substance to be collected first before the more polar components. As the polarity of the solvents increase, more polar compounds travel further faster. This is why it is important for the solvents used to slowly change in polarity throughout the column chromatography procedure for better separation.<ref name="column chromatography">[Mohrig, Jerry R. "Techniques in Organic Chemistry." 2010, W.H. Freeman and Company]

Distillation

One form of distillation is fractional distillation which is a simple technique for accomplishing a large number of distillation/separations, in a single continuous process. A fractional distillation column has an extensive surface area for exchange of heat between ascending vapor and descending liquid. As the condensate accepts heat from the vapor at any point in the column, the condensate is partially vaporized and the vapor is enriched in the more volatile component. At the same time, as the vapor loses heat to the condensate, part of the vapor condenses and the condensate becomes enriched in the less volatile component, returning back to the bottom of the flask. Eventually, the vapor that condenses at the top of the column is pure or enriched in the more pure, volatile component where as the condensate that has returned to the flask contains the pure and enriched less volatile component. The ability to separate these two liquids highly depends on the differences in the boiling points of the two liquids as well as other factors such as rate of distillation, insulation of the column, and the efficiency in the column. In order to achieve good separation, an even heating and low rate of distillation must be applied and maintained as much as possible, creating a high reflux ratio. The type of column also plays an important role as the process of packing the column and the material used all determine the efficiency and ability that a column has for separating the components of a liquid mixture. After the fractions have been collected, TLC can be used to determine the purity of each fraction. After spotting each fraction on a TLC plate and then developing them with a solvent, the presence of one spot in a lane is representative of a pure fraction whereas multiple spots indicate the presence of other compounds in the mixture. After isolating the pure fractions, they can then be evaporated and the pure substances analyzed.

Solid phase Extraction

Solid phase extraction is a separation process by which compounds that are dissolved in a liquid mixture can be separated from other compounds by use of a stationary phase. This separation technique is different than the others in the sense that the compound is not lifted from a solution by varying polarity by means of a solvent, but instead, the adhesion to a stationary phase.

The technique works on the basis of two phases, the mobile phase and the stationary phase. The mobile phase, or the “moving phase”, is the solvent that pushes the mixture of analysis through a stationary phase. Through means of intermolecular interactions, the various components in the mixture will rapidly bind back and forth between the mobile phase and stationary phase while the mobile phase travels up the stationary phase by capillary action. Depending on the type of compound and the interaction it has between the mobile and stationary phase, each compound will display different affinities between the respective phases and demonstrate a different adsorption pattern along the stationary phase.

The stationary phase used most lab settings is amorphous silica gel which contains a high hydrophilic surface. Mobile phases used, examples being eythl acetate, petroleum ether and methyl tertiary butyl ether, possess different polar characteristics that allow for differential “climbing” patterns for varying compounds on a stationary phase. The combination of capillary action and surface tension allow for the mobile phase to travel up the stationary phase and provide for separation of mixtures based on whether the compound of interest would show interaction with the stationary phase or not. As the mobile phase “picks up” the mixture of compounds up the stationary phase, compounds that exhibit a stronger intermolecular interaction will spend more time binding to the stationary phase, traveling a shorter distance and vice versa. It is this differential interaction with the stationary phase that associates a Retardation factor with each compound. The retardation factor is defined as the ratio of the distance a compound travels to mobile phase front. Compounds with a lower retardation factor exhibit a stronger interaction with the stationary phase while compounds with a higher retardation factor exhibits a weaker interaction with the stationary phase.

References

• Mohrig, Jerry R. Techniques in Organic Chemistry. 2006, W.H. Freeman and Company
• Organic Chemistry Lab, Chemistry 143A

Recrystallization

Summary of Recrystallization

Recrystallization is the process of purification that involves dissolution of a solid in a hot solvent, filtration of the heated solution or mixture, crystal formation, and the isolation of the crystalline compound. In order to perform a recrystallization, the solubility of a compound in a hot solvent must be taken advantage of. A saturated solution at a higher temperature normally contains more solute than the same solute/solvent pair at a lower temperature; as a result, the solute precipitates when a warm saturation solution cools. In other words, a solution at a higher temperature will have more dissolved solids and as it cools, the solute will return back to its solid phase, forming a precipitate. Impurities in the solid being recrystallized are usually significantly lower in concentration than the concentration of the substance being purified so as the mixture cools, the impurities remain in solution while the highly concentrated product crystallizes.

Crystal formation of a solute from a solution is a selective process because only solids moving at the right speed and are under the appropriate conditions of concentration and solvent form almost perfect crystalline materials as only molecules of with the right shape fit into the crystal lattice. Recrystallization purifies a compound because dissolution of the impure solid in a suitable hot solvent destroys the crystal lattice of the impure compound and the recrystallization from the cold solvent selectively produces a new, more pure crystal lattice. Slow cooling of the saturated solution promotes formation of pure crystals because the molecules of the impurities that don’t fit too well have time to return to the solution. Crystals that form slowly are larger(not always) and often purer than ones that form quickly because rapid crystal formation traps impurities within the lattice as they are simply surrounded by the crystallizing solute. The most important aspect of recrystallization is the choice of solvent because the solute should have maximum solubility in the hot solvent and minimum solubility in the cold solvent. The relationship of solute and solvent can be best described as “like dissolves like”. This entails that nonionic compounds generally dissolve in water only when they can associate with the water molecules through hydrogen bonding. Hydrocarbons and alkyl halides are virtually insoluble in water whereas carboxylic acids and alcohols are often recrystallize from water solutions. In a miniscale recrystallization experiment several steps are followed to complete the purification process.

1. The solid to be recrystallized must first be weighed and then dissolved in an appropriate hot solvent.
2. The solid impurities must be filtered using a gravity filtration process.
3. The hot recrystallization mixture is then set aside to cool to room temperature.
4. After cooling to room temperature, the solution is then cooled even further by placing it in an ice-water bath for 10-15 minutes to allow further recrystallization.
5. To collect the crystals and to complete the recrystallization, the crystals must be collected by vacuum filtration.

An easy depiction of the recrystallization process

One of the key factors of purification through recrystallization is understanding solubility. In order to proceed with a successful recrystallization process, the mixture must have some important solubility properties. One solvent must be soluble at all temperatures. The next solvent must be soluble at low temperatures. And another solvent must be soluble at high temps, and insoluble at low temps.

Here is a simple illustration which explains how the purification process is conducted.

The blue square is soluble at all temperatures, The orange circle is mostly insoluble at all temperatures, and the green triangle is soluble at high temperatures and insoluble at low temperatures.

1. 1. Add heat: If we heat up the mixture containing the square, circle, and triangle, the square will already be in solution, and we can heat it to the point that the green triangle dissolves. Then we are left with only the insoluble circle.

1. 2. Filter: We can then filter the aqueous dissolved solution and once we separate this we are left with only the triangle and square in solution while the circle was left in the previous vessel.

1. 3. Cool Down: When the newly isolated mixture is cooled, the triangle will begin to crystallize again since it is insoluble at low temperatures, but the square will remain in solution since it is soluble at all temperatures.

1. 4. We separate the aqueous layer(the triangle) and we are left with our compound of interest, the triangle.

Isoelectric Focusing

The Process of Isoelectric Focusing

Isoelectric Focusing is an electrophoresis protein purification process in which proteins are separated by their isoelectric points. The isoelectric point of a protein, the pI, is the characteristic pH at which a protein carries no net charge. The net charge of a protein is determined by the acidity or basicity of the side chains that make up the protein. If a protein has more acidic groups than basic groups then the protein will have a very low pH and be considered acidic. If the protein has more basic groups in its side chains than acidic groups then the overall charge of the protein will cause its pH to be much higher. Isoelectric focusing takes advantage of these properties with the following steps:

1. Create a gel that has a linear pH gradient within it
2. Insert the protein samples into the gel
3. Apply an electric field with an anode (+) at one end and a cathode (-) at the other
4. Allow time for the proteins to migrate toward their neutral pI according to their net charge

The linear pH of the gel allows for the samples to be added arbitrarily because the pH of the environment combined with the electric field will force the movement of the protein regardless of their initial positions within the gel. The proteins do not begin to migrate until the electric field is applied. Upon application of the electric field, the proteins move toward the terminal with the charge opposite them gathering or releasing protons throughout migration until they have reached their neutral, isoelectric point. For example, assume all the protein samples have been applied at the neutral pH location of the gradient where pH=7. When the field has finally been applied the proteins will migrate according to their net charge. If the protein has an isoelectric point of 2 then at the pH of 7, the protein has less hydrogen ions than it needs in order to be neutral, i.e. reach its isoelectric point. This deems the protein as negative and thus it will move toward the anode (+) picking up protons as it travels through the gradient to a lower pH. When the protein has finally gathered enough protons to make it neutral, the protein will no longer have a net charge and cease its migration at its isoelectric point.

Isoelectric Focusing allows for protein purification based on a different protein characteristic, pI. Therefore proteins with similar characteristics such a molecular weight can be purified and separated via their distinct pI's in rather short periods of time.

Sublimation

Sublimation apparatus

Sublimation is the process in which a substance changes directly from a solid to the gas phase without forming an intermediate liquid phase. One example of this is dry ice, which is converted from the solid form of carbon dioxide directly into carbon dioxide gas. In the laboratory, sublimation can be used to purify an organic compound only after meeting four requirements.

1. The compound must vaporize without melting
2. It must be stable enough to vaporize without decomposing
3. The vapors of the compound must be able to condense back to the solid
4. Impurities within the compound do not also sublime.

The apparatus for sublimation consists of an outer vessel and an inner vessel. The outer vessel holds the sample to be purified connected to a vacuum. The inner container known as a cold finger provides a cold surface on which the vaporized compound can recondense as a solid. To perform sublimation, the sample to be sublimed must be placed into a filter flask. Next, an inner tube is placed in the flask and the vacuum is turned on. Afterwards, the sublimation flask is heated gently, using a sand bath, as ice is filled in the inner tube. During sublimation, material will disappear from the bottom of the outer vessel and then reappear on the cool, outside surface of the inner test tube. This is the result of the compound vaporizing as it reaches its sublimation temperature and then recondensing on the cold finger as a result of cooling. After completion, the inner test tube can be removed and the pure solid can be scraped off and analyzed.

It is important to note the distance between the impure compound and the cold finger during sublimation. The components of the sublimation set up need to be close enough to avoid decomposition yet far enough that contamination doesn’t take place. A large distance means that the temperature applied must be very high to keep the compound in the vapor form, which could cause the compound to decompose. A small distance could cause the impurities to easily come into contact with the purified compound on the cold finger. The purification could be ineffective if the distance is not taken into consideration.

Distillation

Distillation is used to separate a two-component mixture and Gas Chromatography is used to identify the constituent compounds. Distillation is a technique that separates compounds based on their vapor pressures and boiling points. When two different compounds are heated, one may boil at a lower temperature than the other. By separating the vapors of the compound with the lower boiling point from the other, one may separate the vapor from the liquid and re-condense it, effectively separating the two. With successive distillations, a high degree of purification is possible. Distillations carried out one a time are called simple distillations. Although high efficiency can be achieved through multiple simple distillations, it would be tedious and require a large initial sample volume. Fractionating distillation, however, simplifies this repeated distillation by providing continuous separations. A fractionating column is utilized to provide extensive surface area to allow for heat exchange between rising vapor and falling condensate; through a recursive mechanism, the upper vapors are more pure in the more volatile compound and the liquid is more pure in the less volatile compound. The degree of separation depends on the different boiling points of compounds as well as the rate of distillation, insulation, and column efficiency. By allowing more time to distill, thermal equilibrium can be reached and higher purities achieved. Likewise, insulation prevents heat loss to preserve initial conditions and column efficiency determines how many distillation “pockets” can occur.

Thin Layer Chromatography (TLC)

Thin Layer Chromatography (TLC) is a simple and quick procedure for separating and identifying components in a mixture. In principle, different components in a mixture have different solubility and differ in their strength of attraction to an adsorbent. This method utilizes this principle and has the mixture to be analyzed performed on a plate with thin layer of a solid adsorbent and then has the plate immersed in a solvent. Components in the mixture will slowly travel up the plate at different rates until they reach the maximum separation for this particular combination of solvent and adsorbent. After the mixture is separated into different colored spots, the plate is dried and the components are examined.

Technique:

1. Choose a solvent to be used to analyze the mixture, and then pour it into a beaker to a depth less than 0.5cm. The entire process is carried out in a beaker with watch glass on the top to prevent solvent vapor from escaping.

2.A TLC plate is prepared. A TLC plate is made with a thin layer of adsorbent, usually silica or alumina. Near the bottom of the plate, use a pencil to draw a line across the plate. This line will be the origin where you spot the mixture to be analyzed.

3. Place the plate in beaker so that only the bottom of the plate is immersed in the solvent.

4. When the solvent rises up by capillary action and past the spot applied, some components of the mixture will travel at faster rate due to their solubility with the solvent and their adsorption strength to the plate.

5. Different separation of colored spots will be seen on the plate. If the spots are not colored, UV lamp is used to visualize the plate.

File:Thin Layer Chromatography.JPG

1: Origin 2, 3: New positions of compound 4: Distance traveled by solvent

To identify the compounds present, the distance travelled by the solvent and the distance travelled by individual spots are then measured from the plate. Using these measurements, a retention factor, Rf is obtained by the following equation:

                        Rf = (Distance traveled by the compound)/(Distance traveled by the solvent)

From the value of Rf, the polarity of a compound can be predicted. Moreover, this value can be used to compare two compounds. If two substances have the same Rf value, there is a high chance that they are the same compound. Otherwise, they are certainly different compounds.

References

Mohrig, Jerry R. Techniques in Organic Chemistry. 2006, W.H. Freeman and Company

Organic Chemistry Lab, Chemistry 143A

Thin-layer Chromatography

Thin Layer Chomatography(TLC)

Thin-layer chromatography (TLC) is useful for determining the composition of compounds in a mixture and also for distinguishing two compounds from one another. In TLC, glass, metal, or plastic plates coated with a thin layer of adsorbent serve as the stationary phase and a pure solvent or mixture of solvents act as the mobile phase. Most nonvolatile solid organic compounds can be analyzed by TLC. To carry out a TLC experiment, a small amount of the mixture being separated is dissolved in a suitable solvent and applied or spotted on the adsorbent near one end of a TLC plate. The plate is then placed in a closed chamber with the edge nearest the applied spot immersed in a shallow layer of the mobile phase called the developing solvent. The solvent is then allowed to rise through the stationary phase by capillary action. As the solvent ascends the plate, the sample is distributed between the mobile and stationary phase, separating as a result of the different polarities between both the phases and the compounds being separated. The more polar a compound is, the more tightly it binds to the adsorbent, and the more slowly it moves on the TLC plate. The developing solvent moves nonpolar substances up the plate most rapidly and polar substances travel up more slowly. The TLC plate is then removed when the solvent front is about 1 cm from the top and its position is marked. The Rf (Retention front or Ratio to front) can be calculated based on how much the compound moves up the plate when compared to the baseline. Rf = distance compound traveled / distance solvent front traveled. The range of this value can go from 0 to 1. An Rf of 0.2 is generally desirable. However, this value can be adjusted by changing the polarity of the solvent used during the mobile phase. The plate is then dried in a fume hood and visualization methods are used to analyze the plates.

To better understand the general principles underlying this process, it is helpful to scale up thin-layer chromatography to sizes of objects we see very often.

Imagine:

Stationary phase as the beach

Mobile phase as waves that wash up to the shore and return back to the sea.

Compounds as people of different mass

Initially, two people of differing mass are sitting at the same position on the beach. As the waves wash up to their position, it will pull in the lighter person closer into the sea. As time goes on, the lighter person will be distanced from the heavier person due to their differing attraction to the sand. The distance they have traveled from their initial location are distinct and serve as a characterization and separation factor of the two individuals. Although in thin-layer chromatography, the mobile phase moves in one direction and the compounds do not differ necessarily by mass but by their differences in affinity to the stationary phase and mobile phase, this example is helpful to picture the general idea of this important process for the identification and separation of mixed compounds.

TLC plate Composition and Preparation

Solid supports to which the stationary phase is bound onto can be made up of glass, plastic, or aluminum sheet. The most common compound used to make the stationary phase is silica gel. Since it is porous, it has a large surface area which facilitates in desirable extensive interactions with compounds. The stationary phase must be bound onto the chosen solid support. This is done by mixing the silica gel with compounds which aid the silica gel particles in adhering to the solid support. The prepared mixture is then spread onto the solid support and allowed to dry.

Visualization Methods for TLC

There are two simple and basic methods of visualize, fluorescence and Iodine visualization. Fluorescence involves the use of adsorbents that contain a fluorescent indicator. The insoluble inorganic indicator is then detected when the outputs from a short-wavelength ultraviolet lamp are used to illuminate the adsorbent side of the plate in a dark box. The separated compounds appear as dark spots on the florescent field because the substances forming the spots usually quench the fluorescence of the adsorbent. These spots can later be analyzed. Another way to visualize colorless organic compounds is to use their adsorption of Iodine vapor. This is done by placing the plate in a bath of iodine vapor. Colored spots are gradually produced from the reaction of the substances with iodine vapor. The spots are dark brown on the white to tan background. After 10–15 minutes, the plate can be removed and the spot should be outlined with a pencil as the spot often disappears before being analyzed later.

Gas Chromatography

Gas Chromatography Apparatus

Gas Chromatography (GC) can quickly assess the purity of a compound but cannot identify a compound by its own unless a known same is available to use as a standard. By comparing retention times, peak enhancements, and chromatogram, the identity of the components of the mixture can be determined. GC is very useful for quantitative analysis of a component mixture as a comparison of relative peak areas on the chromatogram often gives a good approximation of relative amounts of each compound.

Gas chromatography works by essentially having two phases work with each other. The mobile phase, in other words a 'moving phase', is carried out but a inert gas such as helium or an nonreactive gas such as nitrogen. The stationary phase is a microscopic layer of liquid or polymer that is supported by an inert solid that is placed inside a piece of glass tubing which is called a column. The gaseous compounds being analyzed in the mixture dispensed interact with the walls of the column that is coated with a stationary phase. Because of different polarity and affinities that various compounds display with the stationary phase, a retention time is calculated based on how long the compound spends time binding to the stationary phase while the mobile phase moves up the stationary phase. The various compounds continuously bind back and forth to the stationary phase and the mobile phase and are separated by the various strength in intermolecular interactions they share with the stationary phase. It the comparison of the retention times between compounds that displays the efficiency and application to the gas chromatographer.

Infrared Spectroscopy

Infrared spectroscopy is one of the most useful spectroscopic techniques in organic chemistry because it is a rapid and effective method for identifying the presence or absence of simple functional groups within a compound. When IR energy is passed through a sample, absorption bands are observed and these bands are then correlated with types of chemicals bonds which can provide important information about the nature of functional groups in the sample. An IR spectrum has energy measured as frequency of wavelength against intensity of the absorption. This is the result of molecular vibrations. The atoms making up a molecule are in constant motion and the movements of the atoms relative to each other can be described as vibration. The photons of IR radiation absorbed by an organic compound have just the right amount of energy to stretch or bend its covalent bond. An absorption band appears in an infrared spectrum as a result of this vibration. These bands are affected by different vibration modes of the compound, its polarity, as well as its bond order.There are different types of vibration modes at a tetrahedral carbon. They are stretching, bending, scissoring, rocking, twisting, and wagging. For stretching, it can be either symmetric or asymmetric.

Asymmetrical Stretching

Symmetrical Stretching

Bending

Scissoring

Twisting

Wagging

Information on how an IR spectrum comes to be The mid infrared is the region that provides the most information to organic chemists. The region spans from 4000 cm-1 to 600 cm-1. This region is where the absorptions of most of the simple functional groups of organic molecules appear. An IR spectrum plots the wavenumber (inverse of frequency) of a functional group against the intensity of the absorption. This is done by running a sample of interest in an infrared spectrometer and when a certain functional group is recognized, a peak will appear on the spectrum. The wavenumber corresponds to the frequency in which the functional group absorbs energy and the intensity measures how strongly the function group is absorbed. Since the atoms that make up a molecule are in constant motion, they are said to “vibrate.” When these groups absorb the right amount of energy from the photons they are hit with, the molecules will gain enough energy to stretch or bend their covalent bonds. Stretching and bending are two different types of vibration. The energy levels of the molecular vibrations of a molecule are quantized, meaning that vibrations will only occur if the infrared energy matches the frequencies of the molecular vibrations exactly. If the energy does not match, no absorption will occur. Thus, if a function group is not present, there will be no absorption peak on the IR spectrum, making IR spectroscopy a very powerful tool in identifying function groups in a molecule. In addition, bond order can give additional insight into where certain peaks can appear in an IR spectrum. Bond order is the amount of bonding that occurs between two molecules. The bond order between carbon atoms increase as one moves from an alkane to an alkene to an alkyne. The bond order increases with the number of bonds between the two bonding atoms. The higher the bond order of the atoms, the higher the wavenumber the molecule absorbs at, and the greater the energy needed to produce a stretching vibration. Atomic mass also provides information as to the location of the absorption of certain molecules on the IR spectrum. The larger the atomic mass of the vibrating atoms, the higher frequency and the more energy it takes to stretch the bonds of molecule.

Identifying function groups on an IR spectrum After many experiments done with an IR spectrometer, scientists have found regions in which the stretching of certain function groups and bonds occur. An table is shown below:

Bond or functional group	Wavenumber (cm−1)
RO — H (alcohols)	3200 - 3650
RC(=O)O — H (carboxylic acid)	2500 - 3300
R2N — H (amines)	3250 - 3500
RC=C — H (alkynes)	3260 - 3330
C=C — H (alkanes)	2840 - 3000
C—H (alkanes)	2840 - 3000
RC=CH (alkynes)	2100–2260
RC=CN (nitriles)	2220–2260
RC(C=O)H (aldehydes, RC(C=O)R’ (ketones)	1690–1750
RC(C=O)OR’ (esters)	1735–1750
RC(C=O)OH (carboxylic acid)	1710–1760
-C=C- (alkenes)	1620–1680
RC — OR’ (esters, alcohols)	1000–1260

Of the wavenumbers that each functional group can be absorbed at, there are three distinct regions: a functional group region, fingerprint region, and aromatic region. The function group region is between 4000 – 1500 cm-1¬, in which most of the peaks of the major function groups appear. The fingerprint region is between 1500 – 900 cm-1, where many of the bending vibrations appear. It is not as important as the functional group region, but gives you a region to confirm the presence of a certain bond if you are uncertain. The last region, which is between 900 – 600 cm-1, provides information about the presence of aromatic compounds, such as benzenes.

IR spectroscopy alone is not enough to determine the structure of a molecule. It must be coupled with NMR spectroscopy to produce information for structure identification.

Below are some IR spectra for the presence of certain function groups.

IR of an alcohol:

(large wide peak between 3200 – 3400)

IR of a carbonyl group:

(strong peak around 1700)

Nuclear Magnetic Resonance

NMR is one of the most important tools for analysis in organic chemistry and biochemistry. In comparison to other protein-determining techniques, NMR can be advantageous because it analyzes the structure of macromolecules in solution. Other techniques, such as x-ray crystallography, requires macromolecules to be crystallized. NMR is also very useful in that it can show the dynamic side of protein structure, revealing details regarding conformational changes, folding, and interactions with other proteins. NMR technique uses low energy radiation within the radio frequency range. The radiation excites atomic nuclei, however only certain specific isotopes are excited; most commonly ¹H and ¹³C. These atomic nuclei can be modeled as spinning around an axis, this property is known as nuclear spin. Since hydrogen is essentially a proton, its spinning motion creates a magnetic field. For all intents and purposes, the hydrogen can be imagined to be synonymous with a magnet floating freely in space.

When these atomic nuclei are excited by an external magnetic field, they can have one of two orientations, an alpha or beta spin state. These nuclei, 1H and 13C, can be taken as tiny atomic magnets that can align with (alpha) or against (beta) the magnetic field. Alpha being more energetically favorable, and beta being a high energy state. Radiating a sample at just the right frequency to flip between alpha and beta spin states, creates resonance; in which the sample absorbs energy to flip to the beta spin state. After excitation the nuclei return to their original state, so at resonance the nuclei are constantly switching spin states. Nuclei in the beta state give off energy when relaxing to the alpha state. The resonance frequency at which this happens is proportional to how strong the external magnetic field is. The resonance frequency changes from nuclei to nuclei based upon the properties of its nucleus and environment.

Hydrogen (1H) NMR

Resonance frequencies largely depend on the polarity of bonds, the hybridization of attached atoms, and the presence of electron donating or withdrawing groups. When hydrogen is near an electron withdrawing group, such as chloromethane, the hydrogens are said to be deshielded because the polarity of the bonds pulls the hydrogens' electron cloud away from the nuclei. The opposite is known as shielding. These two phenomenon cause different hydrogens to have different resonance frequencies, and therefore different peaks. A peak is said to be at high field, or shifted right when the corresponding nuclei is shielded, and low field, or shifted left when the nuclei is deshielded.

Ester Resonance

From the example NMR above, you can see how the shielding and deshielding affect the peaks. The "C" group hydrogen, is far left because it is deshielded by the carbonyl group. The "B" group hydrogens are slightly less deshielded due to the aromaticity, and the "A" group hydrogens are barely deshielded at all.

¹H NMR Hydrogen Nuclear Magnetic Resonance is one of the most used NMR since ¹H has a nuclear spin. It is positively charged. When it spins, it creates a magnetic field. When an external magnetic field is added, the proton can either have a or B spin. The energy difference between these two states generates resonance. Different atom have different strength of the applied magnetic field. (CH3)4Si is used as a standard. the distance between the frequency of an atom to the frequency of (CH3)4Si is called chemical shift.

Chemical Shift = (distance of peak from (CH3)4Si in Hertz) / ( spectrometer frequency in megahertz) ppm

Here are some examples of commonly used chemical shifts:

Primary alkyl: 0.8-1.0 ppm

Secondary alkyl: 1.2-1.4 ppm

Tertiary alkyl: 1.4-1.7 ppm

Terminal alkene: 4.6-5.0ppm

Internal alkene: 5.2-5.7ppm

Alkyne: 1.7-3.1ppm

Alcohol: 0.5-5.0 ppm

Aromatic: 6.0-9.5ppm

It should be noted that the NMR spectra for the OH functionality of alcohols, the SH functional group of thiols, and the NH2 group of amines possess characteristically broad peaks.

Hydrogens that are in different chemical environments (meaning they have different atoms attached to them thus have different bond strengths and lengths that change how electrons "gather" around them) give different chemical shifts. An environment that is electron poor is considered deshielded and gives low field absorptions (farther to the left in the spectrum). In contrast, an electron rich enviornment is shielded and results in high-field peaks (farther to the right). Equivalent hydrogens give the same signal. Compounds have the same environment if they have the same connectivity and bond strengths/lengths. for example a molecule of benzene has 6 different hydrogens but they are all in the same environment.

Equal Hydrogens on Benzene

It is important to know how to distinguish the equivalence of hydrogen in order to read the ¹H NMR. This could be easy to spot like the electronegativity of the atoms around the nucleus of interest (hydrogen in this case), but the changes could be more subtle like symmetry in the molecule that makes some hydrogens equivalent. For this, the molecule phenol will be shown.

Symmetry makes hydrogen similar

Although the hydroxyl group does change the shielding of the hydrogens there are only three different hydrongens. The ones right next to the OH group are both similar by connectivity but also by symmetry. The line of symmetry makes them alike since nature knows not of right and left. H's connected to the same carbon usually have the same shift but not all the time. if no symmetry can be assessed than the hydrogens are completely different. Besides this, there is another hint that ¹H NMR gives us to help us to deduce the structure of a compound. It is the integration. By comparing the relative areas under the peaks, the ratio of hydrogens can be determined. We can find the relative number of hydrogens by looking at integration. Also, by looking at the splitting, we can find the neighboring hydrogens. The interaction of hydrogens with each other interrupts their magnetic field, thus affecting the shape of their peaks. One sharp peak is called singlet; One of the two-peak set is called doublet; One of the three-peak set is called triplet; One of the four-peak set is called quartet and etc. It follows the n + 1 rule. For instance, if we see one peak, there are no hydrogens attached to the atom next to the atom bonding to the hydrogen. If we see a doublet, that means the neighboring atom of the carbon that this hydrogen or these equivalent hydrogens are attached to has one hydrogen attached to it because the hydrogen affected the peak.

Carbon (13C) NMR

Carbon has the same idea as 1H NMR but 12C does not respond to NMR so the less abundant 13C is what will show up in the NMR spectra. Only about 1% of the carbon present in molecules is 13C and thus the peaks are not as intense as 1H. This lack of abundance is key because now carbons are not affected by the neighboring carbons because the chances of having two 13C atoms next to each other very unlikely. This is actually quite helpful because if there was more 13C not only would there be a disturbances in the 13C NMR but also in the 1H NMR and this would complicate NMR Spectra. The only thing that changes the peaks are the neighboring hydrogens. The same idea applies as in 1H with respect to singlets, doublets, triplets, etc.

13C NMR spectra of propyl benzoate

Propyl Benzoate

Mix Melting Point

The identity of an unknown crystal can be determined by using the process of mix melting point. Mix melting point involves mixing equal amounts of the unknown crystals and another compound of a known melting point. Firstly, the melting point of the unknown crystal is determined; then, one can perform a mix melting point with a compound of known melting point that is closest to that of the unknown crystal. A mixture of the unknown and the known of equal amounts will be grounded together with a spatula and inserted into a capillary tube. Then, the mixture is inserted into a melting point apparatus for the determination of its melting point. In this situation, the unknown is acting like an impurity to the known compound. Impurities generally lower the melting point of the known crystals. A different melting point is observed when two different crystals are mixed because mixing two different compounds will disrupt the crystal lattices of the crystals affecting its physical properties such as melting point. One compound has acted as an impurity toward the other by lowering its melting point. If the melting point of this mixture is identical to that of the known, the mixture is made of two identical compounds meaning the unknown and the known are of the same identity since the same compound cannot act as an impurity to itself.

Example: If the unknown compound(might not be a pure compound) is determined to have a melting point of 110°C, now choose a known compound of the closest melting point to that of the unknown. In this case, it would be fluorine and perform a mix melting point with the unknown. known Compound and Melting Point (Celsius) Benzil – 95 Fluorene-114 9 Fluorenone-84 Unknown+ Fluorene- 112°C

Just in case, perform a mix melting point of the unknown with another known compound, the one which has the 2nd closest melting point next to the unknown, which is benzil. A quick mix melting point shows that a mixture of the two melts nowhere close to the 110s°C range, which rules out benzil as the unknown. Therefore, the unknown compound in this example is determined to be fluorene as the mix melting of the mixture is virtually the same as that of the unknown. Melting point is also crucial is determining the purity of a compound. For instance, if a known compound is pure, the melting point of this known can be used as a standard for measuring purity. By comparing the melting point of this pure compound to a sample compound of the same identity, one can determine the sample compound’s purity. The closer the sample’s melting point is to that of the pure compound, the more pure a sample is. This is because the purer the sample the less impurities. As we discussed earlier, the more impurities that are in a compound lead to decreasing the melting point. Therefore the same compound with the highest melting point is the purest also.

Melting point determinations are done using a melting point apparatus. The compound is loaded into a melting point capillary tube that is closed on one side. Usually some tapping is required to get the compound to the bottom of the tube. After the tube is loaded into the melting point apparatus, heat is applied gradually and the compound will start melting. The experimenter is able to track the progress of the melting by viewing the compound through a scope. The melting point determination apparatus provides an easy and quick way to determine the purity, and identity of a sample!

References

Mohrig, Jerry R. Techniques in Organic Chemistry. 2006, W.H. Freeman and Company

Organic Chemistry Lab, Chemistry 143A

Shore Vollhardt. Organic Chemistry, Structure and Function, 5th Edition. W.H. Freeman and Company. New York.

Ternansky, Robert. "Experiment 4a-b" CHEM 143A Lecture. University of California, San Diego, La Jolla. 03 Feb. 2012. Lecture. Combustion analysis, also called elemental analysis, is one of the oldest analytic methods used to determine the empirical formula of a pure organic compound. A small portion of the compound undergoes combustion in the presence of excess oxygen resulting compounds containing carbon, hydrogen, and nitrogen, which is quantitatively analyzed.

History

Combustion analysis method was invented by Joseph Louis Gay-Lussac in 1810 and was later improved by Justus von Liebig, who studied under Gay-Lussac.

Combustion analysis: - The unknown compound is burned in pure O2 in an apparatus that consists of a combustion furnace and chambers containing compounds that absorb either H2O or CO2. All the H in the unknown is converted to H2O, which is absorbed in the first chamber, and all the C is converted to CO2, which is observed in the second. By weighing the absorbers before and after combustion, we find the masses of CO2 and H2O and use them to calculate the masses of C and H in the compound, from which we find the empirical formula. Many organic compounds also contain oxygen, nitrogen, or a halogen.

Reference: Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change. 5th ed. 2009.

Current Method

The current method for combustion analysis is passing the combustion gases through a chromatography (gas chromatography) column, which detects the carbon dioxide, water, and nitrogen gas by thermal conductivity.

A compound containing carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) is burned completely to form H2O and CO2. The products are drawn through two tubes. The first tube absorbs water, and the second tube absorbs carbon dioxide.

Finding Empirical Formulas

The number of moles of each combustion product must be determined in order to find the empirical formula of the compound. The steps:

1. Weigh the sample being analyzed and place in the apparatus above.
2. Sample undergoes combustion.
3. Weigh the two tubes beforehand.
4. In each of the tubes, there is a substance that absorbs only water and only carbon dioxide.
5. The increased mass in the water tube is the mass of the water absorbed, and the increased mass in the carbon dioxide tube is the mass of the carbon dioxide absorbed.
6. Calculate the mass of carbon from the mass of the trapped carbon dioxide.
7. Calculate the mass of hydrogen from the mass of the trapped water.
8. If there is oxygen, calculate the mass of oxygen by subtracting the mass of carbon and hydrogen from the total mass of the original sample.
9. Now, use these mass to calculate the empirical formula of the pure organic compound sample.

Reference

1. http://en.wikipedia.org/wiki/Combustion_analysis
2. http://en.wikipedia.org/wiki/Joseph_Louis_Gay-Lussac
3. http://preparatorychemistry.com/bishop_combustion_analysis.htm
4. Frederic L. Holmes (1963). "Elementary Analysis and the Origins of Physiological Chemistry". Isis 54 (1): 50–81.

Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change. 5th ed. 2009.

Overview

In a Fischer esterification reaction, a carboxylic acid is exposed to an alcohol and an acid catalyst which results in the production of an ester and water as products. Because the reaction is reversible, the equilibrium can be determined by thermodynamics and there are procedures that can be taken to maximize the yield of the ester. The over reaction is given below:

Esterification

Fischer esterification mechanism

Procedures to maximize the ester include either the removal of water or the excess addition of the reactants. Both of these procedures take advantage of an idea known as Le Chatelier’s Principle and drive the reaction to favor the formation of the ester. When done in a laboratory setting, dry glassware should be used along with excess carboxylic acid. If the glassware is not dry, it may cause the reaction to drive towards the carboxylic acid and away from the ester. The mechanism of the reaction is given to the right:

Methods of Isolation

Reflux through a Condenser

Reflux apparatus numbered

Isolation and purification of the ester is achieved by bringing the reaction to a state of reflux. Because most organic reactions do not readily occur at room temperature, the reaction requires a period of heating and this is why refluxing is needed. Heating the reaction in an open container can result in loss of solvent while heating the reaction in a closed container is dangerous because the closed container can explode. Refluxing is a process that allows for extended heating periods without the loss of any reagents. This is achieved by a continual condensation of the vapor back to a liquid.

When connecting the condenser to the water outlet, it should be made sure that water is coming in from the bottom inlet and leaving the condenser from the top outlet or else the condenser will not be effective. When using magnetic stirring plates, a magnetic spin vane should be added to the vial/container to mix the solution. A state of reflux can be identified if there is condensation building up on the vial or container being heated. Sometimes refluxing can be done under anhydrous conditions where the reflux condenser can be equipped with a drying tube of, for example, calcium chloride. In an esterification reaction, it is essential to use a drying tube because one of the byproducts is water. According to Le'Chatellier's principles, if water is added to the ester after during the reaction, it will drive the reaction in the backwards direction towards the reagents. An anhydrous packed drying tube is used to keep the atmospheric moisture out of the reaction vessel in order to ensure maximum yield.

After a sufficient period of reflux, the ester can be prepared for purification. In order to purify/isolate the ester and obtain a maximum yield, the organic layer containing the ester product must be washed with a base such aqueous 10% sodium carbonate, MTBE (methyl tert-butyl ether) as well as methylene chloride and the MTBE-methylene chloride solvent mixture is to be completely evaporated out. Since the organic layer was produced from an acidic solution, washing it with basic sodium carbonate forms salts and alkaline materials that can be washed out with the aqueous phase. Also, the sodium carbonate will prevent the organic layer from dissolving into the aqueous layer and allow for better separation of the layers.

Column chromatography

Further purification can be achieved through the use of a chromatography column. The column can be made with a dry filter Pasteur pipette filled with a cotton plug, an adsorbent (silica gel), and a mixture compounds such as sea sand. Following elution of the ester product from the chromatography column with methylene chloride, the MTBE-methylene chloride solvent needs to be completely evaporated from the ester. If it is not entirely evaporated, it will negatively affect the purification of the ester. The percent yield of the reaction is only accurate when there is 100% conversion of the starting reactants to the products. If the evaporation is not complete, there will be more product than expected because there will still be some excess methylene chloride and the percent yield will be higher than it should be. Also, because the product will still have traces of the MTBE-methylene chloride solvent, the ester will not be pure and this will result in a impure product. This can affect, for example, the boiling-point of the ester if its physical properties were to be measured. The presence of impurities in the pure liquid decreases the vapor pressure and results in a boiling point higher than that of a pure compound. This can cause confusion in trying to identify the ester. Also, characterization by, for example, IR spectroscopy might show inaccurate functional groups because the ester is not completely pure.

Methods of Characterization and Identification

Ultramicro Boiling-point Determination

Ultramicro boiling-point is a method to determine the boiling point of a liquid and is similar to that of melting point determination. This method utilizes a melting-point capillary tube filled with the liquid that is to be observed and an inverted glass bell. The tube is heated in a Mel-Temp instrument and the boiling-point is determined by observing escaping vapor bubbles until the vapor pressure of the sample is equal to the atmospheric pressure. Smaller TLC capillary tubes can be used to get the liquid into the melting-point capillary tubes. The liquid that is to be observed must be at the bottom of the tube. In order to observe the boiling point, "glass bells" are inserted into the tube with the open end facing downwards. The bells can be made by heating TLC capillary tubes in half with, for example, a Bunsen burner and making sure that one end is open and the other closed. Air should be trapped inside the bell and the temperature can be increased until there is a steady stream of bubble exiting the bell. Eventually, all of the air that was initially inside the bell will be replaced by the vapors of the ester being observed and the bubbles will begin exiting much faster. When they exit at rate that is impossible to count, the heat should be turned off and when the last bubble exits the glass bell, the boiling point has been reached.

At this point, the vapor pressure of the sample is equal to the atmospheric pressure. Samples can be heated more than once to confirm boiling points but they are usually not as accurate as the first one.

Infrared Spectroscopy

PMMI-IR

An IR spectrum of the product can be obtained through a capillary film technique. A liquid sample is placed between two salt plates and run through a spectrometer to obtain a spectrum. The spectrum can reveal certain functional groups with varying intensity peaks and can be used to identify unknown products. The IR results can also be an indication of the reactions progress. If the IR of the product ester still contains an alcohol group stretch in the product, it can mean that the reaction has not gone to completion yet or that the ester was not properly purified.The plates can be made of NaCl or KBr and should not come into contact with water because that can dissolve the plates and destroy them. The plates are made of NaCl or KBr because they are held together by ionic interactions and won't interfere with the IR that only measures the covalent interactions. Capillary tubes can be used to drop small amounts of the liquid that is to be analyzed on one of the plates. The other plate is placed on top and should be used to try and spread the liquid to cover the entire plate. The plates are put into Infrared Spectrometers and a spectrum can be obtained by running the sample. Plates should always be cleaned with Kimwipe and washed with methylene chloride after each use. The IR spectrum to the right is an example of one corresponding to a carboxylate ester.

Nuclear Magnetic Resonance (NMR)

Although IR spectroscopy is a great way to help unravel the structure of a molecule, another technique called NMR (nuclear magnetic resonance) has further progressed the simplicity of deciphering the structure of a studied molecule. NMR, in similar ways to infrared spectroscopy, is also a characterization technique however instead of the use of infrared light, the sample is instead immersed in a magnetic field and hit with radio waves. These radio waves then encourage the nuclei of the molecule to resonate. This resonation is interpreted by a Fourier Transform algorithm which then determines the molecules surroundings, such as the structure. It should be noted that NMR works in the fashion of determining the orientation of how molecules are put together by flip motions caused by varying frequencies. The positively charged nucleus found in each element is a moving charge that creates a magnetic moment. When there is no magnetic field present or being administered to the atom, the microscopic magnets that orient the spin of the nucleus are aligned randomly. However when places in a homogeneous magnetic field, as one would do during NMR, the magnetic moments line up with the administered magnetic field. The motion caused by this alignment, the thermal motion for specifics, creates a torque which makes the magnetic moment "wobble". It is this wobble motion, or resonance, at different frequencies that can help one understand how the molecule is put together.

References

Mohrig, Jerry R., Christina Noring Hammond, and Paul F. Schatz. Techniques in Organic Chemistry. New York: W. H. Freeman and Company, 2010. Print.

Images: Wiki-Media Commons

Trans Esterifcation

This is an organic reaction for the preparation of esters. Instead of Fischer esterification (ester synthesis through carboxylic acid and alcohol) trans-esterification method allows R-substituent to exchange from an alcohol to the ester and vice versa. This is made possible through the use of acid or base catalysts.

Acid Catalyzed Trans-Esterification

Strong acids such as sulfuric, phosphoric, and hydrochloric acids can be used to catalyze the reaction.

Mechanism: In acidic conditions, protonation occurs at the carbonyl and enables a partial positive carbon. This is where the alcohol attaches and then deprotonated by another alcohol. The formation of a tetrahedral complex occurs and an -OR’ group will be protonated due to acidic conditions. It will leave and form a new ester with the R substituent from the alcohol.

Base Catalyzed Trans-Esterification

Bases can be used to catalyze the reaction (for example, K2CO3 or NaOH).

Mechanism: In basic conditions, deprotonation of the alcohol occurs and enables a strong nucleophile to attack the partial positive carbonyl carbon. In response to the new attachment, electrons move from the double bonded oxygen to form a negative charge on the oxygen. To stabilize the tetrahedral complex, the lone pairs of electrons reform and force the OR’ group to leave. The remaining product is an ester with a new R-substituent, and a negatively charged OR’.

Importance of Trans-Esterification

Biodiesel is an ester product that is made possible by trans-esterification. It starts out with triglyceride and excess methanol to form methyl-esters and glycerol. This process is used to make bio-fuel. However, because it is found in oils of food products, it isn’t the best way to mass-produce because of food scarcities. It is a good alternative to fossil fuel and a lot of fast food restaurants convert their cooking oil into useable bio-fuel.

References

Otera, J;Transesterification.Chem.Rev.1993. 93,1449-1470 http://pubs.acs.org/doi/pdf/10.1021/cr00020a004

"Biodiesel Synthesis." (n.d.): 1-3. Web. http://faculty.northseattle.edu/tfurutani/che238_2006/biodiesel.pdf Organic Chemistry Terms

Enantiomers: Two molecules that are mirror images of each other. They are non-superimposable.

Diastereomers: Two molecules that are not enantiomers. Thus, they are not mirror images of each other and non-superimposable.

Epimers: Two molecules that differ in configuration of only one stereocenter.

Stereoisomerism: same structure, but different spatial arrangements

Covalent bonding: formed by sharing of electrons

Ionic bonding: based on electrostatic attraction of two ions with opposite charges

Ionization potential (IP): energy to remove an electron from an atom

Electron affinity (EA): energy released when electron attaches itself to an atom

Net energy input - IP + EA

Pauli's exclusion principle: no orbital may be occupied by more than two electrons.

Hund's rule: degenerate orbitals, s.a. p orbitals, are first occupied by one e- each, all of these e- having the same spin

Aufbau principle: process of adding e- one by one to the orbital sequence

Energy splitting: energy by which the bonding level drops and anti-bonding level is raised

Constitutional or structural isomers: molecules with same molecular formula, but differ in the sequence in which the atoms are held together

Delocalize: tendency of e- to spread out

Chemical Thermodynamics: deals with changes in energy that take place when processes s.a. chemical reaction occurs; controls the extent to which a reaction goes to completion

Chemical Kinetics: concerns the velocity or rate at which the concentration of reaction and products change; describes the speed at which a reaction goes to completion

Reaction Mechanism: step-by-step explanation in molecular terms of how reactants are transformed into products; a mechanism involves the sequence of the breaking and forming of bonds and the identities of any intermediate molecules

Nucleophile ("nucleus-loving"): a species (generally electron rich) which reacts with species that are electron poor (i.e. nuclei)

Electrophile ("electron-loving"): a species (generally electron poor) which reacts with species that are electron rich (has unshared electron pair(s))

Diasteromers: stereoisomers that are not mirror images.

Enantiomers: stereoisomers that are non-superimposable mirror images. They have same melting point, boiling point, refractive index, and same solubility properties.

Pyrolysis: when both C-H bonds and C-C bonds rupture

Bronstead-Lowry acid: proton donor

Bronstead-Lowry base: proton acceptor

Lewis acid: accepts electron pair

Lewis base: donates electron pair

Coulomb's Law: opposite charges attract (electrons to protons); like charges repel (- repel -; + repel +)

Node: region of space where the probability of finding the electron is zero; corresponds to where the wave function has a value of 0

3 Factors that control rate of chemical reactions: energy of activation, temperature, and concentration of reactants

Eclipse conformation: aligned; high energy

Staggered conformation: most stable, furthest away from each other; low energy

"Saturated": all bonds in alkanes are single bonds, all C atoms are sp3 hybridized

"Unsaturated": an organic molecule that contains a carbon-carbon double bond or triple bond.

"Hydrocarbons": only contain H and C

Hash: below the plane

Wedge: above the plane

Hyperconjugation: neighboring C-H bonds can overlap with partially filled p-orbitals; a stabilizing factor. Also, it explains the origin of inductive electron release by alkyl groups.

Petroleum "cracking": generates carbon free radicals catalytically

Radical chain mechanism: initiation step, propagation step (2), termination step (several)

Synthetic Organic Chemistry: science of producing more useful, structurally complex molecules from less structurally complex molecules by successive chemical reaction pathways

Ring strain: eclipsing strain + angle strain

Cycloalkanes: restricted rotation; Cn2n; may be eclipsing + angle strain; cis-trans isomerism

Cyclopropane: one rigid, planar conformation; full eclipsing of bonds; lots of eclipsing strain; lots of angle strain

Cyclobutanes : prefer a "folded conformation"; some angle strain; some eclipsing strain; no specific arrangement; no easily predictable preferred conformation

Cyclopentanes : prefer an "envelope conformation"; no angle strain; minor eclipsing strain; no specific arrangement of bonds; no easily predictable preferred conformation

Cyclohexanes : prefer a "chair conformation"; no angle strain or eclipsing strain; boat and twist boat are minor conformers; chair conformation has: 6 axial bond + 6 equatorial bonds, all bonds are staggered, substituted cyclohexanes have one preferred chair conformation

Halogenation : radical reaction in which a hydrogen atom in an alkane is replaced by halogen

Monosubstitued cyclohexanes : replace one H of cyclohexane by a methyl

Racemic mixture : an equal mixture of 2 enantiomers

Nucleophile substitution : nucleophiles-compounds with an unshared electron pair

Solvolysis : cleavage by solvent

Inductive effect : an effect which spreads out or concentrates electron density due to differences in electronegativity between 2 atoms; a transmissive through-bond phenomenon

Oxidation : an increase in number of bonds from C to a more electronegative element

Spectroscopy : the study of light energy absorption and emission by molecules; a technique for analyzing molecules based on how they absorb radiation. For identification: NMR, IR, UV, and MS.

Quanta : discrete package that organic molecules absorb energy in

Excitation : the absorbed energy produces motion

Chiral center : an atom that contains four different groups that are bonded to it. In order for a molecule to be chiral a chiral center must be present.

Conjugated systems : two double bonds that are separated by a single bond in between. The p-orbitals of the two double bonds overlap with each other across a sigma bond or a single bond.

Steric hindrance: A hindrance that prevents or slows a reaction due to the spatial arrangement of the molecules in the structure. The arrangement of molecules would be arranged in such a way as to block the site of attack.

Synthesis

The synthesis of amino acids is vital to life itself. They are important because they are the basis of proteins and required for the synthesis of more complicated molecules such as vitamins. Amino acid synthesis is the process of creating amino acids from other molecules. Not all animals can synthesize all the amino acids. Non essential amino acids are those that are synthesized while the essential amino acids are those that need to be obtained through diet. Non-essential amino acids are the amino acids that can be synthesized in animals.

The amino acids synthesis pathways are grouped into six categories. They are grouped according to their similar mechanism or the use of similar enzymes to synthesize amino acids. These six categories are simple reactions, branch chain amino acids, aromatic amino acids, threonine/lysine, serine/glycine, and unique pathways.

Synthesis of Amino Acids

Hell-Volhard-Zelinsky

Using Hell-Volhard-Zelinsky Bromination, the creation of racemic alanine can be formed from propanoic acid. This can be done as Hell-Volhard-Zelinsky Bromination allows for an addition of a functional group at the carbon 2 of propanoic acid. Through this reaction, a nucleophile can displace the bromine that is formed on carbon number two. In using a nucleophile such as ammonia, the amino acid Alanine can therefore be formed. Due to the low yields produced by Hell-Volhard-Zelinsky Bromination, Gabriel Synthesis is preferred in the making of primary amines.

Mechanism of Hell-Volhard-Zelinsky Bromination:

938 × 212px

This reaction is followed by an addition of ammonia, which will displace bromine and form Alanine.

Gabriel Synthesis

The formed 2-substitued propanedioate can be alkylated, allowing for the preparation of a variety of substituted amino acids.

Mechanism of Gabriel Synthesis:

Gabriel synthesis mechanism

A propanedioate such as diethyl 2-bromomalonate would be used to form an amino acid such as Glycine. The difference however in the synthesis of Glycine from the Gabriel Synthesis from the regular Gabriel synthesis shown above, is that the group that is attached to the Nitrogen after the first step would be hydrolyzed to form two carboxylic acids. One of the carboxylic acid group would be then decarboxylated and hydrolysis would occur once again to cleave the imide group, which would result in the formation of Glycine.

Strecker Synthesis

This synthesis is based on the cyanohydrin formation that can occur from aldehydes and hydrogen cyanide. When this reaction is carried out in the presence of ammonia or ammonium cyanide with an acetaldehyde, formation of the amino acid Alanine can occur.

Mechanism of Strecker Synthesis of Alanine:

Strecker synthesis

Simple Reactions:The synthesis of Glutamine, Glutamate, Aspartate, Asparagines and Alanine

Glutamate is synthesized by the addition of ammonia to alpha-ketoglutarate.

File:Synthesis of Glutamate.jpg

Synthesis of Glutamate

Using Glutamate, the addition of another ammonia molecule made Glutamine.

File:Synthesis of Glutamine.jpg

Synthesis of Glutamine

Aspartate is made by the combination of Oxaloacetate and Glutamate.

File:Synthesis of Aspartate.jpg

Synthesis of Aspartate

Asparagine can be made by adding ammonia directly to Aspartate.

File:Synthesis of Asparagine.jpg

Synthesis of Asparagine

There are several methods for making Alanine. The most common way is the transferring of amine group from glutamate onto private.

File:Synthesis of Alanine.jpg

Synthesis of Alanine

References

Vollhardt, Peter (2010) Organic Chemistry Structure and Function Sixth Edition. ISBN:142920494X

Introduction

Organometallic chemistry is the chemistry of compounds that contain metal and carbon bonds. They can form various reactions, similar to organic chemistry. For example, the compounds Cr(CO)6 and [Ni(H2O6)]2+ are both octahedral. The CO and H2O ligands are sigma donors. CO is also a pi bond acceptor. Other ligands like CN-, PPh3, and SCN- can act in a similar way as they can function as both sigma donors and pi acceptors. Sandwich compounds are formed by cyclic organic ligands with a delocalized pi system that bonds to metal atoms. Cluster compounds are two or more metal atoms bonded to organic ligands like CO. Cluster compounds can come in all shapes and sizes. Carbon centered clusters are called carbide clusters, which are carbon atoms that are surrounded by five or more metals.

Background

The first organometallic compound to be reported was synthesized by Zeise in 1827. He obtained yellow needle-like crystals after mixing PtCl4 and PtCl2 in ethanol. KCl was then added as well. However, the structure of this compound was not determined until more than 100 years later. The actual ionic formula of the compound was K[Pt(C2H4)Cl3]*H2O. This structure has three chloro ligands occupying the corner of the square with ethylene occupying the fourth. In 1898, Barbier and Grignard performed reactions between magnesium and akly halides, which resulted in the synthesis of alkyl magnesium complexes known as Grignard reagents. These lead to great advances in organic chemistry, but wasn't as significant in organometallic chemistry. However, this was soon changed with the synthesis of sandwich compound ferrocene, which lead to the current modern era of organometallics.

Nomenclature

The number of atoms that a ligand bonds is indicated by (eta) followed by a superscript indicating the number of ligand atoms attached to the metal. For example, the cylopentadienyl ligands in ferrocene bond through all ligands, so it is named as eta 5 - C5H5. This is then named as Pentahaptocyclopentadienyl due to five bonding positions. Some common ligands are named as carbonyl, carbene, carbyne, cyclopropenyl, cyclobutadiene, cyclopentadienyl, benzene, etc.

The 18-electron Rule

In regular chemistry, the octet rule functions as a way for main group compounds to have a full valence shell of 8 electrons. However, in organometallic chemistry, the metal ion can have up to 18 electrons. For example, the compound Cr(CO)6 has 6 electrons since it's in group 6. Each of the CO then donates 2 electrons, for a total of 18 electrons. This configuration is incredibly stable. Other compounds like Cr(CO)5 with 16 electrons and Cr(CO)7 with 20 electrons are far less stable. Two general ways can be used to count the number of electrons in the complex. The first method is the donor pair method, which considers ligands to donate electron pairs to the metal. The charge on each ligand and the formal oxidation state of the metal must be taken into account to determine the charge. For example, Cl- can donate two electrons and CO can also donate 2 electrons whereas C5H5 can donate 6 electrons. The second method is called the neutral-ligand method. This takes into account of the number of electrons that would be donated if the ligands were neutral. For example, Cl has a -1 charge so it can donate 1 electron. O has a charge of 2 electrons, so it can donate two electrons. However, unlike the first method, C5H5 now has a donation of 5 electrons. A metal bonded to another metal counts as one additional electron per metal. The reason for having 18 electrons is due to the complete filling of the electron shell of the transition metal (s2p6d10). Square planar complexes, have both sigma donor and pi acceptor characters, this becomes an exception to the 18 electron rule where a 16 electron configuration is more stable.

Carbonyl

Hundreds of ligands are known to bond to metal atoms through carbon. Carbon monoxide is one of the most common ligand in organometallic chemistry. It is the only ligand in binary carbonyls. CO has some interesting features. The molecular orbital picture of CO is similar to that of N2. First, the highest energy occupied orbital has its largest lobe on the carbon atom. Because this orbital is occupied, the CO gains a sigma donor ability. CO also has two empty pi orbitals, which gives it its ability for pi bonding. This can be confirmed by x-ray crystallography and infrared spectroscopy. First, any change in the bonding between oxygen and carbon can be seen by the vibration by IR. Second, x-ray crystallography measured the distance between C-O bonds at 112.8pm. Weaking of the CO bond by the factors described above would be expected to cause this distance to increase. CO can also act as a bridging ligand. It bridges two metal atoms, both metals can contribute electron density into pi orbitals of the CO to weaken CO bond and lower the energy. Some common reactions that occur with the carbonyl ligand is:

1) Direct reaction of a transition metal with CO. Most of these involve the metal nickel, which interacts in Ni + 4CO -> Ni(CO)4

2) Reductive carbonylation is the reudction of a metal compound in the presence of CO and an appropriate reducing agent. Ex: CrCl3 + 6CO + Al -> Cr(CO)6 + ALCl3.

3) Thermal/photochemical reaction of other binary carbonyls. The most common reaction of carbonyl is CO dissociation. This must be started by a thermal or photochemical reagent. Ex: Cr(CO)6 + PPh3 -> Cr(CO)5(PPh3) + CO

Carbene

Carbene complexes contain metal-carbon double bonds. It was first created in 1964 by Fischer. They typically contain one or two highly electronegative heteroatoms like O, N and S that are directly attached to the carbene. These are called Fischer type carbene complexes. These O, N, S that are attached to the carbene complexes are more stable than the complexes that lack this atom. The stability of this complex is enchanced by the highly electronegative atom that results in pi bonding. When using NMR, it can be seen that this complex is highly dependent on temperature. When the temperature is lowered, it splits into several peaks.

Electrons donated by common fragments

Ligand	Electrons contributed (neutral counting)	Electrons contributed (ionic counting)
X	1	2 (X-; X = F, Cl, Br, I)
H	1	2 (H-)
H	1	0 (H+)
O	2	4(O2-)
N	3	6 (N3-)
NR3	2	2 (NR3; R = H, alkyl, aryl)
CR2	2	4 (CR22-)
Ethylene	2	2 (C2H4)
cyclopentadienyl	5	6(C5H5-)
benzene	6	6 (C6H6)

References

Miessler, Gary. Inorganic Chemistry. 4th Edition. Bioorganometallic chemistry is a science that describes the research of organometallic compounds in biological systems and organisms. It is an interdisciplinary science in that it involves organometallic chemistry, biochemistry, and medicine. Bioorganometallic compounds that are found in nature include enzymes and sensor proteins. Bioinorganometallic chemistry involves toxicology in that some organometallic compounds are toxic.

Vitamin B-12.

Vitamin B12

Vitamin B-12 is a group of very important bioorganometallic compounds. Vitamin B-12 is a class of water-soluble vitamins that are involved with the metabolism of nucleic acids and the development of red blood cells. These molecules all contain a corrin ring structure bonded to cobalt. Vitamin B-12 is not naturally found in humans. The only organism that have the required enzymes to synthesize vitamin B-12 are bacteria. Some sources of vitamin B-12 are meat, eggs, and dairy products. Some diseases that can result from having an abnormal amount of vitamin B-12 (too much or too little) are anemia and some neurological diseases.

Nickel

Some bioorganometallic compounds contain nickel such as the NiFe-hydrogenases. These enzymes also contain iron and sulfur. Another important bioorganometallic compound containing nickel is nickel tetracarbonyl, Ni(CO)4. This compound is not actually a biological molecule but because of its biological effects on humans, it is very important to mention. This compound is very toxic because the metal is toxic and also because it can release carbon monoxide gas. Many other nickel compounds are known to cause allergies and may be carcinogenic.

Cisplatin

Organometallic chemistry is also proving to be very important to medicine. Cisplatin is used in the process of many treatments involving testicular, ovarian, bladder, cervical, head and neck, and small-cell and non–small-cell lung cancers. Cisplatin is a chemotherapy drug that contains a platinum center. Cisplatin works by cross-linking DNA, which leads to apoptosis (programmed cell death). The mechanism in which cisplatin induces apoptosis starts with it binding to mitochondrial DNA, interaction with phospholipids and phosphatidylserine in the membranes, disruption of the cytoskeleton, resulting in inhibition of the polymerization of actin. However, cisplatin has many side effects such as nerve and kidney damage. In addition, some patients can develop cisplatin resistance.

Hydrogenases

Hydrogenases fall in the division of bioorganic metals in that they have active sites that possess Fe-CO functionalities, though the CO ligands only participate as spectator ions. The iron containing hydrogenases have an active site, Fe2(μ-SR)2(μ-CO)(CO)2(CN), that is conntected to an iron sulfide complex through a thiolate bridge. The active site of the nickel-iron hydrogenases is (NC)2(OC)Fe(μ-SR)2Ni(SR)2 (in which SR is cysteinyl).

References

“Bioorganometallic chemistry." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 16 May 2010. Web. 18 November 2010

“Cisplatin." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 26 October 2010. Web. 18 November 2010

“Nickel." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 3 November 2010. Web. 18 November 2010

“Vitamin B12." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 18 November 2010. Web. 18 November 2010

General Info

Alkane: Molecular Formula CnH2n+2

In addition to NMR/IR spectroscopy and mass spectroscopy, the ‘Degree of Unsaturation’ analysis is a tool helpful in identifying molecular structure. To utilize this method, one must understand the meaning of unsaturated and saturated compounds. In the case of acyclic alkanes, a saturated compound is one that has the maximum number of single bonds, following the formula, C_nH_2n+2. In contrast, an example of an unsaturated compound includes an acyclic alkene that follows the formula C_nH_2n (two hydrogens less, result of double bond). Similarly, cycloalkanes follow the same formula: C_nH_2n. From these examples, it can be seen that hydrocarbons with any extra bonds (double or triple bonds) or rings will deviate the saturated formula C_nH_2n+2. This leads to the concept of the degree of unsaturation, which can defined as the sum of numbers of rings and pi bonds present in a molecule.

Each degree of unsaturation refers to a decrease in two hydrogens in the molecule, as a result of the presence of a pi bond or a ring. A degree of unsaturation of 0 means that the molecule follows the formula for an acyclic alkane (C_nH_2n+2). A degree of unsaturation of 1 means that there is a decrease of two hydrogens in the molecular formula and that the resulting formula will CnH2n. Following the same logic, a degree of unsaturation value of 2 will give a molecular formula of C_nH_2n-2. In this situation (degree of unsaturation = 2), there are three possible scenarios. There can be two pi bonds present, one pi bond and one ring present, or just two rings present. Similarly, with a degree of unsaturation value of 3, the scenarios are as follows: three pi bonds present, two pi bonds and one ring present, or one pi bond and two rings present.

Degrees of unsaturation can help us determine how an alkane molecule will act. The more saturated a chain is the less fluidity it has. This can have many implications for structural biochemistry because it changes the way a molecule behaves or its function by changing it's shape and fluidity. Degree of unsaturation can illuminate structure by showing double or triple bonds that exist. This phenomena can also be described as hydrogen deficiency. The more rings and double bonds a molecule has the less saturated the molecule. It is important to remember what these double bonds and rings can do to function and behavior of a molecule. Usually when determining degrees of unsaturation molecules that are not carbons and hydrogens are ignored. These can be molecules such as oxygen, nitrogen or any others.

The examples given above apply to simple situations where there are no heteroatoms present. When these atoms are involved, the degree of unsaturation calculations require a little more work. The same process is involved; however, the presence of halogens reduces the number of hydrogens required for saturation, the presence of nitrogen increases the number of hydrogens required, and the number of oxygen doesn’t affect the hydrogens needed. A general equation can be used to determine the degree of unsaturation from a molecular formula:

Degree of Unsaturation = (2Nc + 2 + Nn – Nx – Nh)/2

Nc = number of carbons, Nn = number of nitrogens, Nx = number of halogens, Nh = number hydrogens

This formula can be verified by drawing out various structures with an assortment of rings, pi bonds, halogens, nitrogens, and oxygens.

Understanding the Equation

Ethane

Ethanol

The presence of nitrogen increases the number of hydrogens required to reach saturation because nitrogen is trivalent. Adding an amine to the compound results in one extra hydrogen, since the nitrogen component is essentially taking away a C-H bond and adding two N-H bonds and a C-N bond. This can be seen in the comparison between ethane (C₂H₆) and ethanamine (C₂H₇N). For halogens, there is essentially one C-H bond being replaced with a C-X bond; therefore, the presence of halogens reducs the number of hydrogens required for saturation: compare ethane (C₂H₆) and chloroethane (C₂H₅)Cl. Oxygen is divalent, and so it will have no effect on the calculation of degree of unsaturation. To verify, compare ethane (C₂H₆) and ethanol (C₂H₆O).

Examples

Butene C4H8. One possible structure from degree of unsaturation value = 1

Cyclobutane C4H8. One possible structure from degree of unsaturation value = 1

A simple example is finding the degree of unsaturation of the molecular formula C₄H₈. Utilizing the formula above, the degree of unsaturation will be (2*4 + 2 + 0 - 0 - 8)/2 = 1. Therefore, the structure of the molecular formula of C₄H₈ has either an alkene present (1 pi bond), or it is a cyclobutane. A slightly more challenging example is a molecular formula of C₅H₉N. In this scenario, the degree of unsaturation will be (2*5 + 2 + 1 – 0 – 9)/2 = 2. Thus, there can be two pi bonds present, one pi bond and one ring, or two rings present in the molecule. A third example is the molecular formula C₁₀H₁₃ClO, which will have a degree of unsaturation of (2*10 + 2 + 0 – 1 – 13)/2 = 4. From this value, it can be seen that there are a variety of combinations of pi bonds/rings (carbonyl group included) that would correspond to the calculated value.

General Info

There are several ways to describe acid-base chemistry. A simple proposed model by Bronsted and Lowry is that an acid is a proton donor, while a base is a proton acceptor. An easy way to understand these definitions it to think about acidity and basicity in regards to water. Whereas a base will remove protons from water to form hydroxide ions, an acid will donate a proton to water, forming hydronium ions. Water is neutral and has a pKa of 15.7. This can be used as a reference point in determining the acidity or basicity of a compound. The pH is defined as the negative log of [H+]. For pure water, the pH is 7, and an aqueous solution below 7 is considered acidic, while a solution above 7 is considered basic. The pKa of a solution can be determined by the equation pKa = -log(Ka) in which Ka is the acid dissociation constant. Generally an acid with a pKa lower than 1 is considered a strong acid, while an acid that has a pKa greater than 4 is considered weak. Some examples of strong acids include HI (pKa = -10), HBr (pKa = -9), and HCl (pKa = -8). Examples of weak acids include acetic acid (pKa = 4.7) and methanol (pKa = 15.5). Molecules can that can act as an acid under certain conditions and act as a base in a different environment are known as amphoteric compounds. Water is a common example of a self-ionizable amphoteric compound.

Another way of depicting acids and bases comes from Gilbert N. Lewis. A Lewis acid is one that is at least two electrons short of a closed outer shell, while a Lewis base contains at least one lone pair of electrons. Moreover, Lewis bases can share one of its lone pairs with a Lewis acid to create a new covalent bond. This concept can help understand the fundamentals of nucleophiles and electrophiles. All Lewis acids are electrophiles. Nucleophiles generally refer to a Lewis base, and always have at least one lone pair of electrons.

The relative strength of an acid HA can be determined by a few general rules:

1) Electronegativity: The more electronegative the atom attached to the acidic proton, the more acidic the proton will be because the bond will be more polar. This trend is very helpful since generally electronegativity increases from left to right in the periodic table. For example, HF is more acidic than H2O, since fluorine is more electronegative than oxygen. Electronegativity is the phenomena of an element to draw electrons toward it. This means that as electronegativity increases, the more likely an element will pull electrons from other elements. Fluorine is the element with the highest known electronegativity levels.

2) Size: With an increasing size of A in the acid HA, the acidity of the compound increases. This is a result of the poor overlap of the outer-shell orbital of A with the 1s hydrogen orbital. Thus, dissociation of HA into H+ and A- is favored for a larger A. Also, the large outer-shell orbital allows electrons to occupy a larger region of space, and thus electron-electron repulsion is diminished in A-. For these reasons, acidity will increase when going down a column in the periodic table. This helps explain why HI > HBr > HCl > HF (decreasing acidity, HI being most acidic of the group).

3) Resonance: Resonance in A- allows for the delocalization of charge over several atoms. An example of this is acetic acid vs. methanol. Acetic acid is more acidic than methanol because its deprotonated form is resonance stabilized, whereas methanol’s is not.

Acetic Acid Resonance

A general rule for determining the acidity of a compound is that the acidity of HA increases to the right and down the periodic table, while the basicity of A- decreases in the same manner.

Real Life Applications

An example of our every day interactions with acid-base chemistry includes stomach acid. The pH of the stomach juice generally remains within a range of 1.0 and 2.5, varying depending on the stimuli of senses. This stomach acid alters the natural folded shapes of protein molecules, allowing them to be broken down by digestive enzymes. Though stomach acid is extremely useful in this manner, it can also be harmful if unregulated, since it can destroy the protein molecules in the stomach tissue itself. To prevent this from happening, the interior of the stomach is coated with a layer of cells known as gastric mucosa, which insulates the stomach wall from acidic gastric juices. Cells beneath the gastric mucosa are activated via stimuli of taste, smell and histamine (a type of signaling molecule) that results in parietal cells releasing HCl into the stomach. Conditions such as hyperacidity, where there is excessive amounts of acid secreted into the stomach, and peptic ulcers, which are sores resulting from bacterial infections, can be regulated by medications that block histamine from signaling the parietal cells. Some common ingredients used in these medications include cimetidine, famotidine, and ranitidine.

Another example of acid-base reactions is the affects of pH on DNA. The formation of DNA occurs readily at a pH of 7.0; however, altering the pH level of a solution containing the double-helical DNA can destabilize the DNA double helix. For example, in a solution with double-helical DNA and a concentrated base (such as OH^-), the DNA will begin to dissociate into its corresponding single strands when pH approaches 9.0. This is a result of the hydroxide ions and their interaction with DNA base pairs, removing specific protons. Similarly, when the pH of this solution drops too low (below 5.0), the DNA double helix is destabilized. This is because some of the hydrogen bond acceptors become protonated and can no longer participate in hydrogen bonding, so the double helix separates. This shows how altering the pH of DNA can disrupt its double-helical structure.

Importance of Inorganic Chemistry in Biochemistry: Bioinorganic Chemistry

Although biochemistry generally focuses on the reactions and interactions of biological and organic molecules within the body, the roles and interactions of various inorganic molecules with the macromolecules in the body are just as important. Of all the elements known, only a few are essential for living organisms: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Sodium (Na), Phosphorus (P), Sulfur (S), Chlorine (Cl), Potassium (K), and Calcium (Ca). Additionally, a small amount of trace elements are essential for certain subsets of organisms. These elements include Lithium (Li), Beryllium (Be), the transition metals Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), and Molybdenum (Mo), and the nonmetals Selenium (Se) and Iodine (I).

As a general overview, metal ions have varying roles in the body. Some examples of the metal ions’ roles are:

1)Linking distant residues or parts of the amino acids in the proteins together. 2)Mediating interactions between the protein ligand. 3)Positioning themselves in the active sites of the macromolecules as nucleophilic catalysts or as a key component in the electron transfer chain.

There are six major elements that are important. They provide the building blocks for nucleic acids, proteins, lipids, etc. They are:

• Sulfur:is a chemical element with a symbol S and its atomic number is 16. It is an abundant and multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8.
• Phosphorus:is a chemical element with a symbol P and its atomic number is 15. Also, it is a multivalent nonmetal of the nitrogen group. Phosphorus as a mineral is almost always present in its maximally oxidised state, as inorganic phosphate rocks.
• Nitrogen:is a chemical element with a symbol N and its atomic number is 7. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.09% by volume of Earth's atmosphere.
• Oxygen: is a chemical element with symbol O and its atomic number is 8.
• Carbon:is the chemical element with symbol C and its atomic number is 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons that is available to form covalent chemical bonds.
• Hydrogen:is a chemical element with a symbol H and its atomic number is 1. With an average atomic weight of 1.00794 u, hydrogen is the lightest element and its monatomic form is the most abundant chemical.

It would appear logical that numerous metal ions would be able to fulfill the biological processes. However, biological processes require a specific metal in order to proceed because of their unique combination of properties such as specific crystal field stabilization energy, complex conformation, and electron transition states. For example:

• coagulation cascade: Ca²⁺ ions
• protein biosynthesis: Mg²⁺ ions
• biomineralization (producing minerals to harden the existing tissues): Ca²⁺, magnesium, iron, etc.
• energy storage: phosphorous as inorganic phosphate P_i, Na⁺, K⁺, iron, etc.
• signaling: Ca²⁺, boron, nitrogen, oxygen
• Lewis acid-base catalysis: zinc, iron, manganese
• numerous proteins, oxidative processes, and enzymes: Zn²⁺ ion

Macromolecules

A macromolecule is a very large molecule that has a polypeptide chain structure. Proteins, rubber, genes, polysaccarides, and synthetic polymers all consist of macromolecules. Macromolecules interact with each other and with small molecules. All of the interactions reflect the complementary between the interacting species. Sometimes, the complementary is general, as in the association of hydrophobic groups, but more often an exact fit of size, shape and chemical affinity is involved.

Inorganic macromolecules can be divided into several categories such as solids formed due to covalent bonds, organosilanes, siloxanes and organosiloxanes. Inorganic molecules are generally simple and are not normally found in living things. Although all organic substances contain carbon, some substances containing carbon, such as diamonds, are considered inorganic.

Read more: http://www.answers.com/topic/inorganic-molecules#ixzz2BnXuBZvS

Examples

Solids Formed by Covalent bonds

Diamond, graphite, silicon, and germanium are some examples of large inorganic macromolecules. However, Zinc sulfide has two forms/phases: the wurtzite and the zinc blende.
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Wurtzite is a mineral that includes zinc sulfide (Fe,Zn)S. ZnO, SiC, AlN, CaSe, BN, C(Hexagonal Diamond) all have the same crystal structure in terms of bonding, symmetry, and packing sequence.

Zinc blende is cubic. Its' structure has the same bonding skeleton as the diamond structure.

Wurtzite and zinc blende structures are the two common structural types of inorganic macromolecules.

Elastic Sulfur

When liquid sulfur is poured into cold water, a long chain of -S-S-S-S-S- is formed. This phase is known as the elastic sulfur phase.

Nucleic Acids

Nucleic acids are polymers made of nucleotides connected via phosphate-sugar backbones. There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). A nucleotide is made up of a 5-membered,sugar (ribose or deoxyribose) which is connected to a nitrogenous base (guanine, adenine, thymine, cytosine, uracil) and phosphorous group(s). DNA is made up of nucleotides containing guanine, adenine, thymine, and cytosine bases. RNA is made up of nucleotides containing guanine, adenine, uracil, and cytosine bases. The nucleotides are attached to each other via a phosphate group along the 5'-3' carbon side of the sugar. In addition, DNA is usually a double-stranded molecule whereas RNA is usually a single-stranded molecule.

"Zinc Fingers" protein forming a complex with DNA

Deoxyribonucleic Acid (DNA)

DNA is a double-helix structure formed by the intertwining of two DNA strands in which the phosphate group is located on the outside of the molecule, whereas the nitrogenous bases are inside the structure and are connected via hydrogen bonds and stabilized by Van der Waals forces, hydrophobic effects, and charge-charge repulsion. DNA binds metal ions to the phosphate groups and the electron donor groups in the nitrogenous bases. By binding to the negatively charged phosphate groups, the metal ions neutralize the negative charges on DNA, thereby stabilizing the double-helical structure of the DNA. High concentrations of metal ions can be deleterious. If there is a high metal ion concentration, then less stabilization is required by hydrogen bonding between the nitrogenous bases which can lead to mis-pairing of bases and errors during transcription.

Ribonucleic Acid (RNA)

RNA polymerase bound to DNA and RNA through the use of a Magnesium²⁺ ion.

Metal ions bind not only with DNA, but also with RNA. Metal ions interact with the phosphate groups and the electron donor groups in the RNA molecule. This interaction is important because it gives structural stability to the RNA. In addition to this contribution, metal ions interact with ribozymes. By nature, RNA is compact, stable, and folded. They are made of:

1)Ribosomal RNA (rRNA) --- their duty is to catalyze and regulate protein synthesis. Also, it carries the peptidyl-transferase activity and it is part of the structure of the ribosome. 2)Small nuclear RNAs molecules --- involved in the nucleus. It is splicing. 3)Signal recognition particle --- transfers the proteins through the membranes of the cell. Basically, it is a protein target.

In all of these RNAs, metal ions play an essential role in the RNA's structure, formation and catalytic mechanisms. These information are explained in more detail as follow: The pathway for the way that the RNA folds is as follows that: First, the RNA changes from a coil to a secondary structure. The second step is the progression to the tertiary structure. In tertiary structures, the long-range interactions determine the tertiary structure of the nucleic acid.

Because of the phosphate sugar backbone, metal ions play a heavy part in the interactions that RNA goes through to fold and function. Different metal ions with different charges play different charges in various roles of the transformation. Ions with a +1 charge have a part in the charge-screening. This allows the RNA molecules transform to the secondary structure. The tertiary structures of the RNA molecules are stabilized, not with monovalent ions, but with metal ions with two charges.

Holistically speaking, the formation of RNA's tertiary structure is contingent on four different criteria: (a) the RNA sequence, (b) metal ion identity, (c) metal ion concentration, and (d) the presence of RNA binding proteins. The preferred metal is Mg²⁺ because Mg²⁺ it is not only helps with stabilizing the tertiary structures, but it also helps the RNA bind to sites of high affinity. However, other metals such as K⁺, Ca²⁺, Mn²⁺, Cd²⁺, Na⁺ and Li⁺ will also suffice. However, their range of reactivity only works for certain RNAs. However, they are vital for RNA metabolism. Trivalent ions besides organic protonated ions are not used.

There are several types of metal ion binding to RNA known:
(a) Diffuse Binding -- performed and done by cations. They are vital to make secondary and tertiary structures.
(b) Site-bound outer-sphere binding of magnesium hexahydrate ion: Water ligands bridge the metal ions and the coordinating atoms on the RNA nucleobase or backbone.
(c) Site-bound inner-sphere binding of Mg²⁺ to the RNA -- The inner-sphere bound metal ions such as Mg²⁺ are involved in RNA formation and function.

Proteins

protein Proteins are molecules that perform many important functions of the cell that keep it alive. Inorganic molecules and ions are a key part of many of these proteins and their interactions with other molecules. Metal ions can influence both the folding processes and the final structures of many important proteins.

Examples

Hemoglobin and Myoglobin

Iron Complex in Hemoglobin

Hemoglobin[[|]] and Myoglobin [[|]]are both oxygen-transport proteins that use metal ions to help carry out their functions. After their polypeptide chains have been sequenced, they bind with Fe²⁺ ions to form their final structures.

• Wikipedia-Hemoglobin
• Wikipedia-Myoglobin

Sodium Potassium Ion Pump

The Sodium Potassium Ion Pump is used to pump sodium ions out of a cell while at the same time pumping potassium ions in. This function can regulate a number of important cell properties, including the size of the cell and the amount of positive charge inside the cell relative to the outside (known as the resting potential).

• Wikipedia-Sodium Potassium Ion Pump

Chlorophyll

Alt text

Heme and Chlorophyll Structures

Chlorophyll is the pigment in many plants that gives them their green color and allows them to create their food from sunlight. Chlorophyll uses a Mg²⁺ ion to start the light reactions of photosynthesis. Chlorophyll a and chlorophyll b are the two different types of chlorophyll that are found in plants. Their structure consists of a porphyrin ring with a central magnesium ion and a long hydrophobic side chain. The difference in the side chain allows chlorophyll to absorb light at different wavelengths. The hydrocarbon tail that is attached to the porphyrin ring makes chlorophyll fat-soluble and insoluble in water.

• Wikipedia-Chlorophyll

More Proteins that contain metals

Fe (heme): peroxidase, catalase, cytochrome P450, cytochrome c

Fe (without heme): ferredoxin, hemerythrin

Hemerythrin without heme

Cu: tyrosinase, nitrite reductase, amine oxidase

ZnII: carbonic anhydrase, carboxypeptidase, DNA polymerase

MgII: DNA polymerase

For more information ---> Miessler and Tarr. Inorganic Chemistry. 3rd ed. Pearson Prentice Hall: 2004.

Carbohydrates

Carbohydrates form complexes with many metal ions. The hydroxyl group carries a slight negative charge which is increased if the hydrogen atom is deprotonated. This allows carbohydrates to attach to metal ions which carry positive charges through ionic interactions[[|]].

Inorganic ions can also oxidize or reduce carbohydrates, determining their reactivity. Carbohydrates can be placed in a cupric ion solution. The ones that react can exist as a ketone or an aldehyde and called reducing sugars. These sugars react readily with many molecules. Those that do not react are non-reducing sugars.

Example

Calcium

In biological systems, carbohydrates have been observed with calcium. Studies show that in an aqueous solution calcium binds to ionic or uncharged carbohydrates. While there is no hard evidence, many believe that carbohydrates may have a role in transporting calcium, calcification, or storage of calcium.

Lipids

Lipids are hydrophobic molecules which in the case of fats and oils have as building blocks fatty acid and triglycerides. Other lipids are steroids.waxes. Phosphorylated lipids are the major component of plasma membrane.Link label Page text.

Inorganic compounds such as PO₄^3- and NaOH play an important role in understanding reactions involving macromolecules and their uses.

Examples

1)phosphate in phospholipids.

Phosphates play a major role in the formation of the lipid bilayer of the plasma membrane. Attaching to fatty acid,they form the hydrophilic end of the bilayer whereas the lipid part form the hydrophobic part.

2)Formation of soap: a mixture of sodium salts of different fatty acids. If the fatty acid salt has potassium rather than sodium, a softer lather is the result.

Soap is produced by a saponification or basic hydrolysis reaction of a fat or oil. Currently, sodium carbonate or sodium hydroxide is used to neutralize the fatty acid and convert it to salt.

General overall hydrolysis reaction:

fat + NaOH ---> glycerol + sodium salt of fatty acid

http://www.elmhurst.edu/~chm/vchembook/images/554hydrolysistrigly.gif

Importance of Metal Ions in Biochemistry

Metal ions play a significant role in the biological systems especially when the ion has empty or half filled d-orbitals. They can positively and negatively affect metabolic processes. Most metals have a strong affinity to accept electrons and form bonds. Thus, a lot of proteins fold into a structure that and bond the metal to form a function(e.g., Hemoglobin). With the help of iron, hemoglobin transports dioxygen O₂. Also, iron (Mg²⁺) holds an essential role in the formation of deoxyribonucleotides by ribonucleotide reductase. Iron in its complexes transforms between two oxidation states by transferring one electron from this state to another state, such as the ferrous (Fe²⁺), and ferric (Fe³⁺). By the process of one electron transfer between two states of iron and storing them, an organism can be able to survive in an aerobic world; however, they should maintain this form of iron in soluble and being ready to use which prevents from presence of uncontrolled redox chemistry.

There are two important proteins that help organisms performing these activities:

a. Transferrin: the iron-transport protein

b. Ferritin: the iron-storage protein.

Trace metal ions are important in biochemical systems. More than one-third of all enzymes require the addition of metal ions or contain a bound metal ion. A protein that contains a metal cofactors is called a metalloprotein[13]. Metal ions have a few characteristics that increase chemical activity: positive charge, ability to form strong bonds that are kinetically favored, and in some cases their ability to be stable in multiple oxidation states.

Many enzymes require metal ions for catalytic activity. Carbonic anhydrase was the first known enzyme to contain zinc, but since then, hundreds have been discovered. A few years after the discovery of Carbonic Anhydrase, scientists discovered this enzyme not only contained a bound zinc ion but that the ion was necessary for catalytic activity. For more information on metal cofactors, see Metal Ion Catalysis .

Carboxypeptidase A contains the Zn⁺² metal in the active site

Less common trace metals are chromium, nickel and vanadium. Other ions are often toxic to organisms, such as barium, beryllium, cadmium, lead, arsenic, selenium, aluminum, thallium and tin. [14] A deficiency of certain metals, such as Zinc(II), have been shown to lead to slowed growth, impaired metabolism and developmental retardation (Ashley & Ridgway, 1970).

Metal ions and Its Role in Bacteria

Bacteria like with any organism has evolved and learned to find their own niche. There has been discovered bacteria that may utilize metal ions for energy. This bacteria or microorganism is known as the Geobacter metallireducens. What it can do is take oxidize organic substances and utilize that to couple with the reduction of metal ions.

The very first of its species was found in the Potomac River in 1987, very close to the nation's capital Washington D.C. This was the first microorganism found that was capable of oxidizing organic compounds into Carbon Dioxide with iron oxidizes (Fe(III)) assisting it as the electron acceptor. This electron acceptor is found on a structure of the bacteria known as the pilus. This added a new perspective on the role of pilus as before it was thought to only for sex conjugation, anchoring to a particular niche, and for twitching motility. This goes to show that metals, which were once thought not to be able to be utilized by any organism, is known to capable of using by bacteria.

Bioremediation

What is bioremediation? It is defined as using microorganisms to help clean up the environment or the contaminated environment back to what it used to be.

With this unique property of the Geobacter species, research has been ongoing in using the Geobacter to assist helping with ameliorating the environment. For instance, the Geobacter may take petroleum contaminants that are in dirty water and converting it into innocuous carbon dioxide by the process of oxidization. Because of their special characteristic, Geobacter are able to speed up the breakdown of pollution in the water and other places. Moreover, it can be used to take away radioactive metals from groundwater.

Not only can be used for bioremediation, Geobacter may also be used as fuel cells by conversion of organic products into electricity. This is made possible by their property of transferring electrons unto the electrode surfaces.

References

1. The Geobactor Project at University of Massachusetts Amherst. 2009. 17 Oct. 2009. <http://geobacter.org>
2. <http://www.ncbi.nlm.nih.gov/sites/entrez?Db=genomeprj&cmd=ShowDetailView&TermToSearch=192> 3. Bertin, Ivano. Biological Inorganic Chemistry: Structure and Reactivity. 2007.

Background

The alkali metals are a series of chemical elements forming Group 1 (IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). (Hydrogen, although nominally also a member of Group 1, very rarely exhibits behavior comparable to the alkali metals). The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group.

Alkali Metals generally exist as cations with a 1+ charge, and exists as lustrous metals that conducts both heat and electricity well. They are water soluble, basic and very reactive. Alkali metals are the least electronegative group, and tend to react best with nonmetals and water.

Potassium and Sodium are vital in biochemical systems. Valinomycin complexes selectively with potassium for transporting molecules across the cell membrane. Crown ethers and cryptands are structures that complex with alkali metal ions; supramolecular chemists are studying these interactions. [15]

The sodium and potassium pump are important in maintaining equilibrium amounts both inside and outside the cell. Sodium is concentrated outside, whereas potassium is concentrated inside. Both of these metals also help to communicate electrical signals in nerves and in the heart. Humans obtain sodium by eating foods with table salt and baking soda. To obtain potassium, we can eat bananas, oranges, and avocados.

Lithium is the lightest of the alkali metals. Traces of lithium ion occur in animal tissues, but it has no known physiological role. Lithium salts were introduced into psychiatry in 1949 for the treatment of mania. It is believed that the lithium ion at concentrations of 1 to 10 mEq per liter inhibits the depolarization-provoked and calcium-dependent release of norepinephrine and dopamine, but not 5-HT, from nerve terminals[1]. Lithium ion is small ( its radius is around 0.6 Angstrom) and mobile ( fast exchange ligands ). However, it weakly binds to ligands and is easily hydrated by water. In medicine, Lithum is used to control bipolar affective disorder such as manic depression.

Sodium reacts spontaneously with water and rapidly with oxygen. It is a silvery, soft solid in its pure form. There is an abundant amount of the sodium element, most commonly recognized as sodium chloride, or table salt. Sodium chloride is an important nutrition for animals.

Potassium reacts very spontaneously with water, igniting the hydrogen, causing it to burst into lavender-colored flames. It is the sixth most abundant element on earth and is used in fertilizer and drain cleaners.

Rubidium has similar characteristics as potassium but is much more reactive. In more than a dozen studies of this element, Rubidium can replace the +1 bond that potassium offers to human/animal intracellular functions in most, if not all cases. More research is being conducted to draw more light on this previously unknown characteristic. One study strongly suggested that rubidium enhances the biological uptake and usage of intracellular lithium.

Cesium is a whitish/yellow soft metal with a surprisingly low melting point of 83 degrees F, making it nearly liquid at room temperature. There has been dozens of clinical trials of this element and it's remarkable possibilities to treat, or even cure cancer. Supposedly, German and French scientists are well-known for their Germanium/Cesium cancer curing treatments, but the FDA does not acknowledge the legitimacy of these claims due to a lack of evidence. Cesium also has similar characteristics to potassium but is extremely reactive (releasing heightened bursts of energy when bonding with positive electrons).

Francium is the second rarest natural and most unstable element. All the francium isotopes are radioactive and have short half-lives.

Applications of Alkali Metals

Most of the alkali metals displayed on the periodic table have many applications aside from being used chemical reactions. Two of the most well known alkali pure metals used are rubidium and caesium. These two metals are used in atomic clocks, which is a device that uses electronic transition frequencies that most accurately depict time and frequency standards for known substances. Compounds such as sodium are also used in daily applications such as in the sodium-vapour lamp, which is simply a lamp that can emit very efficient light.

References

1. Goodman, Louis S, and Alfred Goodman Gilman. The Pharmacological Basis of Therapeutics. 7th ed. New York, N.Y.: Macmillan, 1985. 2. Bertini,Ivano. Biological Inorganic Chemistry : Structure and Reactivity. Structural Biochemistry/Alkali Earth metals/ Transition metals are the elements in groups 3-12 in the d-block of the periodic table. They are known to have 2 or more oxidation states.

General Properties

The general properties of transition elements include:

1. High melting points
2. Several oxidation states
3. Colored compounds

Other properties include high boiling points, high electrical conductivity, and malleability. The transition metals also have d-orbitals which are loosely bound. The first row transition metals generally form high spin complexes. The second and third row transition metals generally form low spin complexes. Transition metals are also able to absorb light. Red light is absorbed at low energy levels while violet light is absorbed at higher energy levels.

Iron

File:Lactoferrin.jpg

Lactoferrin

Iron is a biologically important transition metal as it is also vital to life - it is one of the few trace elements needed for organisms to sustain life. It has three main biological roles: 1. Transport oxygen from lungs to cells It is used to bind to enzymes throughout the body, such as in Hemoglobin to transport oxygen throughout the human body in blood. 2. Energy Production Iron is used in the conversation of sugar, fats, and proteins into adenosine triphosphate, ATP. 3. Catalase Production Iron is involved with the production of catalase and this is important because catalase protects the body from free radical damage.

Rich sources of iron in food include: red meat, soybean, white flour products, seafood, and sunflower seeds Despite its uses in biological systems, an excess amount of iron can be detrimental to the human body. First, iron can cause enzyme dysfunctions by replacing other vital minerals. All these essential minerals compete for binding sites in enzymes, and when iron replaces the competing mineral, it causes the enzyme to malfunction. Second, when iron replaces other elements in the body, it also causes inflammation. Iron attracts oxygen and when in excess, the free radical oxygen damages the surrounding body tissue. In addition, as a carrier for oxygen, iron promotes bacterial growth by feeding it oxygen, leading to chronic infections. Iron can mostly be found in the pancreas, joints, liver, and intestines.

Physical Illnesses Associated with Iron

[16]

• Diabetes
• Nervous System Diseases: Parkinson’s disease, Alzheimer’s disease and behavioral abnormalities, including violence, anti-social behavior, ADHD, and autistic characteristics.
• Hypertension and Cardiac Conditions
• Kidney Problems

Copper

[17]

Copper has a diverse role in the human body.

Connective Tissue and Bones

Copper repairs the calcium in bones and connective tissue. Insufficiency or excess can lead to conditions like osteoporosis, bone spurs, and scoliosis.

Immune System

In the immune system, copper must be in balanced with zinc. When these two elements are not balanced, the body is prone to infection, particularly yeast and fungal infections. Since copper is a critical element in aerobic metabolism, an improper level of copper allows the fungal organisms to flourish.

Reproductive System

Copper also plays a role in the reproductive system as it is required for pregnancy and fertility. An imbalance of copper can lead to premenstrual syndrome, ovarian cysts, miscarriages, and sexual dysfunctions. Studies have shown that woman with deficient estrogen and copper have a higher risk of miscarriage. Correcting the copper level by eating more meats, eggs, poultry, nuts, seeds, and grains can help with a normal pregnancy.

Nervous System

In the nervous system, copper plays a role in triggering the production of neurotransmitters epinephrine, norepinephrine and dopamine. As a result, copper imbalance can be associated with psychological, neurological, and emotional problems in humans.

Copper is used to bind to enzymes throughout the body. It is used to defend the body against damage from free radicals. Foods that contain copper include shellfish (i.e. crab, lobster, etc.), dried beans, and nuts.

File:Hemocyanin color.gif

Hemocyanin

Hemocyanin is an excellent example of the use in proteins. Hemocyanin is an alternative O₂ transport protein that involves the binding of O₂ to the two Cu²⁺, which is then oxidized to Cu³⁺ after binding. It is different from Hemoglobin in that in doesn't "tag along" with red blood cells, but is contained in hemolymph.

Zinc

Zinc is an inorganic compound that play an active role in biological settings. Its ability to adapt to various coordination geometries and its properties as a Lewis acid and redox inert makes it an important compound in structural and catalytic biochemistry.

Zinc undergoes rapid ligand exchange and is regulated by several proteins in cell signaling. For example, in the central nervous system, zinc is released from the synaptic vesicles at some glutamatergic nerve terminals to trigger signaling pathways which affect physiological functions such as synaptic plasticity, potentiation, and cell death. In addition, diabetes studies have shown that zinc is released along with insulin to control glucose levels.

Besides from being regulated, zinc is also capable of regulating other proteins by shifting its concentration. Zinc can influence the productivity of nitric oxide which changes the immune system. Lack of zinc in the body weakens the immune system and leaves the body prone for infections. In the prostate glandular epithelium, a change in the normal concentration level can lead to complications in the prostate. In the nervous system, a concentration of zinc that is too high can mean mitochondrial dysfunctions.

Biochemistry of Mobile Zinc and Nitric Oxide Revealed by Fluorescent Sensors Michael D. Pluth, Elisa Tomat, and Stephen J. Lippard Annual Review of Biochemistry, Vol. 80: 333 -355 (Volume publication date July 2011)

Cobalt

[18] [19]

Cobalt is at the core of B₁₂ vitamins.The structure of this is based on the corrin ring. It is used to treat anemia because it stimulates the production of erythropoietin which makes red blood cells. Like any other element, a high concentration of cobalt is harmful to the human body. Excess intake of cobalt can result in vomiting, nausea, vision problems, heart problems, and thyroid damage. We mainly obtain it from the environment by breathing air, drinking water, and eating food that contain cobalt such as meats, dairy, and leafy green vegetables.

Radioactive cobalt can also cause health concerns. This type of radiation is sometimes used to treat cancer patients. Exposure affects include hair loss, diarrhea, and vomiting.

There are several enzymes that contain cobalt and use it as a ligand to bind to methyls and adenosyl. It is thought that cobalt acts by inhibition of enzymes involved in oxidative metabolism and that the response is the result of tissue hypoxia. More specifically, cobalt blocks the conversion of pyruvate to acetyl coenzyme A (coA) and of α-ketoglutarate to succinate .

Mercury

Mercury was an important constituent of drugs for centuries-as an ingredient in many diuretics, antibacterials, antiseptics, skin ointments, and laxatives. The use of mercury in medicinal preparations has dramatically decreased due to the toxic effects that it has in the human body, such as nausea, vomiting, abdominal pain, bloody diarrhea, kidney damage, and death. Mercury readily forms covalent bonds with sulfur, and it is this property that accounts for most of the biological properties of the metal. When the sulfur is in the form of sulfhydryl groups, divalent mercury replaces the hydrogen atom to form mercaptides, X-Hg-SR and Hg(SR)2, where X is an electronegative radical and R is a protein. Organic mercurials form mercaptides of the type RHg-SR'. Mercurials even in low concentrations are capable of inactivating sulfhydryl enzymes and thus interfering with cellular metabolism and function. Mercury also combines with other ligands of physiological importance, such as phosphoryl, carboxyl, amide, and amine groups[1].

Chromium

[20]

In mammals, chromium, a micronutrient, is only required in small quantities in biological systems. While the exact roles that chromium plays in the body is still unknown, research has proposed that chromium helps maintain proper carbohydrate and lipid metabolism. In the late 1950s. Schwarz and Mertz showed the importance of chromium through experiments involving the diets of rats. When the rats were fed with Torula yeast, a diet lacking chromium, the rats were unable to efficiently remove glucose from the bloodstream. Then when the rats were fed with food rich in chromium, the rats were able to maintain a normal glucose level. This experiment became evidence that chromium depends on insulin.

In the 1980s, Wada and Yamamoto were able to isolate the oligopeptide that binds chromium. This peptide is called chromodulin. Chromodulin is a small molecule of about 1500 Da and can bind four equivalents of chromium ions. The most significant characteristic of chromodulin is its ability of effect insulin by conversion of glucose into carbon dioxide or lipid.

In addition, there has also been some studies that suggests chromium and chromodulin play a role in signal transduction. Analysis of how chromodulin activate or inhibit phosphatase and kinase activity in rat adipocytes reveal an effect of small activation of a membrane phosphotyrosin phosphatase and a significant stimulation of insulin receptor tyrosine kinase activity.

Manganese

[21]

The human body averagely contains about 10 to 20 milligrams of manganese mostly concentrated in the pancreas, bone, liver, and kidneys. Manganese plays a role as a cofactor to important enzymes in the mitochondria and in the synthesis of glycoproteins. It can also act as a catalyst in enzyme processes involved in the synthesis of fatty acids and cholesterol. In skeletal and connective tissue development, manganese is involved in the process of mucopolysaccharide synthesis which is important in skeletal and cartilage structural matrix. Lack of manganese can lead to formation of abnormal cartilage and skeletal tissue, impaired connective tissue, poor muscle coordination, and impaired glucose tolerance and management of blood sugar levels. In the liver, manganese helps enzymes convert arginine to urea. In addition, manganese accompanies the enzyme pyruvate carboxylase which converts various non-carbohydrate substances into glucose for later use.

Metal Homeostasis

Transition metals such as zinc, iron, and copper are relatively essential constituents in the sphere of protein structural stability and functionality. Despite the importance of these metals in biological functions, an overabundance or a deficit of any may issue an action that is harmful to cell growth and viability. As a result, organisms must stabilize metal levels through a homeostatic mechanism. To do this, genes that encrypt the transportation of metals and storage of proteins are often regulated at the transcriptional level when there exist a change in metal concentration.

Many studies have exposed that a bad alteration in zinc, iron, and copper can affect various cancers and diseases like Alzheimer's and Parkinson's. This leads to opportunities where metal levels might invite more complex diseases in the future. Therefore, it is crucial to develop an inclusive understanding of how metal homeostasis can uncover possibilities that are health-sustaining and potentially health-risky.

References

1. http://drlwilson.com/Articles/IRON.htm

2. http://www.drlwilson.com/articles/copper_toxicity_syndrome.htm

3. http://www.scribd.com/doc/63209601/Cobalt-Enzymes

4. http://www.lenntech.com/periodic/elements/co.htm

5. http://jn.nutrition.org/content/130/4/715.full

6. http://www.springboard4health.com/notebook/min_manganese.html

7. Goodman, Louis S.; Alfred Goodman Gilman (1985). The Pharmacological Basis of Therapeutics, 7th Edition. New York, NY: Macmillan.

8. Kate M. Ehrensberger, Amanda J. Bird, Hammering Out Details: Regulating Metal Levels in Eukaryotes, Trends in Biochemical Sciences, Volume 36, Issue 10, October 2011. The Poor Metals are:

• Aluminium (Al)
• Gallium (Ga)
• Indium (In)
• Thallium (Tl)
• Tin (Sn)
• Lead (Pb)
• Bismuth (Bi)

The poor metals are located in the p-block, and are the most electronegative but least reactive [22] of all the metals. They are softer than the transition metals, but cannot be classified as metalloids. [23]

Poor metals also go by the name Post-transition metals. These are elements to the right of transition metals. Poor metals is considered the trivial name for such elements meaning Post-transitional metal name is preferred. Characteristics of poor metals are due to where they lay in the periodic table of elements. Their melting and boiling points are generally lower than normal metals. Their electronegativity and conductivity is also higher, but their texture is softer than usual metals. These elements are distinguished from metalloids by their high boiling points compared to that row of elements in the periodic table.

Aluminum

The third most abundant element on the earth is a light, strong metal that has several types of alloys. One of its alloys is duraluminum, which is stronger than pure aluminum and resistant to corrosion. It contains other elements such as copper, manganese, magnesium, and silicon. It is used for airplane construction and electrical wires.

Aluminum is harmful to the body since it can damage all types of tissues. It is considered a a neurotoxin and protoplasmic poison, meaning that a healthy cell will be damaged or killed as a result of exposure to the aluminum. There has been evidence that accumulation of aluminum can contribute to the cause of Alzheimer's disease. Some research has shown that degeneration of nervous tissue from aluminum in animals are similar to the way the brain degenerates in Alzheimer's disease. Experimentation from Toronto University have shown that Alzheimer's disease patients that were given treatment for the removal of aluminum showed a decrease in the rate of degeneration.

At the molecular level, aluminum sticks to phosphates of our DNA. Other toxic metals, such as mercury and lead, stick to the phosphates as well. However, the biological system has sticky chelators that can remove the toxic metals and send it to the kidney to be detoxified. Aluminum is considered a toxin because it cannot be easily removed from the phsophate, even with chelators. Once it sticks to the phosphate of the DNA, it will stay there until the cell dies.

Some symptoms of aluminum poisoning include forgetfulness, inability to concentrate, and even dementia.

Gallium

Gallium becomes a liquid at a low melting point of 29.8°C. It is a blue-gray metal that can expand upon freezing. Gallium does not exist in its pure form in nature, and commonly found in lead. Gallium is useful as a semiconductor and has similar properties as aluminum. It can also be used to produce mirrors, and is often used in circuits. Pure Gallium does not seem to have any effect on human health, there are traces in the body, and water with traces also seems to have no effect. Gallium compounds on the other hand can be very harmful.

Indium

The melting point of indium is 156.61 °C, boiling point is 2080 °C, specific gravity is 7.31 (20 °C), with a valence of 1, 2, or 3. Indium is a very soft, silvery-white metal. The metal has a brilliant luster and emits a high pitched sound when bent. Indium wets glass. Indium may be toxic, but further research is required to assess its effects.

Thallium

Thallium is a chemical element with atomic number 81, atomic weight 204.37, and symbol Tl. It is a heavy, bluish metal that looks much like lead, it is soft and melts with ease. It will react with oxygen fairly easily, forming a layer of thallium oxide. Approximately two dozen radioactive isotopes of Thallium have been produced. Thallium -201 can be used by doctors to diagnose how well a person's heart is operating. It used to be used as a rodent killer, it is colorless and odorless, so it when undetected to the unsuspecting small animals. But it was soon realized it could no longer be used because of it's detrimental effects on human's health. Its salts are active poisons that may cause alopecia, liver and kidney damage, blindness, and neurologic and psychic symptoms such as ataxia, restlessness, delirium, hallucinations, delusions, and semicoma. Recently it has been looked into as a superconductior, which allows an applied current to pass through it forever.

Tin

Tin has a melting point of 231.9681°C, boiling point of 2270°C, specific gravity (gray) of 5.75 or (white) 7.31, with a valence of 2 or 4. Tin is a malleable silvery-white metal which takes a high polish. It possesses a highly crystalline structure and is moderately ductile. When a bar of tin is bent, the crystals break, producing a characteristic 'tin cry'. Two or three allotropic forms of tin exist. Gray or a tin has a cubic structure. Upon warming, at 13.2°C gray tin changes to white or b tin, which has a tetragonal structure. This transition from the a to the b form is termed the tin pest. A g form may exist between 161°C and the melting point. When tin is cooled below 13.2°C, it slowly changes from the white form to the gray form, although the transition is affected by impurities such as zinc or aluminum and can be prevented if small amounts of bismuth or antimony are present. Tin is resistant to attack by sea, distilled, or soft tap water, but it will corrode in strong acids, alkalis, and acid salts. The presence of oxygen in a solution accelerates the rate of corrosion.

Lead

Lead is virtually ubiquitous in the environment as a result of its natural occurrence and its industrial use. Potential symptoms of overexposure to lead are anorexia, abdominal pain, colic, anemia, paralysis of wrists or ankles, kidney disease, and hypotension. Exposure to lead occasionally produces clearcut, progressive mental deterioration in children. Very low concentrations of lead interfere with the activity of δ-aminolevulinate dehydratase, and ferrochelatase during the biosynthesis of heme[1].

Bismuth

Bismuth has the symbol Bi. It has an atomic number of 83 and its atomic weight is 208.98. It has a melting point of 544.4K and a boiling point of 1837K. In early times bismuth was confused with tin and lead. Claude Geoffroy Jr. showed it to be distinct from lead in 1753.

References

"Gallium." Water Treatment Solutions. Lenntech. 2011. http://www.lenntech.com/periodic/elements/ga.htm

Goodman, Louis S, and Alfred Goodman Gilman. The Pharmacological Basis of Therapeutics. 7th ed. New York, N.Y.: Macmillan, 1985.

http://www.pspinformation.com/nutrition/minerals/aluminum.shtml

The 17 Rare-Earth Elements

[24]:

• Scandium (Sc)
• Yttrium (Y)
• Lanthanum (La)
• Cerium (Ce)
• Praseodymium (Pr)
• Neodymium (Nd)
• Promethium (Pm)
• Samarium (Sm)
• Europium (Eu)
• Gadolinium (Gd)
• Terbium (Tb)
• Dysprosium (Dy)
• Holmium (Ho)
• Erbium (Er)
• Thulium (Tm)
• Ytterbium (Yb)
• Lutetium (Lu)

Rare earth metals are a group of 15 chemically similar elements known as lanthanides that are grouped separately from the periodic table. Rare earth metals are generally found in high concentrations in the earth's crust. Humans use these in many advanced technological devices, such as magnets, superconductors, electronic polishers, flints for lighters, refining catalysts, and hybrid car components. They are also used as active ions in luminescent materials

Rare earth metals or rare earth elements is a set of 17 elements that exhibit similar characteristics. The name of the elements are very misleading because the metals are somewhat easy to find. They are not that rare at all, besides one of them. The only thing rare about these elements is that they are spread thin across the globe. This means that there are no big ore mines of rare earth metals, but instead a fine sprinkling all across the globe. These metals are all heavier than iron which is quite a feat. Rare earth metal prices fluctuate and are sold on a private market making their pricings ambiguous.

Scandium

Scandium has the atomic number 21. It is a silver-white metal. When exposed to air it can develop a faint yellow or pink color. it is most abundant in the stars. It can react with acids rapidly. Scandium was first discovered by Lars Frederick Nilson in 1876 while he was studying the rare earth metals. It is now used in aluminum-scandium alloys in the aerospace industry as well as sports equipment (i.e. golf irons, bicycle frames). It is also used in mercury vapor lamps.

Yttrium

Yttrium was first discovered in 1787 by Carl Arrhenius in a mine near Ytterby, Sweden and was first named ytterbite and later to gadolinite, but with further analysis of the metal by multiple scientists it was discovered to be an impure form of the metal. Friedwich Wohler was the first to obtain the Yttrium metal in 1828 by heating anhydrous yttrium(III) chloride with potassium. Yttrium is a soft, silver metal. It usually exists as Y³⁺.

Yttrium is often used in making alloys. And recently has seen great potential in superconductors as well as lasers. Like other rare earth elements is reacts slowly with cold water and quickly with hot. The solid metal does not react with Oxygen in the air, but in its powdered form it can react explosively at higher temperatures. It is fairly common on earth, and is even largely present in rocks that have been brought back from the moon.

Yttrium has been seen to have toxic effects on rats in laboratories, though not much testing has been done on humans. It is often treated with caution.

Lanthanum

Lanthanum is number 39 in the periodic table. It was first discovered in 1839 by Carl G. Mosander. It can be seen a silver-white metal that is soft enough to be cut by a knife. It most commonly exists as La³⁺. It is ductile and malleable and will easily rust when exposed to air as it will immediately oxidize. It is commonly used in nickel metal hydride rechargeable batteries for hybrid cars. It is also used in making night vision goggles, flame lighter flints, and lenses for high quality cameras and telescopes.

Cerium

Cerium is number 58 in the periodic table. It is a soft and gray metal. It is also reactive, ductile, and malleable. it can rust when exposed to moist air. It was first discovered in 1803 in Sweden by Jöns J. Berzelius and Wilhelm von Hisinger and in that same year was also discovered by Martin Klaproth in Germany. Cerium is used in carbon-arc lighting. Cerium oxide is used in catalytic converters in cars to reduce CO emissions. Flammacerium is used to treat and prevent infection on extensive burn wounds.

Praseodymium

Praseodymium is number 59 in the periodic table. it is a silver-white metal. It is soft and malleable and only slightly toxic. It commonly exists as Pr³⁺ and it's salts usually are a pale green color. It was first identified in 1885 by Carl Auer von Welsbach. It is used in the high-intensity permanent magnets necessary for hybrid cars and wind turbines. It is also used when making specialty glass goggles for welders and glass blowers.

Neodymium

Neodymium is number 60 on the periodic table and has an atomic weight of 144.24 g/mol. It is a silver/white metal that is soft and bright. It has a melting point of 1289K and a boiling point of 3343 K. It existed mostly as Nd³⁺. Its salts usually are a pale purple color. It was first identified by Carl Auer von Welsbach in 1885 when he realized that his earlier discovery of a metal called didymium was actually a mixture of two different elements which were then named neodymium and praseodymium. It is used for NIB magnets used in many electronics and cigarette lighter flints.

Promethium

Promethium is number 61 on the periodic table. It is the only element of the rare earth metals to be radioactive and is very harmful. It's salt forms luminate a pale blue or green when in the dark.

Samarium

Samarium is number 62 on the periodic table. It has an atomic weight of 150.4 g/mol. It was first discovered by Jean Charles Galissard de Marignac in 1853 in Geneva, Switzerland when he found lines in the mineral spectra that he had been studying. It is hard and silver metal. In temperatures that are higher that 150^oC it will ignite in air, otherwise it is stable in normal temperatures. It usually exists as Sm³⁺ and its salts usually have a pale yellow color. It is used in an alloy magnet that is used in headphones, small motors, and pickups for electric guitars and is also used to as an absorber for nuclear reactors. Radioactive ¹⁵²Sm is also used in treating cancer.

Europium

Europium is number 63 on the periodic table. It's atomic weight is 151.96 g/mol. It is slightly toxic but its metal dust is a fire and explosive hazard. It has a melting point of 1095 K and a boiling point of 1873 K. It is found as a soft and ductile silver/white metal. It immediately oxidized when exposed to air. It is considered the most reactive of the rare earth metals and will ignite in temperature of 150-180^oC. It is used in phosphors in anti-forgery marks on Euro bank notes.

The discovery of Europium is accredited to Eugène-Antole Demarçay who, in 1886, had identified spectroscopic lines in Samarium using his own specially developed spectroscope, which was specifically made to study the rare earth metals. Though his results were originally disputed he later proved his findings in 1901 when he was able to isolate europium.

Gadolinium

Gadolinium is number 64 on the periodic table. Its atomic weight is 157.25 g/mol. It has a melting point of 1587 K and a boiling point of 3533 K. It was first found by Jean Charles Galissard de Marignac in 1880 when he recorded spectroscopic lines in an oxide preparation which had been taken from samarskite. His findings were later confirmed by Paul Émile Lecoq de Boisbaudran in 1886. It is a silver/white metal. It is also ductile and malleable. In moist air it will tarnish. It can usually be found as Gd³⁺.

Terbium

Terbium is number 65 on the periodic table. It has an atomic weight of 158.9 g/mol. It has a boiling point of 3493 K and a melting point of 1633 K. It is a grayish metal that is malleable and ductile. It is also soft, enough to cut with a knife. It exists as Tb³⁺ and when excited will emit a green luminescence. It was discovered by Carl Gustaf Mosander in 1843 when, hoping to discover yet another element, he used ammonium hydroxide to precipitate fractions of different basicity from yttria. This produced two different substances which he learned contained the new elements erbium and terbium. It is used in color phosphors in trichromatic lighting and TV and also makes the green colors on Blackberry phones.

Dysprosium

Dysprosium is number 66 on the periodic table. It has a boiling point of 2833 K and a melting point of 1410 K. Its atomic weight is 162.50 g/mol. It is a silver/white metal that is soft and bright. In room temperature it will slowly tarnish. When in acids it will dissolve. It has the highest magnetic strength of all the elements. It usually can be found as Dy³⁺. It was first discovered in 1886 by Paul Émile Lecoq de Boisbaudran when he separated dysprosium oxide from holmium oxide using a time-consuming and intricate procedure. It is used in the cement in control rods in nuclear reactors. It is also used in compact and hard discs.

Holmium

Holmium is number 67 on the periodic table. It has a atomic weight of 164.9 g/mol. It's boiling point is 2973 K. It has a melting point of 1743 K. It is slightly toxic. It is a malleable, ductile, and soft silver metal. When it is heated or exposed to moist air it will oxidize to a yellowish oxide. It is usually found as Ho³⁺ and it's compounds are usually a brown/yellow color. Like Dysprosium, it has the highest magnetic strength and as such it can be used as a flux concentrator for high magnetic fields. It is also used in lasors used for non-invasive medical procedures in treating cancers and kidney stones. Its isotopes are used to color glass and cubic zirconias red and yellow.

Erbium

Erbium is number 68 on the periodic table. It has an atomic weight of 167.26 g/mol. It is a moderately toxic silver-white metal. It is also soft and malleable. It tarnishes slowly when exposed to air. It can also r==eact with water and will dissolve in acids. It can most commonly be found as Er³⁺. Erbium salts are usually found to be a pinkish color. It was first discovered by Carl Gustaf Mosander in 1843 by using ammonium hydroxide to precipitate fractions with different basicity of yttria. From there he found 2 different substances with each contained a new element, erbium and terbium. Erbium is used in amplifiers and lasers and photographic filters.

Thulium

Thulium is number 69 on the periodic table. Its atomic weight is 168.9 g/mol. It is a non-toxic gray metal that is soft and malleable. When exposed in air the metal will slowly tarnish. When reacted with water it will form hydroxide and hydrogen gas. It exists mostly as Tm³⁺. It is one of the least abundant elements. It was first discovered in its oxide form in Uppsala, Sweden in 1879 by Per Teodor Cleve while working with the metal Erbia.

Ytterbium

Ytterbium is number 70 on the periodic table. It has an atomic weight of 173.04 g/mol. It is a ductile and malleable silvery metal. When exposed to air it will immediately tarnish. It was first discovered by Jean Charles Galissard de Marignac in 1878. He managed to extract an unknown powder, which was named ytterbium oxide, after heating erbium nitrate until decomposition. It is used in alloys used for stainless steel. Its compounds are also commonly used as catalysts in organic chemistry reaction.

Lutetium

Lutetium is number 71 on the periodic table. Its atomic weight is 174.97 g/mol. It has a boiling point of 3663 K and a melting point of 1933 K. It is a non toxic silver/white metal. It is the hardest and the densest of the lanthanides. ¹⁷⁷Lu is used for cancer therapy and ¹⁷⁶Lu is used to determine the age of meteorites.

References

<a href="http://www.chemicool.com/">Chemicool Periodic Table</a>

"Yttrium." Chemistry Explained. Advameg, Inc. 2012. http://www.chemistryexplained.com/elements/T-Z/Yttrium.html#b

Overview

Metals such as iron, zinc, and copper all perform important roles in many of the enzymatic reactions that fuel the body’s metabolism. For instance, ions such as Fe²⁺ can bind to the hemoglobin and myoglobin protein to help transport oxygen to organs in the body. Other metals like magnesium and copper act to stabilize the shapes of enzymes. However, there are some metal ions that are highly toxic in excessive amounts. Thus, the body exerts strict control to assure that only one or two free metal atoms are present inside an individual cell.

In trigger and control mechanisms

• Na⁺, K⁺ play an important role in nerve cell membranes. They are electrical charge carries that conduct nerve cell impulses by moving back and forth across the membrane generating an voltage(difference in electrical charge), or potential across the membrane. These ions help communicate electrical signals in the nerves and heart.

• Ca²⁺ plays a critical role in muscle cell contraction and relaxation.

Sodium-Potassium pump generating a concentration gradient

Structural influences

• Ca²⁺, Mg²⁺, Mn²⁺, Zn²⁺ help stabilize a particular protein configuration.

• Mg²⁺ is used for cleavage and replication of DNA, RNA strands
• Bones and shells contain Ca²⁺ like the mineral hydroxyapatite Ca(OH)₂.3Ca₃(PO₄)₂

Mg²⁺ as a co-factor in an enzyme

Lewis acid behavior

• Zn²⁺ and Co²⁺ catalyzes the hydrolysis of phospates by serving as Lewis acids.
• For example carboxypeptidase catalyzes the hydrolysis of C-terminal amino acid residues of proteins. It is released in pancreatic juice of animals for the digestion of proteins.

• Hemoglobin,myoglobin, hemocyanin, and hemerythrin provide O₂ storage and transport.

Redox reactions and General Compounds

• Dehydrogenation; catalyzed by Cu, Mo and Fe by changing their oxidation states.

• Ribose turns into deoxyribose with a catalysis of Co.

Unhealthy Metals

Toxic metals can hinder important enzyme functions.

• Lead inhibits the body's ability to make hemoglobin by interfering with the synthesis of heme which disrupts the blood's oxygen transport system.
• Mercury binds to sulfur-containing molecules which can cause irreversible neurological damage.
• Arsenic can cause cancer in the skin and lungs.

References

Gary Mieesler; Donald A. Tarr; ınorganic Chemistry 3.Edition, 2004.

Background Information

There are many kinds of oxidation states of metals especially transition metals. The stability of a given oxidation state is a comparison between the ionization energy required to remove electrons from the valence orbitals and the solvation or ligation energy obtained by surrounding the metal cation with solvent or ligand atoms. Oxidation-Reduction reactions, otherwise known as redox reactions, are involved in a wide range of vital natural processes. Examples include the rusting of iron, respiration process, and the browning of certain foods. Redox reactions are also used in the operation of batteries.

Oxidation is the loss of electrons, and reduction is the gain of electrons. Each atom in a molecule is assigned an oxidation number or the oxidation state. Besides, the oxidation number for each element in a binary ionic compound equals the ionic charge. The oxidation number for each element in a covalent compound is not obvious because the atoms do not have whole charges. [The Molecular Nature of Matter and Change].

Oxidation states are assigned to metals based on a couple of different rules. These numbers that are assigned are useful in the quest of describing and determining oxidation/reduction reactions and redox equation balancing. It is important to remember that oxidation states do not suggest the compound's charge, but rather for use in balancing reactions. Some metals have fixed oxidation states while others such as iron have numerous oxidation states that are possible!

History

The current concept of "oxidation state" was introduced by W. M. Latimer in 1938. Oxidation itself was first studied by Antoine Lavoisier, who believed that oxidation was always the result of reactions with oxygen, thus the name. Although Lavoisier's idea has been shown to be incorrect, the name he proposed is still used, albeit more generally. Oxidation states were one of the intellectual "stepping stones" Mendeleev used to derive the modern periodic table.

Oxidation States

In order to determine whether a chemical reaction is a oxidation-reduction reaction, we must look at all the oxidation numbers (oxidation states) at all the elements involved in the reaction. To do so there are a set of rules to follow when assigning oxidation numbers.

• An atom in its pure/elemental form, the oxidation state is always zero.
• Monoatomic ions have an oxidation state equal to the charge of the ion. For example, K+ would have an oxidation number of +1, S^2- would have an oxidation state of -2.

1. keep in mind that alkali metal ions always have a +1 charge which means all group 1A metals always have an oxidation number of +1. The same rule applies to Group 2A having a +2 state and Aluminum possessing a +3 oxidation number.

• Nonmetals will usually have a negative oxidation number, however there are exceptions:

1. Oxygen usually has a -2 oxidation number in both ionic and molecular compounds. However, compounds with peroxide, O2^2-, gives each oxygen an oxidation state of -1.
2. Hydrogen has a +1 charge when bonding to nonmetals and a -1 charge when bonding to metals.
3. The oxidation state of Fluorine is -1 in all compounds.

• The sum of all oxidation numbers within a neutral compound must be zero. The sum of the oxidation numbers of a polyatomic ion must equal the number of the charge.

For example, consider the reaction: zinc metal added to a strong acid

Zn(s) + 2H+(aq) -> Zn2+(aq) + H2(g) The oxidation state of Zinc metal = 0. The oxidation state of H+ = +1. The Oxidation state of Zn2+ = +2. The oxidation state of H2 = 0. The sum is equal to zero because there are 2 H+ ions.

References

Gary Mieesler; Donald A. Tarr; ınorganic Chemistry 3. Edition, 2004.

Theodore L. Brown; H. Eugene Lemay, Jr.; Bruce E. Bursten; Catherin J. Murphy; Chemistry: the Central Science 11. Edition, 2009.

A "free" copper is a thermodynamic term that corresponds to aquo Cu(I) or Cu(II) complexes without binding to ligands such as amino acids or biopolymers. Free intracellular copper ions can also be defined in kinetic terms: less than 0.01 percent of total cellular copper can become free in the cytoplasm during the cell's lifetime. Copper chelation capacity becomes potent when copper detoxification systems such as metallothioneins are induced. Metallochaperones succeed in acquiring the metal and donating it to enzymes that need it. There are three different copper trafficking pathways discovered for copper.

Three Pathways

Three Pathways

Pathway 1: Copper to Mitochondria A mitochondrial enzyme named Cytochrome oxidase requires three copper ions to be inserted into two subunits: a binuclear copper site protruding into the inner membrane space of the mitochondria and a mononuclear site buried within the inner membrane. Among the host of assembly factors required for cytochrome oxidase activity, two proteins have an effect on copper utilization.

Pathway 2: ATX1 Pathway of Copper to Golgi

Proposed pathway for copper transfer from ATX1 to CCC2

ATX1 was found to deliver copper to an intracellular copper transporter located in the Golgi compartment. Then the copper transporter can pump the metal into the lumen of the Golgi for insertion into copper enzymes destined on cell surface. Specially, copper delivered by ATX1 are targeted by P-type copper transporters which conserved in eukaryotes. P-type copper transporters can transport ATPase. Human beings specially have two types of transporters which are ATP7A and ATP7B.

Pathway 3: Copper to Cytosolic Superoxide Dismutase

Capture and release model for copper transfer from CCS to SOD1

Cytosolic Superoxide Dismutase specially targer copper with enzyme SOD1. The enzyme SOD1 protects cells from oxidative damage under toxic superoxide anion radicals in redox reactions of copper ions. The insertion of copper into SOD1 requires a copper metallochaperone involved in the lysine biosynthetic pathway, known as LYS7. CCS is the largest of copper metallochaperones discovered so far. Whereas ATX1 and COX17 are defined in domain with single protein, CCS can be defined in three different domains.

Metallochaperones in Prokaryotes

Prokaryotes do not have intracellular compartmentalization usually presents in eukaryotes; therefore metal carriers such as COX17 may not be useful. A homologue to ATX1 was discovered against enteric bacteria. CopZ was originally proposed to function as a copper transcription factor, it is also a copper metallochaperone. Purified CopZ can transfer copper displaces the zinc ion needed for CopY binding to DNA.

The analogous trafficking pathways also exist for other metals. Other than copper for the delivery of iron to the sites of iron-sulfur cluster assembly, other groups such as the IscA family of proteins also perform the similar functions. Other than that, prokaryotic nickel-binding proteins may also facilitate the insertion of the metal into nickel-requiring enzymes, such as urease and cobalt dehydrogenase.

Conclusion

Metallochaperones are critical for determing cell function under copper conditions. Metallochaperones ensure the safe delivery of the metal ion to proper intracellular destination and also protect the cargo from adventitious reactions.

References

Thomas V. O'Halloran and Valeria Cizewski Culotta "Metallochaperones, an Intracellular Shuttle Service for Metal Ions" Bioremediation is a technology that utilizes microorganisms to reduce, eliminate, contain, or transform pollutants present in sediment, water, and air. The microorganisms used to perform bioremediation are called bioremediators.

Metals are significant natural components of all soils, where their presence in the mineral fraction constitutes a pool of potentially mobile metal species, many essential nutrients for plants and microbes, and important solid components that can have a fundamental effect on soil biogeochemical processes; for example: clays, minerals, iron and manganese oxides. Metals are also present in organic fractions--frequently in bound forms--with some metal recycling occurring as a result of organic matter degradation. The aqueous phase provides a mobile medium for metal transfer and circulation through the soil and to organisms, and to the aquatic environment. Metals such as mercury, lead, and arsenic, potentially can be toxic to the kidneys, decrease mental capabilities, and cause weakness, headaches, abdominal cramps, diarrhea, and anemia.

Mobility of Metal Contaminants

The immobility of metals is primarily caused by reactions that cause metals to precipitate or chemical reactions that keep metals in a solid phase. Chemical and physical properties affect the mobility of metals in soils and groundwater. Under acidic conditions (pH ranging between 4.0-8.5), metal cations are mobile while anions tend to transform to oxide minerals. At high pH levels, cations adsorb into mineral surfaces and metal anions are mobilized. Hydrous metal oxides of iron, aluminum, and manganese can affect metal concentrations because these minerals can subtract cations and anions.

When a microorganism oxidizes or reduces species, this reaction causes metals to precipitate. Mercury is an example of a metal that can be precipitated. The process begins when mercury (Hg2+) is reduced to mercuric sulfide causing mercury to transform to a precipitated form. Chromium is another metal that can convert to a precipitated form with the use of microorganisms. The process involves the reduction of hexavalent chromium (Cr 6+) to trivalent chromium (Cr 3+), which then can precipitate to chromium oxides, sulfides, or phosphates. Research today is focusing on other metal and radioactive contaminants that can undergo precipitation processes.

References

http://mic.sgmjournals.org/content/156/3/609.full http://www.cluin.org/download/studentpapers/bio_of_metals_paper.pdf

Semimetals

• Boron (B)
• Silicon (Si)
• Germanium (Ge)
• Arsenic (As)
• Antimony (Sb)
• Tellurium (Te)

The semimetals are parts of group 13-16 of the periodic table of the elements. The semimetals, or metalloids, resemble metals in some aspects and nonmetals in other aspects. As such, most of their properties are intermediate among the elements. For example, their electronegativities and ionization energies generally lie between those of metals and nonmetals. Other properties such as boiling points, melting points, and densities vary widely. Semimetals are especially important for their intermediate level of conductivity, which makes them the basis of semiconductors.

Introduction of Porous Silicon and Etching Procedure

Introduction of Porous Silicon

Arthur Uhlir Jr. and Ingeborg Uhlir discovered porous silicon by chance in 1956. It took place at Bell Labs in the United States. Their experiments involved developing the surfaces and shapes of silicon; however, under several test trials, the silicon being tested formed unwanted colored layers on its surface, such as black and red. This mishap was ignored and went unrecognized for over 20 years.

Towards the end of the 1980's, the discovery of porous silicon was noticed by the scientific community. A research analyst named Leigh Canham commented that the discovery of porous silicon proved to have some effects and advantages to the quantam confinement. After experiments and tests were ran in the 1990's, results showed that the porous silicon was able to emit light in chemical dissolution. More interest evolved in the scientific community and a large amount of experiments were conducted throughout the rest of the 1990's.

The application of research area of Porous Silicon is actually based on Porous silicon dioxide chip, which normally has 40nm-70nm irregular holes on chip. Inside of holes, there are negative charges on the surface of oxidized porous Si. These negative charges have been used to bind protein using for medical purpose. The size of holes mainly depends on currency strength during the etching process. The resistance of silicon chip and surface area of silicon chip also affect the size of holes. The silicon chips are unstable in high pH. The pH condition higher than 7 will cause silicon chip dissolve. Silicon chip has stable and negatively charges surface between pH4~7.

Picture of porous silicon:

http://sailorgroup.ucsd.edu/research/images/pSi_xsectionTHN.JPG

Classification of porous silicon

Porosity

The definition of porosity is the fraction of void within the pSi layer, which can be determined by weight measurement. The porosity depends on current density, HF concentration, and thickness of silicon layer. The range of porosity of silicon chip is between 4% to 95%. The porous silicon layer is more stable in medium to low porosity condition. The formula of porosity is: P=[(V0-V)/V0 ]=[1-V/V0 ] =(1-P0 /P)×100 %

Pore size

Based on the sizes of silicon pore, porous silicon layer are divided to three categories: macroporous, mesoporous, and microporous.

macroporous is defined as pore width less than 2nm.

mesoporous is defined as pore width between 2nm and 50nm.

microporous is defined as pore width larger than 50nm.

Procedure of Etching Silicon Chip

Example for 1.3cm*1.3cm silicon chip

Picture of Etching instrument of porous silicon:

http://sailorgroup.ucsd.edu/research/images/etch_cell_schematic.gif

Preparation:

Preparing Aqueous HF (48%), Ethanol (99.9%), highly doped p-type Si chip (normally has resistivity ranging from 0.0008 to 0.001 Ω-cm), a piece of aluminum foil, and Teflon etching cell with a platinum counter electrode.

Procedure:

1, Obtaining 30ml HF and 10ml Ethanol, gently mixing the solution.

2, Cutting silicon chip into 1.3cm*1.3cm pieces, aluminum foil into 5.0cm*2.0cm piece.

3, Setting up the silicon chip in the order as picture shows and checking airtightness of the Teflon etching cell.

4, Putting the Teflon etching cell on the operation platform in etching area and adjusting position of etching ring and the Teflon etching cell.

5, Adding HF/ethanol mixing solution into the Teflon etching cell until it totally submerges etching ring.

6, Using computer software to set up time and currency strength, and then beginning to etch. (Normally researchers will etch twice times. First time only etching 5 seconds, and then rinsing chip, letting chip react with NaOH to clean the surface of silicon chip. On the second time, researchers will set up computer data as what they really want to obtain in research. As the chip introduced in the example, the currency strength that may polish chip is higher than 800mA.)

7, Rinsing silicon chip twice in ethanol and drying the chip with nitrogen gun. (In order to reduce the damage caused in evaporate process, sometimes researchers will use pentane instead of ethanol, which has higher surface tension.)

8, Using computer software Fringe to obtain the data, and saving pictures for all chips in both air and ethanol situation.

9, Oxidizing silicon chip into a tube furnace at 750 °C for 1 hour in air and then let it cool to room temperature.

---

Reference

1. Canham, L. T. 1993, A glowing future for silicon, New Scientist.

2.Sailor Group website: http://sailorgroup.ucsd.edu/research/porous_Si_intro.html

3. Elizabeth C. Wu, Ji-Ho Park, Jennifer Park, Ester Segal, Fre#de#rique Cunin, and Michael J. Sailor, "Oxidation-Triggered Release of Fluorescent Molecules or Drugs from Mesoporous Si Microparticles", ACS Nano, 2008, 2 (11), 2401-2409 • Publication Date (Web): 08 November 2008

4. Michelle Y. Chen and Michael J. Sailor, "Charge-Gated Transport of Proteins in Nanostructured Optical Films of Mesoporous Silica"

5. Jennifer S. Andrew, Emily J. Anglin, Elizabeth C. Wu, Michelle Y. Chen, Lingyun Cheng, William R. Freeman, and Michael J. Sailor, "Sustained Release of a Monoclonal Antibody from Electrochemically Prepared Mesoporous Silicon Oxide"

6.http://en.wikipedia.org/wiki/Porous_silicon

7.http://baike.baidu.com/view/653736.htm

8. Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.

Semimetals

• Boron (B)
• Silicon (Si)
• Germanium (Ge)
• Arsenic (As)
• Antimony (Sb)
• Tellurium (Te)

The semimetals are parts of group 13-16 of the periodic table of the elements. The semimetals, or metalloids, resemble metals in some aspects and nonmetals in other aspects. As such, most of their properties are intermediate among the elements. For example, their electronegativities and ionization energies generally lie between those of metals and nonmetals. Other properties such as boiling points, melting points, and densities vary widely. Semimetals are especially important for their intermediate level of conductivity, which makes them the basis of semiconductors.

Introduction of Porous Silicon and Etching Procedure

Introduction of Porous Silicon

Arthur Uhlir Jr. and Ingeborg Uhlir discovered porous silicon by chance in 1956. It took place at Bell Labs in the United States. Their experiments involved developing the surfaces and shapes of silicon; however, under several test trials, the silicon being tested formed unwanted colored layers on its surface, such as black and red. This mishap was ignored and went unrecognized for over 20 years.

Towards the end of the 1980's, the discovery of porous silicon was noticed by the scientific community. A research analyst named Leigh Canham commented that the discovery of porous silicon proved to have some effects and advantages to the quantam confinement. After experiments and tests were ran in the 1990's, results showed that the porous silicon was able to emit light in chemical dissolution. More interest evolved in the scientific community and a large amount of experiments were conducted throughout the rest of the 1990's.

The application of research area of Porous Silicon is actually based on Porous silicon dioxide chip, which normally has 40nm-70nm irregular holes on chip. Inside of holes, there are negative charges on the surface of oxidized porous Si. These negative charges have been used to bind protein using for medical purpose. The size of holes mainly depends on currency strength during the etching process. The resistance of silicon chip and surface area of silicon chip also affect the size of holes. The silicon chips are unstable in high pH. The pH condition higher than 7 will cause silicon chip dissolve. Silicon chip has stable and negatively charges surface between pH4~7.

Picture of porous silicon:

http://sailorgroup.ucsd.edu/research/images/pSi_xsectionTHN.JPG

Classification of porous silicon

Porosity

The definition of porosity is the fraction of void within the pSi layer, which can be determined by weight measurement. The porosity depends on current density, HF concentration, and thickness of silicon layer. The range of porosity of silicon chip is between 4% to 95%. The porous silicon layer is more stable in medium to low porosity condition. The formula of porosity is: P=[(V0-V)/V0 ]=[1-V/V0 ] =(1-P0 /P)×100 %

Pore size

Based on the sizes of silicon pore, porous silicon layer are divided to three categories: macroporous, mesoporous, and microporous.

macroporous is defined as pore width less than 2nm.

mesoporous is defined as pore width between 2nm and 50nm.

microporous is defined as pore width larger than 50nm.

Procedure of Etching Silicon Chip

Example for 1.3cm*1.3cm silicon chip

Picture of Etching instrument of porous silicon:

http://sailorgroup.ucsd.edu/research/images/etch_cell_schematic.gif

Preparation:

Preparing Aqueous HF (48%), Ethanol (99.9%), highly doped p-type Si chip (normally has resistivity ranging from 0.0008 to 0.001 Ω-cm), a piece of aluminum foil, and Teflon etching cell with a platinum counter electrode.

Procedure:

1, Obtaining 30ml HF and 10ml Ethanol, gently mixing the solution.

2, Cutting silicon chip into 1.3cm*1.3cm pieces, aluminum foil into 5.0cm*2.0cm piece.

3, Setting up the silicon chip in the order as picture shows and checking airtightness of the Teflon etching cell.

4, Putting the Teflon etching cell on the operation platform in etching area and adjusting position of etching ring and the Teflon etching cell.

5, Adding HF/ethanol mixing solution into the Teflon etching cell until it totally submerges etching ring.

6, Using computer software to set up time and currency strength, and then beginning to etch. (Normally researchers will etch twice times. First time only etching 5 seconds, and then rinsing chip, letting chip react with NaOH to clean the surface of silicon chip. On the second time, researchers will set up computer data as what they really want to obtain in research. As the chip introduced in the example, the currency strength that may polish chip is higher than 800mA.)

7, Rinsing silicon chip twice in ethanol and drying the chip with nitrogen gun. (In order to reduce the damage caused in evaporate process, sometimes researchers will use pentane instead of ethanol, which has higher surface tension.)

8, Using computer software Fringe to obtain the data, and saving pictures for all chips in both air and ethanol situation.

9, Oxidizing silicon chip into a tube furnace at 750 °C for 1 hour in air and then let it cool to room temperature.

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Reference

1. Canham, L. T. 1993, A glowing future for silicon, New Scientist.

2.Sailor Group website: http://sailorgroup.ucsd.edu/research/porous_Si_intro.html

3. Elizabeth C. Wu, Ji-Ho Park, Jennifer Park, Ester Segal, Fre#de#rique Cunin, and Michael J. Sailor, "Oxidation-Triggered Release of Fluorescent Molecules or Drugs from Mesoporous Si Microparticles", ACS Nano, 2008, 2 (11), 2401-2409 • Publication Date (Web): 08 November 2008

4. Michelle Y. Chen and Michael J. Sailor, "Charge-Gated Transport of Proteins in Nanostructured Optical Films of Mesoporous Silica"

5. Jennifer S. Andrew, Emily J. Anglin, Elizabeth C. Wu, Michelle Y. Chen, Lingyun Cheng, William R. Freeman, and Michael J. Sailor, "Sustained Release of a Monoclonal Antibody from Electrochemically Prepared Mesoporous Silicon Oxide"

6.http://en.wikipedia.org/wiki/Porous_silicon

7.http://baike.baidu.com/view/653736.htm

8. Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.

Carbon Nanomaterials

There are four types of carbon nanomaterial, graphene (a two dimensional sheet of benzene rings), carbon nanotube (a nanosize tube that can be imagined as a rolled-up graphene sheet), fullerenes (a zero dimensional ball), and dimondoid.

The properties of these materials are summarized in the table below.

Graphene sheet

The figure shown below is a graphene sheet. It is made up by perfectly align benzene rings. The carbon to carbon bond length is approximately 1.41 angstrom, which is really small. There are rarely any impurities present in graphene sheet.

Graphene sheet is a perfect conductor. As can be seen from the figure below, graphene sheet has an idealized density of state. There is no gap between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) such that electrons can be promoted easily from HOMO to LUMO, thus, making it a perfect conductor. The unit hexagonal cell of graphene contains two carbon atoms and has an area of 0.052 nm^2. We can therefore calculate its density as being 0.77 mg/m^2. The strength of a 1 m^2 graphene sheet is incredibly high. 1 m^2 graphene sheet, weight less than 1 mg, can hold an object that is up to about 4 kg before it would break.

Carbon nanotube

Carbon nanotube can be imagined as a “rolled-up” graphene sheet. Different orientation of rolling, or different chirality, determines the conductivity of the carbon nanotube. There are three types of carbon nanotubes, depending on the chirality, armchair tube, zigzag tube, and chiral tube.

(n,m) represent the unit vector of a graphene structure. The zigzag tube has unit vector of (n,0), the armchair tube has unit vector of (n,n), and the chiral tube has unit vector of (n,m)

The figure above illustrated how the chirality affects the conducting property of the carbon nanotube. If n – m = 3q, where q is an integer, then the tube is conducting Otherwise, the tube is semiconducting.

As can be seen from the figure above, Figure a represents the DOE of conducting, and Figure b represents the DOE of semiconducting carbon nanotube. In Figure a, there is clearly no gap in DOE, making it very easy to promote electrons from lower molecular orbital to higher molecular orbital, hence conducting electricity. However, in the case of semiconducting nanotube in Figure b, there is clearly a gap from approximately -1 to 1 unit energy, making it hard to promote electrons.

References

Tao, Andrea. Lecture 7. University of California, San Diego, La Jolla. 14 Oct. 2010. Nonmetals are generally dull and brittle. They are poor conductors of electricity. Compared to metals, they have lower densities, melting points, boiling points, and higher electronegativity.

Carbon

Carbon is one of element in the earth, which has symbol C and atomic number 6. Its electron configuration is [He]2s2 2p2. Carbon is a very useful element mainly due to its bonding abilities. It is able to form single, double, and triple bonds to other elements (usually Hydrogen). All living organisms contain carbon, including humans who have about 18% carbon by weight.

Carbon has 12 isotopes, which from C8 to C19. C12 and C13 are stable; all other isotopes are radioactive. Carbon is very stable under room temperature. it is not soluble in water, organic solution, or low concentration acid or base.Carbon can react with oxygen in high temperature; in all halogen, only fluorine can react with carbon directly; carbon is very reducible element, which can reduce other metals.

Common uses of carbon include diamonds, gasoline, kerosene, smoke detectors, radiocarbon dating, and graphite used for cooking and artwork. The most major organic compound in nature is soot.

In plants, carbon dioxide and water combine to form simple sugars called buffer carbohydrates . The process of this formation is called photosynthesis and the driving force that provides energy is the sun.

Nitrogen

Nitrogen is an element in group (V) of period table which has symbol N and atomic number 7, discovered in 1772 by Scottish physician Daniel Rutherford.

Nitrogen has 17 isotopes. N14 and N15 are stable.

At standard conditions, nitrogen is a colorless, odorless, tasteless inert diatomic gas. It occurs in all living organisms and is a constituent in amino acids, which make up proteins, which make up nucleic acids(DNA and RNA). Nitrogen also plays an important role in plant growing and fruit maturing. Increasing the percentage of Nitrogen in the soil can improve the production of farm plant.

Nitrogen is the largest constituent of the Earth's atmosphere, occupied 78% volume. It makes up about 4% of dry weight of plant matter, about 3% of weight of the human body, and 0.0046% in Earth's shell.

Oxygen

Oxygen is one of elements in group (VI) of period table, which has symbol O, atomic number 8 and electron configuration 1s2 2s2 2p4, discovered in 1774. At standard condition, Oxygen is a colorless, odorless, and tasteless diatomic gas. Oxygen has second highest electronegativity in all elements and is a strong oxidizing agent. So, all elements except inert gases can form compound with Oxygen.Oxygen has 3 stable isotopes: O16, O17 and O18.

Oxygen is the most abundant element in Earth's shell, occupied 48.6%. The Oxygen gas also makes up 23% in the air.

All major structural molecules in living organisms (i.e. proteins, carbohydrates , lipids,amino acid, etc.) contain oxygen. It is used in both photosynthesis as well as cellular respiration in order to sustain life. The overall formula for photosynthesis is 6 CO2 + 6 H2O+ sunlight --> C6H12O6 + 6 O2

It also makes up the ozone layer in the form of O3, which protects the earth from UV rays radiating from the sun.

Sulfur

Picture of natural sulfur crystals

Sulfur is one of elements in group (VI) of period table, which has symbol S, atomic number 16 and electron configuration [Ne] 3s2 3p4. At room temperature, elemental sulfur is a bright yellow crystalline solid formed by cyclic octatomic molecules with formula S8. Sulfur can oxidize most metals and several nonmetals; it also can reduce several strong oxidants, such as oxygen and fluorine. As sulfur burns, a blue flame shows with formation of sulfur dioxide. Sulfur is insoluble in water but solve in some nonpolar solvent, such as carbon disulfide and benzene.

Sulfur has 18 isotopes. Four of them are most stable: S-32 (95.02%), S-33 (0.75%), S-34 (4.21%) and S-36 (0.02%).

Some amino acids contain sulfur in their structure, which form Disulfide bonds, playing very significant roles in proteins.

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Phosphorus

Phosphorus is an element in group (V) of period table which has symbol P, atomic number 15, and electron configuration [Ne] 3s2 3p3, discovered in 1669 (white phosphorus) by a German businessman whose name is Henning Brand. Phosphorus has high reactivity and is toxic. There is no free element phosphorus found in earth.

The major role of phosphorus is fertilizers. Phosphorus also build detergents, pesticides and nerve agents, and matches. In biology, phosphorus is a component of DNA, RNA, ATP and also forms phospholipids in all membranes.

There are four forms of phosphorus existing in the world:

White phosphorus consists of tetrahedral P4 molecules, in which each atom is bound to the other three atoms by a single bond. When temperature raises to 800 °C, P4 molecules decompose to P2 molecules, forming liquid and gaseous phosphorus. There are 2 forms of solid White phosphorus, which at low temperature is β form and high temperature is α form.White phosphorus is the least stable, the most reactive, more volatile, less dense, and more toxic than the other form of phosphorus. White phosphorus doesn’t dissolve in water, but dissolve in Benzene and ether, stored in water.

Picture of white Phosphorus

Red phosphorus is viewed as one bond in P4 broke and formed single bond with P on neighboring P4 molecule. Red phosphorus is red powder and nontoxic, formed by white phosphorus at 250 °C .

Picture of red Phosphorus

Structure of red Phosphorus:

http://en.wikipedia.org/wiki/File:RedPhosphorus.jpg

Black phosphorus is least reactive form of phosphorus, which is stable at temperature below 550 °C. Known as β-metallic phosphorus, it also has a similar structure as graphite. High pressure is required to form black phosphorus.

Structure of black Phosphorus:

http://en.wikipedia.org/wiki/File:BlackPhosphorus.jpg

Violet phosphorus is formed by red phosphorus above 550 °C.

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Nitric Oxide

Some physical properties for nitric oxide: 1)exists as a gaseous form in nature 2)has limited water solubility 3)has diverse oxidation chemistry and shifted biological concentration

Some previous techniques that involved to develop nitric oxide’s structure such as electron paramagnetic resonance spectroscopy has disadvantage of determining its biological selectivity, sensitivity and toxicity. Comparing to previous methods, optical tools to detect Nitric Oxide was more accurate to discover the forms of Nitric Oxide.

Ratiometric fluorescent probe is the technique to detect the biological structure of nitric oxide using light to provide a high spatial and temporal resolution of the structure.

Fluorescence imaging of biological analytes act as excellent tools to monitor biological processes in real time with a relatively high resolution and accurate detection.

There are three strategies that can be used to recognize the accurate structure of Nitric Oxide. The first strategy is using organic probes. This method involves an oxidation product of Nitric Oxide, such as Dinitrogen trioxide, to react with a functional group to regulate the fluorescence. This reaction results in bright emission enhancement with the presence of Nitric Oxide under the existence of oxygen, the aerobic condition. The second strategy associates with transition metals to mediate reactivity of an emissive dye with Nitric Oxide. With this technique, a paramagnetic Copper ion carrying a secondary amine emission of the fluorophore reacts with Nitric Oxide in coordination. During the reaction, the Cooper ion is being reduced with deprotonation, more positive charges were released, and also the secondary amine with nitrosation. Therefore, the diminishing paramagnetic electric technique in the N-nitrosated product was than produced. In this strategy, the Copper mediated reactivity does not depend on the presence of oxygen. Without the requirement of oxygen gas, the technique with probes is conceivably useful to resolve the imaging Nitric Oxide in hypoxic situation. Some advantages with this method are: faster reaction with Nitric Oxide due to metal medicated reactivity; directly offers a circumstance for devising reversible probes to determine Nitric Oxide. One weakness of this strategy is less bright compared to pure organic probes in term of the on-and-off state. The last technique to encounter the Nitric Oxide formation is to encode probes genetically. It involves the use of encoding of Nitric Oxide reactive proteins with transition metal genetically to further identify the biological formula of Nitric Oxide gas in nature under light. One example using the technique implies two mutant fluorescent proteins connect to an MT domain-charge, react with Nitric Oxide, which than can therefore provide a clear result of formation.

The figure below illustrates the fluorescence probe with emission of light:

References

http://baike.baidu.com/view/23998.htm

http://en.wikipedia.org/wiki/Carbon

http://baike.baidu.com/view/24009.htm

http://en.wikipedia.org/wiki/Nitrogen

http://baike.baidu.com/view/13797.htm

http://en.wikipedia.org/wiki/Oxygen

http://baike.baidu.com/view/39160.htm

http://en.wikipedia.org/wiki/Sulfur

http://en.wikipedia.org/wiki/Phosphorus

http://baike.baidu.com/view/39167.htm

Halogens

The Halogens are:

• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
• Astatine (At)

The halogens are a part of the 17th group of the periodic table of the elements. The halogens have varying physical properties; at room temperature fluorine and chlorine are gasses, bromine is a liquid, and iodine is a solid. On the other hand, they are very similar in their chemical properties. Any of the halogens will react with any of the alkali metals in a 1:1 ratio to form an alkali halide. Examples of some of these salt compounds are NaCl, LiF, and RbI.

Reactivity:

Halogens are extremely electronegative which means that they are highly reactive since they are only one electron away from filling their outer most electron shell. The halogen fluorine is one of the most reactive elements and has to be handled with care. The storing of fluorine is very difficult because it could react with glass if it has any water in it so it is usually stored in a container lined with a fluoride compounds. Chlorine and Bromine can be used as a sterilizer by their high reactivity. Iodine could also be used as sterilization for the skin.

Besides, halogens mean "salt formers" and the compounds that contain halogens are also called "salts." All of the halogens have seven electrons in their outer shells, which give them an oxidation number of -1. Also, at room temperature, the halogens exist in all three states: liquid, solid, and gas. The gas state include fluorine and chlorine, liquid state include bromine, and the solid state include iodine and astatine.

Fluorine

Fluorine's atomic number is 9. It can make a single bond with itself. It's characteristics are a poisonous yellow brown gas that is very reactive. At room temperature Fluoride in the gas phase.

Fluoride is the reduced form of fluorine and consumption of this ion can have toxic affects on the human body. Despite its toxicity, fluoride can be found in many everyday products including, toothpaste, vitamin supplements, baby formulas, and even public water. Many dental products contain fluoride in order to prevent tooth decay, but an over consumption can be fatal. Effects include birth defects, osteoarthritis, bone and uterine cancer, and skeletal fluorsis. Fluoride poisoning can be tested in medical laboratories by measuring the amounts of potassium and calcium in blood samples. Too little calcium and too much potassium may be signs that a person has been exposed to too much fluoride.

Applications of Fluorine

There are tons of compounds that involve the element fluorine and many of those compounds are useful for research. One such example is cryolite. Cryolite is a sodium-aluminium-fluoride compound that is used in the Hall-Heroult Process, where one takes cryolite and combine it with molten alumina followed by electrolysation. This leads to a collection of aluminum metal at the cathode of the electrophersis. This is an industrial method to produce pure aluminum by using a fluoride precursor. Another important daily use of a fluorine compound is sodium fluoride. This compound serves two purposes in the sense that it is present in the enamel of the tooth in the form of fluoroapatite. By fluorinating the water, people who use the water would have the benefits of fluorine, providing extra strength to the teeth. The other major use is the active ingredient in toothpaste. In toothpaste, the sodium fluoride acts as an active ingredient to dissociate water. This dissociation causes the separation of the fluorine atom enabling it to be absorbed into the bone of your teeth, enhancing the strength. more complex compounds containing fluorine which are used both in the industrial setting and in household environments include polytetrafluoroethylene, most commonly known as Teflon. In addition many refrigerants use fluorine, as well as in fire extinguishing reagent and propellant in aerosol sprays.

Chlorine

Chlorine gas.

Chlorine's atomic number is 17. It is abundant in nature since it is part of common salts, such as sodium chloride (NaCl). In its elemental form it can be seen as a pale green gas. Chlorine is also in the gas phase at room temperature.

In biological systems, chlorine accounts for about 0.15% of the human body weight. Chlorine is primarily used in the production of hydrochloric acid which is secreted from the parietal cells in the stomach and is used in maintaining the acidic environment for pepsin. It plays a vital role in maintaining the proper acid-base balance of body fluids. It is absorbed in the intestine and secreted through urine, sweat, vomit and diarrhea.

A deficiency in chlorine can lead to a condition called metabolic alkalosis where the pH of the blood is higher than normal. Symptoms include: decrease in ventilation, acidic urine, and excessive excretion of potassium.

Atomic chlorine can also be linked to the depletion of ozone. Chlorofluorocarbons (CFCs), which are originally used in refrigerators and air conditioners, escape and rise into the stratosphere are then decomposed by UV light from the sun to form atomic Cl. The newly formed atomic chloride is then able to react with ozone (O₃), thus depleting the ozone concentration in the stratosphere and forming oxygen gas (O₂).

Bromine

Bromine.

Bromine's atomic number is 35. At room temperature it is a reddish-brown liquid. The vapors from bromine are corrosive and toxic. Bromine is the only halogen that is liquid at room temperature. It has a similarly colored vapor with an offensive and suffocating odor. It is the only nonmetallic element that is liquid under ordinary conditions, it evaporates easily at standard temperature and pressures in a red vapor that has a strong disagreeable odor resembling that of chlorine. Bromine is less active chemically than chlorine and fluorine but is more active than iodine; its compounds are similar to those of the other halogens. Bromine is soluble in organic solvents and in water.

Read more: http://www.lenntech.com/periodic/elements/br.htm#ixzz2Aem37Fo2

Iodine

Iodine

Iodine's atomic number is 53. At room temperature it is a dark purple-grey solid and is the heaviest element in a living organism.

Iodine is an essential mineral for the body. It is heavily used in the thyroid gland, but also can be found in breast tissue, salivary gland, and adrenal gland. Without iodine, thyroid hormones cannot be produce and this can lead to a condition called hypothyroidism. Without treatment, the thyroid gland will swell and produce a visible goiter. Children with hypothyroidism may develop mental retardation. In women, hypothyroidism can lead to infertility, miscarriages, breast, and ovarian cancer. Thyroid problems have been common issue for many years particularly in middle aged women. Correlated studies have shown that iodine levels in the general population have been significantly decreased in recent years. Most salt has iodine and with current health problems, many people have been consuming less salt, and therefore in-taking less iodine.

Applications of Iodine

Due to its lower oxidizing power compared to fluorine and chlorine, there not as much applications of this halogen as compared to the ones mentioned above, as most of the applications of halogens are derived from their oxidizing power. Nonetheless, iodine has its applications in the medical field. The solution of iodine and potassium iodine forms the solution better known as "Lugol"s iodine, which is present in emergency safety kits for the purpose of disinfecting wounds and disinfecting water by killing bacteria that may be present. The other use of iodine in the medical field is te generation of certain thyroid hormones from iodine derivatives. Thyroid hormones such as thyroxine and triiodothronine are dependent on a source of a small amount of iodine. a deficiency of iodine would usually involve in the development in "goitre" or a swelling in the thyroid gland and the laryx. In order to prevent such a swelling, it is recommended to consume more seafood seeing as how the ocean water naturally contains more iodine, the sea life would contain more percent iodine if consumed and absorbed in the body.

Astatine

Astatine's atomic number is 85. This element is radio active and was only discovered because radioactivity in nature. Is is also the heaviest halogen but only small amounts are found at a time because it has a short half life. It is just presumed to be a black solid. At room temperature Astatine exists as a solid. An image of Astatine can be found here :Astatine

Hydrogen halides

Hydrogen halides are HF, HCL, HBr, HI, and HAt which are very strong acids because the can dissociate completely into ions.

References

http://www.globalhealingcenter.com/natural-health/how-safe-is-fluoride/ http://www.springboard4health.com/notebook/min_chloride.html

Bradley, Rachelle. "IODINE DEFICIENCY AND YOUR HEALTH: Hypothyroidism, Thyroid Disease and Fibrocystic Breast Disease." Heartland Naturopathic Clinic. n. page. Web. 30 Nov. 2011. <http://www.heartlandnaturopathic.com/Iodine.htm>

Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.

"Production and use of Chlorofluorocarbons". CIESIN Thematic Guides. http://www.ciesin.org/TG/OZ/prodcfcs.html

Background Information

The Noble Gases are:

• Helium (He)
• Neon (Ne)
• Argon (Ar)
• Krypton (Kr)
• Xenon (Xe)
• Radon (Rn)

The inert gases are the 18th group in the periodic table. They are also sometimes called the "noble gasses" due to their lack of reactivity with other chemicals. The elements in this group are typically inert because they possess a full valence shell. All of the noble gases are monatomic, as opposed to other gases (i.e. H₂, O₂, N₂, F₂, Cl₂) which exist as diatomic at room temperature and atmospheric pressure. The inert gas helium has been employed in respiratory obstruction, in investigative and diagnostic testing, and in hyperbaric applications[2].

The noble gases have weak interatomic force, and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements. Helium has several unique qualities when compared with other elements: its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit superfluidity; it is the only element that cannot be solidified by cooling under standard conditions—a pressure of 25 standard atmospheres (2,500 kPa; 370 psi) must be applied at a temperature of 0.95 K (−272.200 °C; −457.960 °F) to convert it to a solid. The noble gases up to xenon have multiple stable isotopes. Radon has no stable isotopes; its longest-lived isotope, 222Rn, has a half-life of 3.8 days and decays to form helium and polonium, which ultimately decays to lead.

The noble gases are colorless, odorless, tasteless, and nonflammable under standard conditions. They were once labeled group 0 in the periodic table because it was believed they had a valence of zero, meaning their atoms cannot combine with those of other elements to form compounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse. Very little is known about the properties of the most recent member of group 18, ununoctium (Uuo).

The noble gases show extremely low chemical reactivity; consequently, only a few hundred noble gas compounds have been formed. Neutral compounds in which helium and neon are involved in chemical bonds have not been formed (although there are some theoretical evidence for a few helium compounds), while xenon, krypton, and argon have shown only minor reactivity. The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn.

Neon, argon, krypton, and xenon are obtained from air using the methods of liquefaction of gases, to convert elements to a liquid state, and fractional distillation, to separate mixtures into component parts. Helium is typically produced by separating it from natural gas, and radon is isolated from the radioactive decay of radium compounds.[11] The prices of the noble gases are influenced by their natural abundance, with argon being the cheapest and xenon the most expensive. As an example, the table to the right lists the 2004 prices in the United States for laboratory quantities of each gas.

Helium

Helium is colorless and odorless. Of all the elements in the periodic table, it has the lowest boiling and melting points. It only exists in gas form at extreme environmental conditions. Pierre Janssen and Norman Lockyer first observed this element in 1868 when they found there existed a yellow line of light in a solar eclipse. Helium is the second lightest element and second most abundant element in the universe. This is due to the high binding energy of helium to lithium, beryllium, and boron. Helium has two electrons in the orbital around the nucleus with two protons.

Applications of Helium

Helium was initially used in dirigible-balloons, however, nowadays it is used in the production of inert gaseous during magnesium, aluminum and titanium welding. Helium is also used in the cooling of nuclear reactors as a transfer media because of its inert/unreactive properties. Helium mixed with oxygen is used for asthma treatment because of its capability of diffusing through lungs so easily. Helium can also be used in respiratory mixtures for high depth divers because of its deprived solubility in blood as apposed to nitrogen. In the liquid state, helium can also be used to achieve extremely low temperatures in electronic devices or for studies in regions of extremely low temperature.

Krypton

Krypton, found oftentimes in fluorescent lamps, is also colorless and odorless and is isolated for research or industrial purposes by distilling air. Krypton can be used to construct high power lasers or krypton fluoride lasers. It has very unique spectral signatures because of its strong spectrum lines. The amount of krypton available in the history of the universe was derived from meteors and solar wind. It acts as a great light source for high end photography.

Applications of Krypton

Due to the rarity of krypton its applications are dramatically diminished. However krypton is used commercially in the illumination industry where together with argon can be used in fluorescent lamps. Krypton is also used in flashed for high-speed pictures because it allows for an emission of intense light in a reduced time. This is because when krypton is excited, it emits an intense flash of photons that last only 1/50000 of a second. In smaller amounts, krypton can be used to increase the life of tungsten filaments and in addition be in the medical industry as an absorber of x-ray emissions.

Argon

Argon is more soluble in water than nitrogen gas. During room temperature, argon will not form any stable formations. Argon was isolated in 1894 by William Ramsay when he removed oxygen, carbon dioxide, water, and nitrogen from clean air. Earth's atmosphere is composed of 1.29% of argon. Argon exists in the most common isotopes as Argon 40, Argon 36, Argon 38, and Argon 40.

Applications of Argon

The primary use of argon of specific isotopes can be using to date metamorphic and igneous minerals. In addition, different isotopes of argon can be used to date the movement on fault systems. The dating of such minerals and systems may provide the age information on a rock but generally assumptions must be made. In addition to dating, argon can be used in electric light and fluorescent tubes, photo tubes, glow tubes and in lasers. Being an inert gas, it can be used fr welding and cutting, blanketing reactive elements, and act as a protective atmosphere for growing crystals of silicon and germanium due to its nonreactive properties.

Xenon

Xenon served as an important tool to gain insight of the solar system. Xenon exists in nine stable isotopes but there are actually over 40 stable isotopes that undergo half life and decay. Xenon exists in trace amounts in the earth's atmosphere and in mineral springs. The light properties of xenon have a broad spectrum of visible light and emits bluish light in a gas tube. Nuclear reactors oftentimes expel xenon as well.

Applications of Xenon

Similar to krypton, due to its high cost and limited abundance, it has very little applications. It is used in photography flashed and, in proportional amounts, used in filling gas mixtures for radiation detection. It is capable of doing so because of its high cross section of ionization of x-rays and gamma rays. AS a stretch, xenon has been used in the past as analytical oxidizing agents and fluorizing agents for specific purposes.

Neon

Of all the elements, Neon has the smallest range of liquid state. Neon emits red light in neon lamps and in discharge tubes. It exists in trace amounts in the earth's atmosphere and in air.It is the second lightest noble gas, and its density is only 2/3 that of air. Neon emits the strongest discharge of light at normal conditions. Glow-discharge lamps are typically very small but give off a good amount of light. Although all noble gases are unreactive, neon is considered the least reactive. The rarity of neon gas makes attaining small quantities for research very expensive.

Radon

Radon is a very dense gas at room temperature and is a health hazard. It has high radioactivity therefore it's hard to study it. Radon is formed from the decay of a chain or uranium. High concentrations of radon can cause lung cancer therefore it is considered a very toxic air contaminant and facilities must be evacuated if radon is released. Miners are most exposed to radon. Lung cancer and bad ventilation occurred among many miners in Schneeberg, Germany during the early years of the Cold War.

Applications of Radon

Radon has been used as a radiation source in cancer therapy with advantages over the currently used radium. It is also used as a radioactive tracer to spot gas leaks and in fluid measurements.

References

1. Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.

2. Goodman, Louis S, and Alfred Goodman Gilman. The Pharmacological Basis of Therapeutics. 7th ed. New York, N.Y.: Macmillan, 1985. Linear Combination of Atomic Orbitals (LCAO) is the sum of atomic wave functions that lead to the formation of molecular orbitals.

Parameters

-The symmetry of the orbitals must have the same sign overlap

-The energy of the atomic orbitals must be smaller

-The distance between the atoms must be short enough to provide good overlap of the orbitals.

The Linear combination of atomic orbitals are essential for molecular orbital understanding and building the molecular orbital diagram. The Linear combination of atomic orbitals or LCAO are tools that have described groups of orbitals as linear combinations of basic functions. The LCAO helps us understand symmetry of a molecule and how the ligands surround the center.

s Orbital Bonding

s Orbital bonding is when their two electron clouds overlap and merge into a large molecular electron cloud. This results in a linear combination of the atomic orbitals. Bond order is defined by

${\text{B.O.}}={\frac {{\text{number of bonding electrons}}-{\text{number of antibonding electrons}}}{2}}\$

This has to be positive in order for a stable bond to be created. The overall number of bonding and antibonding electrons determine the number of bonds

Bonding molecular orbital is where both atomic wave functions contribute, which has a lower energy than the original atomic orbitals Antibonding molecular orbital is the cancellation of waves functions, at a higher energy than the original atomic orbitals. Nonbonding molecular orbital is when the orbital of one atom has a symmetry that does not match the orbital of any other atoms

p Orbital Bonding

p Orbital bonding occurs when the two orbitals overlap, the overlapping regions have the same sign. As a result, the sum of the two orbitals has an increased electron probability in the overlap region. The overlap of two opposite signs leads to a node of zero electron density. As a result, the orbitals of Px, Py, and Pz are to be considered separately.

d Orbital Bonding

The transition metals bond in a similar way as p orbitals, but results in Dyz, Dz^2, Dyz, Dxz, Dx^2-y^2, and Dxy. The Dxz and Dyz orbitals form pie bonds. The Dx^2-y^2 and Dxy orbitals are collinear to the z axis and form delta bonds. The Dz^2 orbital forms a sigma bond.

Paramagnetic vs. Diamagnetic

Paramagnetic compounds are attracted by an external magnetic field due to one or more unpaired elections that are attracted to the magnetic field Diamagnetic compounds are not attracted by an external magnetic field because all the electrons are paired. This can have profound effects on metal compounds.

Reference

Miessler, Gary. Inorganic Chemistry. 4th Edition.

Molecular orbital theory uses group theory to describe the bonding patterns of molecular orbitals. This is determined by the symmetry and energies of the orbitals involved in bonding.

Requirements for overlap that result in bonding

1. Symmetry of the orbitals of the overlap must have the same sign of ϕ. 2. The energies between the orbitals must be similar for a bonding to occur. This energy difference is ~14-15eV max for two orbitals to bond. 3. The distance between the atoms must be short enough that they provide good overlap. However, this can't be too short because it would lead to an interference from repulsive forces. 4. When two orbitals bond, their new overall energy orbital must be lower than the two original bonding orbitals.

Molecular Orbitals Combinations

The two orbitals gradually move closer to each other that their electron clouds overlap, leading to a larger electron cloud. The signs of the orbitals are determined by the shading (dark vs. light) between the two orbitals. The bonding molecular orbital is where this larger electron cloud resides that was combined from the two smaller original orbitals. This is lower in energy than both original bonding orbitals due to stabilization. The opposite of this, called the anti-bonding molecular orbitals, involves a region called the node where there's no electron density from the cancellation of two wave functions. This results in a higher energy than the two original bonding orbitals. An asterisk is usually used to designate an anti-bonding orbital. Nonbonding orbitals can also result from the incompatible symmetry between two atomic orbitals.

Molecular orbital combinations are great when trying to understand how molecules interact and bond! As stated above, molecular orbitals try to show how two electron densities overlap together ultimately binding two molecules. There is always either a positive charge or negative charge on the densities, or one might call it an antibonding property and bonding property due to two different spins. In essence, when two electron densities meet, they either repel or attract. The molecular orbital combinations try to explain how this happens.

Molecular orbitals need to be considered in order to understand how electrons move and exist in a bonded structure. An orbital is a quantum mechanical description of wave function, or in other words where electrons reside. Due to the unpredictable nature of electrons, the molecular orbital theory is not absolutely stable, but we use it more as a tool to help us understand rather than as the absolute rule in chemistry. It is important to remember molecular orbitals are used as tools to explain rather than being the explanation itself!

Diatomic Molecular Orbital Energies

Table 1. Calculated MO energies for diatomic molecules in Hartrees
	H2	Li2	B2	C2	N2	O2	F2
1σg	-0.5969	-2.4523	-7.7040	- 11.3598	- 15.6820	- 20.7296	-26.4289
1σu		-2.4520	-7.7032	-11.3575	-15.6783	-20.7286	-26.4286
2σg		-0.1816	-0.7057	-1.0613	-1.4736	-1.6488	-1.7620
2σu			-0.3637	-0.5172	-0.7780	-1.0987	-1.4997
3σg					-0.6350	-0.7358	-0.7504
1πu			-0.3594	-0.4579	-0.6154	-0.7052	-0.8097
1πg						-0.5319	-0.6682
1s (AO)	-0.5	-2.4778	-7.6953	-11.3255	-15.6289	-20.6686	-26.3829
2s (AO)		-0.1963	-0.4947	-0.7056	-0.9452	-1.2443	-1.5726
2p (AO)			-0.3099	-0.4333	-0.5677	-0.6319	-0.7300

Correlation Diagram

Correlation diagrams, also known as Walsh diagrams, show the mixing of orbitals of the same energy. This shows the effect of moving two atoms together and combining two nuclei into one nucleus. Non-crossing rules also result from the correlation diagram. This rule states that orbitals of the same symmetry will never have their energies interact.

HOMO vs. LUMO

HOMO stands for highest occupied molecular orbital and LUMO stands for the lowest unoccupied molecular orbital. These are called frontier orbitals because they are the first of their respective occupied and unoccupied orbitals. HOMO and LUMO are significant in molecular orbital bonding because it has a greater contribution from the lower energy atomic orbital. Their electron density is also concentrated on the atom with the lower energy level.

Group Orbitals

Group orbitals are collections of matching orbitals on the outer atoms. The ion FHF- is a good example of group orbitals because of the strong hydrogen bonding. The lowest energy group orbitals come from the 2s orbitals of fluorine. The two 2s fluorine orbitals can have matching signs or they can have opposite signs in term of their wave functions. The 2px and 2py orbitals are the same, except they are separated by hydrogen's 1s orbital. As a result, H will only bond to the 2s orbital and pz orbital of fluorine. As a result, five out of the six p orbitals of fluorine will be shown as nonbonding.

Steps to Drawing MO Diagram

1. The point group of the molecule must first be determined. If the molecule is linear with infinite rotation such D∞h, it's often useful to use a similar point group like D2h to approximate.

2. x, y, z axis are assigned to each of the atoms. In general, the highest rotation axis is known as the z-axis. In most non-linear molecules, the y axis is used to point directly at the central atom.

3. A reducible representation is then formed from the s-orbitals of the central atoms. It is then found for all other outer orbitals if the s-orbital is not the most outer orbital. A value of 1 is returned from the reducible representation when the symmetry operation returns exactly the original position of the orbitals. A value of 0 is returned if the symmetry operation changes the location of the orbitals. A value of -1 is returned if the symmetry operation inverts the sign of the orbitals (eg: a p orbital's phase is changed but location stays the same). These values are additive, so a value of E (same position) on four orbitals, it will return a value of 1 x 4 = 4.

4. The reducible representation is then converted to its irreducible representations to determine the symmetry-adapted linear combinations (SALCS) of the orbitals. The total values for the irreducible representations must add up to the reducible representation.

5. The reducible and irreducible representation is then determined for the central atom.

6. The atomic orbitals of the central atom is then matched with the atomic orbitals of the group orbitals based on their symmetry and overlap.

Example (CO2):

1. Since CO2 has a D∞h, the D2h point group is used as a substitute. 2. The z-axis chosen as the primary rotation axis - C2 3. The reducible representation of the outer atom orbitals are determined. There will be four total, 2s, 2px, 2py, 2pz.

	E	C2 (z)	C2 (y)	C2 (x)	i	σ(xy)	σ(xz)	σ(yz)
Reducible Representation (s)	2	2	0	0	0	0	2	2

	E	C2 (z)	C2 (y)	C2 (x)	i	σ(xy)	σ(xz)	σ(yz)
Reducible Representation (2pz)	2	2	0	0	0	0	2	2
Reducible Representation (2px)	2	-2	0	0	0	0	2	-2
Reducible Representation (2py)	2	-2	0	0	0	0	-2	2

4. From the reducible presentation, the irreducible representation is then determined from the character table of D2hhere

This is the irreducible representation for the outer 2s orbitals

	E	C2 (z)	C2 (y)	C2 (x)	i	σ(xy)	σ(xz)	σ(yz)
Ag	1	1	1	1	1	1	1	1
B1u	1	1	-1	-1	-1	-1	1	1

Ag and B1u are the unique sets in the character table. It should be known that the irreducible representation adds up to the reducible representation.

What these symbols mean: A - singly degenerate (only 1 orbital transform): Symmetric with respect to the primary rotation axis B - singly degenerate (only 1 orbital transform)): Anti-symmetric with respect to the primary rotation axis E - doubly degenerate (two orbitals transform together) T - triply degenerate (three orbitals transform together) Subscript 1 - symmetric with respect to perpendicular C2 Subscript 2 - anti-symmetric with respect to perpendicular C2 ' - symmetric with respect to σh '' - anti-symmetric with respect to σh

5. The symmetry of the outer orbitals is matched to the symmetry of the central atom. The central atom has: 2s - Ag 2px - B3u 2py - B2u 2pz - B1u

6. Finally, the molecular orbitals are formed. In the group 1 and 2 orbitals, the oxygen 2s have Ag and B1u symmetry, which can be matched with the Ag symmetry of the 2s carbon orbital. In the group 3 and 4 orbitals, the oxygen 2pz orbital has Ag and B1u symmetry. This allows group 3 to bond with 2s of carbon and group 4 with the 2pz of carbon. This is then done for the rest of the orbitals, matched and drawn in the MO diagram. Like all bonding orbitals, their energies decrease when they bond due to stabilization.

Reference

Miessler, Gary. Inorganic Chemistry. 4th Edition.

General steps

1. First, determine the point group of the molecule.

2. Draw out the orbitals of the central atoms.

3. Determine the irreducible representation of the orbitals of the central atoms

4. Draw out the possible bonding interaction of the orbitals from the peripheral atoms

5. Determine the reducible representation of the orbitals of the peripheral atoms

6. From the reducible representation, determine the irreducible representation of the orbitals of the peripheral atoms

7. Determine the orbital energies from a table.

8. Place the orbitals on the energy scale, qualitatively.

9. Start forming bonding, non-bonding, and antibonding interaction . Remember ONLY ORBITALS THAT HAVE THE SAME IRREDUCIBLE MULLIKEN SYMBOL AND POINT AT THE RIGHT DIRECTION CAN THEY FORM A BONDING INTERACTION.

10. Fill in the electrons

Things to remember:

1. The left side is the metal center. The right side of the diagram is for ligands.

1. Non bonding pairs do not bond. Therefore they move right into the center of the diagram, not changing height. This is for non bonding electrons and such.

1. Count all electrons donated by ligands and metal center before filling in the center bonding electrons.

1. Remember that for every non bonding orbital there will be a bonding orbital.

1. Symmetry adapted linear combination models will illuminate how the molecule looks while bonding. What electron densities are charged etc.

Molecular orbital diagrams are done to help us gain a better understanding of the molecule in terms of binding and interactions. There is Bond Order that is determined in a Molecular orbital diagram. The bond order describes number of bonds between a distinct pair of atoms. Lewis dot structures are used to calculate a bond order, or there is also a method using a molecular orbital diagram and it's orbitals.

Bond order = [bonding electrons - non bonding electrons] / 2

Molecular orbital diagrams also tell if a specific molecule is magnetic or not. A molecule is either paramagnetic or diamagnetic. Paramagnetic means the molecule has an electron without the opposite spin pair. This ultimately leads to the molecule displaying magnetic characteristics. Diamagnetic means all of the electrons in the molecular orbital diagram is paired. Therefore having no magnetic qualities. This is important because it shows how we can use the molecular orbital diagram as a tool to tell us real characteristics of a molecule. The molecular orbital diagrams is a tool that was created to help us understand and think of molecules binding in a new light. Ultimately it does not show the overall complexity involved in bonding, but helps provide foundations of understanding to students.

Example Water

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1. Water belongs to C2v point group. Refer to point group. Below is the character table of C2v point group

2. The orbitals of central atom, oxygen, is as followed,

3. In order to determine the irreducible representations of these s and p orbitals, the symmetry operations must be done individually on the orbitals. If the orbital does not change sign, the character returns a +1 for the symmetry operation. If the orbital changes sign, the character returns a -1 for the symmetry operation. a. S orbital has A1 for the Mulliken symbol

b. Px orbital has B1 for the Mulliken symbol

c. Py orbital has B2 for the Mulliken symbol

d. Pz orbital has A1 for the Mulliken symbol

These Mulliken symbols for the P orbitals are also given by the second to last column by the character table.

4. Since hydrogen atom belongs to the first period, there is only s orbital involved in bonding. S orbitals exhibit solely sigma bonding.

5. Now, we need to determine the reducible representation of the orbitals above. Remember, there are TWO orbitals in the figure above, so there a total of TWO SALCS. If the symmetry operation does not change the orbital position = +1 If the symmetry operation changes the orbital position = 0 If the symmetry operation does not change the position, but changes sign = -1 Remember, changing position takes precedence than changing sign

6. To determine the irreducible representation from the reducible representation, some educated guess can be done. First of all, the central S orbital must almost always involve in sigma bonding, so one of the irreducible representation must be A1. If A1 is determined, then B1 can be determined by simple arithmetic.

7. The S orbitals from hydrogen atom have -13.61eV of energy. The S orbital from oxygen has -32.38 eV of energy, and the P orbitals have -15.85 eV.

8. The scale is in electron volts (eV)

9. Now we are able to connect the lines to form the complete molecular orbital diagram

I. The A1 SALC together with A1 on the central atom form the lowest 1a1 bonding orbital and 3a1 antibonding orbital. The bonding orbital must have lower energy than both A1 from the central atom and A1 from the SALC. The antibonding orbital must have higher energy than both A1 from the central atom and A1 from the SALC.

II. The B1 SALC together with B1 on the central atom form 1b1 bonding orbital and 2b1 antiboding orbital.

III. Because 2S and 2Pz on the central atom have the same Mulliken symbol, they mix to form 2a1. The mixing lower the energy of Pz orbital.

IV. Because there is no corresponding B2 orbital of the SALC, B2 on the central becomes nonbonding.

On the right, are the corresponding figures of the orbitals. Note that 1a1 has more central atom contribution than the SALC because the bonding molecular orbital is closer in energy than the SALC. On the contrary, 2b1 has more SALC contribution than the central atom because the antibonding molecular orbital is closer in energy to the SALC.

10. On the left side, the central atom, oxygen has 6 valance electrons. Fill the electrons from bottom to top according to Hund’s Rule. On the other hand, the SALCs only have 2 electrons because each H atom donates 1 electron. As the result, there are 8 electrons in the system. Then, they can be filled from bottom to top in the molecular diagram.

Now we have a complete molecular orbital diagram of the water molecule.

Note that there are two lone pairs in water molecule in Lewis dot structure.

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So, how do we locate these two lone pairs in the molecular orbital diagram? The obvious one is the one in non-bonding orbital, which is the b2 orbital. The other one would be the bonding orbital, 1a1. Even though 1a1 is a bonding orbital, it also has some non-bonding character because the S orbital on the central atom is used way less than 50% in forming the 1a1 bonding. The S orbital only contributes less than 50% of its orbital in forming 1a1 bonding orbital because it is too low in energy.

Because 1a1 and b2 are really close in energy to the S orbital and the P orbitals in oxygen, these two molecular orbitals are more oxygen in character. As the result, the electrons located in these two orbitals are located on the oxygen, not on the hydrogen. This theory corresponds really well with Lewis’ theory.

References

Figueroa, Joshua. "Character Tables, Irreducible Representations of Central Atom." Lecture 9. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "SALCs, Molecular Orbital Diagrams, and High Symmetry Point Groups." Lecture 10. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "More SALC’s Molecular Orbital Diagrams and High Symmetry Point Groups." Lecture 11. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "Molecular Orbital Diagrams Featuring and Pi-Bonding and High Symmetry Point Groups." Lecture 13. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "Degenerate Orbitals, Methods for Determining Irreducible Representations and SALCs." Lecture 14. University of California, San Diego, La Jolla. Lecture.

Reduction Formula

When the system gets far too complicated, i.e. more than 3 peripheral atoms, then it is hard to reduce the reducible representation to irreducible representations. To overcome this, a formula can be used to calculate the irreducible representations. The formula is as followed: (# of irreducible representations of a given type) = (1/order)*sum of [ (# operations in class) * (character of reducible representation) * (character of irreducible representation)]

where order = sum of operation of all classes

Example: Ammonia

Below is the sigma framework of ammonia peripheral atoms.

Ammonia belongs to C3v point group.

C3v point group character table is given below.

The reducible representation are as followed:

Because there are three orbitals going in, there are three SALCs coming out.

Now the reduction formula can be applied to find the irreducible representations of the SALCs. nA1 = 1/6 [(1)(3)(1) + (2)(0)(1) + (3)(1)(1)] = 1 This means that there is one A1 SALC in the irreducible representation The order of this point group is 6 because the coefficient of E, C3, SigmaV are 1, 2, and 3, respectively, and the sum of these coefficients is 6. 1. The first 1 in first term represents the coefficient of E in the character table.

2. The 3 in first term represents the character in the reducible representation

3. The second 1 in first term represents the character in the character table

4. The 2 in second term represents the coefficient of C3 in the character table.

5. The 0 in the second term represents the character in the reducible representation

6. The 1 in the second term represents the character in the irreducible representation

7. The 3 in the third term represents the coefficient of SigmaV in the character table

8. The first 1 in the third term represents the character in the reducible representation

9. The second 1 in the third term represents the character in the irreducible representation

The number of irreducible representation of A2 and E can be found with the same fashion. nA2 = 1/6 [(1)(3)(1) + (2)(0)(1) + (3)(1)(-1) ] = 0 nE = 1/6 [(1)(3)(2) + (2)(0)(-1) + (3)(1)(0) ] = 1

Although there is A1 and only one E, this E SALC actually composed of two SALCS. As the result, in total, we have three SALCs.

Projection Operator

Now that we know that ammonia molecule is made up by one A1 and one E SALC sets. How do we know how they look like? Projection operation is a tool to help us visualize the SALCS. It maps the SALCs into its correct orientation according to its symmetry.

The formula for projection operator is as followed:

The SALCs in ammonia, for example, can be found by using this projection operator formula. There are a few rules to followed in order to use this formula correctly

1. Name the atoms alphabetically.

2. Decide whether the primary axis of rotation is counterclockwise or clockwise.

3. Expand all the symmetry operation

＝Example: Ammonia=

The axis of rotation in this case is along the z-axis counterclockwise. Hydrogen A is align with the x-axis. It is important to remember to put one peripheral atom along x or y axis in a three-fold symmetry for convenience.

Now, we first determine how A1 should look like. (You should be able to immediately visualize A1. However, here is a simple demonstration)

(Note that the symmetry operation is expanded to its unique operation.)

First row is focusing on hydrogen A atom. 1. When E operation is done on hydrogen A, hydrogen A is returned. Then A is multiply by the character from the irreducible representation, which is 1.

2. When C3 operation counterclockwise is done on hydrogen A, hydrogen B is returned. Then B is multiply by the character from the irreducible representation, which is 1.

3. When C32 operation counterclockwise is done on hydrogen A, hydrogen C is returned. Then C is multiply by the character from the irreducible representation, which is 1.

4. When vertical reflection plane operation along hydrogen A is done on hydrogen A, hydrogen A is returned. Then A is multiply by the character from the irreducible representation, which is 1.

5. When vertical reflection plane operation along hydrogen B is done on hydrogen A, hydrogen C is returned. Then C is multiply by the character from the irreducible representation, which is 1.

6. When vertical reflection plane operation along hydrogen C is done on hydrogen A, hydrogen B is returned. Then B is multiply by the character from the irreducible representation, which is 1.

Using similar operation focusing on hydrogen B and C, the outcome is the same. (Note: this is because A1 is singly degenerate)

The sum of these operations is 2A + 2B + 2C

Which can be further reduced to A + B + C As the result, the SALC looks exactly like:

Now, we will determine how the E SALCs would look like, which is not as trivial as the A1 SALC.

Below is the projection operation of the E SALCs

(Note that the coefficients are different because the coefficients from the character table for E is 2 -1 0)

We need to figure out two unique E SALCs from these three equations. This is why we need to put a peripheral atom along one of the axis. Since hydrogen A is along the x-axis, the first equation is a unique equation. However, the second unique equation should not contain hydrogen A. In order to get an equation without hydrogen A, the other two equations must subtract each other. As the result, one of the unique equations is 2A – B – C, while the other one being 3B -3C, which further reduces to B – C. Now we are ready to draw the E SALCs.

Below is the complete molecular orbital diagram of ammonia.

In ammonia, the 2S orbital of nitrogen not only forms 1a1 molecular orbital with the A1 SALC but also mixes with 2Pz to form the 2a1 molecular orbital. This orbital is hence known as sp hybridization. Because 2a1 molecular orbital is formed by mixing with the 2S orbital, the orbital is stabilized and hence has lower orbital energy.

The electrons in the 2a1 molecular orbital are the lone pair on ammonia. This sp hybridized orbital has an inversion center. This inversion center can readily flip the nitrogen as the figure shown below. As the result, there is never a chiral center in amine groups, even though four different constituents are attached to nitrogen.

References

Figueroa, Joshua. "Character Tables, Irreducible Representations of Central Atom." Lecture 9. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "SALCs, Molecular Orbital Diagrams, and High Symmetry Point Groups." Lecture 10. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "More SALC’s Molecular Orbital Diagrams and High Symmetry Point Groups." Lecture 11. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "Molecular Orbital Diagrams Featuring and Pi-Bonding and High Symmetry Point Groups." Lecture 13. University of California, San Diego, La Jolla. Lecture. Figueroa, Joshua. "Degenerate Orbitals, Methods for Determining Irreducible Representations and SALCs." Lecture 14. University of California, San Diego, La Jolla. Lecture. Each molecule has a set of symmetry operations that describes the molecule's overall symmetry. This set of symmetry to classify molecules is known as point group Group theory is a powerful mathematical tool in determining the symmetry, properties, and data of complex molecules. Molecules are said to belong to the same point group if they have the same symmetry response under a set of operations. That is to say, two molecules can have totally different shapes but belong to the same point group as long as the respond the same to some symmetry operations. A molecule can only belong to one point group. It cannot belong to more than one point group.

It is important to understand that point groups is a tool for categorizing the different symmetries a molecule might have. This tries to classify molecules based on their molecular shapes and symmetries. There are various symmetry elements used to illuminate the shape of the molecule. The shape and symmetry of a molecule is highly important especially in structural biochemistry because structure determines functions of proteins and molecules. It is also important to note that many molecules are constantly going through conformational change, therefore its point group can also change. Point group is a tool for us to understand on a molecular level, symmetry.

Types of Point Groups

Molecules can be separated by the symmetry elements that they contain. The different Symmetry operations can be classified into:

Ammonia

-Rotation operation (C_n): is rotation of 360/n degrees about a rotation axis that renders the object unchanged.. For example, ammonia (NH₃) contains a threefold rotational axis, which gives it a C₃ operation. Objects or molecules can have more than one rotational axis. An example of an object with more than one rotational axis is a snowflake or octahedron.

Snowflake

-Reflection operation (σ): is a reflection across the mirror plane where when the object remains unchanged when reflected. There are three different σ's. The first is σ_vwhere the mirror plane is parallel to the primary axis of rotation and an outer atom. A σ_d is a mirror plane that is also parallel to the primary axis of rotation but it does not pass through an outer atom, rather it is between two outer atoms. An exception to this is in T_d and D_nd, in which all mirror planes are referred to as σ_d. The last mirror plane is such that is a horizontal reflection σ_h where the plane of reflection is perpendicular to the primary axis of rotation. A primary axis of rotation is the axis with the greatest number of rotational symmetry folds. Objects and molecules can also have more than one plane of reflection.

File:Sigma v.jpg

σ_v

File:Sigma d.jpg

σ_d

File:Sigma h.jpg

σ_h

-Inversion operation (i): is a combination of a C₂ rotation and a σ_h operation to invert the molecule across a point of inversion, which is usually taken to be the center of the molecule. It is a reflection (or inversion) through the mid-point (center) of the object or molecule.

-Improper Rotation (S_n): is the rotation of 360/n, followed by reflection through a plane perpendicular to the axis of rotation, σ_h. An S₂ operation is identical to an inversion (both being a 180 degree rotation, followed by a reflection across the perpendicular horizontal axis).

-Identity (E): is merely a rotation by 360 degrees around an arbitrary axis that returns an equivalent configuration. Every molecule has at least an E operation.

Examples

O_h – octahedral point group – contains all symmetry elements and operation of a perfect octahedron.

Figure I. Octahedron.

C_∞v : An infinite number of mirror plane parallel to an infinite number of rotation axis.

D_∞v : not only has an infinite number of rotation axis and mirror planes but also has a two-fold rotation perpendicular to these infinite number of rotation axis. D_∞v also has an inversion center.

C_3v There is a 3-fold primary rotation axis and 3 mirror planes parallel to that axis.

D_3h Designates that there are 3 two-fold rotational axes perpendicular to a three-fold primary axis and a mirror plane perpendicular to that primary axis.

How to Work a Point Group Flow Chart

All molecules have some type of symmetry or lack thereof, and can be grouped into categories (called point groups) where all molecules with the same symmetries have the same point group. To facilitate the determination of a molecule's point group, without having to exhaustively determine every one of its symmetry properties, a point group flow chart can be used. A molecule's point group can be easily determined by finding how many rotational and mirror planes it contains in a logical manner. The less symmetrical an object is, the more difficult symmetry operations are needed to determine its point group, where as the higher symmetries are more easily determined. This allows users to answer simple questions based on the shape of the molecule. If one is able to understand the point group flowchart, there requires no memorization. Therefore point group determination is quite simple, all it requires is knowledge of types of point group and some visualization.

There are many other point groups such as C_4v, C_5v, D_4h, D_5h, etc. Memorizing the definition of all the point-groups is overwhelming. To simplify this, point group of a molecule can be easily found by following a flow chart. First, start with determining if the molecule is linear or not. If it is, then determine if there is an inversion center. If the molecule is not linear, check if there are two or more C_n, where n is greater than two. If yes, then determine if there is an inversion center and if there is a C₅ rotation axis. If there are two or more C_n's, then check if there are C_n's at all. If so, then check if the C_n with the higher n is perpendicular to nC₂. If so, then determine if the molecule has a σ_h or σ_d. If there aren't two or more C_n's, then check if there is a σ_h, nσ_v, or S_2n. These are the cases where there is a C_n, but if there aren't any C_n's then check if there is a σ_h of inversion center. If this is done correctly, the arrival at the end of a chain will be the point group for the observed molecule.

Point Group Flow Chart

Example of how to use the flow chart

File:Water molecule like Micky mouse.png

Water molecule

Above is a figure of water molecule. Following the flow chart: 1. The molecule is not linear 2. The molecule does not have more than 2 axis of rotation other than the primary axis 3. The molecule has a proper C₂ primary rotation 4. But the molecule does not have 2 other proper rotation axis perpendicular to the primary rotation axis 5. The molecule does not have a mirror plane perpendicular to the primary axis 6. The molecule has two mirror plane parallel to the primary axis 7. Thus, water molecule belongs to C_2v point group

File:BF5.png

BF₅ molecule

Above is a figure of BF₅ molecule. Following the point group flow chart: 1. The molecule is not linear 2. The molecule does not have more than 2 rotation axis other than the primary rotational axis 3. The molecule has a 4-fold proper rotational axis 4. The molecule does not have 4 C₂ axis that is perpendicular to the primary rotational axis 5. The molecule does not have a mirror plane that is perpendicular to the primary rotational axis 6. The molecule has 4 mirror planes that are perpendicular to the primary rotational axis 7. The molecule hence belongs to C_4v point group

File:Cyclopropane molecular orbital.png

Cyclopropane

Above is a figure of a cyclopropane molecule. Following the point group flow chart: 1. The molecule is not linear 2. The molecule does not have more than 2 rotation axis other than the primary rotational axis 3. The molecule has a proper 3-fold rotational axis 4. The molecule has 3 perpendicular C₂ axis 5. The molecule has a mirror plane perpendicular to the primary rotational axis 6. The molecule hence belongs to D_3h point group

File:Cyclopropane molecular orbital.png

Octachloride osmium

Above is a molecule octachloride osmium with an eclipse conformation. Following the flow chart 1. The molecule is not linear 2. The molecule does not have more than 2 rotation axis other than the primary rotational axis 3. The molecule has a C₄ proper rotational axis 4. The molecule has 4 C₂ proper rotational axis that is perpendicular to the primary rotational axis 5. The molecule does not have a mirror plane that is perpendicular to the primary rotational axis 6. The molecule has 4 mirror planes that are parallel to the primary rotational axis 7. The molecule hence belongs to D_4d point group

High Symmetry Point Groups

Molecules of high symmetry are of certain geometric shapes like Linear, Octahedral (O_h), Tetrahedral (T_d), and Icosahedral (Ih) shapes. Although there are some variations in these symmetries, they are often rare in nature.

-C_∞v are linear molecules with an infinite number of rotations along the primary rotation axis, and an infinite number of reflection planes containing the rotation axis. They have no center of inversion, and are usually two atom molecules.

-D_∞h are also linear molecules with an infinite number of rotations along the primary axis and an infinite number of reflection planes containing the rotation axis. The three main difference from a C_∞v are that a C₂ axes and a reflection plane exist perpendicular to the primary axis of rotation, and an inversion center.

-T_d molecules have a tetrahedral geometry (but don't always have the shape of a tetrahedral). They have 4 C₃ axes, 3 C₂ axes, 3 S₄ axes, and 6 mirror planes. However, they lack C4 axes.

-O_h molecules are most commonly those with octahedral structure. They typically have 4 C₃ rotations, 3 C₄ rotations, and an inversion center

-I_h molecules are identified by their C₃ and C₅ axes, along with its inversion center.

Low Symmetry Point Groups

C₁ is a molecule with no symmetry other than the identity operation, E. C_s is a molecule with only a mirror plane and the identity operation, E. C_i contains only an inversion center and the identity operation, E.

Examples of Specific Point Groups

Group	Intl	Orbifold	Coxeter	Order	Description
Cn	n	nn	[n]+	n	Cyclic: n-fold rotations. Abstract group Zn, the group of integers under addition modulo n.
Dn	nm	*nn	[n]	2n	Dihedral: cyclic with reflections. Abstract group Dihn, the dihedral group.

Finite isomorphism and correspondences

The subset of pure reflectional point groups, defined by 1 or 2 mirrors, can also be given by their Coxeter group and related polygons. These include 5 crystallographic groups. The symmetry of the reflectional groups can be doubled by an isomorphism, mapping both mirrors onto each other by a bisecting mirror, doubling the symmetry order.

Reflective					Rotational			Related polygons
Group	Coxeter group		Coxeter diagram		Order	Subgroup	Coxeter		Order
D1	A1	[ ]	Template:CDD	Template:CDD	2	C1	[]+	1	Digon
D2	A12	[2]	Template:CDD	Template:CDD	4	C2	[2]+	2	Rectangle
D3	A2	[3]	Template:CDD	Template:CDD	6	C3	[3]+	3	Equilateral triangle
D4	BC2	[4]	Template:CDD	Template:CDD	8	C4	[4]+	4	Square
D5	H2	[5]	Template:CDD	Template:CDD	10	C5	[5]+	5	Regular pentagon
D6	G2	[6]	Template:CDD	Template:CDD	12	C6	[6]+	6	Regular hexagon
Dn	I2(n)	[n]	Template:CDD	Template:CDD	2n	Cn	[n]+	n	Regular polygon
D2×2	A12×2	[[2]] = [4]	Template:CDD	Template:CDD = Template:CDD	8
D3×2	A2×2	[[3]] = [6]	Template:CDD	Template:CDD = Template:CDD	12
D4×2	BC2×2	[[4]] = [8]	Template:CDD	Template:CDD = Template:CDD	16
D5×2	H2×2	[[5]] = [10]	Template:CDD	Template:CDD = Template:CDD	20
D6×2	G2×2	[[6]] = [12]	Template:CDD	Template:CDD = Template:CDD	24
Dn×2	I2(n)×2	[[n]] = [2n]	Template:CDD	Template:CDD = Template:CDD	4n

Intl* Geo Orbifold Schönflies Conway Coxeter Order 1 1 1 C1 C1 [ ]+ 1 1 22 ×1 Ci = S2 CC2 [2+,2+] 2 2 = m 1 *1 Cs = C1v = C1h ±C1 = CD2 [ ] 2 2 3 4 5 6 n 2 3 4 5 6 n 22 33 44 55 66 nn C2 C3 C4 C5 C6 Cn C2 C3 C4 C5 C6 Cn [2]+ [3]+ [4]+ [5]+ [6]+ [n]+ 2 3 4 5 6 n 2mm 3m 4mm 5m 6mm nmm nm 2 3 4 5 6 n *22 *33 *44 *55 *66 *nn C2v C3v C4v C5v C6v Cnv CD4 CD6 CD8 CD10 CD12 CD2n [2] [3] [4] [5] [6] [n] 4 6 8 10 12 2n 2/m 3/m 4/m 5/m 6/m n/m 2 2 3 2 4 2 5 2 6 2 n 2 2* 3* 4* 5* 6* n* C2h C3h C4h C5h C6h Cnh ±C2 CC6 ±C4 CC10 ±C6 ±Cn / CC2n [2,2+] [2,3+] [2,4+] [2,5+] [2,6+] [2,n+] 4 6 8 10 12 2n 4 3 8 5 12 2n n 4 2 6 2 8 2 10 2 12 2 2n 2 2× 3× 4× 5× 6× n× S4 S6 S8 S10 S12 S2n CC4 ±C3 CC8 ±C5 CC12 CC2n / ±Cn [2+,4+] [2+,6+] [2+,8+] [2+,10+] [2+,12+] [2+,2n+] 4 6 8 10 12 2n

Intl Geo Orbifold Schönflies Conway Coxeter Order 222 32 422 52 622 n22 n2 2 2 3 2 4 2 5 2 6 2 n 2 222 223 224 225 226 22n D2 D3 D4 D5 D6 Dn D4 D6 D8 D10 D12 D2n [2,2]+ [2,3]+ [2,4]+ [2,5]+ [2,6]+ [2,n]+ 4 6 8 10 12 2n mmm 6m2 4/mmm 10m2 6/mmm n/mmm 2nm2 2 2 3 2 4 2 5 2 6 2 n 2 *222 *223 *224 *225 *226 *22n D2h D3h D4h D5h D6h Dnh ±D4 DD12 ±D8 DD20 ±D12 ±D2n / DD4n [2,2] [2,3] [2,4] [2,5] [2,6] [2,n] 8 12 16 20 24 4n 42m 3m 82m 5m 122m 2n2m nm 4 2 6 2 8 2 10 2 12 2 n 2 2*2 2*3 2*4 2*5 2*6 2*n D2d D3d D4d D5d D6d Dnd ±D4 ±D6 DD16 ±D10 DD24 DD4n / ±D2n [2+,4] [2+,6] [2+,8] [2+,10] [2+,12] [2+,2n] 8 12 16 20 24 4n 23 3 3 332 T T [3,3]+ 12 m3 4 3 3*2 Th ±T [3+,4] 24 43m 3 3 *332 Td TO [3,3] 24 432 4 3 432 O O [3,4]+ 24 m3m 4 3 *432 Oh ±O [3,4] 48 532 5 3 532 I I [3,5]+ 60 53m 5 3 *532 Ih ±I [3,5] 120

(*) When the Intl entries are duplicated, the first is for even n, the second for odd n.

Reflective subset groups

Finite isomorphism and correspondences

The subset of pure reflectional point groups, defined by 1 to 3 mirror planes, can also be given by their Coxeter group and related polyhedra. The [3,3] group can be doubled, written as [[3,3]], mapping the first and last mirrors onto each other, doubling the symmetry to 48, and isomorphic to the [4,3] group.

Schönflies	Coxeter group		Coxeter diagram		Order	Related regular and prismatic polyhedra
Td	A3	[3,3]	Template:CDD		24	Tetrahedron
Td×Dih1 = Oh	A3×2 = BC3	[[3,3]] = [4,3]	Template:CDD	= Template:CDD	48	Stellated octahedron
Oh	BC3	[4,3]	Template:CDD		48	Cube, octahedron
Ih	H3	[5,3]	Template:CDD		120	Icosahedron, dodecahedron
D3h	A2×A1	[3,2]	Template:CDD		12	Triangular prism
D3h×Dih1 = D6h	A2×A1×2	[[3],2]	Template:CDD	= Template:CDD	24	Hexagonal prism
D4h	BC2×A1	[4,2]	Template:CDD		16	Square prism
D4h×Dih1 = D8h	BC2×A1×2	[[4],2] = [8,2]	Template:CDD	= Template:CDD	32	Octagonal prism
D5h	H2×A1	[5,2]	Template:CDD		20	Pentagonal prism
D6h	G2×A1	[6,2]	Template:CDD		24	Hexagonal prism
Dnh	I2(n)×A1	[n,2]	Template:CDD		4n	n-gonal prism
Dnh×Dih1 = D2nh	I2(n)×A1×2	[[n],2]	Template:CDD	= Template:CDD	8n
D2h	A13	[2,2]	Template:CDD		8	Cuboid
D2h×Dih1	A13×2	[[2],2] = [4,2]	Template:CDD	= Template:CDD	16
D2h×Dih3 = Oh	A13×6	[3[2,2]] = [4,3]	Template:CDD	= Template:CDD	48
C3v	A2	[1,3]	Template:CDD		6	Hosohedron
C4v	BC2	[1,4]	Template:CDD		8
C5v	H2	[1,5]	Template:CDD		10
C6v	G2	[1,6]	Template:CDD		12
Cnv	I2(n)	[1,n]	Template:CDD		2n
Cnv×Dih1 = C2nv	I2(n)×2	[1,[n]] = [1,2n]	Template:CDD	= Template:CDD	4n
C2v	A12	[1,2]	Template:CDD		4
C2v×Dih1	A12×2	[1,[2]]	Template:CDD	= Template:CDD	8
Cs	A1	[1,1]	Template:CDD		2

Representations

Irreducible representation is used to create a character table for that point group.

-A value of 1 means that the symmetry does not change its orbital position

-A value of 0 means the operation changes orbital's position

-A value of -1 means the orbital does not change its symmetry but inverts its sign.

Reducible representation is simply a combination of irreducible representations.

Character Table

Character tables are designated by these characters which means different operations.

-x,y,x is the transformation of the x,y,z coordinates

-R_x, R_y, R_z is the rotation about the x, y, z axes

-R is any symmetry operation like C₂

-X is the character of operation

-i, j is the designation of different representations such as A₁ or A₂

-h is the total number of symmetry operations in the group (order)

References

http://en.wikipedia.org/wiki/Point_group

Miessler, Gary. Inorganic Chemistry. 4th Edition.

Character Table

Below is an example of a character table.

File:D4h.png

D4h Character Table

The higher the symmetry of the point group, the more complicated the character table is. As the result, sometimes, the complexity and symmetry of the point group can be reduced and approximated by a lower symmetry point group. For example, in D4h, the character table is shown above. However, to simplify the table, the point group can be approximated by C4v point group. As the one below.

File:C4v.png

C4v Character Table

A character table is made up of 5 parts.

1. The symbol of the point group is given at the upper left hand corner of the character table.

2. The operations that belonged to the point group is given at the top row, organized into classes.

3. Mulliken symbols for irreducible representations are given at the left column.

4. The characters of the irreducible representations are given at the center of the table. These are the outcome of the basis function in response to the operations of the group

5. Some certain functions are listed on the right. These functions show the irreducible representations for which the function can be served as basis.

Mulliken symbols

A – singlely degenerate, meaning only one orbital has that particular symmetry and level of energy. Symmetric with respect to the primary rotational axis.

B – singlely degenerate, meaning only one orbital has that particular symmetry and level of energy. Anti-symmetric with respect to the primary rotational axis.

E – doubly degenerate, meaning that two orbitals have the same symmetry and the same level of energy. These orbitals transform together.

T – triply degenerate, meaning that three orbitals have the same symmetry and the same level of energy. These orbitals transform together.

Subscript g – symmetric with respect to inversion center.

Subscript u – anti-symmetric with respect to inversion center.

Subscript 1 – symmetric with respect to perpendicular C2 axis

Subscript 2 – anti-symmetric with respect to perpendicular C2 axis.

‘(prime) – designates symmetric with respect to horizontal reflection plane

“(double prime) – designates anti-symmetric with respect to horizontal reflection plane

How to use a character table

An orbital of a central atom is placed at the origin. The symmetry operations are perform on the orbital.

If the sign of the orbital does not change, a positive 1 is returned for the character.

If the sign inverts, a negative 1 is returned for the character.

Example

S orbital at the origin

Above is an S orbital of the central atom sitting at the origin. Remember, it is very critical to always place the orbital of the central atom at the origin. Refer to Figure VIIII, there are a total of five operations in C4v point group.

1. If an E operation is performed on this S orbital, a positive 1 is returned for the character since the sign of the S orbital remains the same.

2. If a C4 operation is performed on this S orbital, a positive 1 is returned for the character (for convention, the primary rotation axis is always placed along the z axis).

3. If A C2 operation is performed on this S orbital, a positive 1 is returned for the character.

4. If a mirror plane reflection through the bonds is performed on this S orbital, a positive 1 is returned for the character.

5. If a mirror plane reflection between the bonds is performed on this S orbital, a positive 1 is returned for the character.

Px orbital at the origin

Above is a Px orbital of a central atom sitting at the origin. Refer to C2v character table, there are a total of 4 symmetry operations in this point group.

1. If an E operation is performed on this Px orbital, a positive 1 is returned for the character.

2. If a C2 operation along the z axis is performed on this Px orbital, a negative 1 is returned because the sign inverts

3. If a vertical mirror reflection on xz plane is performed on this Px orbital, a positive 1 is returned.

4. If a vertical mirror reflection on yz plane is performed on this Px orbital, a negative 1 is returned.

Since 1, -1, 1, -1 are returned for the character, the corresponding Mulliken symbol is B1.

Since 1, 1, 1, 1, 1 is returned for all the character, matching the Mulliken symbol on the left of the table, A1 is the irreducible representation of this S orbital. S orbital is a perfect sphere, so it is immediately obvious that it is a perfectly symmetrical orbital.

Reference

Figueroa, Joshua. "Intro to Symmetry and Symmetry Element." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Symmetry operation and character table." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Character tables, irreducible representations of central atom." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Symmetry elements and point groups." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "SALCS, molecular orbital diagrams and high symmetry point groups." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

"Symmetry Resources." Otterbein University. N.p., n.d. Web. 20 Nov. 2012. <http://symmetry.otterbein.edu/>.

"Point Group Symmetry Character Tables - Chemistry Online Education." Point Group Symmetry Character Tables - Chemistry Online Education. N.p., n.d. Web. 20 Nov. 2012. <http://www.webqc.org/symmetry.php>.

Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. Upper Saddle River, NJ: Pearson Prentice Hall, 2011. Print.

Symmetry Adapted Linear Combination (SALC)

SALC is a method to combine TWO OR MORE orbitals into one group and to see how they behave as a group. The behavior is influenced by each other and by the orbital from the central atom. The treatment of SALC is somehow similar to that of central orbital.

The rule is as followed: If the symmetry operation does not change the orbital position = +1 If the symmetry operation changes the orbital position = 0 If the symmetry operation does not change the orbital position but changes the sign = -1 (It is very important to remember that changing position has the priority over changing sign)

Symmetry Adapted Linear Combinations are helpful in getting a three dimensional picture of how all aspects of binding works. This means it shows how the s, p and d bindings occur with respect to electron densities and their respective spin states. Different models are matched to see how a molecule will bond in an energetically favorable path and an equally unfavorable path. These are called binding and anti-bonding pairs. When there are lone pairs, there is not really a SALC because no combination of electron densities occur.

Symmetry adapted linear combination models are done with knowledge of a molecular orbital. With both the use of a molecular orbital and symmetry adapted linear combination models, there can be a great deal of knowledge about how a molecule bonds with each ligand. The molecular orbital diagram helps to show all the states in which the molecule fills electrons, and their respective energy levels with respect to each other. In essence, using molecular orbital diagrams and symmetry adapted linear combination models, we can predict where electrons are most likely to be found and why. SALCs allow us to get a better understanding of how molecules exist in terms of how binding and spin states relate.

Example

The example showing here is hydrogen atoms on water molecule.

File:Water salc table.png

The reducible and irreducible representation of the hydrogen atoms on water molecule

E = A stays at A, B stays at B = 1 + 1 = 2 C2 = A changes position to B, B changes position to A = 0 + 0 = 0 Sigma v (xz) = A statys at A, and B stays at B = 1 + 1 = 2 Sigma v’ (yz) = A changes position to B, and B changes position to A = 0 + 0 = 0 So the reducible representations are 2 0 2 0 With simple reduction, one can tell that this can be reduced to A1 ( 1 1 1 1) and B1 ( 1 -1 1 -1)

Below is the outcome of the SALCs

File:The salcs.png

The SALCs made up of water s orbitals

Reference

"Fourier Transform Infrared Spectroscopy (FTIR)." UC Davis Chem Wiki. N.p., n.d. Web. 8 Nov. 2012. <Fourier Transform Infrared Spectroscopy (FTIR)>.

"Symmetry Elements." N.p., n.d. Web. 09 Nov. 2012. <http://myphayao.com/symmetry-elements-examples/>.

Figueroa, Joshua. "Intro to Symmetry and Symmetry Element." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Symmetry operation and character table." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Character tables, irreducible representations of central atom." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "Symmetry elements and point groups." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

Figueroa, Joshua. "SALCS, molecular orbital diagrams and high symmetry point groups." Inorganic Chemistry. University of California, San Diego, La Jolla. Oct. 2012. Lecture.

"Symmetry Resources." Otterbein University. N.p., n.d. Web. 20 Nov. 2012. <http://symmetry.otterbein.edu/>.

"Point Group Symmetry Character Tables - Chemistry Online Education." Point Group Symmetry Character Tables - Chemistry Online Education. N.p., n.d. Web. 20 Nov. 2012. <http://www.webqc.org/symmetry.php>.

Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. Upper Saddle River, NJ: Pearson Prentice Hall, 2011. Print. Chemical reactions occur everyday in life, in our body, in the atmosphere, in oceans, etc. Understanding chemical reactions helps scientists understand how things works, such as how photosynthesis is achieved to something as simple as how NaCl (table salt) is made. Several factors effect the rate of chemical reactions such as concentration of reactants, pressure, temperature, catalyst, nature of reactants, orientation of reacting species, surface area, intensity of light, and nature of solvent. In general a chemical reaction will either be exothermic, releasing energy, or endothermic, gaining energy.
Factors affecting the rate of chemical reactions
Concentration
The more concentration of reactants one has the faster it will go, meaning the rate of reaction is directly proportional to the concentration of the reactants. It will go faster because there is a higher probability for the reactants to collide and interact with each other. Equation: rate (r)= c^n, where c=concentration and n=order of the reaction.
Pressure
This mainly affects the rate of reaction in gasses. In is very similar to the concentration that is if you increase the partial pressure of the gasses there is a higher probability for the gas molecules to collide and react.
Temperature
This affects the rate of reaction because the average kinetic energy of the molecules is higher thus there will be a higher frequency of the molecules to collide and react with one another.

Catalyst
Catalyst is a substance that increases the rate of reaction by lowering the activation energy required for the reaction to proceed. It is not consumed or changed throughout the reaction.
Nature of reactants
In general ionic bonds are formed faster than covalent bonds because in covalent compounds there has to be a bond breakage that will slow down the entire reaction, whereas in ionic bond it is more or less a substitution of ions.
Orientation of Reacting species
If the reactants are more complex it will make it difficult and lower the probability for the reactants to collide with the correct orientation for the reaction to occur. Opposing that is simple reactants where the probability for the reactants to collide with the correct orientation is high. Therefore complex reactants will have a slower reaction time than simple reactants because the probability for the reactants to collide in the correct orientation and react is low.

Reference
http://www.adichemistry.com/physical/kinetics/factors/factors-affecting-rate-reaction.html When studying Chemistry, it is important to consider both the chemical properties of the reactants and the conditions under which the reaction occurs, the mechanism by which it takes place, the rate at which it occurs and the equilibrium toward which it proceeds. The law of mass action states that the rate of a chemical reaction at a constant temperature depends only on the concentrations of the substances that influence the rate, which are usually one or more of the reactants, but can occasionally be a product. Another influence on the rate can be caused by the presence of a catalyst that does not appear in the balances chemical equation. The rate law can only be experimentally determined and can be used to predict the relationship between the rate of a reaction and the concentrations of reactants.

Rate Equation

For almost all forward and irreversible reactions, the rate is proportional to the product of the concentrations of the reactants, each raised to some degree. For example, for the reaction aA+bB-> cC +dD, the rate is proportional to [A]^m[B]^n, that is: rate = k[A]^m[B]^n The k is the rate constant. Multiplying the units of k by the concentration factors raised to the appropriate powers give the rate in units of: concentration/time. The proportionality factor, k called the rate constant, is a constant at a certain temperature. There are dimensions in the constant and it can be easily determined by using dimensional analysis of a particular rate law.. It is commonly known that as the value of the k value increases, the reaction speed increases.

Rate Law Determination through Experiment

The value of k (constant), x and y in the rate law equation (rate= [A]^m[B]n) must be determined experimentally for a given reaction at given temperature. the rate is measured as a function of the initial concentrations of the reactants A and B.

Order or Reactions

Chemical reactions are often classified on the basis of kinetics as zero-order, first-order, second-order, mixed order, or higher-order reactions. The general reaction aA + bB → cC + dD will be used in the discussion next. First lets note what each of these orders means in terms of initial rate of reaction effect:

1. Zero-order in the reactant—there is no effect on the initial rate of reaction
2. First-order in the reactant—there initial rate of reaction doubles
3. Second order in the reactant—the initial rate of the reaction quadruples
4. Third order in the reactant—the initial rate of reaction increases eightfold

The equation of rate law

$-r_{A}=k_{A}(T)fn(C_{A},C_{B},...)$

$k_{A}$ is the specific rate of reaction, which also called the rate constant.

Generally, four reactions are involved with rate laws. The homogeneous reactions include only one phase. The Heterogeneous reactions include two or more phases, and usually appear at the interface between the phases. The reversible reactions depend on the concentrations of reactants and products that can react in both directions. The irreversible reactions react in only one direction until one of the reactants is exhausted.

Generally, there are four types of reaction order. The most common form is

$-r_{A}=k_{A}C_{A}^{a}C_{B}^{b}$

The units of the specific reaction rate constant are

$k=(Concentration)^{1-n}/Time$

The four types of reaction order:

• Zero-order (n=0): $-r_{A}=k_{A}$

The unit of k of zero-order is $mol/dm^{3}/s$

• First-order (n=1): $-r_{A}=k_{A}C_{A}$

The unit of k of first-order is $s^{-1}$

• Second-order (n=2): $-r_{A}=k_{A}C_{A}^{2}$

The unit of k of second-order is $dm^{3}/mol/s$

• Third-order (n=3): $-r_{A}=k_{A}C_{A}^{3}$

The unit of k of third-order is $(dm^{3}/mol)^{2}/s$

References

Fogler, H. Scott. "Chapter 3." Essentials of Chemical Reaction Engineering. Upper Saddle River, NJ: Prentice Hall, 2011. N. pag. Print.

Background

Product ratios of reactions can be changed according to the conditions in which the reaction occurs. Depending on the kinetic and thermodynamic control of the reaction, product ratios can vary dramatically. An example of this is the hydrobromination of 1,3-butadiene at 40^oC v.s 0^oC. At 0^oC, the reaction is under kinetic control and the products favor 3-Bromo-1-butene over 1-Bromo-2-butene in a 70:30 mixture. However at 40^oC, the reaction is under thermodynamic control and favors the 1-Bromo-2-butene product with a 15:85 ratio.

Chemical reactions can be controlled by thermodynamic or kinetic means. When a reaction can yield two different products, the reaction conditions change the outcome and therefore the products. When conditions of a reaction changes, the reaction itself may change. Some conditions to be considered are temperature, pressure, light, or even solvent. The difference between thermodynamic and kinetic conditions is that thermodynamic factors associate with the relative stability of the molecules while the kinetic factors are associated with the rate of product formation. This phenomena of different products can only occur when there are two different paths with two different activation energies or energy required to make the reaction progress. In essence, when temperature is low the reaction undergoes kinetic control where the major product is the one that comes from the quickest reaction. When temperature is high, the reaction undergoes thermodynamic control which provides the more stable system and product. Under kinetic control the reaction is rate based and is not reversible, but under thermodynamic control the reaction is in equilibrium and is also reversible under the correct conditions.

Thermodynamic Control

When the product ratio of a reaction depends upon the thermodynamic stability of the products, a reaction is said to be under thermodynamic control. In the hydrobromination of 1,3-Butadiene at 40^oC, the ratio of the products favors the more stable isomer, as it is under thermodynamic control. At higher temperatures or over long periods of time, the more stable isomer will dominate as the two isomers will be in equilibrium and rapidly interconvert.

Thermodynamic control of a reaction favors the more thermodynamically stable molecule. This reaction pathway is selected for by using long reaction times or high temperatures. Long reaction times or high temperatures will give the molecules more energy to surpass the lower kinetic pathway to eventually make its way over to the more stable thermodynamic product. Even though molecules have already taken the kinetic pathway, with enough time or high enough temperatures, the molecules will have the energy to go back and overcome the higher activation energy in order to rest in the more stable state or product. Essentially, with enough time or temperature molecules will select for the thermodynamically controlled pathway and end as the more stable product.

Kinetic Control

A product ratio that follows the relative rates of formation, such as the hydrobromination of 1,3-butadiene at 0^oC, is said to be under kinetic control. The reason 3-Bromo-1-butene dominates is because at 0^oC, the reverse formation is much slower than at 40^oC so the isomers are not rapidly inter-converting.

Kinetic control favors shorter reaction times. This is also the path that requires less energy or has a lower activation energy. This is because at the start of a fast reaction more molecules will go towards the kinetic lower energy path and if stopped short, the molecules will not have a chance to return and take the thermodynamic path. Also, low temperature favors the kinetic pathway because it does not give the molecules that already took the kinetic path enough energy to return back to the starting material and then take the thermodynamic path. The way to get the kinetic product is to use short times for reactions to trap the kinetic product or use high temperatures.

References

Types of chemical reactions

Synthesis

In a synthesis reaction, two or more reactants combine to form one complex product. The general equation to represent this reaction is: A + B→AB

Decomposition

The decomposition reaction is the opposite of the synthesis reaction. A complex compound breaks down into simpler substances. The general equation to represent this reaction is: AB→ A + B

Dissociation

Dissociation looks like and can be mistaken for decomposition but they are very different reactions. When a compound dissociates, it breaks down into ions instead of atoms. The products are therefore charged. The general equation looks like: AB→ A⁺ + B^-

Single Replacement

A non-combined element replaces another in a compound and the element that it replaces has to be of the same group (e.g.metal replaces metal). The general equation: A + BC→ AC + B

Double Replacement

In a double replacement, two compounds each trade one element of the same type over to the other. The general equation: AB + CD→ AC + BD

Combustion

In a combustion reaction, a compound is combined with oxygen to produce carbon dioxide and water as products. Example: C_nH_x + O₂→CO₂ + H₂O

Acid-Base

The acid base reaction is a type of double replacement reaction. When the acid and base react, the H⁺ ion reacts with the OH^- ion to form H₂O (water). The products of this reaction are typically water and an ionic salt. HA + BOH→ H₂O + BA

References

1. "Types of Reactions." ThinkQuest. Oracle Foundation, n.d. Web. 20 Nov. 2012. <http://library.thinkquest.org/2923/react.html>.

2. "TheSix Types of Chemical Reaction." The Six Types of Chemical Reaction. N.p., n.d. Web. 20 Nov. 2012. <http://misterguch.brinkster.net/6typesofchemicalrxn.html>. Molecular Geometry

Definitions

Molecular geometry is the 3-dimensional shape that a molecule occupies in space. It is determined by the central atom and the surrounding atoms and electron pairs. The shape of most molecules can be predicted using the Valence Shell Electron Pair Repulsion (VSEPR) method. This method states a few rules to help one determine the shape of a substance without using high technology methods such as X-ray crystallography, NMR Spectroscopy, or electron microscopy. Some of the most common shapes that can be taken are linear, trigonal planar, tetrahedral, pyramidal, and angular (or bent).

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory Electron pairs around a central atom arrange themselves so that they can be as far apart as possible from each other. The valence shell is the outermost electron-occupied shell of an atom that holds the electrons involved in bonding. In a covalent bond, a pair of electrons is shared between two atoms. In a polyatomic molecule, several atoms are bonded to a central atom using two or more electron pairs. The repulsion between negatively charged electron pairs in bonds or as lone pairs causes them to spread apart as much as possible. The idea of "electron pair repulsion can be demonstrated by tying several inflated balloons together at their necks. Each balloon represents an electron pair. The balloons will try to minimize the crowding and will spread as far apart as possible. According to VSEPR theory, molecular geometry can be predicted by starting with the electron pair geometry about the central atom and adding atoms to some or all of the electron pairs. This model produces good agreement with experimental determinations for simple molecules. With this model in mind, the molecular geometry can be determined in a systematic way. Molecules can then be divided into two groups: Group 1: Molecules with NO lone electron pairs. In this case the molecular geometry is identical to the electron pair geometry. Group 2: Molecules with one or more lone electron pairs. In this case an extra step is needed to to translate from electron pair geometry to the final molecular geometry, since only the positions of bonded atoms are considered in molecular geometry.

The VSEPR theory says that:

1) The electron pairs and atoms around a central atom will give a molecule its shape.

2) Electron pairs around a central atom in a molecule will attempt to be as far away from each other as possible.

3) Lone pairs around the central atom will have an effect on the shape of the entire molecule, just like an atom would.

4) Lone pairs of electrons repel each other more strongly than a lone pair and a shared pair, which in turn repel each other more than two shared pairs (or bonds between the central atom and another atom).

These are just a few of the basic postulates of the VSEPR theory that, along with the lewis dot structure of a molecule, allow us to determine the shape it will have.

VSEPR geometries

Ring Molecular Geometry

Many organic compounds contain rings of carbon atoms or other atoms such as oxygen or nitrogen. The simplest ring compound contains 3 carbons as in cyclopropane. The most common ring compounds contain either 5 or 6 carbons. These compounds are also called cyclic.

Bent Molecular Geometry

The molecule that is made up of 4 equally spaced sp3 hybrid orbitals forming bond angles of approximately 109.5o. The shape of the orbitals is tetrahedral. Two of the orbitals contain lone pairs of electrons. The two atoms connected to the central atom form a molecule with a bent shape.

Hybridization and Geometry

There is a simple relationship between the hybridization of an atom and its geometry, i.e., the way or ways in which other atoms can be disposed about it in space. This module examines the allowed bonding patterns for the representative elements B, C, N, O, and F, as well as their geometries as a function of their hybridization and formal charge.

Finding Hybridization For Hybridization of molecules: 1. Count total number of valence Electrons 2. Divide by 8, if no of e- > 8. Divide by 2, if no of e- ≤ 8.

File:Hybridization and molecular shape, png

Hybridization and Molecular Shapes

Polarity and Geometry

The geometry of a molecule determines many properties such as polarity, reactivity, state, and much more. The polarity is directly related to the shape because many molecules with polar bonds can be non polar as a whole due to symmetry of the molecule that has equal electron pull outwards or inward. One such example is carbon dioxide. The electronegative oxygens on either side of the carbon pull atoms toward them selves, cause a partial negative charge on each end, while the less electronegative carbon is left with a partial positive charge. The oxygens are both double bonded to the carbon leaving no lone pairs to bend the molecule, which is linear and has 180 degree bond angles. The equal pull of both partial negative charges balance each other out because they are of equal magnitude and exact opposite directions and the whole molecule is non polar. .

CO2

On the other hand, a molecule can have polar bonds and a different shape that does not balance out the electronegative pull. For example, the hydrogen dioxide molecule, which also contains three atoms but has a different shape. Water is tetrahedral and bent because it has two extra electron pairs which push the hydrogen atoms into a 109.5 degree angle. As a result, the partial negative charge on oxygen is balanced on each side, but is pointed up and inwards, giving the whole molecule polar properties.

Water with 4 single electrons

The polarity in turn then affects many properties itself, such as melting point, boiling point, vapor pressure, solubility and the types of bonds that can be formed, just to name a few. Polar molecules will have higher melting and boiling points and lower vapor pressure due to the attraction between the molecules. Polar molecules also tend to dissolve other polar molecules while non polar molecules also dissolve other non polar substances. This is often referred to as "like dissolves like" as a quick way to remember this property.

Reactivity and Geometry

Another property determined by molecular geometry is reactivity. The reactivity is affected in many ways. One example is the type of bonds the molecule makes based on polarity. For example, some polar molecule can react in hydrogen bonds while non polar molecules react in london dispersion forces. But there are other ways that the geometry directly affects sections that a molecule partakes in. Large bulky molecules often react using the unhindered side, even if it is not as favorable of a product, because of the easier reaction. Another example is enzymes and the way the "fit" certain substrates based on their own sake and the shape of the substrates. These are just a couple examples of the many ways that molecular shape can help determine reactivity.

Whole Enzyme

Determination of Geometry

Although VSEPR and a few similar methods can be used to determine the geometry of a molecule, there are high tech methods that can show us the shape of much larger molecules, usually proteins, without a doubt. These methods include X-ray crystallography, NMR Spectroscopy, and electron microscopy. X-ray crystallography is one of the best methods because of the very high resolution that is a result of the wavelength, which is very close to that of the bonds between the atoms.

1. X-ray Crystallography

The first step in this method is to turn the protein into a crystal, which is done by adding certain salts that crystalize the macromolecule. Once it is in its crystal form, an X-ray beam is aimed at the protein and what happens to the light is observed. Some light will be absorbed, some will be scattered, and some will be diffracted. All of the rays are studied closely and their information, such as amplitude for example, give biochemists information about the atom in the molecule.

X-ray crystallography

2. Nuclear Magnetic Resonance Spectroscopy

Another technique, called Nuclear Magnetic Resonance Spectroscopy, allows protein structure to be studied without being first converted into a crystal. It is one of the very few methods that allows the substance to be studied in solution, which is usually CDCl3. The two types of NMRs commonly used are proton NMR and carbon NMR, due to the magnetic resonance of the elements. For H-NMR, when a molecule is exposed to a magnetic and proton get excited and line up, four things can be observed that tell us about the environment of each identical group of protons. The chemical shift (how far downfield the signal is) tells what functional groups are surrounding the proton(s). The signal integration gives us a ratio that compares the amount of protons in each family. The splitting(amount of peaks in one signal, such as a doublet, triplet, etc) tells us how many neighboring hydrogens there are. And finally, the number of signals itself tells us how many different families of protons there are. C-NMR works the same way except for integration, which is not always clear as it is in H-NMR. So all this information is gathered for a molecule and its exact connectivity can usually be determined, even if it was an unknown molecule.

Espectro NMR C-13

3. Electron Microscopy

The third method is electron microscopy, which uses uses an electron microscope that conducts an electron ray. The electron beams are aimed at the macromolecules and then impose an image of the molecule and its structure upon fluorescent or photographic film for us to see. Sometimes they are even sent directly to a screen, such as a computer screen. This image is a black a white picture but is still very useful in determining structure.

Electron microscopy

Sources

http://chem-guide.blogspot.com/2010/04/valence-shell-electron-pair-repulsion.html

http://www.rcsb.org/pdb/101/static101.do?p=education_discussion/Looking-at-Structures/methods.html

Biochemistry 6th Edition by Berg, Tymoczko, and Stryer

http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Molecular_Geometry

Periodic trends are specific patterns that are present in the periodic table, which illustrate different aspects of a certain element, including its size and its properties with electrons. When comparing the properties of the chemical elements, recurring ('periodic') trends are apparent. This led to the creation of the periodic table as a useful way to display the elements and rationalize their behavior. When laid out in tabular form, many trends in properties can be observed to increase or decrease as one progresses along a row or column.

These period trends can be explained by theories of atomic structure. The elements are laid out in order of increasing atomic number, which represents increasing positive charge in the atomic nucleus. Negative electrons are arranged in orbitals around the nucleus; recurring properties are due to recurring configurations of these electrons.

The main periodic trends include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, and metallic character. The periodic trends that arise from the arrangement of the periodic table provide chemists with an invaluable tool to quickly predict an element's properties. These trends exist because of the similar atomic structure of the elements within their respective group families or period and the periodic nature of the elements.

Reference
http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Table_of_the_Elements/Periodic_Trends

Sizes of Atoms

Definition

The atomic radius is defined as the measured length from the atomic nucleus to the electron orbital that is in the outermost shell of the atom. Typically in practice, this length is measured by obtaining the distance between identical, adjacent atomic nuclei of the sample element, and this distance is then divided by half. In many cases, the size of the atom of a compound varies depending on the substances that are near it, hence it can be said that the atomic radius can vary slightly depending on what is around it.

The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value. Atomic radius may refer to the ionic radius, covalent radius, metallic radius, or van der Waals radius. In all cases, the size of the atom is dependent on how far out the electrons extend. The atomic radius for an element tends to increase as one goes down an element group. The electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius actually may decrease.

Ionic radii are difficult to measure with any degree of certainty, and vary according to the environment of the ion. For example, it matters what the co-ordination of the ion is (how many oppositely charged ions are touching it), and what those ions are.

There are several different measures of ionic radii in use, and these all differ from each other by varying amounts. It means that if we are going to make reliable comparisons using ionic radii, they have to come from the same source.

Trends

Across the Period

The atomic radius usually decreases in a period from left to right, due to the fact that nuclear charge increases, which then attracts the electrons that are orbiting and pulls them closer to the nucleus.

Across the Group

The atomic radius usually increases in a group from top to bottom, due to the fact that an additional energy level is added to the shell, which increases the n value.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Definition

Electron affinity is the energy change or gain that is accompanies an atom when an electron is added to it. When a neutral chlorine atom in the gaseous form picks up an electron to form a Cl- ion, it releases an energy of 349 kJ/mol or 3.6 eV/atom. It is said to have an electron affinity of -349 kJ/mol and this large number indicates that it forms a stable negative ion. Small numbers indicate that a less stable negative ion is formed. Groups VIA and VIIA in the periodic table have the largest electron affinities. Alkali earth elements (Group IIA) and noble gases (Group VIIIA) do not form stable negative ions.

The sign of the electron affinity is associated with the potential energy that is created with the addition of the electron. If the addition of the electron were to create stability within the atom, it is considered to have higher electron affinity, which decreases the potential energy and this energy gained is negative. If the addition of the electron were to make the atom less stable, the potential energy increases, and the energy gained is found to be negative.

Electron affinity is essentially the opposite of the ionization energy. Instead of removing an electron from the element, we add an electron to the element to create an anion. Also, the noble gases, alkali metals and alkali earth metals have electron affinity.

Trends

Across the Period

Typically, electron affinity increases from left to right across the period, but it is definitely not a regular or steady increase. Factors such as charge and the atomic size both affect electron affinities, hence the trend across the period is not as regular as the other periodic table trends.

Across the Group

Electron affinity tends to decrease down a group, but as mentioned previously, it is not a regular trend, and there are several exceptions to this rule of thumb down the group.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Electronegativity

Electronegativity increases going up the periodic table and to the right of it. Meaning the most electronegative atom is Fluorine and the least electronegative atom is Cesium. What electronegativity means is the ability of an atom to attract electrons. For example, in a H-Cl bond, Cl is more electronegative than H. Therefore, the electrons will be closer to Cl.

Besides, because the nucleus of a smaller atom is closer to the shared pair than that of a larger atom, it attracts the bonding electrons more strongly. The electronegativity is inversely related to atomic size. In generaly, the nonmetals are more electronegativity than the metals. One of the important uses of electronegativity is determining the oxidation number of an atom. The atom's electronegativity refers to its ability to pull bonded electrons towards it. The greater the change in the electronegativity for the two atoms in a bond, the more polar the bond is and also the greater its ionic character.

Electronegativity is the ability of an atom in a molecule to attract a shared electron pair to itself, forming a polar covalent bond. The negative side of a polar covalent bond corresponds to the more electronegative element. In addition, the more polar a bond, the larger the difference in electronegativity of the two atoms.

There is no direct way of measuring electronegativity. Dipole-moment measurements tell us about the electrical behavior of all electron pairs in the molecule, not just the bonding pair in which we are interested. Also, the polarity of a bond depends on whether the bond is a single, double, or triple bond and on what the other atoms and electron pairs in a molecule are. Therefore, the dipole moment cannot tell us quantitatively the difference between the electronegativities of two bonded atoms.

Metals are the least electronegative of the elements.

History

Electronegativity was first introduced by Pauling in the 1930s to describe bond energies. Pauling's calculations of electronegative from bond energies requires averaging over a number of compounds to cancel out experiemental errors. Jaffe used this idea to develop a theory of the electronegativity of orbitals rather than just atoms.

Number	Symbol	Name	Electronegativity
1	H	hydrogen	2.300
2	He	helium	4.160
3	Li	lithium	0.912
4	Be	beryllium	1.576
5	B	boron	2.051
6	C	carbon	2.544
7	N	nitrogen	3.066
8	O	oxygen	3.610
9	F	fluorine	4.193
10	Ne	neon	4.789
11	Na	sodium	0.869
12	Mg	magnesium	1.293
13	Al	aluminium	1.613
14	Si	silicon	1.916
15	P	phosphorus	2.253
16	S	sulfur	2.589
17	Cl	chlorine	2.869
18	Ar	argon	3.242
19	K	potassium	0.734
20	Ca	calcium	1.034
21	Sc	scandium	1.19
22	Ti	titanium	1.38
23	V	vanadium	1.53
24	Cr	chromium	1.65
25	Mn	manganese	1.75
26	Fe	iron	1.80
27	Co	cobalt	1.84
28	Ni	nickel	1.88
29	Cu	copper	1.85
30	Zn	zinc	1.59
31	Ga	gallium	1.756
32	Ge	germanium	1.994
33	As	arsenic	2.211
34	Se	selenium	2.434
35	Br	bromine	2.685
36	Kr	krypton	2.966
37	Rb	rubidium	0.706
38	Sr	strontium	0.963
39	Y	yttrium	1.12
40	Zr	zirconium	1.32
41	Nb	niobium	1.41
42	Mo	molybdenum	1.47
43	Tc	technetium	1.51
44	Ru	ruthenium	1.54
45	Rh	rhodium	1.56
46	Pd	palladium	1.59
47	Ag	silver	1.87
48	Cd	cadmium	1.52
49	In	indium	1.656
50	Sn	tin	1.824
51	Sb	antimony	1.984
52	Te	tellurium	2.158
53	I	iodine	2.359
54	Xe	xenon	2.582
55	Cs	caesium	0.659
56	Ba	barium	0.881
71	Lu	lutetium	1.09
72	Hf	hafnium	1.16
73	Ta	tantalum	1.34
74	W	tungsten	1.47
75	Re	rhenium	1.60
76	Os	osmium	1.65
77	Ir	iridium	1.68
78	Pt	platinum	1.72
79	Au	gold	1.92
80	Hg	mercury	1.76
81	Tl	thallium	1.789
82	Pb	lead	1.854
83	Bi	bismuth	2.01
84	Po	polonium	2.19
85	At	astatine	2.39
86	Rn	radon	2.60
87	Fr	francium	0.67
88	Ra	radium	0.89

References

Miessler, Gary. Inorganic Chemistry. 4th Edition.

Definition

Ionization energy is the minimum energy necessary to remove one mole of electrons from one mole of atoms in the gaseous state. The first ionization energy refers to the energy necessary to rid of one electron, so it can be noted that the "nth" ionization energy is the energy required to rid of the atom's nth electron, after (n-1) electrons before it have been removed. The first ionization energy is a significant factor in chemical reactivity, due to the fact that atoms with low first ionization energies tend to become the cations during reactions, while those with high ionization energies (with noble gases being an exception) tend to convert to anions.

If the ionization energy is high, that means that it takes a lot of energy to remove the outermost electron. If the ionization energy is low, that means that it takes only a small amount of energy to remove the outermost electron.

As the nuclear charge increases, the attraction between the nucleus and the electrons increases and it requires more energy to remove the outermost electron and that means there is a higher ionization energy. As you go across the periodic table, nuclear charge is the most important consideration. So, going across the periodic table, there should be an increase in ionization energy because of the increasing nuclear charge.

Trends

Ionization energies have an inverse relationship with atomic radius. As size decreases for an atom, more energy would be required to remove the atom.

Across the Period

File:Ionization Energy 3.png

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Ionization energies tend to increase across the period, due to the fact that there are a greater number of protons that that gain the attraction to the orbiting electrons more effectively, which would increase the energy necessary to remove an electron.

File:Ionization Energy 2.png

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Across the Group

Ionization energies tend to decrease down a group, due to the fact that it takes less energy to remove an outer-shell electron as opposed to an inner-shell electron. An exception to this pattern occurs in Group 3A (the group after the transition elements) of the periodic table, since it does not increase from aluminum (Al) to thallium (Tl).

As we go down the periodic table, the electrons are further from the nucleus, causing the atoms to be larger and the ionization energies to be lower.

Importance of Ionization Energy and Why it Matters

In most chemical reactions, a clear understanding of ionization energy is crucial in order to comprehend why certain bonds occur and the energy that is associated with them. It also provides for the understanding how of various atoms make covalent or ionic bonds with each other. For example, the alkali metal of sodium has an ionization energy of roughly 5000 kJ/mol. The ionization energy of the chlorine atom is roughly 1200 kJ/mol. When creating the sodium chloride complex, the difference in their ionization energy is so great that chemically they can combine as an ionic bond, the strongest of bonds known to chemistry. However, when you start to see compounds of atoms that have ionization energies that are much closer to each other, then we start seeing weaker and weaker bonds. For example, the ionization energies of both carbon and chloride are fairly close to each other which dictates their bond type as covalent, the second strongest bond in chemistry. The general rule regarding bond energies and bond strengths is that the closer the two elements are on the periodic table, the weaker the bond will be between the two.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Definition

Metals are typically located within the left and lower areas of the periodic table, and they can be defined as solids that have relatively high melting points. In addition, another characteristic of metals is that they are typically shiny and act as good thermal and electrical conductors. Nonmetals, on the other hand are located within the upper right side of the periodic table. Metalloids can be observed in the region between these two classes and hold properties that exemplify traits of both metals and nonmetals.

Metal behavior is described as chemical properties of elements that are metals. These traits are all based on one fact. That fact is the relative ease of metals to lose electrons or to be oxidized. The trends can be explained through chemistry. Across a period, metallic character decreases because atoms more readily accept electrons to fill a valence shell. Down a group, metallic character increases because as the atomic radius becomes larger, the electrons are more easily lost or reduced!

Chemical Properties of Metals: Metals combine with other metals and some non-metallic elements to form a vast number of alloys that enhance the properties of metals in specific applications. For example, the combination of iron, nickel and chromium provides a series of stainless steel alloys that are in common use. Metals such as nickel, vanadium, molybdenum, cobalt, rare earths and the platinum group metals enable the catalytic reactions for the synthesis of many organic chemicals from petroleum. A wide variety of metal compounds and salts impart beneficial properties to products like plastics in terms of colour, brightness, flame resistance and resistance to degradation. Photography has been made possible by the effect of light on metal salts.

Mechanical Properties of Metals: The properties of strength and ductility enable the extensive use of metals in structures and machinery. Metals and alloys exhibit ductility, malleability and the ability to be deformed plastically (that is, without breaking), making them easy to shape into beams (steel beams for construction), extrusions (aluminum frames for doors and windows), coins, metal cans and a variety of fasteners (nails and paper-clips). The strength of metals under pressure (compression), stretching (tensile) and sheer forces makes them ideal for structural purposes in buildings, automobiles, aircraft frames, gas pipelines, bridges, cables, and some sports equipment.

Conductivity of Metals: Metals are excellent conductors of both heat and electricity. In general, conductivity increases with decreasing temperature, so that, at absolute zero (-273°C), conductivity is infinite. To emphasize, metals become superconductors. Thermal conductivity is harnessed in automobile radiators and cooking utensils. Electrical conductivity provides society with the ability to transmit electricity over long distances to provide lights and power in cities remote from electrical generating stations. The circuitry in household appliances, television sets and computers rely on electrical conductivity. Resistance to Wear, Corrosion, Fatigue and Temperature: Metals are hard and durable. They are used in applications sensitive to corrosion such as chemical plants, food preparation, medical applications, plumbing and lead in storage batteries. Wear resistance is critical in bearings for all modes of transportation and in machine tools. Fatigue resistance is the ability to resist breaking after repeated deformation such as bending, which enables the use of metals in springs, levers and gears. Temperature resistance makes metals suitable for jet engines and filaments in light-bulbs. Optical characteristics: Metals are uniformly lustrous and, except for copper and gold, are silvery or greyish. The reason is because all metals absorb light at all frequencies and immediately radiate it. Metals impart mirrors with their reflective surface. The lustre of metals gives them the attractive appearance that is so important in jewellery and coins. Metals provide the intangible, distinctive metallic ring that is associated with coins.

Magnetic Properties: Ferromagnetism is exhibited by iron and several other metals. In addition, other metals and alloys can be magnetized in an electrical field to exhibit paramagnetism. Magnetic properties are employed in electric motors, generators, and speaker systems for audio equipment. Emission Properties: Metals emit electrons when exposed to radiation (for example: light) of a short wavelength or when heated to sufficiently high temperatures. These phenomena are exploited in television screens, using rare earth oxides and in a variety of electronic devices and instruments. Conversely, the ability of metals such as lead to absorb radiation is employed in shielding. For instance, in the apron provided by dentists during an X-ray examination.

Trends

Across the Period

Metallic behavior tends to decrease from the left of the period to the right, due to the increasing number of valence electrons and the decreasing atomic radius.

Across the Group

Metallic behavior tends to increase from top to bottom, due to the increase in the number of electron shells and atomic radius.

Coordination Compounds

The interpretation of spectra of coordination compounds is important in identifying the lowest energy form. First, the energy levels are sketched to show the d electrons. Then the spin multiplicity of the lowest-energy state is equal to the number of unpaired electrons. The maximum possible M1 values for the configuration are determined, which determines the ground term. The selection rules are stated as: 1) The bonds in transition metal complexes vibrate so that they may temporarily change their geometry. This is called vibronic coupling, which provide a vibration that distorts the central atom. This is called the Laporte selection rule. 2) Tetrahedral complexes often absorb more strongly than octahedral compleses of the same metal. The sigma bonding in transition metal complexes can be described as a mixing of p-orbital character. 3) Spin-orbit coupling provides a mechanism for the second selection rule, which states that transitions between states of different spins are not allowed. This is called the spin selection rule.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Miessler, Gary. Inorganic Chemistry. 4th Edition.

Melting Point

Melting point is defined as the temperature at which the bond within a solid collapse and the solid is converted into a liquid. The stronger the force of attraction between molecules or atoms, the higher the melting point will be, because more energy is required to break these bonds and change the state of the molecule.

Since melting points are varied across the periodic table, there is not a distinguishable trend.

1. As the atomic number of elements increases, the melting point mostly increases because there are more electrons around the nucleus, which creates stronger intermolecular forces. With stronger forces, the melting point rises, as it takes more energy to break these forces. But the melting point also varies according to the type of structure, with giant covalent or metallic structures having a higher melting point than simple molecular structures.

2. Metals usually have high melting point due to the metallic bonding. This is an electrostatic attraction between positive ions and delocalised electrons (strong bonds).

3. Non-metals usually have low melting points, as they normally have simple molecular structures.

Chart of Melting Points of Elements

The melting point of a molecule or compound is a very important characteristic. Many times we can determine that identity of a compound or molecule using just this characteristics. This would be called melting point determination where a compound is slowly heated to find the temperature at which it melts. The unknown compound's melting temperature is compared to other known melting points, where it can be either extrapolated that it is the correct compound or a different one. In order to further investigate, a mixed melting point determination is done. This is done by mixing two compounds and testing the resulting melting point. If the compounds are different then the melting point will always lower. If the two compounds are identical the melting point stays the same. The use of a melting point apparatus and care is needed for this. This experiment shows the importance of melting point characteristic in chemistry. Not only as a characteristic, but also as a means to identifying unknown compounds.

Examples

Melting points and boiling points of the first eight carboxylic acids (°C) For most substances, melting and freezing points are approximately equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvin (−38.83 °C or −37.89 °F). However, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F, 273.15 K); this is also known as the ice point. In the presence of nucleating substances the freezing point of water is the same as the melting point, but in the absence of nucleators water can supercool to −42 °C (−43.6 °F, 231 K) before freezing. The chemical element with the highest melting point is tungsten, at 3683 K (3410 °C, 6170 °F) making it excellent for use as filaments in light bulbs. The often-cited carbon does not melt at ambient pressure but sublimes at about 4000 K; a liquid phase only exists above pressures of 10 MPa and estimated 4300–4700 K. Tantalum hafnium carbide (Ta4HfC5) is a refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F). At the other end of the scale, helium does not freeze at all at normal pressure, even at temperatures very close to absolute zero; pressures over 20 times normal atmospheric pressure are necessary.

What is Medicinal Chemistry

Medicinal chemistry is a field in which scientists use chemistry techniques to develop pharmaceuticals for use in medicine. In the beginning, medicinal chemists wanted to be able to extraction useful molecules from plants. Today, scientists want to create analogs of molecules found in plants and synthesize new products that promote high potency levels and good pharmokinetics. These new synthetic drugs are created through new organic chemistry mechanisms that involve molecular biology and biochemistry. Medicinal chemistry focuses on drug development and discovery.

History of Medicinal Chemistry

Medicinal chemistry is the application of chemical research techniques to the synthesis of pharmaceuticals. During the early stages of medicinal chemistry development, scientists were primarily concerned with the isolation of medicinal agents found in plants. Today, scientists in this field are also equally concerned with the creation of new synthetic drug compounds. Medicinal chemistry is almost always geared toward drug discovery and development.

Carrying Out Basic Research

Medicinal chemistry research is an important area of research that is performed in many university labs. As an assistant professor at the University of Maryland School of Pharmacy, Alex Mackerell, Jr. has done research on cocaine and cocaine analogs to develop drugs for the treatment of cocaine addiction. His research, however, was not solely focused on just getting a product, but also on understanding basic chemical reactions and their properties. "We were interested in the physical properties and in the underlying mechanisms of cocaine," he says. The purpose of the research was to develop a cocaine antagonist that would cause ill effects when cocaine is ingested. This type of research characterizes the research being conducted in academic environments.

Drug discovery method

Medicinal chemists use hits to find molecules in which they want to synthesive and develop. Hits are generated from assays that contains the activity that the scientist wants to promote. They come from various places such as, investigation of molecules in pathology, from plants, fungi and other natural products. The way hits are created is from the structural interactions of the molecules with the receptors in the body and each hit that is created has different properties. Scientist use these properties to either mimic or inhibit such responses. After hits are generated, scientist then create many compounds. From these newly created compounds, scientists test the characteristics of the compounds to find out the potency and if it continues to provide what the scientist is looking for in terms of usefulness. After this is done, usually the scientist narrows down the hit and searches for an optimized molecule that they would like to continue to develop. This molecule is then given many studies and is implemented on a large scale to test, in-vitro, in-vivo and finally ran into clinical trials on animals and eventually on to humans. During these tests, scientist look for side-effects and effects on the animals before moving onto human clinical trials. Each process is arduous and roughly takes anywhere from 10-15 years on average.

References

http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_ARTICLEMAIN&node_id=1188&content_id=CTP_003395&use_sec=true&sec_url_var=region1&__uuid=e5325b42-1f6a-4334-bba8-88cd2e224f3e

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A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions. In addition to their microscopic structure, large crystals are usually identifiable by their macroscopic geometrical shape, consisting of flat faces with specific, characteristic orientations.

The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.

Examples of large crystals include snowflakes, diamonds, and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics.

Crystal structure (microscopic)

Halite (table salt, NaCl): Microscopic and macroscopic

Microscopic structure of a halite crystal. (Purple is sodium ion, green is chlorine ion.) There is cubic symmetry in the atoms' arrangement.

Macroscopic (~16cm) halite crystal. The right-angles between crystal faces are due to the cubic symmetry of the atoms' arrangement.

The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement. (Quasicrystals are an exception, see below.)

Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals (called "crystallites" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries. Most macroscopic inorganic solids are polycrystalline, including almost all metals, ceramics, ice, rocks, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids, also called glassy, vitreous, or noncrystalline. These have no periodic order, even microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does.

A crystal structure (an arrangement of atoms in a crystal) is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal.

The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups. These are grouped into 7 crystal systems, such as cubic crystal system (where the crystals may form cubes or rectangular boxes, such as halite shown at right) or hexagonal crystal system (where the crystals may form hexagons, such as ordinary water ice).

Crystal faces and shapes

As a halite crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many dangling bonds. Therefore these parts of the crystal grow out very quickly (yellow arrows). Eventually, the whole surface consists of smooth, stable faces, where new atoms cannot as easily attach themselves.

Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see.

Euhedral crystals are those with obvious, well-formed flat faces. Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid.

The flat faces (also called facets) of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index. This occurs because some surface orientations are more stable than others (lower surface energy). As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. (See diagram on right.)

One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying crystal symmetry.

A crystal's habit is its visible external shape. This is determined by the crystal structure (which restricts the possible facet orientations), the specific crystal chemistry and bonding (which may favor some facet types over others), and the conditions under which the crystal formed.

Occurrence in nature

Ice crystals.

Fossil shell with calcite crystals.

Rocks By volume and weight, the largest concentrations of crystals in the earth are part of the Earth's solid bedrock.

Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock. The vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized; but many kinds of lava were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous or glassy matter is common. Other crystalline rocks, the metamorphic rocks such as marbles, mica-schists and quartzites, are recrystallized. This means that they were at first fragmental rocks like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution, but the high temperature and pressure conditions of metamorphism have acted on them by erasing their original structures and inducing recrystallization in the solid state.

Other rock crystals have formed out of precipitation from fluids, commonly water, to form druses or quartz veins. The evaporites such as halite, gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in arid climates.

Ice Water-based ice in the form of snow, sea ice and glaciers is a very common manifestation of crystalline or polycrystalline matter on Earth. A single snowflake is typically a single crystal, while an ice cube is a polycrystal.

Organigenic crystals

Many living organisms are able to produce crystals, for example calcite and aragonite in the case of most molluscs or hydroxylapatite in the case of vertebrates.

Introduction

Crystalline solid material is one of the two major types of solid materials. Another is amorphous solid. Generally, crystalline solids are composed of atoms or ions. These atoms, ions, or molecules pack in regular geometric arrays called unit cell. The unit cell is a structural component that, when repeated in all directions, result in a macroscopic crystal.

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Simple structures

Seven crystal systems are listed in the following table, which can describe all crystals.

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The positions of atoms, ions or molecules are described in lattice points in unit cell. By putting points at the corners of the unit cell, the seven crystal systems can be obtained.

However, there are other arrangements of points. In 1848, Bravais, a French crystallographer, demonstrated that there are 14 point lattices. These 14 possible crystal structures are called Bravais lattices showed in the following table.

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Structures of binary compounds

Binary compounds, containing two elements, can be described in seven different specific crystal structures.

1. Sodium chiloride, NaCl (rock salt)

NaCl: FCC Cl with Na in all octahedral sites. There are 4 Na and 4 Cl in each unit cell. Each unit cell has 4 formula units of NaCl structure.

2. Cesium chiloride, CsCl

CsCl: simple cubic Cl with Cs in all cubic sites. There are 1 Cs and 1 Cl in each unit cell. Each unit cell has 1 formula unit of CsCl structure.

3. Zinc blende, ZnS

Zinc blend: FCC S with Zn in half tetrahedral sites. There are 4 Zn and 4 S in each unit cell. Each unit cell has 4 formula units of ZnS structure.

4. Wurtzite, Zns

Wurtzite: HCP S with Zn in half tetrahedral sites. Both Zn and S are in tetrahedral hole of the other lattice. Each type of ion forms a hexagonal closed=packed lattice. There are 4 Zn and 4 S in each unit cell. Each unit cell has 4 formula units of wurtzite structure.

5. Fluorite, CaF2

Fluorite: FCC Ca with F in all tetrahedral sites. There are 4 Ca and 8 F in each unit cell. Each unit cell has 4 formula units of fluorite structure.

6. Nickel arsenide, NiAs

NiAs: HCP As with Ni in all octahedral sites. There are two Ni and 2 As in each unit cell. Each unit cell has 2 formula units of NiAs structure.

7. Rutile, TiO2

Rutile has a primitive tetragonal unit cell. The coordination number for Ti is 6, for O is 3. There are two Ti and four O in each unit cell. Each unit cell has 2 formula units of TiO2 structure.

Reference

Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. Upper Saddle River, NJ: Pearson Prentice Hall, 2011. Print.

General chemistry (sometimes called "gen chem" for short) is a course often taught at the high school and introductory university level. It is intended to serve as a broad introduction to a variety of concepts in chemistry and is widely taught. At the university level, it is also sometimes used as a "weed out" course for disciplines (sometimes related, sometimes not) which are perceived to require a high level of intellectual rigor or large course loads. It is also one of the few chemistry courses in most universities that does not explicitly explore a particular discipline such as organic chemistry or analytical chemistry.

General chemistry courses typically introduce concepts such as stoichiometry, prediction of reaction products, thermodynamics, nuclear chemistry, electrochemistry, chemical kinetics, and many of the rudiments of physical chemistry. Though the list of subjects covered is typically broad, leading some to criticize both the class and the discipline as encouraging memorization, most general chemistry courses are firmly grounded in several fundamental physical rules for which the primary challenge is understanding when the rules are applicable.

Concepts Taught

1. Properties of Matter: An explanation of the most fundamental concept in chemistry: matter.
2. Atomic Structure: While technically in the domain of physics, atoms determine the behavior of matter, making them a necessary starting point for any discussion of chemistry.
3. Compounds and Bonding: Chemical bonding is introduced, which explains how less than one hundred naturally-occurring elements can combine to form all the different compounds that fill our world.
4. Chemical Reactions: Things get interesting once chemical reactions start making and breaking bonds.
5. Aqueous Solutions: Substances dissolved in water have special properties. This is when acids and bases are introduced.
6. Phases of Matter: A detailed look at the organization of substances, with particular focus on gases.
7. Chemical Equilibria: Chemical reactions don't go on forever. Equilibrium is the balance that reactions seek to achieve.
8. Chemical Kinetics: Kinetics explain why it takes years for an iron nail to rust, but only a split second for a hydrogen-filled hot air balloon to explode.
9. Thermodynamics: Two things decide which reactions can occur and which reactions cannot: heat and chaos. Or enthalpy and entropy, as they are called in thermodynamics
10. Chemistries of Various Elements: An exploration of the elements that make up all substance. Includes an introduction to nuclear chemistry and carbon, the essence of organic chemistry.

Overview

Significant Figures are the significant number of digits used to take measurements or used in calculations. Most digits are actually significant. Zero is the only digit that is not significant. They are only used to position the decimal points.

There are three steps needed to satisfy significant figures.
1. The measured quantity must have a decimal point
2. Start left, then move right to the first nonzero digit
3. That digit must be counted and every digit to its right is considered significant

Complication

Zeros that end a number & lie either before or after the decimal points are considered significant.

Examples of Significant Figures

4 significant figures
5.080 kM
9820. kM
5.080 x 10^3 M

3 significant figures
3.00 x 10^2 mL
900. mL
0.300 L

2 significant figures
9800 mL
5.90 x 10^3 L

1 significant figure
500 mL
4 L

Significant Figures in Calculations

Many times there are too many significant figures, so we round off the answer to get the proper number of them.
The least measurement usually sets the limit for the entire calculation which determines the number of significant figures in the final answer.

Significant Figures in Arithmetic Operations

1. Multiplication & Division
The answer has the same amount of significant figures as the measurement with the fewest significant figures.

Example: 2 mL x 4.3 mL x 22.29 mL = 2 x 10^2 mL
2000. mL / 10. = 20. X 10^1 mL

2. Addition & Subtraction
The answer has the same amount of significant figures as there are in the measurement with the fewest decimal place.

Example: 22.2 mL + 22.22 mL = 44. 4 mL
23.2 mL – 10.00 mL = 13.2 mL

Rounding Off Rules

1. If the number is >5, the next number increases by 1.
986.567 rounds to 986.57 for 5 significant figures or 985.6 for 4 significant figures.

2. If the number is <5, the next number stays the same.
.2921 rounds to .292 for 3 significant figures or .29 for 2 significant figures.

3. If the number is =5, the next number increase by 1 if it is odd and stays the same if it is even.
18.75 rounds to 18.8, but 18.65 rounds to 18.6

4. One or more additional significant figures must be carried throughout calculations. Only final answer should be rounded.

Exact Numbers

Exact numbers have no uncertainty and many times are constants.
They do not limit the number of significant figures in the answer.
Many times the exact number will have as many significant figures as the calculation is required.

Example: 48 hours in 1 day. 60 minutes in 1 hour. 60 seconds in 1 minute. 26 letters in the English alphabet.

Resources

Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Properties of Matter

Matter is…
consisted of “stuff”
anything that has mass and volume

Example:
Cups, Identification Cards, Humans, Sweaters, Air

Composition

Composition is the simpler substances that make up matter.

Substance

Substance has a fixed composition and is a type of matter.

Properties

Properties are the characteristics of each substance.

Physical Properties
Physical property is a substance that is shown by itself. There does not need to change into or interact with other substances. They usually require physical change, which is when the substance alters its physical form and not its composition. They are usually the same before and after any changes.
Water (solid form) -> water (liquid form)

Chemical Properties
Unlike physical property, chemical property is a substance that requires be changing into or interacting with other substances. They usually have chemical changes or chemical reactions. This is when the substance is converted into a different substance.
Water -> hydrogen gas + oxygen gas

3 States of Matter

1. Solid
It has its own fixed shape.
2. Liquid
It conforms to the container; however, it all depends on how much liquid you have in the container.
3. Gas
It also conform the shape and fits to the entire container. It, however, does not form at the surface.

Periodic Table

Atomic Number
The atomic number is the amount of protons present in the nucleus of the atom.
Example: Carbons have 6 protons. Therefore, it’s atomic number is 6.
Oxygen has 8 protons. Therefore, it’s atomic number is 8.

Mass Number
The mass number is the total amount of protons and neutrons in the nucleus of the atom.
Example: Carbon has 6 protons and 6 neutrons. The total number is 12, which is also known as the mass number.

Atomic Symbol
The atomic symbol is the symbol for each element in the periodic table.
Example: Carbon’s atomic symbol is C.

Resources

Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.

Structural Biochemistry/Biological Bases/

Definition of Living Organisms

living organism can be anything which can breath, respond to stimuli, motion or locomotion,reproduce, etc examples are humans,plants,viruses,bacteria etc

Characteristics of Living Organisms

1. High degree of chemical complexity and microscopic organization

Living organisms are composed of cells with highly organized systems, starting from the smallest cellular unit to the largest. Some organisms are unicellular (composed of one cell), free-living and contain organelles to help carry out their life processes. More complex organisms are multicellular (composed of more than one cell). These cells are organized into specialized roles, like tissues or muscles, where they each live co-dependently. Tissues are organized into organs. The organ system in composed of multiple organs that function together. An organism is a complex series of organ systems.

2. Systems to extract, transform and use energy from the environment

Living organisms that convert organic matter from the environment and convert into energy that chemical material inside our cells use in metabolism are called heterotrophs. Other organisms such as algae and plants that use photosynthesis to extract energy from the sun along with carbon dioxide and water to produce sugars are called autotrophs. Certain types of bacteria use chemosynthesis to couple the transformation of organic molecules into from their energy with inorganic molecules. Without this ability to extract, transform and use energy, organisms will quickly become unable to power their life functions.

3. Self-replication and self-assembly

Living organisms have the ability to reproduce. In plants and simple animals, reproduction is through the process of asexual reproduction. Asexual reproduction is defined as a single individual reproducing without the help of another individual. Examples of asexual reproduction include splitting or budding. More complex organisms participate in sexual reproduction, where two individual contribute genetic information to create a new individual with a unique combination of traits.

4. Sensing and responding to changes in environment

Living organism have the ability to respond to stimuli in their environment. Changes may include light, heat, sound or chemical contact. In response to stimuli, an organism uses hormones to change their behavior in response to the surrounding environment.

5. Define function for each component and regulation among them

Living organisms possess specific conditions that they must maintain for survival. The ability to regulate conditions such as temperature, nutrients and water are essential for survival. Homeostasis, a stable balance of internal conditions against external forces is a condition that must be kept.

6. History of evolutionary change

Populations of living organisms adapt to changes in their environment through evolution. As changes occur in the population, the organism adapts, using their abilities to metabolize respond and reproduce. Characteristics that are more useful in survival are passed to offsprings and allows survival for the species.

Growth of Living Organisms

All living organisms grow. An increase in mass and increase in number of individuals are twin characteristics of growth. A multicellular organism grows by cell division. In plants, this growth by cell division occurs continuously throughout their life span. In animals, this growth is seen only up to a certain age. However, cell division occurs in certain tissues to replace lost cells. Unicellular organisms also grow by cell division. One can easily observe this in an invitro cultures by simply counting the number of cells under the microscope. In a majority of higher ordered animals and plants, growth and reproduction are mutually exclusive events. One must remember that increase in body mass is considered as growth. Non-living objects also grow if we take an increase in body mass as a criterion for growth. Mountains, boulders and sand mounds grow, however this kind of growth exhibited by non-living objects is by the accumulation of material on the surface. In living organisms, growth is from inside. Growth, therefore cannot be taken as a defining property of living organisms. Conditions under which growth can be observed in living organisms have to be explained in order to understand that it is a characteristic of living systems. Dead organism do not grow.

Reproduction

Reproduction is a characteristic of living organisms.--- In multicellular organisms, reproduction refers to the production of progeny possessing features more or less similar to those of parents. Invariably and implicitly we refer to sexual reproduction. Organisms reproduce by asexual means also. Fungi multiply and spread easily due to the millions of asexual spores they produce. In lower organisms like yeast and hydra, we observe budding. In Planaria (flat worms), we observe true regeneration, i.e., a fragmented organism regenerates the lost part of its body and becomes, a new organism. The fungi, the filamentous algae, the protonema of mosses, all easily multiply by fragmentation. When it comes to unicellular organisms like bacteria, unicellular algae or Amoeba, reproduction is synonymous with growth, i.e., increase in number of cells. We have already defined growth as equivalent to increase in cell number or mass. Hence, we notice that in single-celled organisms, we are not very clear about the usage of these two terms - growth and reproduction. Further, there are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc). Hence, reproduction also cannot be an all-inclusive defining characteristic of living organisms. Of course, no non-living object is capable of reproducing or replicating by itself.

Metabolism

Another characteristic of life is metabolism. All living organisms are made of chemicals. These chemicals, small and big, belonging to various classes, sizes, functions, etc., are constantly being made and changed into some other biomolecules. These conversions are chemical reactions or metabolic reactions. There are thousands of metabolic reactions occurring simultaneously inside all living organisms, be they unicellular or multicellular. All plants, animals, fungi and microbes exhibit metabolism. The sum total of all the chemical reactions occurring in our body is metabolism. No non-living object exhibits metabolism. Metabolic reactions can be demonstrated outside the body in cell-free systems. An isolated metabolic reaction(s) outside the body of an organism, performed in a test tube is neither living nor non-living. Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things but surely living reactions.

Adaptation of Environments

The most obvious and technically complicated feature of all living organisms is this ability to sense their surroundings or environment and respond to these environmental stimuli which could be physical, chemical or biological. We sense our environment through our sense organs. Plants respond to external factors like light, water, temperature, other organisms, pollutants, etc. All organisms, from the prokaryotes to the most complex eukaryotes can sense and respond to environmental cues. Photoperiod affects reproduction in seasonal breeders, both plants and animals. All organisms handle chemicals entering their bodies. All organisms therefore, are 'aware' of their surroundings. Human beings are the only organisms known to be self aware, i.e., have self-consciousness.

Consciousness

Consciousness therefore, becomes the defining property of living organisms. When it comes to human beings, it is all the more difficult to define the living state. We observe patients lying in coma in hospitals virtually supported by machines which replace heart and lungs. The patient is otherwise brain-dead. The patient has no self-consciousness. Are such patients who never come back to normal life, living or non-living? In higher classes, you will come to know that all living phenomena are due to underlying interactions. Properties of tissues are not present in the constituent cells but arise as a result of interactions among the constituent cells. Similarly, properties of cellular organelles are not present in the molecular constituents of the organelle but arise as a result of interactions among the molecular components comprising the organelle. These interactions result in emergent properties at a higher level of organization. This phenomenon is true in the hierarchy of organizational complexity at all levels.

Self-Replicating, and Self-Regulating

All living organisms are self-replicating, evolving and have self-regulating interactive systems capable of responding to external stimuli. Biology is the story of life on earth and is the story of evolution for living organisms on earth. All living organisms - present, past and future, are linked to one another by the sharing of common genetic material, but to varying degrees of assortments.

References

http://wiki.answers.com/Q/What_are_the_8_characteristics_of_living_organisms_and_describe_each Viadiu, Hector. Chem 114A Lecture Notes 1. Fall, 2012.</ref> http://www.cliffsnotes.com/study_guide/Characteristics-of-Living-Things.topicArticleId-8741,articleId-8578.html</ref> http://www.essortment.com/six-characteristics-life-47733.html http://lisacruz2.tripod.com/id30.html http://www.ehow.com/facts_5535157_living-organisms.html

Retinoid and chemistry of vision

Retinoids and carotenoids are molecules whose chemistry and metabolism in conjunction with specific processing proteins, help explain the chemical basis of vision. Since both retinoids and carotenoids are classified as isoprenoids, they have limited chemical transformation possibilities. Interestingly, because these genes have found to be highly conserved in the formation of insect and vertebrate vision and are involved in chromophore production and recycling, the concept of a common ancestral origin for the chemistry of vision has been proposed and developed.

Summary of chemicals involved in the human visual cycle.

Sensory rhodopsin II (rainbow colored) embedded in a lipid bilayer (heads red and tails blue) with Transducin below it.

Chromophore isomerization

Cycles of cis to trans isomerization of the visual chromophore is an intrinsic mechanism for animal vision.

G protein signaling is a signal transduction pathway where heptahelical transmembrane receptors such as rhodopsin respond to a variety of chemical signals (hormones, neurotransmitters, etc.), activate the heterotrimeric G proteins, and help carry out a cascade of events that are responsible for many physiological processes throughout the body.

It has been discovered that visual pigments make up one class of G protein-coupled receptors and have components such as opsin (integral transmembrane protein) and a covalently attached retinylidene chromophore that is involved in the process of phototransduction.

Visual GPCR signaling requires a diet-derived chromophore that is generated naturally through an oxidative cleavage of carotenoids (C40) to retinoids (C20). The retinoid then is converted to 11-cis-retinal derivatives (2-dehydro-retinal for vertebrates, 3-hydroxy-retinal for insects). These retinal derivatives would then form a Schiff-base linkage with a Lys residue in opsin to create functional visual pigments. When light is absorbed, all the cis chromophores would isomerize to the trans isomers, which subsequently transforms rhodopsin into an activated state called Meta II. Meta II binds transducin (photoreceptor specific G protein), initiating a cascade that results in the hyperpolarization of the plasma membrane.

Chemical structure of beta-carotene. The eleven conjugated double bonds that form the chromophore of the molecule are highlighted in red.

In order to regenerate the cis-chromophores, an enzymatic pathway called the retinoid cycle has been studied. This cycle involves the rod photoreceptors and cone photoreceptors. Rod photoreceptors consume the cis isomers despite being saturated by cone photoreceptors under bright light. Thus, a cone specific regeneration pathway has been proposed to avoid this competition between the two receptors. If mutations occur at the genes that code for important components such as proteins that facilitate the absorption, transportation, metabolism, and storage pathway of dietary precursors for chromophore (carotenoids and retinoids), blinding diseases and more fatal diseases such as Matthew-Wood syndrome may develop.

Recycling Visual Chromophore

The first step of the recycling process is carried out by retinol dehydrogenases (RDHs), where all the trans-retinal is reduced to trans-retinol. The main RDHs are the RDH8 in outer segments (OS) photoreceptor and RDH12 in photoreceptor inner segments (there are other RDHs). All the trans-retinol is transported from the OS to the RPE, where they are esterified. This process is facilitated by two retinoid-binding proteins called the interphotoreceptor retinoid-binding protein (IRBP) and cellular retinol-binding protein-1 (CRBP1). These trans-retinyl esters form a stable storage form for vitamin A and oil droplet structures called retinosomes. Then, the enzyme RPE65 catalyzes the endothermic reaction that converts all trans-retinoids to 11-cis-retinols. In the final step, enzymes such as RDH5, RDH10, and RDH11 catalytically oxidize the 11-cis-retinols to the original 11-cis-retinals that are needed to sustain vision. These 11-cis-retinals then bind to cellular retinaldehyde-binding protein (CRALBP), which mediates its transport back to the photoreceptor OS and opsin.

In the cone regeneration pathway, all the trans-retinol released from cone OS is transported to Muller cells rather than RPEs. There, they are isomerized to 11-cis isomeric form and esterify to 11-cis-retinyl esters by acyl-CoA: retinol acyltransferase (ARAT). These esters are converted to 11-cis-retinol with the aid of 11-cis-retinyl ester hydrolase (REH), then they bind to CRALBP and taken back to cone receptors. In the final step, NADP+/NADPH dependent 11-cis-RDH activity facilitates the regeneration of visual chromophore.

Structures of Visual Cycle Enzymes and Retinal/Retinol-binding Proteins

Visual cycle enzymes are the series of microsomal enzymes that facilitate the conversion of trans-retinol to cis-retinal. They are typically membrane-bound enzymes, which makes them harder to study because detergent is needed before X-ray crystallography could proceed. Apocarotenoid oxygenase (ACO), a water-soluble homolog of the RPE65 enzyme, belongs to the carotenoid cleavage oxygenase (CCO) family. The structure of ACO demonstrates that the CCOs contain a 7-bladed β-propeller general fold and that the ferrous ion cofactor is coordinated by four highly conserved His residues. Structure of native RPE65 from Bos Taurus shows that there is only one way of inserting the protein into the active site. This is identified by the discovery of a single tunnel in RPE65 that allows both the entry of substrates and the release of products. Deducing from this structure, scientists have proposed that retinoid substrates enter the active site from the membrane and the products leave the active site into another component in the membrane (RDH5) for further processing. These processes happen to take place in the endoplasmic reticulum membranes without involvement of retinoid-binding proteins.

There are four main retinol/retinal binding proteins; they are the RBP, CRBP, IRBP, and CRALBP. RBP and CRBP are cup shape proteins with a single domain. The active site where ligands are bound is comprised of hydrophobic anti-parallel β-barrel folds that have an affinity for only the trans-retinol molecules. The orientations of the bounded retinols are different in RBP and CRBP. When found in RBP, they tend to cluster around the cavity entrance. In contrast, they are found in the cavity base in CRBP.

IRBP is a soluble lipoglycoprotein made by photoreceptor neurons. Its major function is thought to be facilitating the transport of retinoids between the cell layers of photoreceptors and RPE. In contrast to the single domain observed in RBP and CRBP, the protein contains approximately three to four retinoid binding sites. The protein adopts a rod-shape structure and upon ligand binding, conformational changes to a bent molecular structure. Upon further structural analysis, two hydrophobic cavities are revealed to be potential ligand binding sites.

Retinol binding protein 1RBP

CRALBP belongs to a family of proteins that in their natural state, bind hydrophobic ligands and is consisted of a cluster of highly basic amino acid residues. High-resolution structural analysis has revealed that the 11-cis-Retinal binds deeply in the cavity of the protein, with the center of the ligand closest to the cavity entrance. In contrast to its cis double bond being twisted in rhodopsin (which converts to trans when triggered by light), the retinal molecule adopts a perfect cis configuration when bound to CRALBP. This makes cis-to-trans isomerization highly unfavorable. Thus, the retinal can preserve its cis configuration while being transported to opsin and photoreceptor OS.

Photochemical and chemical retinoid isomerization

The conjugated double bond in retinoids, when triggered by light, proceeds to isomerize. Under light perception, 11-cis-retinylidene is converted to all-trans-retinylidene. In this reaction mechanism, the double bond is transiently broken by the energy supplied by the photon. In order to regenerate the light-sensitive chromophore, a process called isomerohydrolase (so called retinoid isomerase) activity is required. The enzyme catalyzes the reaction of water with a carbocation. In the case of photochemical pathway, this enzyme catalyzes the process of regenerating the cis conformation retinylidene. However, from a theoretical chemical mechanism perspective, isomerization and hydrolysis of this molecule by water does not seem to be plausible. Although many reaction mechanisms have been proposed, Sn1 nucleophilic substitution seems to be the most plausible mechanism. In this reaction, a very stable double-bond-conjugated carbocation is generated after the alkyl-cleavage of the ester group by a ferric/ferrous ion cofactor and RPE65. The addition of a nucleophile completes the transformation of all-trans-retinyl esters to 11-cis-retinol.

References

1. Von Lintig J, Kiser PD, Golczak M, Palczewski K. "The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision." Trends Biochem Sci. 2010 Jul;35(7):400-10. Epub 2010 Feb 24.

2. Wikipedia contributors. "Transducin." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Nov. 2012. Web. 6 Dec. 2012.

3. Wikipedia contributors. "Chromophore." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 26 Aug. 2012. Web. 6 Dec. 2012.

4. Wikipedia contributors. "Retinol binding protein." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Jul. 2012. Web. 7 Dec. 2012. Structural Biochemistry/Evolutionary Bases/=

Beginnings

The Earth was formed about 4.5 billion years ago as theorized by the Big Bang. Life did not arise on the primordial earth due to reducing conditions and super-hot climates. After billions of years; however, signs of molecules began to arise. It was not until 3.9 billion years ago did these molecules begin to form into protobionts. The early reducing atmosphere and conditions on earth also promoted organic synthesis. Energy was everywhere on earth due to climate, lightning, ultra-violet rays and volcanoes. The Oparin-Haldane hypothesis was proposed to explain earth's early conditions and help shed light on how the first molecules were formed. Other theories have hypothesized that the Earth could have been bombarded by asteroids that contained both D and L amino acids, but as billions of years passed the L-isomer amino acid predominated and is in all organisms today.

Evidence of Life's History: By studying fossils and through relative dating and absolute dating, geologists were able to establish a geologic record of the Earth’s history. Fossils are the remains of ancient life or activity. Types of fossils include imprint fossils, trace fossils, petrification, and amber. Relative dating provides a geologic time scale. The principle of superposition states that in sedimentary rocks, the highest stratum is the youngest and vice versa. Absolute dating is a technique that use radioactive isotopes and half life of elements to determine the actual ages of strata in geological column.

The current geologic record divides Earth’s history into three eons: the Archaean, Proterozoic, and Phanerozoic eons. The Archaean and Proterozoic eons lasted approximately 4 billion years together leaving the Phanerozoic eon to encompass the most recent 500 million years of Earth’s history [2].

The Phanerozoic eon represents most of the time for the existence of animals on Earth. This eon is split into three eras on the geologic record: the Paleozoic, Mesozoic, and Cenozoic eras.

The geologic record depicted as a clock analogy [3].

Early Earth - Precambrian Era (4.5 Billion Years ago - 545 Million Years Ago)

Conditions on the earth before the Cambrian explosion and prior to the three domains of life we know today were very extreme and harsh. However, protobionts (organic molecules that formed abiotically) began to form through time and were theorized to be the first molecules, and the precursor to the prokaryotic cells. After a few billion years prokaryotic cells formed and along with these prokaryotes, cyanobacteria began to grow and photosynthesize, thus releasing oxygen into the atmosphere and changing the oxygen level of the earth. Prior to this earth had little to no oxygen. With newly diversified cells, bacteria began to conjugate. Conjugation of bacteria over time therefore led to single celled eukaryotes, the protist. As protist began to replicate and grow, multicellular eukaryotes began to arise and led to multicellular organisms. These conditions over time paved way for the massive Cambrian explosion where diversity of life on a massive scale took place.

Precambrium - Archean: 3.5 bya (billions years ago) - oldest fossils (of bacteria) found. There were only prokaryotes for the first 1.5 billions years of life. Fossil of cyanobacteria were found, they were oxygenic and could carry out photosynthesis. 2.7 bya - atmosphere accumulated Oxygen and iron precipitated out of the oceans.

Precambrium - Proterozoic: 1.9 bya - oldest Eukaryotes found. 1.2bya - multicellular eukaryotes -alga- found. 700mya, there were diverse algae and soft-bodied animals. 650mya - there was a massive glacier (snowball earth) cooling of earth. 540mya - Cambrian explosion - diverse animals and algae come about.

The Oxygen Revolution

During Earth’s history, there were many key events that resulted in major evolutions. One such event was the “oxygen revolution”. Most of the oxygen gas in the atmosphere was of a biological origin, particularly from the water-splitting step of photosynthesis in ancient cyanobacteria. The oxygen gas was first dissolved by the surrounding water. The oxygen gas then started to react with dissolved iron once the concentration of oxygen was high enough. This reaction produced iron oxide which precipitated to form sediments. As the oxygen gas concentration kept rising, the seas and lakes soon became saturated with oxygen gas and the oxygen started to “gas out” of the water and into the atmosphere [2]. The amount of atmospheric oxygen gas was increasing at a gradual rate between 2.3 and 2.7 billion years ago; however, the rate of oxygen gas being added to the atmosphere spiked up rapidly causing an increase of 1% to 10% of its present level [2]. This is known as the “oxygen revolution” and it had a huge impact of the life inhabiting Earth at the time. Many prokaryotic groups became extinct due to oxygen being toxic to these groups by attacking chemical bonds, inhibiting enzymes, and damaging the cells. The species that survived did so by inhabiting an anaerobic environment. Diverse adaptations also resulted from this revolution, one of which was cellular respiration. The oxygen revolution also brought about the first eukaryotes. The origin of the eukaryotes can be further explained by the endosymbiont theory.

Paleozic Era (545 Million Years Ago - 251 Million Years Ago)

The Paleozice Era is marked by the cambrian explosion in which organismic diversification exploded, forming different life forms on earth. Due to lack of a fossil record prior to the Paleozic era, little can be said about when the cambrian explosion actually occurred but new novelty structures such as jaws, appendages, bones, and other features on today's organisms began to form. There were also marine diversification. The first vertebrates appeared 525 mya. THere were invasion of land by many. The first land plants and fungi formed 460mya. THe first land animal Arthropods appeared. Following this, the first insects was found 396mya. Furthermore, the first land vertebrate (amphibians) appeared 360mya. The appearance of amniotes (terresterial animal) in the late Paleozoic.

Also during this time the earth due to continental drift began to move together and form a supercontinent, pangea. There were a reduction of shallow coastal habitats and along came cold harsh interior climates. However, the Paleozoic Era ended with a mass extinction termed the Permian Crisis. During the Permian Period about 251 million years ago, 96% of species went extinct. This occurred because massive volcanic eruptions released SO2 and CO2, causing global warming feedback. This massive release of methane by the underwater volcanoes led to a change in ocean circulation and an anoxic ocean, or an ocean depleted of oxygen.

Mesozoic Era (251 - 65 Million Years ago)

The Mesozoic era, also known as the age of the dinosaurs shows more growth and diversification in organisms. Gymnosperms appeared in the early Mesozoic. During the mid-mesozoic, dinosaurs were introduced. The late Jurassic introduced birds. Origin of mammals are from early Mesozoic. 127 mya angiosperms (flowering plants) appeared. Also during this time the earth due to continental drift Pangea began to break. The end of the Mesozoic era is also marked with another mass extinction (especially of dinosaurs),the Cretaceous Crisis, theorized to have been caused by a gigantic asteroid, hitting the area of Yucatan. Layers of iridium are found around this area, thus proving that there could have been a catastrophic asteroid hitting this area affecting the plants and organisms that lived around that area during that time. The global effects included (1) crustal material ejected into stratosphere, (2) widespread fires nearer to impact, (3) reduced photosynthesis for several years, and (4) reduced global temperatures.

Cenozoic Era(65 Million Years Ago - Present)

The Cenozoic also known as the age of mammals began after the extinction of all the dinosaurs (except birds). The first grass appeared 40mya. At the beginning of Pleistocene, 2mya, there were glaciations. Human evolution occurred during this time. Homo-sapiens as we know today began to populate the earth 100,000 years ago. There are many misconceptions: our ancestors are not chimps, not half-stooped cave dwellers, and are not contemporaries of dinosaurs.

References

1. Campbell, Neil A. (2005). Biology. Pearson. ISBN 0-8053-7146-0 .

2. Campbell, Neil A. (2010). Biology 9th Edition. Pearson. ISBN-10: 0321558235.

3. By Woudloper derivative work: Andrew Colvin (Geologic_clock.jpg) [Public domain], via Wikimedia Commons

The RNA World Hypothesis speculates that the origin of life began with ribonucleic acid (RNA) because of its ability to serve both as a storage for genetic information and enzymatic activity. It is proposed that RNA preceded the current genetic material, deoxyribonucleic acid (DNA), and led the evolution of the DNA → RNA → protein world.

There are two schools of thought that both support the RNA World hypothesis:

1. According to the Genetic Takeover Hypothesis, an earlier form of life on earth used RNA as its only genetic component. This proposes that there may be a pre-RNA molecule that used RNA or by change created RNA as a side product.

2. The first form of life on earth used RNA as its only genetic component. This theory requires that RNA came from inanimate matter.

History

The book, The Genetic Code, written in 1967 by Carl Woese, was the first published material that supported RNA World Hypothesis. Francis Crick and Leslie Orgel proposed the idea that RNA once did the work of DNA and proteins in 1968. Their theories were not validated until the work of Nobel Prize laureate Thomas R. Cech. In the 1970s, Cech was studying the splicing of RNA in a single-celled organism, Tetrahymena thermophila, when he discovered that an unprocessed RNA molecule could splice itself. He announced his discovery in 1982 and became the first to show that RNA has catalytic functions. The phrase “RNA World Hypothesis” was then coined later in 1986 by Harvard molecular biologist and Nobel Prize laureate Walter Gilbert as he commented on the recent observations of the catalytic properties of RNA. Another major milestone occurred in 2000 when it was published in Science that "The Ribosome is a Ribozyme" and the proteins in the ribosomes exist primarily on the periphery.

Theory

The primordial soup that made up Earth had compounds including nucleotides. These nucleotides sequenced spontaneously and randomly, eventually forming an RNA molecule (or a similar molecule) with catalytic characteristics. RNA has properties of autocatalytic self-replication and assembly, contributing to its exponential increase in number. Scientists today have assumed that replication was not perfect in the time of primitive life, and therefore variations of RNA developed. RNA's catalytic properties do not only apply to itself, but it also catalyzes transesterification—a process necessary for protein synthesis that allows specific peptide sequences and proteins to arise. Some of the peptides formed may have supported the self-replication of RNA and provided the possibility of undergoing modifications. Those modifications led to more efficient sequences of RNA molecules.

A possible model for forming purine and pyrimidine bases was proposed by Urey and Miller's experiment. This experiment brought about evidence that organic molecules originated from inorganic ingredients such as carbon dioxide, ammonia, and water etc. Such products were mixed under a reduced environment and subjected to electric shock (to simulate lightning), which led to the creation of more reactive molecules, such as hydrogen, cyanide, and aldehydes, as well as some amino acids and organic acids over a certain period of time. These amino acids contributed to the formation of peptide sequences.

http://www.smithlifescience.com/MillersExp.htm (see apparatus)

Further evidence that the RNA World Hypothesis has clout is found through the function of present day ribosomes. RNA is the ribosome's tool for synthesizing proteins and catalyzing the formation of peptide bonds. A form of RNA known as transfer RNA (tRNA) is responsible for delivering free amino acids to the ribosome and growing peptide chain. Therefore, this points to the fact that RNA is multifunctional, and can act as a synthesizer, transporter, messenger, and ribosome molecule.

One may ask, if RNA was the precursor of DNA and proteins, how did this evolution occur? DNA complements the RNA sequence and stores genomic information. Since DNA is a more stable molecule than RNA, it makes sense for DNA to adapt to the environment and take over this job of RNA. And how is DNA more stable than RNA? The difference between the general structure of DNA and RNA is found in the sugar. DNA has a deoxyribose sugar while RNA has a ribose sugar. The missing 2'-OH group on the deoxyribose sugar is what makes DNA more stable, since there is no hydroxyl group for other molecules to react with. Otherwise, RNA does not remain in a helical ring, as does DNA, since the chain of nucleotides would be easily broken apart. Another possibility scientists are exploring is the idea that reverse transcriptase (RT) played a role in the transformation from RNA to DNA. Reverse transcriptase catalyzes the formation of DNA from an RNA template, and RT is the defining feature of retroviruses like HIV. RT, along with RNA replicase, may be the enzyme that performed this transition. Furthermore, the combination of cyanoacetaldehyde and urea formed uracil (U) and cytosine (C)-- components of the primordial soup. This belief was supported by another of Miller's experiments. There is no evidence at this time that thymine (T), the nitrogenous base in DNA that takes the place of uracil (U) in RNA, was formed from this atmosphere. This infers that RNA was a predecessor of DNA. In addition, proteins that had formed from RNA were found to be versatile structures, allowing them to take over what was initially RNA's catalytic functioning.

RNA instability due to 2'OH.

Properties Supporting Hypothesis

Properties that support the RNA Hypothesis became more clear in the 1980s when it was discovered that RNA can activate and deactivate other molecules by binding with them while folding into specific structures. Before this discovery, researchers believed that RNA only had a few functions. Consideration of RNA as the pre-component of cellular life has since been studied extensively.

Major evidence that has the scientific community believing that RNA predates DNA and proteins are as follows:

- RNA has the ability to store genetic data, and pass down hereditary information.

- It is the main component linking up DNA and gene formation to amino acids and protein synthesis via transcription and translation.

- RNA's ability to duplicate itself as well as the genetic information it carries, very much like DNA.

- RNA's complexity is less than that of DNA, and involves fewer types of molecules in order to self-replicate.

- DNA requires an RNA primer in order to replicate while RNA does not need any such primer. This shows how DNA seems to depend more on RNA for its continued existence rather than the other way around.

- RNA is able to catalyze reactions as proteins do. The formation of a protein is also administered by RNA which points heavily to its preexistence over the proteins.

- RNA's ability to form double helices similar to DNA, and tertiary structures similar to those of catalytic proteins.

- The structure of RNA, with an hydroxyl group in the 2' position of the sugar molecule, makes it a less stable molecule which is capable of attacking a phosphodiester bond near it as long as the RNA molecule is in a flexible position and not constrained. This made it susceptible to breakdown and allowed an adaptation of different conformations which perhaps was beneficial to early life.

- RNA's different set of bases such as Uracil, which is “1 product of damage to cytosine” made RNA more prone to mutations thus making it more suitable to primitive life in early times.

The Miller-Urey experiment

An experiment supporting the RNA World Hypothesis was, the Miller-Urey Experiment. The Miller-Urey Experiment was an experiment tested by Stanley L. Miller and Harold C. Urey in 1953 to see which molecules were present in the origin of life. This experiment specifically tested the hypothesis of Alexander Oparin's and J.B.S. Haldane's hypothesis that stated the conditions of prebiotic Earth favored chemical reactions, synthesizing inorganic compounds into organic ones. Both experiments helped scientists around the world better understand the evolution of the Earth and how organic compounds formed. The gases used by Miller and Urey were Methane (CH₄), Ammonia (NH₃), Hydrogen (H₂) and water (H₂O). After putting these gases (which were presumed to be present in prebiotic Earth), Miller and Urey continuously ran an electric current throughout the closed vessel system to stimulate lightning, which was thought to be extremely common on early Earth. These compounds were put inside a sterile array of glass tubes and flasks connected in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired between the electrodes to simulate lightning through the atmosphere and water vapor, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle. Within a day, the mixture had turned pink in color,[1] and at the end of one week of continuous operation, Miller and Urey observed that as much as 10–15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids that are used to make proteins in living cells, with glycine as the most abundant. Sugars and liquids were also formed. Nucleic acids were not formed within the reaction. But the common 20 amino acids were formed, in various concentrations. This experiment thus showed that organic compounds that are vital to cellular function and life were easily made under the conditions of prebiotic Earth.

In an interview, Stanley Miller stated: "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids."[2] This further supported the RNA World Hypothesis.

Other Experiments

The Miller-Urey and Oparin experiments helped launch other experiments to further show that organic compounds formed in early Earth and to also confirm the RNA World Hypothesis.

In 1961, Juan Oro conducted an experiment that concluded amino acids could be formed from HCN, hydrogen cyanide, and Ammonia (NH₃) in an aqueous solution. This experiment also produced Adenine, one of the nucleotide bases. This became a major breakthrough because adenine is one of the four bases in DNA and RNA, the genetic material of a cell. Adenine is also used during the process of ATP (adenosine triphosphate), which is an energy releasing molecule in cells.

This experiment led to more showing that the other three RNA and DNA bases could be formed through similar experiments of simulated chemical environments with reducing atmospheres.

Properties Opposing Hypothesis

Most of the opponents of the RNA World concentrate on dispelling the idea that RNA was the first form of genetic material, although they do agree that there may have been some other pre-RNA form of genetic material. In summary:

1. Ribose is relatively unstable and difficult to form in a prebiotic mixture. Despite the favorable and controllable conditions that are available in laboratory settings, pre-cellular life has never been created from inanimate matter.

2. The origin of life began roughly 300 million years ago. Some believe that this is too short of a time period for the prebiotic soup to evolve in to a pre-RNA or RNA World.

3. Lack of evidence of large amounts of polyphosphates in primitive Earth makes it unlikely that it was the source of prebiotic energy or that it was involved in the first genetic material. .

Others believe that RNA is not a likely pre-DNA form of genetic material. Their arguments include:

1. The limited catalytic capabilities of RNA. Theorists say that RNA needed to have had a multitude of catalytic abilities to be able to survive the prebiotic world, but RNA has not shown this. Proteins, on the other hand, do have those catalytic abilities via their varying, enzymatic abilities.

2. The prebiotic simulation of the formation of the RNA molecule has shown some difficulty in that the bases and the sugar molecule do not readily react in water.

3. Opponents advocate proteins over RNA because they are easily formed.

4. The probability of the right components of pre-cellular life to exist at the same place and time, without contaminates, and with the correct catalytic reactions is next to improbable .

5. Recent research shows that non-coding RNA regions have well-adapted and very specialized roles in the cell. Examples include siRNA and miRNA—they work well in an environment where RNAi and mRNA already exist. Because of their usefulness that we are just beginning to understand, it makes it less likely that there are "relics" of the RNA World present in our DNA as Gilbert originally mentioned in 1986 .

Alternative Theories

The difficulty of RNA formation has caused other propositions of alternative theories on precursor materials for cellular life:

- Peptide Nucleic Acid theory (PNA), a nucleic acid with a backbone of peptide bonds, made a likely theory because it overcame the problem in RNA theory regarding the difficulty of RNA to attach ribose and phosphate groups together.

- Threose nucleic acids are proposed as a more likely starting material than RNA.

- Glycold nucleic acids are proposed as precursors rather than RNA because they are easily formed.

- Double origin theory suggests that both RNA and proteins existed around the same time independently.

Support

The RNA World hypothesis is supported by RNA's ability to store, transmit, and duplicate genetic information, as DNA does. RNA can also act as a ribozyme (an enzyme made of ribonucleic acid). Because it can reproduce on its own, performing the tasks of both DNA and proteins (enzymes), RNA is believed to have once been capable of independent life. Further, while nucleotides were not found in Miller-Urey's origins of life experiments, they were found by others' simulations, notably those of Joan Oro. Experiments with basic ribozymes, like the viral RNA Q-beta, have shown that simple self-replicating RNA structures can withstand even strong selective pressures (e.g., opposite-chirality chain terminators) (The Basics of Selection (London: Springer, 1997)).

Additionally, in the past a given RNA molecule might have survived longer than it can today. Ultraviolet light can cause RNA to polymerize while at the same time breaking down other types of organic molecules that could have the potential of catalyzing the break down of RNA (ribonucleases), suggesting that RNA may have been a relatively common substance on early Earth. This aspect of the theory is still untested and is based on a constant concentration of sugar-phosphate molecules.

Difficulties

The base cytosine does not have a plausible prebiotic simulation method because it easily undergoes hydrolysis.

Prebiotic simulations making nucleotides have conditions incompatible with those for making sugars (lots of formaldehyde). So they must somehow be synthesized, then brought together. However, they do not react in water. Anhydrous reactions bind with purines, but only 8% of them bind with the correct carbon atom on the sugar bound to the correct nitrogen atom on the base. Pyrimidines, however, do not react with ribose, even anhydrously.

Then phosphate must be introduced, but in nature phosphate in solution is extremely rare because it is so readily precipitated. After being introduced, the phosphate must combine with the nucleoside and the correct hydroxyl must be phosphorylated, in order to create a nucleotide.

For the nucleotides to form RNA, they must be activated themselves (meaning that they must be combined with two more phosphate groups, as in adenosine triphosphate). Activated purine nucleotides form small chains on a pre-existing template of all-pyrimidine RNA. However, this does not happen in reverse because the pyrimidine nucleotides do not stack well.

Additionally, the ribose must all be the same enantiomer, because any nucleotides of the wrong chirality act as chain terminators.

A.G. Cairns-Smith in 1982 criticized writers for exaggerating the implications of the Miller-Urey experiment. He argued that the experiment showed, not the possibility that nucleic acids preceded life, but its implausibility. He claimed that the process of constructing nucleic acids would require 18 distinct conditions and events that would have to occur continually over millions of years in order to build up the required quantities.

Alternative Hypothesis

As mentioned above, a different version of the same hypothesis is "Pre-RNA world", where a different nucleic acid is proposed to pre-date RNA. A candidate nucleic acid is peptide nucleic acid (PNA), which uses simple peptide bonds to link nucleobases.53 PNA is more stable than RNA, but its ability to be generated under prebiological conditions has yet to be demonstrated experimentally.

Threose nucleic acid (GNA), and like PNA, also lack experimental evidence for their respective abiogenesis.

An alternative—or complementary— theory of RNA origin is proposed in the PAH world hypothesis, whereby 57

The iron-sulfur world theory proposes that simple metabolic processes developed before genetic materials did, and these energy-producing cycles catalyzed the production of genes.

Yet another alternative theory to the RNA world hypothesis is the panspermia hypothesis. It discusses the possibility that the earliest life on this planet was carried here from somewhere else in the galaxy, possibly on meteorites similar to the Murchison meteorite.58 This does not invalidate the concept of an RNA world, but posits that this world was not Earth but rather another, probably older, planet.

Implications of the RNA World

The RNA world hypothesis, if true, has important implications for the very definition of life. For the majority of the time following the elucidation of the structure of DNA by Watson and Crick, life was considered as being largely defined in terms of DNA and proteins: DNA and proteins seemed to be the dominant macromolecules in the living cell, with RNA serving only to aid in creating proteins from the DNA blueprint.

The RNA world hypothesis places RNA at center-stage when life originated. This has been accompanied by many studies in the last ten years demonstrating important aspects of RNA function that were not previously known, and support the idea of a critical role for RNA in the functionality of life. In 2001, the RNA world hypothesis was given a major boost with the deciphering of the 3-dimensional structure of the ribosome, which revealed the key catalytic sites of ribosomes to be composed of RNA and for the proteins to hold no major structural role, and be of peripheral functional importance. Specifically, the formation of the peptide bond, the reaction that binds amino acids together into proteins, is now known to be catalyzed by an adenine residue in the rRNA: the ribosome is a ribozyme. This finding suggests that RNA molecules were most likely capable of generating the first proteins. Other interesting discoveries demonstrating a role for RNA beyond a simple message or transfer molecule include the importance of small nuclear ribonucleoproteins (SnRNPs) in the processing of pre-mRNA and RNA editing and reverse transcription from RNA in Eucaryotes in the maintenance of telomeres in the telomerase reaction.

References

1. ^ Gesteland, R.F., Cech, T.R., Atkins, J.F., 2006, The RNA World: the nature of modern RNA suggests a prebiotic RNA, Cold Spring Harbor Laboratory Press, United States of America, 768 p.
2. ^ Gilbert, W., "The RNA World". Nature 618.
3. ^ Altman, S. The RNA World
4. ^ Lazcano, A, Miller, S.L., "The Origin and Early Evolution of Life: Prebiotic Chemistry, the Pre-RNA World, and Time" Cell, Vol. 85, 7930798, June 14, 1996.
5. ^ RNA World Hypothesis at NationMaster.com
6. ^ RNA World Hypothesis at ExperienceFestival
7. ^ Eddy, S.R., "Non-Coding RNA Genes and the Modern RNA World" Nature Reviews: Genetics Vol. 2, Dec. 2001.

8.http://www.smithlifescience.com/MillersExp.htm

9.Nelson, David L. Principles of Biochemistry, 4th ed. W. H. Freeman, 2004.

10.http://www.helsinki.fi/~matpitka/articles/prebiotic.pdf

11.http://www.nationmaster.com/encyclopedia/RNA-world-hypothesis#History

12. Asimov, Isaac (1981). Extraterrestrial Civilizations. Pan Books Ltd. pp. 178. http://www.accessexcellence.org/WN/NM/miller.php

13.http://www.experiencefestival.com/a/RNA_world_hypothesis_-_The_base_pair/id/4697160

14.http://www.nationmaster.com/encyclopedia/RNA-world-hypothesis#Support

15. http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html

http://www.fantastico.herobo.com/?media=RNA_world_hypothesis In order for evolution to occur, three different processes are required and must be met:

1. Generation of a diverse population

2. Selection of members based on criterion of fitness

3. Reproduction to enrich the population in more-fit members

Nucleic acids are such molecules that are capable of undergoing all three processes. Combinatorial chemistry is the concept of rapidly producing an abundant amount of a specific molecule of interest. Because of this process, diverse populations of nucleic acid molecules can be synthesized. A common example of this process involves attempting to create an RNA molecule that specifically binds ATP and other nucleotides. First, a randomized pool of RNA sequences, called apimers, are placed into an ATP affinity column. Some RNA molecules are then found bounded to ATP. These molecules are released from the affinity column by adding excess ATP. This collection of molecules are allowed to replicate into DNA by using the enzyme reverse transcriptase. Once transcribed into DNA, the molecules undergo PCR, and ultimately transcribe back into RNA.

References

Evolutions of Humans

Humans come from the genus Homo and species sapien. Through the widely accepted classification system of Kingdom, phylum, class, order, family, genus and species. Humans all come from vertebrates which have a great line of divergence, some of the vertebrates are Lancelets, urochordtaes, craniates, tunicates, ray finned fish, lobed finned fish and from the lobed finned fish emegered the terrestrial vertebrates and mammals, and out of the mammals in a group called primates came humans. Humans are considered to be members of apes and come from the old world monkeys as opposed to new world monkeys. Old world monkeys have tails but are weak and unlike new world monkeys are unable to use their tails to do tasks such as hang on trees. Many primates share derived characteristics. They have hands and feet which allows them to hold things and reach for them. They have opposable thumbs useful for grabbing and holding, they are developed and conditioned culturally and socially. They have vision different from other animals which allows them to perceive depth .They also have larger brains. From the old world monkeys hominoids diverged around 20-25 million years ago. Humans are in this group and are called bipedal hominoids. The human species we know today, Homo sapiens are about 160,000 years old and are relatively young for an Earth that existed about 4.6 billion years ago. Hominoids have derived characteristics as well. They shared similar derived characteristics such as hairlessness, have larger brains, stand upright are able to communicate through language, shorter jaws and are able to create tools and instruments. It is correlated that hominoids came from Africa about 6 million years ago. The earliest of hominoids had small brains and walked upright.

Hominin Evolution: Approximately 6-7 mya, the last common ancestor between humans and chimpanzees diverged into at least two separate lineages. Hominins refer to the group of organisms in the lineage that gave rise to humans. The candidates for earliest hominin include Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba, and Ardipithecus ramidus (Klein 2009).

As its name suggests, Sahelanthropus tchadensis was found in Toros Menalla, Chad (6-7 mya). The fossil that belongs to this species was nicknamed, “Toumai,” and consisted of an almost complete cranium. Based on the anterior position of the foramen magnum, it is likely that Sahelanthropus tchadensis was a bipedal organism. However, its brain size of 360-370 cc is far smaller than that of the average person. Because the Toumai fossil had no postcranial features and the skull was not completely intact, there is debate as to whether Sahelanthropus tchadensis was an ape and not an ancestor of modern humans.

Orrorin tugenensis was found in the Tugen hills in Kenya and is dated to approximately 6 mya. The curved phalanges on this species indicate that it was skilled at tree-climbing. However, the long neck of the femur and the location of the obturator externus groove support bipedalism.

The last 2 candidates for earliest hominin belong to the genera Ardipithecus. Most of the specimens for Ardipithecus kadabba are isolated teeth that date 5.2-5.8 mya. In contrast, there is an almost complete skeleton specimen for Ardipithecus ramidus, which dates to 4.4 mya. Ardipithecus ramidus had traits in between those of apes and humans. The anterior position of the foramen magnum suggests that Ardipithecus ramidus was bipedal. In addition, unlike some apes, there was no evidence of knuckle-walking adaptations. However, while its legs were adapted for bipedalism, Ardipithecus ramidus also had an opposable big toe and partially grasping feet to facilitate an arboreal lifestyle, both of which are traits associated with apes. Therefore, based on current evidence, it is unclear whether or not any of the proposed candidates are hominins that eventually gave rise to humans or if they merely represent ape lineages that went extinct.

Australopithecines: Australopithecines represent a subfamily that has enough fossil data to support them as the oldest known hominins. They existed from 1.2-4.4 mya and had human bipedalism and other traits as well as ape-like traits, including a smaller brain size and adaptations for climbing trees. Australopithecines can be divided into 4 genera: Ardipithecus (ramidus), Australopithecus, Kanyanthropus, and Paranthropus. Ardipithecus ramidus is sometimes included in the Australopithecine subfamily because its placement in the fossil record remains unclear.

One of the earliest Australopithecines was Australopithecus anamnesis dated 3.9-4.2 mya. While foot bones suggest bipedalism, curved phalanges and other hand traits indicate an ability to knuckle walk and climb trees. Other hominin features include large molars and thick enamel on its dentition.

Australopithecus afarensis is one of the better documented Australopithecines. Some of the more famous finds include “Lucy,” “The First Family,” and “Dikika Child.” Lucy was an almost complete skeleton found in 1974 that dates to 3.2 mya. The First Family includes 13 individuals found at the same site in 1975. Finally, the Dikika Child is a fossil of a 3 year old with a brain size of 300 cc, which represented about 75 % of the adult brain size. This indicates that Australopithecus afarensis matured more slowly than chimpanzees. In general, Australopithecus afarensis shared features with both apes and humans. Like humans, Australopithecus afarensis had a longitudinal arch in its foot and a broad, basin-shaped pelvis that facilitated bipedalism. However, they also had small brains (400 cc) and had a diastema in between that of chimpanzees and humans (who lack it completely).

Bipedalism vs. Brain Size: The finding of hominin fossils in Africa demonstrate that bipedalism came earlier in evolution than an increase in brain size comparable to that of modern humans. Therefore, it is important to understand why bipedalism was chosen as the mode of locomotion in human evolution. There are currently many theories as to why hominins became bipedal.

One of the theories on why hominins became bipedal was that it was easier to carry tools, foods, children, and other resources from one location to another. Another idea states that bipedalism allowed people to see over tall grass so that they could avoid predators and search for prey. Finally, it is possible that it was more energetically efficient to walk on two feet. It is possible that a combination of all of these reasons as well as other reasons led humans to become bipedal. However, whatever the reason may be, there are certain drawbacks to bipedalism that had to be considered. For example, the restructuring of the pelvis that allows for bipedalism also leads to difficult births. Furthermore, since the weight increases as you move down the spine, bipedalism also causes lower back pains. Therefore, the benefits of bipedalism must have outweighed the costs in order for hominins to adapt to this type of locomotion.

o Homo

 Homo habilis • Lived anywhere from the range of 2.4 – 1.6 mya, they used tools to hunt.

 Homo ergaster • Lived about 1.9 to 1.6 million years ago. They were able to walk on two, and had large brains.

 Homo erectus • Came from Africa, existed around 1.8 mya. They were the first hominoid group to leave Africa .

 Homo neanderthalensis

•Neandertal and Anatomically Modern Human Co-existence: The relation, if any, between Neandertals and Homo sapiens remains unclear. While some have suggested that Neandertals are members of the genus Homo, others contend that the hominid species belongs to a different genus. The debate centers on the level of interaction between Neandertals and anatomically modern humans (AMH), with particular emphasis to their co-habitation of Europe, which may have lasted as little as 5,000 years. Based on the current fossil evidence, it is supported that AMH had spread across Europe by 38 kya. In addition, there is fossil data that suggests that Neandertals may have existed as recently as 34-38 kya (Joris et al., 2011; Doronichev et al., 2011). Therefore, there is evidence of overlap both in time and space between the two groups. However, what is less clear is the level of interaction and interbreeding that took place. How much of the human genome comes from Neandertal genes and what role did humans have in Neandertal extinction? In addition, if humans did cause the Neandertals to go extinct, what evolutionarily favorable traits enabled them to outcompete a hunter species with larger brains and an anatomy built to combat the cold climate of the time.

Hypotheses on Neandertal Extinction: As stated above, the fossil record shows Neandertal extinction around 34-38 kya. However, fossil evidence also reveals that Neandertals may have lived as long ago as 130 kya. So, soon after their encounter with AMH, they went extinct. The hypotheses that involve human action include competition, intergroup violence, or disease.

1) The first hypothesis that places AMH as the cause of Neandertal extinction is competition. Anatomically modern humans outcompeted Neandertals for resources and utilized the environment more efficiently. Early evidence suggested that AMH had a more varied diet that included plant foods as well as meat whereas Neandertals were exclusively carnivores. Furthermore, AMH used more complex tools that could kill game at a distance (La Pierre, 2008). In contrast, Neandertals had simpler weapons that put them up close with their prey. As a result, their mortality rate was suggested to be higher than that of AMH. However, recent evidence supports a more varied diet for Neandertals that included plant foods as well as meat (Brooks et al., 2011). Therefore, it is unlikely that completion was the primary cause of extinction, although it may have been a contributing factor.

2) Intergroup violence is the hypothesis that most directly links humans with Neandertal extinction. In essence, humans hunted down Neandertals and drove them to the brink of extinction. The surviving Neandertals were too scattered and too few in numbers to survive and reproduce. While the evidence for this is minimal, some fossil evidence exists. For example, there is a punctured rib in a Neandertal individual at Shanidar that resembles a projectile weapon used by AMH (Churchill et al., 2009). There are also other wounds that have been attributed to intergroup violence (McCall and Shields, 2008). Finally, in support of this hypothesis, intergroup violence fits the short time frame of co-existence that ended with Neandertal extinction.

3) Another hypothesis involves AMH bringing to Europe with them a virus that decimated the Neandertal population. After years of co-evolution in Africa, AMH had become resistant to the virus, but the Neandertal population was immunologically naïve (Greenwood and Wolff, 2010). Support for this hypothesis is largely based off historical evidence of foreigners bringing with them new diseases that killed off a large part of the indigenous population (i.e. Europeans bringing small pox to the New World and killing millions of Native Americans).

4) Other hypotheses have also been put forth that do not rely much on interaction between the two groups, such as humans being better adapted to the climate changes. However, whatever the case may be, it is clear that Neandertals remain an important part of human evolution. In conclusion, sequencing of the Neandertal genome has revealed that Asian and European modern human populations share 1-4 % of their genome with Neandertals. In contrast, present-day Africa populations do not have this genetic relation. Therefore, while the intergroup interbreeding was probably minimal, a part of the Neandertal genome exists today in the modern human population.

 The theory is that Homo’s all came from Africa and gave rise to the Neanderthals and Homo erectuses. Scientist have used the molecular clock as evidence that the out of Africa hypothesis is the best estimate/theory of how the humans of today came about. The molecular clock theory states that the rate at which proteins and DNA change relative to itself and is constant for whichever substance it is. For example, that rate at which HIV replicates and mutates is constant and the rate at which proteins are being replicated in humans and the DNA and RNA running the transcription then translation of the central dogma.

References:

Banks, W.E., d’Errico, F., Kageyama, M., Peterson, A.T., Sanchez-Goni, M.F., Sima, A. (2008). “Neanderthal Extinction by Competitive Exclusion.” PLoS ONE, 3 (12): 39-72. Berger, T.D. and Trinkaus, E. (1995). “Patterns of Trauma among the Neandertals.” Journal of Archaeological Science, 6 (22): 841-852. Brooks, A.S., Henry, A.G., Piperno, D.R. (2011). “Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I

	and II, Belgium).” PNAS, 2 (108): 1-6.

Churchill, S.E., Daniel, J.A., Franciscus, R.G., McKean-Peraza, H.A., Warren, B.R. (2009). “Shanidar 3 Neandertal rib puncture wound and Paleolithic weaponry.” Journal of Human Evolution, 2 (57): 163-178.

Doronichev, V.B., Golovanova, L.V., Higham, T.F.G., Pinhasi, R. (2011). “Revised age of late

Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus.”

PNAS, 21 (108): 8611-8616.

Green et al. (2010). “A Draft Sequence of the Neandertal Genome.” Science, 328 (5979): 710-

722.

Greenwood, A.D. and Wolff, H. (2010). “Did viral disease of humans wipe out the Neandertals?” Medical Hypotheses, 1 (75): 99-105.

Joris, O., Street, M., Terberger, T., Weninger, B. (2011). “Radiocarbon Dating the Middle to

Upper Paleolithic Transition: The Demise of the Last Neanderthals and the First

Appearance of Anatomically Modern Humans in Europe.” Continuity and Discontinuity

in the Peopling of Europe: One Hundred Fifty Years of Neanderthal Study. Condemi, S.

and Weniger, G. Vol.1. New York: Springer, 2011. P. 239-298. Print.

Klein, Richard G. The Human Career: Human Biological and Cultural Origins. 3rd Ed. Chicago and London: The University of Chicago Press,

       2009. Print.

La Pierre, Y. Neandertals: A Prehistoric Puzzle. Minneapolis: Twenty-First Century Books,

2008. Print.

McCall, G.S. and Shields, N. (2008). “Examining the evidence from small-scale societies and

early prehistory and implications for modern theories of aggression and violence.”

Aggression and Violent Behavior, 1 (13): 1-9.

Trinkaus, E. (2011). “Late Pleistocene adult mortality patterns and modern human

establishment.” PNAS, 4 (108): 1267-1271. Although it is true that natural selection takes an effect on individual organisms, the evolutionary influence of natural selection is only evident in a population of organisms over time in their changes as a whole.

Genetic Variation

Genetic variations are the distinctions among individuals in the makeup of their DNA segments or genes; individual variations commonly reflect genetic variation. Genetic variation is the reason why evolution exists in the first place; it gives the raw material for evolutionary change. Often, the genetically determined components of phenotypic variation can have evolutionary effects.

Variation allows for survival of the population by adaptation or flexibility to the environment. Genetic variation is advantageous for this reason. It allows for a population to survive despite a changing environment. A great amount of variation exists due to being able to draw from a huge pool of alleles and genes. Some new alleles allow for greater survivability and therefore get selected fore, while others are detrimental and die out. Also, there are neutral alleles that do not contribute to overall survivability so they exist due to their neutrality. Genes and alleles are mixed to contribute to variation through breeding. There are two types of breeding methods that are non-random. They are non-random because individuals choose potential mates based on specific qualities. Inbreeding describes how mates choose partners with similar traits to themselves while outbreeding is how mates choose for qualities polar to their own.

Variation within a Population

There are two different types of characters that will vary within a population. The first of the two is a discrete character, which can be categorized on an either-or-basis. The discrete characters are commonly decided by a single gene locus with differing alleles that create discrete phenotypes. The other type of characters are the quantitative characters, which differ along the range within a population. As opposed to discrete characters, quantitative characters are commonly decided by two or more genes on a single phenotypic character.

Variation Between Populations

Species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or simply geographically located elsewhere with different environments. One example of geographic variation are clines, or graded changes in a character down a geographic axis.

Sources of Genetic Variation

Genetic variation stems from when gene duplication, mutation, or other processes produce new genes and alleles. New genetic variants can be created in little phases of time in organisms that rapidly reproduce. However existing genes can be arranged in new ways as a result from genetic variation by sexual reproduction. So overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction.

Hardy-Weinberg equilibrium

One of the most important tools population genetics gave to the study of evolution was the principle of Hardy-Weinberg equilibrium. This principle states that there is nothing in gene replication, meiosis, fertilization, or reproduction that changes the frequency of gene alleles over time. As long as no other forces act upon the population, a gene in that today has 25% allele "A" and 75% allele "a" will still have 25% A and 75% in a million years.

The Hardy-Weinberg equilibrium is based in five assumptions, which, if they held true in nature, would create a situation where no other forces were acting upon a population and no change in gene frequencies (evolution) would occur. These assumptions are:

1. The population is infinite, isolated, and panmictic, where all individuals have an equal chance of mating with any other individual in the population. 2. There is no mutation in the genetic material of the population. 3. There is no gene flow in the population (no individuals leave or join the population). 4. There is no genetic drift. 5. There is no difference in reproductive success between members in the population (natural selection is not acting on the population).

Since these are the assumptions that must hold true for the Hardy-Weinberg equilibrium to be maintained, their opposites are the causes of evolution, or the change of allele frequencies in populations. For example, mutation can cause changes in allele frequency by creating new, altered genetic material while gene flow can change allele frequencies by introducing new alleles to the population through immigration or removing alleles through emigration. Of these five causes of evolution, only the last one involves natural selection, or directional change imposed by survival of the fittest in a harsh environment. The first four, all of which can play critical roles in evolution, involve chance events.

By studying the allele frequency in a population, and doing some math, ecologists can determine whether allele frequencies occur as predicted by the Hardy-Weinberg equilibrium principle. Hardy-Wienberg allele frequencies are likely with large populations that have lots of gene flow and random mating, and are unlikely if populations are small, isolated, or have non-random mating patterns. If a study reveals allele frequencies that do not match Hardy-Weinberg predictions, the population is probably structured in some way (e.g., is made up of more than one isolated subpopulation or has non-random mating). This is what two teams of researchers have done with monarch butterflies.

References

http://www.ndsu.edu/pubweb/~mcclean/plsc431/popgen/popgen1.htm The Oparin-Haldane Hypothesis

The Oparin-Haldane hypothesis is a hypothesis independently developed by two scientists: Russian chemist A.I. Oparin and British scientist John Haldane.

Both independently suggested that if the primitive atmosphere was reducing (as opposed to oxygen-rich), and if there was an appropriate supply of energy, such as lightning or ultraviolet light, high temp of around 1073 K then a wide range of organic compounds might be synthesized.

Oparin came up with the hypothesis in 1924 that Earth’s atmosphere was extremely reducing in its early stages of development. This means that the atmosphere had an excess of negative charge and could cause reducing reactions by adding electrons to compounds. Oparin suggested that these organic compounds could have undergone a series of reactions leading to more and more complex molecules. Under these circumstances, Oparin hypothesized that organic molecules could have formed from simple inorganic molecules.He proposed that the molecules formed colloid aggregates, or 'coacervates', in an aqueous environment. These coacervates were able to absorb and assimilate organic compounds from the environment in a way reminiscent of metabolism. They would have taken part in evolutionary processes, eventually leading to the first lifeforms.

Similarly, in 1929, before Haldane read about Oparin’s theory of a reducing atmosphere, Haldane also hypothesized that the early stages of Earth’s atmosphere was reducing, which could catalyze reactions that would form more complicated organic molecules from simpler molecules. Haldane hypothesized that the oceans served as a huge cooking pot where powered by the sun or lightning, chemical reactions could occur in an aqueous environment to form a huge diversity of organic compounds. Haldane proposed that the primordial sea served as a vast chemical laboratory powered by solar energy. The atmosphere was oxygen free, and the combination of carbon dioxide, ammonia and ultraviolet radiation gave rise to a host of organic compounds. The sea became a 'hot dilute soup' containing large populations of organic monomers and polymers. Haldane envisaged that groups of monomers and polymers acquired lipid membranes, and that further developments eventually led to the first living cells.

Haldane coined the term 'prebiotic soup' or 'prebiotic atmosphere' that consisted of an abundance of methane, ammonia, and water. This term became a powerful symbol of the Oparin-Haldane view of the origin of life.

Miller-Urey Experiment

In 1953, two scientists set out to test Oparin and Haldane's hypothesis. Harold Urey and his student Stanley Miller tried to calculate the chemical constituents of the atmosphere of the early Earth. They based their calculations on the view that the early atmosphere was reducing. In order to do this, they simulated early earth atmospheric conditions by creating a closed system which contained water, methane gas, ammonia, and hydrogen gas in 2:1:2 ratio along with water vapours and along with it kept a high temperature of 1073 K.Urey suggested that his student, Miller should attempt to synthesize organic compounds in this type of atmosphere.

Miller carried out an experiment in which he passed a continuous spark discharge at 75,000 volts through a flask containing the gases identified by Urey along with water. Furthermore, this electrical current was run through the laboratory set up to simulate the catalytic source of lightning that was present in the early atmosphere.

Miller found that after 18 days of circulating the mixture, most of the ammonia and much of the methane had been consumed. The main gaseous products were carbon monoxide (CO) and nitrogen (N₂). In addition, there was an accumulation of dark material in the water. Few of the specific constituents of this could not be identified, but it was clear that the material included a large range of organic polymers. From the results of their experiment, they found that up to 15% of the carbon in the system was inorganic compounds that had formed in the system. This conclusion proved that organic molecules could be formed from inorganic molecules in Earth’s early atmosphere. In addition, out of the organic molecules produced, Miller and Urey showed that some of the organic compounds were amino acids, which are necessary for living organisms.

Analysis of the aqueous solution showed that the following had also been synthesized:

1. 25 amino acids (the main ones being glycine, alanine and aspartic acid)

2. Several fatty acids

3. Hydroxy acids

4. Amide products.

Aftermath of the Miller-Urey Experiment

The Miller-Urey experiment was immediately recognized as an important breakthrough in the study of the origin of life. It was received as confirmation of the Oparin-Haldane hypothesis in that several of the key molecules of life could have been synthesised on the primitive Earth in the kind of conditions envisioned by Oparin and Haldane. These molecules would then have been able to take part in prebiotic chemical processes, leading to the origin of life.

Other similar experiments have been done to mimic early Earth conditions in an attempt to find other ways organic molecules could have formed. One experiment was done to mimic deep underwater volcano conditions. At these underwater volcanoes, catalytic heat as well as many minerals were constantly supplied. This provided an ideal system for organic molecules to be formed. This system was also found to produce amino acids, which is essential for living organisms to process into proteins.

Since the Miller-Urey experiment, a great deal of effort has been spent investigating prebiotic chemistry. It has become apparent that organizing simple molecules into assemblies capable of reproducing and evolving is a far greater task than was generally realized during the excitement that followed the experiment. In addition, the view that the early atmosphere was highly reducing was challenged towards the end of the twentieth century, and is no longer the consensus view.

Although the significance of specific details of the Miller-Urey for the origin of life may now be in question, it began the new scientific discipline of prebiotic chemistry, and has been enormously influential in the development of ideas about the origin of life.

References:

"Origin Of Life: Twentieth Century Landmarks." Origin Of Life: Oparin-Haldane Hypothesis. N.p., n.d. Web. 28 Oct. 2012. <http://www.simsoup.info/Origin_Landmarks_Oparin_Haldane.html>.

"The Miller/Urey Experiment." The Miller/Urey Experiment. N.p., n.d. Web. 28 Oct. 2012. <http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html>.

Reece, Jane B., and Neil A. Campbell. Campbell Biology. Harlow: Pearson Education, 2011. Print.

Overview

Molecular evolution is in part a process of evolution at the scale of DNA, RNA, and proteins. Molecular evolution emerged as a scientific field in the 1960s as researchers from molecular biology, evolutionary biology and population genetics sought to understand recent discoveries on the structure and function of nucleic acids and protein. Some of the key topics that spurred development of the field have been the evolution of enzyme function, the use of nucleic acid divergence as a "molecular clock" to study species divergence, and the origin of noncoding DNA. Recent advances in genomics, including whole-genome sequencing, high-throughput protein characterization, and bioinformatics have led to a dramatic increase in studies on the topic. In the 2000s, some of the active topics have been the role of gene duplication in the emergence of novel gene function, the extent of adaptive molecular evolution versus neutral processes of mutation and drift, and the identification of molecular changes responsible for various human characteristics especially those pertaining to infection, disease, and cognition.

In order for evolution to occur, three different processes are required and must be met:

1. Generation of a diverse population

2. Selection of members based on criterion of fitness

3. Reproduction to enrich the population in more-fit members

Nucleic acids are such molecules that are capable of undergoing all three processes. Combinatorial chemistry is the concept of rapidly producing an abundant amount of a specific molecule of interest. Because of this process, diverse populations of nucleic acid molecules can be synthesized. A common example of this process involves attempting to create an RNA molecule that specifically binds ATP and other nucleotides. First, a randomized pool of RNA sequences, called apimers, are placed into an ATP affinity column. Some RNA molecules are then found bounded to ATP. These molecules are released from the affinity column by adding excess ATP. This collection of molecules are allowed to replicate into DNA by using the enzyme reverse transcriptase. Once transcribed into DNA, the molecules undergo PCR, and ultimately transcribe back into RNA.

References

Class Project
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Introduction

The three domains of life

The different parts of a Eukaryote and Prokaryote

In the past thirty years, scientists were able to use technological discoveries to redefine the classifications of life on earth. In 1977, American microbiologist, Carl Woese altered the previous two-domain system of Eukaryota (Eukarya) and Prokaryota. The Prokaryota domain was split into the two separate domains of Bacteria and Archaea. Woese was able to look at the similarities and differences of living organisms at the genetic sequencing level. More specifically, Woese analyzed how closely organisms were related based on the 16S ribosomal RNA or rRNA present in all organisms. With the new knowledge from the study of organisms' biochemical differences, scientists were able to classify life on earth into three distinctive groups, or domains: Eukarya, Bacteria, and Archaea. Archaea is more closely related to Eukaryotes than Eubacteria

Classification

Eukarya

All multicellular organisms are made of eukaryotic cells

Most living plants and animals are composed of eukaryotic cells. Eukaryotic cells receive their name from the Greek eu meaning true and karyon suggesting that they have a true nucleus which contains their DNA. The determining factor for this domain is the presence of a distinct membrane-bound nucleus in each cell. The nucleus in eukaryotes consist of nuclear material enclosed with a double membrane, the nuclear envelope. Another distinctive feature of Eukarya is the presence of membrane-bound organelles in many of the cells. This type of cell can divide in two processes mitosis and meiosis. It keeps its DNA in chromosomes. This domain includes all multicellular organisms: plants, animals, fungi and protists. Interestingly, human beings and yeast are both classified under this same domain. Ciliates, fungi, flagellates, and microsporidia also belong in this domain.

Eukaryotic cells can be categorized in to two types of cells, animal and plant cells.

Genes from natural communities continually reveal new species of microbial eukaryotes in previously unknown divisions. The genetic analysis shows that similar miniaturized eukaryotes branch from all the groups in the phylogenetic tree.

Microbes include members of the three domains of life. The three domains of life are bacteria, archaea and eukarya.

Prokaryota

Out of the two prokaryotic cells are considerably simpler than eukaryotic cells. Bacteria and archaea are considered prokaryotes. Prokaryotic comes from the Greek prefix pro which means before and the Greek root karyon which means kernel. They tend to be smaller in size and are not as complex as eukaryotic cells. As their names suggests, prokaryotic cells lack any organelles enclosed with a membrane. Their DNA is in an area called the nucleoid which also is not enclosed by a membrane. The nucleoid is not separated from the cytoplasm. Generally speaking, their chromosomes and plasmids are circular and naked, they have no nuclear envelope. In addition, prokaryotic cells control genes via operons. But, like eukaryotic cells, they have a plasma membrane surrounded by a cell wall. However, their cell wall contains petidoglycans which can't be found in Eukaryotic cells.

Prokaryotic cells have been split in to two distinct groups, bacteria and archaea.

Bacteria

This domain was considered prokaryote. It consists of all the unicellular prokaryotic microorganisms that do not have a cell nucleus or contain any organelles in their cells. Bacteria can be spherical, rod-like, or spiral in shape. Although bacteria contain ribosomes also found in Eukaryotes and Archaebacteria, bacterial ribosomes are much different in structure. Bacteria are characterized as having a cell wall (except for mycoplasma). Their cell walls are called peptidoglycans, which is composed of peptides and sugars. The cell wall gives the cell structure and shape. It also gives rigidity to prevent water from flowing into the cell membrane of the bacteria and causing the cell to burst. The cell wall usually lies exterior to the cell membrane as in Gram Positive cells. But in the case of Gram Negative bacterial cells, there is a second cell membrane that is exterior to the cell wall. This gives these types of bacteria unique properties to help it survive. Bacteria has straight chain fatty acids ester linked to glycerol. The membrane of bacteria is bilayer. The wall of bacteria has peptidoglycan or none. An example of bacteria can be mycoplasma. Mycoplasma has no wall.

Bacteria also have flagella that help with their motility. The bacteria rotate their flagella in a counterclockwise manner and swim towards an attractant. To move away from a repellent, the flagella rotates clockwise and tumbles to change directions. Bacteria also have pili, which are used mainly for attachment and exchange of genetic information.

The gram stain differentiates bacteria into two fundamental varieties of cells based on staining properties of the cell wall. The two common types are: 1) Bacteria that retain the initial CV stain (purple) = gram +. 2) Those that are decolorized and stain pink with safranin = gram -. Gram positive has thick walls that resists de-colorization and is composed of PG and teichoic acids. Teichoic acids anchor layers of peptidoglycan together. Lipoteichoic acid connects peptidoglycan to the membrane. Some species have capsules: Sugar (polysaccharides)coating. They can protect against phagocytosis. The capsule is slippery, so when a phagocyte tries to eat the bacteria, it can slip away. It may also contribute to virulence and help cells stick to surfaces. Some species have S-layers. S-layers consist of protein or glycoprotein. Its function is not totally clear, but its tough proteins may deter predators. It may contribute to cell shape and aid in protection against osmotic stress.

Gram negative has thin peptidoglycan layer outside the inner membrane (cytoplasmic membrane). It has lipoprotein (murein lipoprotein). Lipoprotein binds peptidoglycan to the inner leaflet of the outer membrane. The outer membrane has lipid-protein bilayer, which can be disrupted by de-colorization. The outer membrane has phospholipids, porins and LPS. The outer membrane's function is to protect permeability barrier (impermeable to large molecules and hydrophobic compounds from the environment). The porins of gram negative are channels that allow for transport of molecules across the membrane (sugars, amino acids, vitamins, etc, some antibiotic get through them as well. Porins provide permeability for entering and exiting compounds. The LPS of gram negative coats the surface of the cell. It increases the negative charge of the cell membrane-excludes large hydrophobic compounds such as bile salts and antibiotics (permeable only to low molecular weight, hydrophilic molecules). LPS may play a role as an adhesin used in colonization of the host. It helps protect cell against phagocytosis. The LPS of gram negative has three parts. The three parts are lipid A, core oligosaccharide and O polysaccharide. 1) Lipid A is important pathogenic factor (endotoxin- when released into a host's blood stream or GI tract causes toxicity of gram - infection. 2)The core oligosaccharide component that attaches directly to lipid A. One of the glucosamines of lipid A is linked to the core polysaccharide, extending out from the cell. The core oligosaccharide consists of 5 sugars. 3)O polysaccharide is the outermost domain of the LPS molecule. It is a repetitive glycan polymer attached to the core oligosaccharide. The composition of the O chain varies from strain to strain. Greater 160 different O antigen structures produced by different E. coli strains. These chains form a layer that helps bacteria resist phagocytosis by white blood cells. O-polysaccharides serves as an antigen or cell surface marker. It is exposed on the very outer surface of the bacterial cell, and, as a consequence, is a target for recognition by host antibodies. Variations in LPS structure provide for the existence of different antigenic strains of a pathogen that may be able to bypass a previous immunological response to a related strain .

Some bacteria have walls of intermediate structure and they stain in a variable manner.

Mycobacteria have a cell envelope which is not typical of Gram positives or Gram negatives. The mycobacterial cell envelope does not consists of the outer membrane characteristic of Gram negative bacteria, but has a significant peptidoglycan-arabinogalactan-mycolic acid wall structure which provides an external permeability barrier. Moreover, mycobacteria contain waxy mycolic acids. They have thickest cell walls known. They are highly resistant to antimicrobials due to thick waxy envelope. The envelope also makes it hard to take up nutrients: slow growers. Mycobacteria have no dyes permeate thus neither gram positive nor gram negative .

Mycoplasma is considered to be bacteria. The lack of a cell wall makes these species resistant to antibiotics that target cell wall synthesis. There are no cell walls to protect from osmotic pressure, so they use sterols to strengthen membrane or they live inside other cells.

Bacteria inhibit soils, surface waters, tissues of other living or decaying organisms. Thermotoga, flavobacteria, cyanobacteria, purple bacteria, gram-positive bacteria, and green nonsulfur bacteria belong in this domain.

Archaea

Archaea is considered to be prokaryote. Archaea is referred to as "archaebacteria." They have the appearance of bacteria with their unicellular rod and spiral-like structures. They are prokaryotes and lack a cell nucleus and other organelles. Archaea have the most extreme variations in phospholipid side-chain structures. Archaea have a branched hydrocarbon and ether linked to glycerol. Ether links between glycerol and fatty acids. Hydrocarbon chains are branched terpenoids (polymeric structures derived from isporene-every 4th carbon extends a methyl branch). The branches strengthen the membrane by limiting movement of the hydrocarbon chains. The membrane of archaea is bilayer or monolayer. In the wall of archaea, there are peptidoglycan and S-layer. Unlike bacteria, Archaea possess genes and pathways that evolutionarily link them closer to eukaryotes than to bacteria. Archaea contain the enzymes used for transcription and translation which reflects their closer similarity to eukaryotic organisms rather than bacteria. However, they are unlike eukaryotes in that they use a larger range of sources in order to gain energy. As a result of their differences from bacteria and eukaryotes, Archaea was placed into its own category. Archaea can live in extreme and harsh conditions not suited for other living organisms. For example, thermophiles occupy hot springs or volcanoes, while halophiles inhabit salty bodies of water such as the Dead Sea. Archaea were later discovered in a broader range of habitats. The extreme environments that many archaea live in are at high pressures, salt concentrations or temperatures. Archaea are the best extremophiles. Archaea have such variability of wall structure that the Gram stain is not a useful differentiating tool.

There are other several traits of Archaea. Archaea has membrane lipids (diethers or isoprenoid L-glycerol ethers; the membrane lipid chains stiffen by covalent cross-links or by forming pentacyclic rings. Also, Archaea has Pseudomurein sacculus that contains talosaminuronic acid whereas the peptide bridges has only the L-amino acids.

In the Archaea gene regulation, the genomes of Archaea is similar to Bacteria in size and gene density, and genes of related function are often arranged in operons. Some tRNA gene sequences are interrupted by nontranslated sequences. These nontranslational sequences are called introns. So, the transcription factors is similar to eukaryotes.

Archaea includes two phyla: Crenarchaeota and Euryarchaeota. In the phylum Crenarchaeota, it has a substantial proportion of soil, marine, and benthic microbial communities. The metabolic crenarchaeotes metabolize sulfur, and it is either by aerobic oxidation or anaerobic reduction (which means no oxygen is required). However, in the phylum Euryarchaeota, it has members in soil and water. They have more metabolism than that in Crenarchaeota. Methanogens are the divergent group of Euryarchaeota.

Thermoplasma can be considered as archaea. Thermoplasma are thermoAcidophiles that thrive at pH<2. They are tetraether lipid monolayer membrane. Extreme halophiles, methanogens, and extreme thermophiles belong in this domain.

References

Slonczewski, Joan L. Microbiology. 2nd ed. New York, 2009.

Introduction

Archaea are a branch of the three-domain system of life that contains single-celled microorganisms. In 1977, Carl Woese was studying recently discovered prokaryotes in hot springs. With much observations, he noticed that there was a unique sequencing of 16S rRNA genes from many of the organisms. He noticed that the sequencing of the prokaryotes were a distinct form of life in itself- he called them archaea. This transformed the previous 5-kingdom system (plantae, animalia, fungi, protists, and prokaryotes) drawn by Lynn Margulis into 3 equally distinct groups (bacteria, archaea, and eukarya). Woese argued that the bacteria, archaea, and eukaryotes each represent a primary line of descent that diverged early on from an ancestral progenote with poorly developed genetic machinery. This hypothesis is reflected in the name archaea, from the Greek archae or ancient. Later he treated these groups formally as domains, each comprising several kingdoms. This division has become very popular, although the idea of the progenote itself is not generally supported. Some biologists, however, have argued that the archaebacteria and eukaryotes arose from specialized eubacteria and actually are from the same origin. Archaea are a class of prokaryotes.

Similarities and Differences to Bacteria and Eukarya

It is thought that archaea and bacteria diverged early in their existence because of all the dissimilarities between the two groups. Archaea are similar to other prokaryotes in most aspects of cell structure and metabolism. Both bacteria and archaea are enclosed in cell membranes. But unlike most bacteria, they have a single cell membrane that lacks a peptidoglycan wall and their genetic transcription and translation - the two central processes in molecular biology - do not show the typical bacterial features, but are extremely similar to those of eukaryotes. Archaea are evolutionarily more related to eukaryotes than they are to eubacteria, even though eubacteria and archaea are both prokaryotic groups.

Eukarya and archaea are noted to have many similarities especially in regards to metabolic pathways. For example, enzymes present in transcription and translation of archaea are more closely related to those of Eukarya than bacteria. They both use elongation factors, and their transcription involves TATA-binding proteins and TFIIB as in eukaryotes.

Several other characteristics also set the archaea apart. Both bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids, whereas archaea have membranes composed of glycerol-ether lipids. These differences may be an adaptation on the part of archaea to hyperthermophily. Archaea also have flagella that are notably different in composition and development from the superficially similar flagella of bacteria.

The relationship between archaea and Eukarya remains an important problem. Aside from the similarities noted above, many genetic trees group the two together. Some place eukaryotes closer to Eurarchaeota than Crenarchaeota are, although the membrane chemistry suggests otherwise. However, the discovery of archaean-like genes in certain bacteria, such as Thermotoga, makes their relationship difficult to determine. Some have suggested that eukaryotes arose through fusion of an archaean and eubacterium, which became the nucleus and cytoplasm, which accounts for various genetic similarities but runs into difficulties explaining cell structure.

Properties

Individual archaeans range from 0.1 to over 15 μm in diameter. They occur in various shapes, such as spherical, rod-shaped, spiral, lobed, or rectangular. They also exhibit a variety of different types of metabolism. Of note, the halobacteria can use light to produce ATP, although no archaea conduct photosynthesis with an electron transport chain, as occurs in other groups.

thermophile bacteria

Archaea are bound by a plasma membrane that is layered by pseudopeptidoglycans instead of peptidoglycans. So unlike for the latter, the membrane is resistent to antibiotics that target and prevent the synthesis of peptidoglycan wall. The pseudopeptidoglycans consist of polysaccharides that give the archaea a rigid structure. The archaeal membranes consist of a glycerol-1-phosphate backbone with ether linkages between glycerol and fatty acids. The fatty acids are hydrocarbons that are strengthened due to branched terpenoids (polymeric structures derived from isoporene. This limits the movement of the chains). The membranes can exist as bilayers or monolayers. They contain cytosol, a nucleoid, metabolites, coenzymes, inorganic ions and enzymes. Archaea reproduce asexually; they divide by binary fission, fragmentation and budding. Unlike the other two branches, they do not spore. Unlike bacteria, no pathogenic archaea have ever been identified.

The most common cell wall type for archaea is the S-Layers. The S-Layers are made up of proteins and glycoproteins in a hexagonal symmetry. But archaea does not always have a wall. For example, thermoplasmas thrive at acidic environments lower than 2. These archaeans have a unique tetraether lipid monolayer membrane that is not acid labile. This unique membrane structure is what allows it to withstand extreme environments.

Pseudopeptidoglycan

Unlike bacteria that have their cell walls made of peptidoglycan, archaea, mostly methanogens, have cell walls made of pseudopeptidoglycan. The difference is in the sugars that make up the peptidoglycan backbone. In peptidoglycan, the two sugars are N-acetylglucosamine (NAG) and N-acetylmuramic (NAM) acid. While in pseudopeptidoglycan, the NAM is replaced by N-acetyltalosaminuronic acid (NAT). NAG and NAT are bonded by a β (1,3) sugar linkage instead of a β (1,4) linkage.

This is significant, because it makes these archaea resistant to the enzyme, lysozyme, which only breaks down β (1,4) sugar linkages like those found in peptidoglycan.

Pseudopeptidoglycan also has different amino acids used for their peptide cross links. The order of the attached amino acids is D-glutamine, L-alanine, L-lysine, and D-glutamine. The peptide bond that forms is between the L-lysine a of one NAT and the second D-glutamine of a parallel NAT. These different amino acids make it so antibiotics such as vancomycin and penicillin have no effect on these cell walls.

Ecology

Archaea are commonly known as "extremeophiles", prevailing in extreme environments for they love to live under extreme, harsh environments of pH, temperature and salinity. Certain Archaea are called thermophiles because they are found in extremely high temperature environments, such as in the hot springs of volcanoes. There are psychrophiles that thrive in low temperatures, such as Antarctica. Others can live under highly acidic conditions (acidophiles), as well as highly saline environments (halophiles). This characteristic is very specific to Archaea. Out of the three domains, they are generally the only ones known to live and thrive under these extreme conditions. However, a large number of them also live in non-extreme environments, with the plankton community among the ocean waters.

Archaea can survive in both aerobic and anaerobic environments. Aerobic means in the presence of oxygen, while anaerobic means very a very small amount of oxygen available. In the past they have been known to inhabit extreme environments such as high-acidity bogs and ocean depths. However, it is now known that they inhabit soils, ocean and marshland, and might be one of the most abundant organisms on Earth. Under the archaea group, there are four phyla based on rRNA trees: Crenarchaeota, Euryarchaeota, Korarchaeota, and Nanoarchaeota. Crenarchaeota and Euryarchaeota are the only two that have been heavily researched. The other two other groups have been tentatively created for certain environmental samples and the peculiar species Nanoarchaeum equitans, discovered in 2002 by Karl Stetter, but their affinities are uncertain.

Crenarchaeota

Crenarchaeota consists of these following divisions:

a. Hyperthermophiles: are thermophilic (love hot weather) and acidophilic ( can live under low pH environment)...organisms in this type are usually found in hot sulfur spring, can even live in pH 0.9...example : Sulfobus, Pyrolobus fumarii, etc..

b. Extreme halophiles: include organisms living in highly salty environments...example: halococcus,

c. Thermoplasma: similar to hyperthermophiles type (love high temperature and low pH environments) but they lack cell wall... They were found in coal deposits.

Euryarchaeota

Methanogens is single division under this clade. As its name implies, methanogens release CH4 (methane) as waste product by reducing CO2 (carbon dioxide)...Methanogens are obligate anaerobes which are poisoned by Oxygen. Examples of methanogens are Methanobacterium bryantii, Methanopyrus, etc...

http://www.bio-medicine.org/biology-definition/Archaea/

Groups of Archaeabacteria

There are 3 groups of archaebacteria: the methanogens, the halophiles and the thermoacidophiles. Methanogens produce energy by converting H2 and CO2 into methane gas. They are found in the intestinal areas of humans and some animals such as cows and in the marshes. Halophiles live in a high salt atmosphere. Therefore, they are found in the Great Salt Lake, Dead Sea and other areas with a high salt concentration. Thermoacidophiles are found in the areas with a very high temperature and very acidic circumstances. They can be found in hydrothermal vents and volcanic vents.

Characteristics of Archaebacteria

Archaebacteria are force anaerobes and they live only in oxygen-free circumstances. They are known as extremophiles, as they are capable to live in a variety of atmosphere. Some species can live in the temperatures over boiling point at 100 degree Celsius. They can also live in acidic, alkaline or saline aquatic surroundings. Some can endure the pressures of more than 200 atmospheres.

The size of archaebacteria varies from 1/10th of a micrometer to more than 15 micrometers. Some of archaebacteria have flagella. Like all prokaryotes, archaebacteria don't have the membrane-bound organelles. They don't contain nuclei, endoplasmic reticula, Golgi complexes, mitochondria, chloroplasts or lysosomes. The cells consist of a thick cytoplasm that includes all the compounds and molecules needed for metabolism and nutrition. Their cell wall doesn't contain peptidoglycan. The rigid cell wall backings the cell and allows archaebacterium to hold its shape. It also defends the cell from overflowing when present in a hypotonic environment. Archaebacteria have lipids in their cell membranes. They are self-possessed of branched hydrocarbon chains, linked to glycerol by ether linkages.

Since these organisms don't have a nucleus, the genetic material drifts freely in the cytoplasm. They contain rRNA. DNA contains a single, circular molecule, which is compact and tightly twisted. None of the protein is associated with DNA. The archaebacterial cell may contain plasmids, which are small, circular pieces of DNA. They can replace independent of larger, genomic DNA circle. Plasmids often code for antibiotic resistance or particular enzymes.

Archaebacteria duplicates by an asexual procedure known as binary fission. During this process, the bacterial DNA replicates. The cell wall pinches off in the center, due to which the organism is divided into two new cells. Each cell consists of a copy of circular DNA. It is quite fast method. Some species separate every 20 minutes. Although genetic material can be exchanged between the cells by three various processes, sexual reproduction is not seen in archaebacteria.

During transformation, DNA fragments unrestricted by one bacterium are taken up by another bacterium. In the process of transduction, a bacterial phage transfers genetic material from one organism to another. In the process of conjugation, genetic material is interchanged between two bacteria. These mechanisms lead to genetic recombination, causing the continual evolution of archaebacteria.

Reference

http://www.ehow.com/list_7419364_cell-characteristics-archaebacteria.html http://science.jrank.org/pages/476/Archaebacteria.html
Slonczewski, Joan, and John Watkins. Foster. Microbiology: An Evolving Science. New York: W.W. Norton, 2011. 717-21. Print.

General information

Post translational modifications of proteins were first found in eukaryotes, and recently, similar modifications were also found in some bacteria. Discovery of these proteins that have similar structure as that of ubiquitin can create polymeric chains and proteins that can result in covalent modification brings in new view to the three domains of life.

Ubiquitin are related modifiers

Some of the modifications of proteins are phosphorylation, acetylation, and glycosylation. Some of the eukaryotic proteins used to study these modifications include ubiquitin and ubiquitin-like derivatives. Ubiquitin is a somewhat small protein, which contains 76 amino acids. In the target protein, Gly at the C-terminus of ubiquitin often involves in conjugation with Lys residue. Through the use of ATP, Gly at the C-terminus is adenylated by an activated enzyme, and the adenylated Gly is then transported to a cysteine active site in the activated enzyme itself. Then, ubiquitin moves to a conjugating enzyme and the conjugated ubiquitin moves to a protein ligase. At the ligase, a bond forms and connects an isopeptide to the Lys residue on the target substrate. Further modification of ubiquitin can be carried out using excess ubiquitin in the N-terminus or Lys residue. A regulatory complex called the 26S proteasome recognizes proteins that need degradation. At the complex, ubiquitin is prepared for further ubiquitylation via the deubiquitylating protease (DUBs). There are several ubiquitin like proteins, containing the same β grasp fold as that of ubiquitin, found in eukaryotes. All sequence of the ubiquitin like proteins end in Gly-Gly. They are often used for changing the interaction between proteins, transferring target proteins to certain locations of the cell. Due to their functions, ubiquitin and ubiquitin like proteins play a significant role in eukaryotes.

A bacterial small protein modifier and Ubl proteins

In recent discovery, protein post translational modification is found in archaea and bacteria. Researchers are looking to understand the mechanism for targeted protein degradation. It was found that in mycobacteria a prokaryotic ubiquitin like protein (Pup) acts as the starting signal for proteasomal degradation. Different from the ubiquitin folding, Pup is a disordered protein, meaning that despite its ending sequence at the C-terminus is Gly-Gly, and it attaches to substrate through Glu rather than the usually used Gly. No polypupylation is observed, so it is assumed monopupylation may be sufficient for protein degradation. Only a few organisms contain Pup homologues whereas bacteria and archaea contain ubiquitin. For example, proteins like ThiS and MoaD have β grasp folding like ubiquitin, and they undergo similar reaction as that of ubiquitin. However, the difference is that ThiS and MoaD transfer sulfur instead of forming conjugation in proteins. Another protein called Urm 1, has similar structure as ThiS and Moad, carries sulfur in tRNA modification pathway. From these protein properties, one proposal is that bacterial protein that has similar structure as that of ubiquitin may covalently modify other proteins. An experiment was conducted to test the formation of conjugates in a halophilic archaeon. The criteria for the experiment were that the proteins used are small and those proteins must have an ending sequence of Gly-Gly. Out of the tested proteins, three out of five were in conjugate with the ubiquitin like protein, while the two remaining small proteins made conjugate with other proteins. From the obtained result, the two conjugated proteins are referred to as archaeal modifier protein. The factors that affect the modification properties are the growth conditions and the presence of proteasomal subunits. Result from the experiment suggested the archaeal modifier proteins may be involved in certain feedback inhibition if there is no full repertoire of proteasome function. Removal of the ending sequence Gly-Gly from the archaeal modifier proteins abolished SAMPlyation for the two proteins are each named SAMP1 and SAMP2. SAMP1 sites are revealed by the addition of Arg or Lys at the C-terminus of the protein. However, it seems that Lys is the target amino acid in the conjugation process, even though it might be the case that SAMP1 conjugates with other amino acids rather than Lys. Under high concentration of salt, the protein folding and protein activity are maximized. In contrast to SAMP1, SAMP2 conjugates with Lys58, a feature of polyubiquitylation. It remains unknown how many SAMP2 can fit into one chain of polymers for proteasomal degradation. Recent research suggests that phosphorylation is significant for target proteins in the process of proteasomal degradation since protein can regulate ubiquitylation in eukaryotic substrates.

SAMP enzymology

An isopeptide bond is formed between SAMP2 and MoeB, a homologue in the Urm 1 pathway, at Lys113. The formation of this bond suggested that the activity and stability of MoeB are regulated by SAMP2. The enzymes involved in forming conjugates might have similar structure as eukaryotic conjugating enzymes. The SAMPs proteins forms conjugates with other proteins via Gly rather than Glu. Furthermore, SAMPylation involves a homologue of the eukaryote. For the above two conditions, it suggested that the archaeal system has more similarities to the eukaryotic ubiquitin system and Pup system of bacterial has less similarities. The proteins that are involved in the deconjugation process of ubiquitin like proteins in eukaryotes have homologues in both bacteria and archaea.

Prospects

The newly discovered ubiquitin like proteins in archaeans gives new direction to the study of proteins targeting to a proteasome. There is no strong evidence to support that SAMPs are involved in the targeting protein to proteasome as in eukaryotic and bacterial species. To finding the function of proteasome having on SAMPylation, closely examine the transcriptional changes in the mutants of proteasome. The mutated proteasome function facilitates the translation of specific transcripts, thus leading to protein accumulation. One more finding is that all prokaryotes contain the ubiquitin like proteins, meaning that the ubiquitin like protein is not limited to be found in archaea. Despite that the post translational modifiers were thought to be found only in eukaryotic domain, the new discovery lead to new understanding that post translational modifiers are also found in every type of organisms.

Reference

K. Heran Darwin and Kay Hofmann. "SAMPyling proteins in archaea". http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2892244/pdf/nihms-188535.pdf

Scanning electron microscope image of Vibrio cholerae bacteria, which infect the digestive system.

Bacteria makes up one of the three domains of life. It is part of the prokaryotic domain and is often referred to as "Eubacteria" to distinguish its domain from "Archaebacteria" or Archaea. Unlike eukaryotes, bacteria have nucleoids instead of nuclei. They are diverse metabolically, and their cell walls are composed of peptidoglycan. Bacteria is often found in tissue of other organisms, soils, or water surfaces. Bacteria have specific structural characteristics including a cell envelope, ribosomes, nucleoid, pili, and flagella.

The term "bacteria" has been associated with many negative functions in life, such as human disease. However, bacteria are vital for many processes. In fact, many more bacteria cells exist than human cells in the body, especially on the skin and digestive tract. Bacteria can also be used to produce food, such as yoghurt. Bacteria are also important in the fields of biotechnology and gene therapy due to their possession of circular DNA called plasmids, which contain the genes that encode antibiotic resistance. As such, the study of prokaryotic plasmids grants scientists a deeper understanding of the genes and proteins they encode.

The cell wall, which is made of peptidoglycan, is essential for the survival of bacteria. The way an antibiotic like penicillin is able to kill bacteria is to inhibit a step in the peptidoglycan synthesis. There are two different types of cell walls - gram positive and gram negative. Most bacteria have gram negative.

Molecular studies suggest the existence of 50 or so bacterial phyla (considered kingdoms by some). However, the structural and metabolic features of about half of these are unknown. Though some members of domain bacteria live in extreme environments, many more favors moderate conditions. Many bacteria form symbiotic associations with eukaryotes and are thus of concern in medicine and agriculture. The most important two phyla that are important to eukaryotic cell evolution, global ecology, and human affairs, are Proteobacteria and Cyanobacteria.

Cell Wall

Peptidoglycan Structure

The cell wall in bacteria serves as a physical barrier between the cell and its surroundings. It maintains the shape of the cell, protects from physical penetration and prevents the cell from lysing under hypotonic conditions. The rigidity of the cell well is due to Peptidoglycan, a grid of sugar polymers connected by short amino acid chains, usually 3-5 amino acids long. Peptidoglycan is unique to the cell walls of bacteria, as eukaryotic cell walls are generally made of chiten or cellulose, and archaea bacteria have cell walls composed of other polysaccharides and proteins. The cell wall of bacteria falls into two categories, Gram-positive and Gram-negative, named after the gram strain test that initially separated the two categories.

Gram Stain

Gram staining uses the structure of the cell wall to differentiate two main types of bacteria: Gram-positive and Gram-negative. It is often one of the first steps used in identifying prokaryotes. However, there are exceptions that make this method fallible, as there are some organisms that are Gram indeterminate or Gram variable.

All bacteria have a cytoplasmic membrane that encloses the cell. On top of that, only Gram-positive bacteria have thick peptidoglycan cell wall. Although Gram-negative bacterial cells have very thin peptidoglycan, they have an extra layer called the outer membrane. Gram staining focuses on the differences of such structure in its method of identification. There are four main steps to this staining. The first step is to carry out the primary stain using crystal violet (deep blue) after the bacterial cell is heat fixed. This dye stains the surface of both Gram-positive and Gram-negative blue. After residual dye is rinsed off, Gram's iodine or Lugol's solution is added to help increase the crystal violet's interaction with the negatively charged surface, increasing the binding. Although Gram's iodine is of dark brown color, it does not change the color of the primary stain. Crystal violet-iodine (CV-I) complex may form in this step. Adding the decolorizer is the crucial step of this process. The decolorizer solubilizes lipids and dissolves the outer membrane structure. After the outer membrane is gone, it can easily pass through the thin, porous layer of peptidoglycan as well as the cell membrane. It flushes out the CV-I complex, resulting in the Gram-negative cell being destained and colorless. The thick peptidoglycan layer of the Gram-positive bacteria slows down the entry of decolorizer. Since the decolorizer is also a dehydrating agent, it shrinks the pores in the peptidoglycan layer so as the CV-I complex is going to be flushed out, they are trapped within the layer. The timing of decolorization is important and can be halted by rinsing with deionized water. The last step is the counterstain. This provides a contrasting color to the primary stain, and the staining depends on the amount of crystal violet already present. This will stain the Gram-negative cells readily but cause little to no effect to the blue coloration of Gram-negative cells.

Gram-Positive

Gram-Positive vs. Gram-Negative

Gram-Positive bacteria is characterized by its much thicker layer of peptidoglycan in the cell wall, usually 20 to 80 nanometers thick and composing about 90% of the dry cell weight. In most gram-positive bacteria it is the only component of the cell wall, located just outside the plasma membrane. This makes Gram-positive bacteria susceptible to common antibiotics such as penicillin. Penicillin works by preventing the protein cross-linking of polysaccharides in Peptidoglycan, thus rendering the cell defenseless after its failure to produce Peptidoglycan.
Other characteristics of a gram-positive bacteria are often capsules, which are polysaccharides layering outside the cell walls that are slippery and prevent the bacteria from phagocytosis. There is also the S-Layer that's made of proteins. Unfortunately it is hard to culture the S-Layer of bacteria in laboratory. Something very unique and specific to gram-positives are teichoic acids. These are acid substances embedded in the cell walls that strengthen the gram positive cell walls of bacteria. They are made up of chains of phosphodiester-linked glycerol/ribitol, with sugars or amino acids linked to the middle. They are largely negatively charged is responsible for the negative charge of the bacteria surface as a whole. Certain teichoic acids, lipoteichoic acids, are covalently bound to membrane lipids.

Gram-Negative

Gram-negative bacteria is characterized by its thinner layer of peptidoglycan in the cell wall, usually only 7 to 8 nanometers thick and composing about 10% of the dry cell weight. This reduction in peptidoglycan is due to the fact that Gram-negative bacteria have another protection layer in the form of lipopolysaccharides, located outside both the plasma membrane and peptidoglycan layer. The lipopolysaccharides (LPS) greatly enhance the cell rigidity as well as protect the cell from certain chemical attacks. The LPS has many useful functions to the gram negative bacteria: increases the negative charge of the cell membrane, excludes large hydrophobic compounds, and protects from phagocytosis.

The LPS is made up 3 components: Lipid A, Core oligosaccharide, and O polysaccharide. The Lipid A is an important pathogenic factor. It contains endotoxins of pathogens such as E. Coli and Salmonella. The core oligosaccharide is a non-varying linker component that attaches directly to lipid A. It is made up of about 5 sugars. Finally, the O polysaccharide is the outermost part of LPS.The layers resist phagocytosis by white blood cells. It serves as an antigen, but is also a target for recognition by host antibodies.

The LPS layer in gram-negative bacteria protects the peptidoglycan layer from being deteriorated by penicillin. Other drugs, such as ampicillin, have been created to attack the LPS layer in certain gram-negative bacteria. This is significant as without the LPS layer gram-negative bacteria dies. Gram-negative bacteria are generally much more pathogenic than Gram-positive bacteria, as LPS is an endotoxin.Treating gram negative bacteria is hard because the increasing concentration of lipid A, a part of LPS, in patient's blood would lead to peptic shock and dead.

Quorum Sensing

In order to regulate gene expression in a cell population-dependent manner, Gram-negative bacteria uses quorum-sensing signal in response to the lack of nutrients, competition with other bacteria which use same nutrients, and increase of toxic material in their surroundings. The major molecule of quorum-sensing signals is N-Acyl-L-homoserine lactones (AHLs). When this molecule is accumulated to threshold concentration, AHLs bind to and activate the transcription of genes.

Peptidoglycan

Peptidoglycan is a three dimensional mesh, made up of repeating disaccharide subunits cross-linked by stem peptides, which surrounds the entire bacterial cell wall. Peptidoglycan helps in maintaining cell shape, protecting against distress due to difference in osmotic pressure, and helping in the formation of daughter cells during cell division. Biosynthesis of peptidoglycan occurs in two different cellular compartments: the cytoplasm and the periplasm.

Cytoplasm

Biosynthesis of peptidoglycan begins in the cytoplasm which contains the ATP-dependent amino acid ligases of the Mur family of enzymes. First to occur is the generation of UDP-MurNAc by MurA and Mur B with the addition of a stem peptide to the UDP-MurNAc by MurC, D, E and F creating the substrate UDP-MurNAc-pentapeptide. This substrate is then ligated to an undecaprenyl phosphate carrier lipid by the integral membrane protein Mra Y, creating the membrane bound substrate Lipid I. The last step is the addition of a GlcNAc molecule from another UDP-MurNAc to Lipid I creating Lipid II. Lipid II is then transferred to the periplasm where the next stage begins. Because all peptidoglycan biosynthesis intermediates are soluble, in order to prevent them from diffusing across the membrane, a cytoplasmic complex forms which regulate their diffusion and facilitates the transfer of Lipid II to the periplasm. This cytoplasmic complex is made up of the soluble MurA-F proteins, the integral membrane MurG and MraY proteins as well as the cytoskeletal MreB. MreB forms the helical cables along the entire cell and helps maintain the cell’s shape. MreB also affects where the MurB-G enzymes localize in the cytoplasm.

Periplasm

Once in the periplasm, Lipid II undergoes both polymerization of glycan chains, called glycosyltransfer (GT), and cross-linking of the stem peptides, called transpeptidation (TP). Both reactions are catalyzed by penicillin-binding proteins (PBP’s), a form of peptidoglycan synthases. There are two types of PBP’s working to catalyze the GT and TP reactions. High molecular mass PBP’s have either both GT and TP domains (Class A PBP’s) where the GT and TP reactions occur or a TP domain preceded by an N-terminal pedestal (Class B). Low molecular mass PBP’s on the other hand regulate cross-linking reactions by cleaving the peptide bonds in the stem peptide of Lipid II. In the TP reactions, the C-terminus of the peptide chain is mostly in the active site cleft of the PBP while the N-terminus points out towards the solvent. The freeing of the N-terminus allows for TP reactions to occur between the amino acid of the third stem peptide residue and a neighboring peptide. In studies with E. coli bacteria, class A PBP’s have shown to interact with the outer membrane bound enzyme MltA. Both interact with one another via the scaffolding protein MipA which is also linked to the outer membrane, implying that peptidoglycan-synthesizing macromolecule complexes are able to anchor themselves to both the inner and outer membrane. Interestingly MreB has also shown to participate in the biosynthesis of peptidoglycan in the periplasm. MreB determines the helical pattern in which the peptidoglycan precursors are inserted into the cell wall as well as promoting the GT activity of PBP’s. In order for this to occur MreB forms a cytoplasmic complex with the inner membrane protein RodZ. Depending on the cyplasmic region of RodZ, it co-localizes with MreB in a helical manner. The MreB and RodZ complex acts as a ‘trans-membrane’ link between the cytoplasm, cytoskeleton and the periplasm which not only stabilizes cell wall elongation but also ensures that the peptidoglycan precursors are inserted in the cell wall in a spiral-like fashion.

Penicilin Binding Proteins

PBP’s are found between the membrane and the periplasm where both Class A and Class B PBP’s form a complex with the membrane-associated protein MreC via MreC’s β sheet-rich C-terminal region. It is MreC which acts as the framework for the periplasmic peptidoglycan-biosynthetic proteins and who also participates in cell wall elongation, interacting with RodZ. Along with participating the biosynthesis of peptidoglycans, PBP’s are also the targets of β-lactam antibiotics which covalently bonds to the bacterial cell wall by mimicking the structure of the stem peptide substrate. Over the years bacteria’s have become more resistant to β-lactam antibiotics. This is mostly due to mutations in the PBP’s sequence along with structural modifications of the enzymes around their catalytic cleft. These mutations and modifications decrease the binding stability of the antibiotics stopping them from binding to the bacterial cell wall and killing the bacteria.

MreB Cytoskeleton as spatial coordinator of cell wall biosynthesis complex

MreB is a cytoskeleton element that assembles into filamentous structures within the bacterial cytoplasm.MreB and its homologs have been shown to interact and co-localize with cytoplasmic protein( MurB-G), membrane-imbedded proteins ( MreD, MraY and RodA) as well as other molecules with large periplasmic domain in organism. Recent research shows that peptidoglycan precursors are inserted into cell wall following helical pattern which is dependent on MreB and It’s also reported that MreB also promote the GT activity of PBPs. This ability of MreB is because of RodZ, an inner membrane protein containing an 80-residue, N-terminal cytoplasmic region and a 200-amino acid periplasmic C-terminal tail. RodZ co-localizes with MreB helices in a manner that is strictly dependent on its cytoplasmic region. MreB- RodZ complexes act as a major stabilizing factor in bacterial cell wall and ensure s the insertion of new peptidoglycan in a spiral like fashion into the cell wall.

Internal Structure

Average Bacterial Prokaryote Cell Structure

Bacteria are simple organisms with no membrane-bound organs.

Nucleoid

the area that contains all of the DNA for the bacteria. It has a circular structure and is not bound

Ribosome

has the same function as those eukaryotes. It translates the genetic material from RNA to produce proteins.

Storage Granuoles

stores nutrients and reserves.

Endospore

the formation of a proteinaceous coat surrounding bacterial spore providing protection against enzymatic and chemical changes in the environment such as high temperatures, change in pH, or UV irradiation.

Flagella

Types of Flagella

A- monotrichous, B- lophotrichous, C-amphitrichous, D- peritrichous

There are four types of bacterial flagella arrangements. The first is monotrichous flagella, in which the bacterium has only one flagella. The second is amphitrichous, in which there are two flagella on opposite ends of the bacterium, but only one flagellum works at a time. The third is lophotrichous, in which there are multiple flagella located on one spot. The fourth is peritrichous, in which there are flagella pointing in all directions.

Flagella Structure

The structure of the flagella is made up of four rings, the L ring, the P ring, the MS ring, and the C ring. The MS and C rings make up the basal body, which anchors the flagella to the cell. Around these two rings there are Fli and Mot proteins. The Fli proteins act as switches that rotate the flagellum clockwise or counterclockwise. The Mot proteins generate torque through a proton channel that generates electrostatic forces on the MS and C rings, causing them to spin. The hook connects the flagella filament to the motor. The filament is made of the protein flagellin. When a flagellum is made, the flagellin from inside the cell goes up through the hook and is added to the tip of the growing flagella.

Proteobacteria

Diagram of Helicobacter Pylori Causing Stomach Ulcers

Though Proteobacteria share molecular and cell wall features, this phylum displays amazing diversity of form of metabolism. Genera of this phylum consists of five subgroups: alpha (α), beta (β), gamma (γ), delta (δ), and epsilon (ε). The ancestry of mitochondria can be traced to the α – Proteobacteria, which also include several genera noted for symbiotic relationships with animals and plants. For example, Rhizobium and related genera of α – Proteobacteria form nutritionally beneficial associations with the roots legume plants and are thus agriculturally important.The genus Nitrosomonas, a soil inhabitant, a soil inhabitant important in the global nitrogen cycle, represents the β-Proteobacteria. Vibrio cholerae, a γ-Proteobacteria, causes cholera epidemics when drinking water becomes contaminated with animal waste during floods and other natural disasters. The δ-Proteobacteria includes the colony-forming myxobacteria and predatory bdellovibrios, which drill their way through the cell walls of other bacteria in order to consume them. Helicobacter pylorus, which causes stomach ulcers, belongs to ε-Proteobacteria.

Vibrio fischeri

Vibrio fischeri are bioluminescent marine gram negative proteobacterium. They can live freely by themselves or in communities that are symbiotic with Hawaiian bobtail squid. In this relationship the squid gets light to be able to camouflage from predators. The bacteria get a home and food. Together they are a model system for studying animal and microbial symbiosis. During the day the bacteria are expelled and are gathered up again during the day, because the bobtail squid is a nocturnal feeder. These symbionts are acquired horizontally, meaning they come from the environment. The squid live in shallow coastal waters, are 1.2 inches long, and have their light organ in the mantle.

Quorum Sensing

The mechanism by which the bacteria are able to produce bioluminescence is quorum sensing, which is a cell to cell communication process used to facilitate gene expression as a response to changes in population density. There are two paths that quorum sensing can take, low cell density and high cell density. The low cell density path is for asocial behaviors, while the high cell density path is for social and group behaviors. Cells, in this case, Vibrio fischeri are, produce molecules called autoinducers. These autoinducers will then reach a minimum threshold required for detection within the community, and they will start gene expression together, in this case: bioluminescence.

Lux Genes

The protein, luxI, makes the autoinducer, acyl homoserine lactone (AHL). The autoinducer of luxR enters the cell and binds to LuxR, which is a regulatory molecule in Vibrio fischeri. This makes a luxR-autoinducer complex, which binds to the Lux operon and promotes transcription of more luxI, which leads to more autoiducers, further expressing the lux operon. The luxA and B genes encode for luciferase which is responsible for bioluminescence.

Cyanobacteria

Cyanobacteria: Guerrero Negro, Baja California, Mexico

The phylum Cyanobacteria contains photosynthetic bacteria that are abundant in fresh water, oceans, and wetlands and on the surfaces of arid soils. Cyanobacteria acquired their name from the typical blue-green (cyan) coloration of their cells. This coloration is a result of the presence of accessory phycobilin pigments that help chlorophyll absorb light energy. Cyanobacteria are the only prokaryotes that generate oxygen as a product of photosynthesis. Ancient Cyanobacteria produced Earth’s first oxygen-rich atmosphere which allowed the rise of eukaryotes. The plastids of eukaryotic algae and plants arose from Cyanobacteria. Cyanobacteria display the greatest structural diversity found among bacterial phyla. Some occur as single cells, while others form colonies of cells held together by a thick gluey substance called mucilage. Many Cyanobacteria produce specialized cells and display intracellular chemical communication, the hallmarks of multicellular organisms. Many Cyanobacteria that grow in conditions of high light intensity produce protective brown sunscreen compounds at their surfaces. Cyanobacteria play an essential ecological role by producing organic carbon and fixed nitrogen. However, several kinds of Cyanobacteria, especially the genera Microcystis, Anabaena, and Cylindrospermopis, form nuisance growths in freshwater lakes during the warm season. Such growths, known as blooms, give the water a pea-soup appearance. Blooms develop when natural water receives excess fertilizer from sewage discharge or agricultural runoff. Such blooms are becoming more common every year and are of serious concern because they may produce toxins in amounts sufficient to harm the health of humans and other animals. Consequently, it is inadvisable for people or pets to swim in or consume water that has a visible Cyanobacterial boom.

Mollicutes

Mollicutes are from the phyla, Firmicutes, which are gram positive low guanine and cytosine bacteria. Most of them lack a cell wall which gives them their name since mollis means "soft" in Latin. Since they have no cell wall, they do not stain gram positive or negative, and are not affected by antibiotics such as penicillin and vancomycin that target cell walls. They are also one of the smallest groups of bacteria ranging from 0.2 – 0.3 μm long. In addition, having no cell wall makes them vulnerable, so they are often found living within another cell, which allows them to have reduced genome size. Because of this fact they often cause contamination in culturing cells in the laboratory.

Mycoplasma

File:Mmobile.png

Movement apparatus of M. mobile

Mycoplasma is a genus of the Mollicutes class. Since they have no cell wall, and thus no pili, flagella, or any related movement system, it is mysterious how they move. One of the fastest members of this genus is Mycoplasma mobile, which is a pathogen to freshwater fish. They move fast relative to its size, moving at about ten body lengths per second. To put that in perspective, Usain Bolt moves at 5.3 bl/s. Under a microscope it almost looks as if they are vibrating and it causes to the propel in a random direction. It is known that their shape is closely related to how it moves. It is shaped like a light bulb, with a polar head on the narrower part. It glides unidirectionally with its head, which allows it to attach to surfaces. Scientists compared wild types and mutants that couldn't glide to figure out what proteins are needed for gliding. Antibodies were used to inhibit gliding and localize the specific proteins. Along with electron microscopy, scientists were able to make a model of the gliding proteins that looked like a leg extending out from the cell body. This structure is made out of four proteins. The "foot" is able to interact with surfaces, allowing it to look like it is gliding. The fourth protein, p42, in this "leg" uses ATP hydrolysis to move the structure. The whole structure almost looks like a centipede in motion. This process makes it look like M. mobile can "walk" across surfaces.

Deep Branching Thermophiles

File:Bacterialphylogeny.png

Bacterial Phylogeny

These bacteria are extremophiles that consist of the phyla: Aquificae, Thermotogae, Thermus-Deinococcus, and Chloroflexi. These phyla are the most deeply branched. They are similar to Archaea, in that they share some of the same environments, and have similar physiology. They also have the fastest doubling rates of all cells, making them have higher rates of mutation.

Aquificae

These bacteria can be found in hot springs, sulfur pools and deep sea vents. They can survive in temperatures from 85-95°C. They are chemolithoautotrophs, making their own energy from inorganic compounds in the environment, that are the primary carbon fixers in their respective environments. These bacteria produce waster as a byproduct, hence their name.

Thermus-Deinococcus

This phylum of bacteria is are cocci, and are special for their high resistance to environmental extremes such as high temperature and even radiation. These bacteria have a think cell wall, which makes them resemble gram positive bacteria, because they retain the violet stain. However, they have an outer membrane on their cell wall, so they are more closely related to gram-negative bacteria.

One of the reasons that these bacteria are so resistant to their environment is that an amino acid in their peptidoglycan wall is substituted with a special amino acid. Instead of having a peptide cross link, between diaminopimelic acid (DAP) and D-alanine, DAP is replaced by L-ornithine.

One notable member of this phyla is Thermus aquaticus. This species is important in PCR, because their DNA polymerase, Taq DNA polymerase, is resistant to heat that is used in PCR.

Thermotogae

These bacteria are also thermophiles and hyperthermophiles. They can be found in hot springs and hydrothermal vents. They stain as gram negative, but lack a second cell membrance, and are anaerobic, do not need oxygen. Their phylum name is derived from the fact that they live in hot environments and have a "toga" sheath surrounding the cell.

This phyla has possible uses in biotechnology, because of its ability to function at high temperatures. They are currently being used in research for alternatives to fossil fuels, because they can make hydrogen gas from carbohydrates.

Chloroflexi

Bacteria in this phyla have a variety of extremophile traits. Some are moderate thermophiles, aerobic, anaerobic, and photoheterotrophs. These bacteria mostly stain gram negative, but have one cell membrane. The Chloroflexus species contain photosynthetic apparatus within chlorosomes.

Type IV secretion System

The type IV secretion system spans through the bacterial cell envelope with ATPases, VirB4, VirB11, and VirD4 powering substrate secretion and assisting in the assembly. According to biochemical data, the inner membrane channel consist of inner membrane protein VirB6 and VirB8. The protein VirB10 is a major component of the outer membrane of the type IV secretion system. In different types IV secretion systems, an extracellular pilus comprised of VirB2 and VirB5 may be present. It was recently discovered that a core complex is created from VirB7, VirB9, and VirB10 forming in the inner membrane and inserting itself into the outer membrane. The core complex that forms in the inner membrane is usually comprised of VirB7, VirB8, VirB9, and VirB10 which are referred to as core proteins in a type IV secretion system. Tests have been studied on the core complex showing that VirB7, VirB9, and VirB10 form a 1.05-MDa complex that is inserted in both the outer and inner membranes. The structure of the outside membrane with the O-layer contains 14 copies of the CTD of VirB10, VirB9, and full-length VirB7. VirB10 proteins have the ability to inserts itself into the inner and outer membranes. When the core complex is viewed at different sites, the VirB10CTD forms an inner ring that surrounds the VirB9-VirB7 complex and it also forms the inner wall of the complex. The outer-membrane complex contains three proteins that are essential for complex assembly and channel formation. The protein VirB10CTD directly interacts with VirB9CTD while VirB7 interacts with VirB9CTD only. The inner-membrane of the core complex consists of an I-layer that is made of the protein VirB9 and VirB10. The inner-membrane protein VirB8 composed of an N-terminal transmembrane segment is inserted into the inner-membrane along with the protein VirB6. Various type IV secretion systems contain ATPases that supply energy for substrate translation and apparatus assembly. In Gram-negative or Gram-positive bacteria, coupling proteins are found in the type IV secretion system. The coupling proteins in the type IV system are used in substrate recruitment during secretion. VirB11 are peripheral inner-membranes proteins that can be found in a type IV secretion system. Type IV secretion systems also contain tubular pilus structures.

Virulence Factors

Virulence factors are properties that allow for a microorganism (in this case bacteria) to establish itself within the host cell and enhance its ability to cause disease.

The first step in bacterial infection is colonization of bacteria in host cell aided by adherence factors. In general, adherence to a host cell depends on a eukaryotic acceptor and a bacterial ligand. The eukaryotic acceptor is usually a carbohydrate or peptide residue on the membrane surface while the bacterial ligand, or adhesin, is a macromolecular component of the cell surface that interacts with the receptor. A common adhesin is the fimbrae, which are proteins filaments on the bacterial surface. A bacterial cell will first approach the host cell in nonspecific adherence, which includes possible hydrophobic interactions and electrostatic attractions. This is the docking phase of the bacterial cell leading to specific adherence, which involves permanent formation of bonds between the bacteria and host cells that are mostly irreversible under physiological conditions.

After colonizing a host cell, bacteria will utilize invasion factors to break down the host’s defenses. These substances are termed invasins and act in short distances to break down primary and secondary cell defenses. Many bacteria are also surrounded by capsules that protect them from phagocytosis or opsonization. Spreading factors affect the physical properties of tissues and intercellular components of cells helping to spread the bacteria. Hyaluronidase is in enzyme that attacks the connective tissue by depolymerizing hyaluronic acid. Collagenase breaks down collagen, which is a major building material for muscles and tissue. Neuraminidase breaks down sialic acid, which gives structure to the epithelial cells in the intestines.

Programmed Cell Death

In a multicellular organisms, it is not only essential to control the rate of cell division but also to control the rate of cell death of cells that are no longer needed. Programmed Cell death (PCD) is a bacterial stress response which leads to cell suicide mediated by an intracellular program and is responsible for eliminating unwanted or potentially harmful cells.

Chromosomal toxin-antitoxin module mazEF

mazEF is one of the toxin-antitoxin systems that have been found on the chromosomes of many bacteria including Escherichial coli that was discovered to play an important part in bacterial programmed cell death to regulate the amount of cells and to assist bacteria on coping with a stressful environment change.

The mazEF module consists of two adjacent genes, mazF and mazE. MazF is a stable, long-lived toxin while MazE is a labile antitoxin that is antagonizes MazF and are degraded in vivo by ClpPA serine protease. These two genes are co-expressed and the mazEF system is negatively autoregulated by the combined action of those two proteins on the mazEF promoter P2.

Under stressful condition change such as perpetual alterations in temperature, osmolarity, pH, and nutrition levels, mazEF coexpression is inhibited because of the degradation by of MazE. As a labile protein, MazE is easier to be degraded than MazF. This trend leads to the difference between the two proteins in cellular concentration in which MazF is dominated and thus, triggers program cell death.

Modes of MazF action Because MazE has failed to inhibit MazF, its toxic effect continuously exerts. Under stressful conditions, the activity of MazF will bring up 2 issues that coordinately produces a distinct pool of stress proteins to response to the condition: 1/ the formation of leaderless mRNA (by removing the 5’-unstranslated region of specific mRNAs) and inhibition of protein synthesis: mazF inhibits the synthesis of about 90% and but selectively enables the specific synthesis of about 10% of proteins for the survival of a small subpopulation of proteins under stressful condition. 2/ formation of the stress translation machinery: MazF removes 43 nucleotides from the 3’ terminus of the 16S rRNA to from stress-ribosomes which are responsible for translating leaderless mRNAs. The pathway using the specialized translation machinery provides a fast and efficient response to a variety of stressful conditions that cells have to cope with.

References

Mattei, Pierre-Jean, David Neves, Andrea Dessen."Bridging cell wall biosynthesis and bacterial morphogenesis." Structural Biology (online). Nov. 2010.

Slonczewski, Joan, and John Watkins. Foster. Microbiology: An Evolving Science. New York: W.W. Norton, 2011. 717-18. Print.

Moll, Isabella, and Hannah Engelberg-Kulka. "Trends in Biochemical Sciences." Trends in Biochemical Sciences. 37.11 (2012): 493-498. Web. 28 Oct. 2012. <http://ac.els-cdn.com/S0968000412001119/1-s2.0-S0968000412001119-main.pdf?_tid=c0222dc8-215e-11e2-a403-00000aacb361&acdnat=1351470340_177eb1398fa277f3bb5dea0eea48db79>.

http://schaechter.asmblog.org/schaechter/2012/10/fa.html Daniel P. Haeusser

History

Thought to be more than 1.7 billion years ago, the Domain Eukarya was the last to evolve. The Domain includes all organisms with a eukaryotic cell which contain membranous organelles such as mitochondria and chloroplasts. Through comparison of ribosomal RNAs and nucleotide sequences, the Eukarya domain finds itself more closely related to the Archaea rather than Bacteria domain. Eukaryotic organisms are characterized by a membrane-enclosed nucleus containing the organism's genetic material, or DNA, and corresponding proteins. All animals, plants, fungi, and protists fall under this taxonomic group.

Eukaryotes are classified according to how they obtain energy from the surrounding environment. Phototrophs obtain energy from light, whereas chemotrophs obtain energy via oxidation of chemical fuels. All phototrophic eukaryotes derive carbon from CO₂. All non-phototrophic eukaryotes depend on organics for their carbon source, and are therefore termed organotrophs.

Four Kingdoms of Eukaryotes

Animal Diversity

Animals

Animals belong to the kingdom Animalia. They are heterotrophs and therefore require the ingestion of organic compounds for energy. Animals are usually multi-cellular. About 95% are invertebrates. Unlike other classes of eukaryotes, animals contain cells that lack cell walls.

Kingdom Animalia is divided into two subkingdoms:

a. Parazoa: lack tissues and organs.

- Porifera (Sponges): mostly live in marine, though some live in freshwater. They are asymmetrical. Their shape and size depends on the environment or substrate to which they are attached. Sponges are sessile and contain stationary life. They are made up of two layers: the epidermis and an inner layer of flagellated choanocytes. They capture food suspended in the water. Individual sponges are capable of producing both eggs and sperm.

b. Eumetazoa: remains as the rest of animal phyla. They are true multicellular animals with differentiated tissues.

- Cnidaria: Some examples of this phylum are coral, sea anemores, and jelly fish. They have radial symmetry. The body is divided into 2 layers of tissues: outer epidermis and inner gastrodermis. Furthermore, there is also mesoglea, which is a gelatinous layer between these top layers.

- Ctenophora: Comb jellies. They are radially symmetrical and have two layers with a very thick mesoglea between them. The mouth is located downward. They move by using 8 vertical rows of cilia or combs. They also posses two long tentacles.

- Protostomes: tripoblastic eumetazoans, possess mesoderm but lack a body cavity. Platyhelminthes (flatworms) falls into this phyla. They are bilaterally symmetrical and flattened animals with distinctive heads. Platyhelminthes are divided into three classes: class Tuberllaria (free-living flatworms), class Trematoda (flukes), class cestoda (tapeworms).

- Pseudocoelomate Protosomes: have mesoderm lines outside of the body cavity and possess pseudocoelom.

+ Rotifera (Wheel Animals) : some live in fresh water ponds and streams, while others are marine and live in soil. They are multicellular animals. They posses a corona, which is used for feeding and locomotion. They have complete digestive tracts consisting of a mouth and anus.

+ Nematoda (Roundworms): are cylindrical worms that taper at both ends. Although some live in soil and water, most are parasitic.

- Annelida (Segmented Worms): Segmented. They have complete digestive system with specialized compartments: segmented excretory system, closed circulatory systems, and well-developed nervous system. The Annelida are divided into 3 classes:

+ Class Obligochaeta (Earthworms): they have pairs of setae on each segment for locomotion

+ Class Polychaeta (Marine Worms): they have numerous setae (spines). Parapodia, well-developed head but lacks sedentary forms.

+ Class Hirudinea (Leeches): are ectoparasites which feed on blood and tissue fluid of their hosts. The external segmentation is expansion of wrinkles for feeding. They are found in terrestrial, fresh water, and marine environments.

- Mollusca (Clams, Snail, Squid): Most live in marine while some in fresh water and terrestrial environments. They lack segmentation but have well-developed circulatory system (with a heart), well-developed nervous system, possess radula (rasp-like plate in mouth used for scraping food), mantle ( a layer of cell that secrete a shell CaCO3 and create mantle cavity), feet (muscular organs for locomotion) and visceral mass.

+ Class Polyplacophora (Chitons): marine mollusks that have shells made up of eight plates.

+ Class Gastropoda (Snails): have single, spiraled shell and a distinct head with well-developed eyes and sensory tentacles. This class exhibit torsion which is the coiling shell and internal organs.

+ Class Cephalopoda (Octopus, Squid and Nautilus): The foot is modified into grasping tentacles with suckers. The shell is reduced and internal. All have well-developed eyes and complex nervous systems.

- Arthropoda (Insects, Arachnids, Crustaceans): They have distinguishing features due to the presence of chitinous, a joined exoskeleton. Also, they have circulatory system, external segmentation, complex sensory organs, and nervous system even though they are invertebrate. Furthermore, they have complex musculat system and exoskeleton that cannot grow but be shed and replaced.

- Trilobitomorpha (Trilobites): they were once the most numerous arthropods. They have three-lobed shield dorsally. The second segment had a pair of antennae and all following segments have a pair of biramous appendages used as walking legs. Unfortunately, they went extinct.

- Cheliceriformes (Arachnids): lack antennae but have chelicerae as feeding apparatus. Their body is composed of a cephalothorax and abdomen. The appendages consists of chelicerae, pedipalps, and four pairs of walking legs.

+ Class Arachnida (Scorpions, Spiders, and Ticks): Most of them are terrestrial.

+ Class Merostomata (include Horseshoe Crabs): they all live in marine. The cephalothorax is covered by a single carapace while the abdomen is covered by a shield. Moreover, they have a stout, which is a spike-like telson.

- Myriapoda: Terrestrial. They have single pair of antennae. This phylum is divided into 3 classes:

+Centipedes: the body is segmented with one pair of walking legs per segment.

+ Millipedes: the body is made up of fused double segments and each segment hold two pairs of walking legs. The first four segments that are not fused make up the thorax (which don't have any legs).

- Hexapoda (insects): are considered as the most successful group. The body consists of three parts : head, thorax, and abdomen. They have three pairs of walking legs on the thorax. Generally, they have wings on the thorax.

+ Class Coleoptera (Beetles)

+ Class Diptera (Flies)

+ Class Hymenoptera (Ants, Bees, Wasps)

+ Class Lepidoptera (Butterflies, Moths)

+ Class Orthoptera (Grasshoppers, Crickets, Katydids)

- Phylum Crustacea: live in aquatic environments, or marine. The head and thorax form a cephalothorax. They have two pairs of antennae, mouth bearing mandibles.

+ Class Cirripedia (Barnacles): they are the only sessile group in this phylum.

+ Class Decapoda (Crabs, Lobsters, Shrimp, Crayfish, Krill): An elongated abdomen and telson ( for swimming)followed by cephalothorax. They are capable of crawling forward slowly.

- Echinodermata: All marine. They have pentaradial symmetry while their larvae have bilateral symmetry. They have CaCO3 endoskeleton. Also, they have vascular system and associated tube feet.

+ Class Asteroidea (Sea Star) : Marine animals. They have central disc and five tubular feet.

+ Class Holothuroidea (Sea cucumbers) they have bilateral symmetry and pentaradial structure. However, they lack endoskeleton and used the thickened epidermis as a protective covering.

+ Class Ophiuroidea (Brittle Stars): they have a small, round disc with five distinct arms which are easily broken. These arms are flexible and used to move in snake-like motion.

+ Class Echinoidea (Sea Urchin, Sand dollar): They have flattened test and short spines.

Some Representative Classes of the Phylum Chordata:

• Chondrichthyes: cartilaginous fish
• Osteichthyes: bony fish
• Sauropsida: reptiles
• Amphibia: amphibians
• Aves: birds
• Mammalia: mammals (mammalary gland)

Plants

Plants belong to the Plantae kingdom. The Plantae kingdom consists of over 300,000 species of multicellular autotrophs that convert sunlight into usable chemical energy through the process known as photosynthesis. Essential to photosynthesis are the pigments chlorophyll-a and b that absorb the light energy. All plants have cell walls made of cellulose. Plants' life cycle involves an alternation of generations in which diploid plants (2n) give rise to haploid plants (n), which through a sexual process, produces diploid plants again. All land plants can be divided into two categories: bryophytes (non-vascular plants) and tracheophytes (vascular plants).

All plants have a life cycle that alternates between a diploid organism and a haploid organism. The diploid form of the life cycle is called the sporophyte, which contains two sets of genetic DNA while the haploid form of the life cycle is the gametophyte, comprising of only one set of DNA. As a result, this phenomenon of a life cycle which switches between a diploid and a haploid organism is called an alternation of generations. The haploid gametophyte cells reproduce through mitosis to form the gametes of the plant, the egg and sperm; when they are fertilized, they form the diploid zygote, which becomes the diploid sporophyte. Later, when the diploid sporophyte reproduces by meiosis, haploid reproductive cells are developed as the gametophytes.

Nonvascular Plants

Bryophytes are thought to be the first land plants. They do not have lignin-impregnated cells and are limited in size because of this. They also lack roots and have structures called rhizoids instead which are somewhat similar. Bryophytes are mainly in the gametophyte (haploid) phase for the majority of their life cycle. This group mainly consists of liverworts, hornworts, and mosses.

Nonvascular plants lack cells that are joined together that function together to transport nutrients and water throughout the whole plant. As a result, these plants are unable to grow tall, giving them the nickname ‘ground hugging plants’ since they only grow close to the ground and form carpets of mosses. The gametophyte stage dominates the life cycle of nonvascular plants, and the sporophyte grows on top of the gametophyte. Some nonvascular plants are bisexual, and they produce both male and female gametophytes. Mosses require a film of water in order for the sperm to get to the egg for fertilization and reproduction. As a result, many mosses are found in environments that remain moist, allowing for ideal moss reproduction. The Sphagnum moss is a collection of moss in the environment that forms a bog, which holds a lot of water. Because of this, sphagnum mosses are oftentimes used in soil fertilizers to help the sandy soil retain more water.

The sporophytes of nonvascular plants are usually attached on top of the gametophyte and it contains three distinct parts, the foot, the seta, and the capsule. The foot is responsible for attaching to the gametophyte, but more importantly, its main function is to absorb nutrients from the gametophyte. The seta is a stalk that is able to transport these nutrients to the capsule. The capsule uses these nutrients to produce spores through meiosis, which will fertilize the eggs and reproduce more moss. One evolutionary adaptation that nonvascular plants have obtained is the use of the peristome, which is a structure that opens and closes based on the environment to allow the spores to be discharged from the capsule. When the environment is dry, the peristome opens, allowing the spores to travel far by the wind, and when the environment is wet, the peristome closes until stronger gusts of winds are present.

Vascular Plants

Tracheophytes tend to be larger and more complex than bryophytes. Unlike the bryophytes, the tracheophytes contain the xylem and phloem, which are used to transport water and sugars throughout the plant. Tracheophytes are mainly in the sporophyte (diploid) phase for the majority of their life cycle. This group consists of ferns, conifers, and flowering plants.

Vascular plants contain the key vascular tissues, xylem and phloem, that allow them to grow taller. On the contrary to nonvascular plants, these plants have dominant sporophytes instead of gametophytes, and the sporophyte is no longer dependent on the gametophyte for nutrients. Vascular plants have roots that absorb nutrients from the soil and transport them to the rest of the plant. Xylem is the plant tissue that is responsible for transporting water and nutrients from the soil to the whole plant. The cell walls of this tissue are strengthened by lignin. The second tissue, phloem, is responsible for transporting the amino acids, sugars, and organic products to other parts of the plant to allow it to grow. With these vascular tissues, plants were able to grow taller and stronger, giving the plant support for more sunlight. As more plants were able to grow taller, they competed to grow taller than each other in order to get more sunlight and produce more sugars and starches. Additionally, these taller plants were able to disperse their spores farther, allowing the population to spread into new environments.

The sporophytes of vascular plants evolved roots and leaves that helped the plant produce more sugars. The roots helped anchor the plant into the ground by forming a huge network of branching roots in the soil. This helped the plant absorb more nutrients from the soil. The leave evolved, allowing the plant to capture more sunlight and produce photosynthesis and producing sugars for itself to grow taller. These seedless vascular plants dominated the forests, growing to hundreds of feet competing for sunlight. Ferns were a dominant plant that was abundant in many varieties.

The seed plant can be divided into two categories, gymnosperms and angiosperms. Seed plants contain a seed, which is the whole ovule which is an embryo that is packed with a food supply and encased in a protective coating. This allows the seed to be dispersed farther distances and still have the nutrients to thrive. Unlike vascular plants, seed plants do not need water in order for fertilization to occur. Gymnosperms have seeds that are exposed and are not enclosed in chambers, such as pine trees, gingko trees, and cycads. Angiosperms are plants with flowers. The reproductive part of the plant is called the flower, and when it is fertilized, it becomes the fruit. Flowering plants have evolved over time in order to increase their chances of getting fertilized by animals or other forces of nature. For examples, some flowering plants evolved red petals, which stand out to birds’ eyes. As a result, when the birds reach with their beaks for the nectar in the flower, the pollen is rubbed onto the bird to be transported to different flowers for fertilization. Another example is that flowers have evolved to have light colored and fragrant flowers, attracting nocturnal animals to pass the pollen on to other flowers for fertilization.

Various Fungi

Fungi

Fungi is a member of the kingdom Fungi includes molds, mushrooms, and mildews. Most of fungi are photosynthetic and are either saprotrophic or parasitic. This class of Eukaryotes are heterotrophic and is comprised of hyphae (which forms mycelium) and has cell walls made of chitin. The reproduction of fungi usually involves spores made in specialized structures. Examples of fungi would be yeasts, molds, and mushrooms. They are actually closely related to animals than to plants in the evolution sense. But in studies, fungi are classified under botany. Reproduction can be sexual or asexual. Asexual reproduction generally involves the formation of spores while sexual reproduction involves the production of complex structures. Fungi are divided into four clades base on the sexual structure:

- Zygomycota: are mostly terrestrial but some live in soil or on decaying plants. The hyphae lack septa ( cross wall) so the mycelium is multinucleate mass. Examples of this clade are Rhizopus, Pilobus, and etc.

- Ascomycota: are most diverse. These fungi asexually reproduce through conidia. Sexually, two mating strains come together and form asci within ascocarp. In each ascus, eight ascospores are produced. Yeast, which is a unicellular type, is an example of this clade. Furthermore, Penicillium is used in modern medicine is another example of this clade.

- Basidiomycota: don't have asexual structure. Sexually reproduce by basidiospore, which are located in basidiocarp. Example of this clade is mushroom.

- Chytridiomyota

Fungi can live in a wide range of habitats. But of course, different species lives in various habitats. There are about 70,000 species that are formally found, but the actual number of fungi species is still unknown.

Fungi perform the most important task for ecosystems. They are decomposers, returning nutrients they absorb back into the ground for circulation. Without fungi, there would be a shortage of nutrients in the soil, and plants and entire ecosystems would be completely different. Mycorrhizae is a mutual relationship between a fungi and a plant. The fungi’s hyphae either penetrate the roots of a plant or grow close to the roots of a plant. As a result, the fungus helps absorb nutrients for the plant, and the plant receives sugars from the plant as well. Together, they both benefit from this relationship. This relationship helps the plant increase its intake of nutrients, allowing it to grow taller and faster. In an experiment between a plant and a plant with a beneficial relationship with a fungus, the plant with the fungus grew to a taller height over the same period of time.

Another beneficial relationship is between a fungus and an ant. The ants gather leaves for the fungus while the fungus digests the leaves and produces sugars that the ants can eat. In this case, both the fungus and the ant depend on each other. The ant cannot eat the leaves, and the fungus cannot reach the leaves. As a result, this relationship helps the fungus and the ant thrive together.

Human uses many fungi as food and medicine.

Various Protists

Protists

Protists are eukaryotic microorganisms that are unicellular, colonial, or multicellular; but the one specific property that sets them apart from each other in Eukarya domain is that they don't have specialized tissues. They do not have any of the distinctive characters of plants, animals, or fungi. All algae except blue-green algae, protozoans, and other organisms that were thought to be fungi have now been correctly identified in the kingdom Protista. The organisms usually composed of Protists are characterized by their ability to photosynthesize (like algae), produce molds (mildews, slime molds and water molds), and forage. Algae and other photosynthetic protists are of higher importance to our lives mainly because they produce a substantial amount of oxygen. They thrive only in aquatic environments. Some protists have flagella that are involved in locomotion. They reproduce in several ways: Isogamy, in which the motile gametes are the same size. Anisogamy, in which the female gametes are larger than the males. Lastly, oogamy, in which the larger female gametes are none motile and the males are motile. The Protists are divided into 12 divisions. This was based on modes of nutrition, pigments, carbohydrate food reserves, flagella, cell wall components, and the environment in which they live in. Protists are an important part of the food chain. For example, kelp beds are one of the most productive ecosystems on the earth.

a. Stramenopila: include diverse autotrophs and heterotrophs possess two flagella : one flagellum has numerous hair-like projection and the other is smooth, lacking hair.

- Bacillariophyta (diatoms): live in fresh and salt water. They possess chlorophyll a and b and carotenoids ( they have golden color). They contribute oxygen to atmosphere. Diatoms live in a silica shell and resembles an agar plate.

- Phaeophyta (brown seaweed): live in temperate water. They possess chlorophyll a and c and brown pigment fucoxanthin. Their cellwall are made of cellulose and algin. For example, macrocystis.

b. Rhodophyta: marine seaweed. They have chlorophylls a and d and pigment phycobilins. They have red color.

c. Chlorophyta (green algae): unicellular and colonial. They have chlorophylls a and b. Cell wall is made of cellulose and stores starch. Examples of this class are Spirogyra and Desmids.

d. Euglenozoa

- Euglenophyta: one or two anterior flagella. They can be autotrophic or heterotrophic. Example: Euglena.

- Kinetoplastida: have single large mitochomdrion along with kinetoplastid which store DNA. They are symbiotic or pathogenic. Example: Trypanosoma gambiense.

- Trychonympha: Beneficial endosymbionts that develops mutualistic relationship with termites.

e. Alveolata:

- Dinoflagellata: Found in marine waters. They have paired flagella lying in grooves which give mobility. Example: Ceratium.

- Apicomplexa: Parasitic and none motile as adults. Examples: Plasmodium vivax ( cause malaria)

- Ciliophora: Move by cilia and have large macro-nucleus and micro-nuclei.They are holotrophic. Most of them are single-celled organisms. Example: Paramecium, Vorticella.

f. Protists with Pseudopodia: they have pseudopodia which are used in locomotion and feeding.

- Amoebozoa (Amoebas): live in marine and fresh water or in soil. They have no fixed body and lack the shell.

- Radiolaria : live in ocean. They are enclosed by porous tests of silica dioxide. Using axopodia to feed and slowly move.

- Cercozoans (forams): live in marine, enclosed by coiled tests of CaCO3.

[32]

References

Reece, Jane B., and Neil A. Campbell. Campbell Biology. Harlow: Pearson Education, 2011. Print.

Community Ecology:

A community is a group of different species of organisms that live close enough together that they influence each other. There are two different hypothesis of why there are communities of organisms in ecosystems. The first hypothesis, integrated hypothesis, was made by F.E. Clements, who’s idea was that a community depended on the interactions between the organisms that allowed organisms to survive together. The second hypothesis, independent hypothesis, was made by Henry Allan Gleason, who believed that a community consisted of organisms who have the same living essential requirements. For example, this means that animals who share the same food source will live close to each other since they both must live near the food source in order to survive.

Species Interactions:

In a community, organisms of different species interact with each other, resulting in positive or negative effects for each species. These types of interactions are divided into four groups: competition, predation, herbivory, and symbiosis.

1. Competition. Competition is an interaction between different species when they compete for the same limited resource in the environment. Competition brings negative effects for both competing parties since each species does not have full control of the resource. In fact, when competition is strong between two species for the same limited resource, one species will adapt to use the resource more efficiently and reproduce rapidly. Over time, this species will eventually out produce the competing species, leading to the death of the other species. This phenomenon is called competitive exclusion. If two species require the same limiting resource in an ecosystem, these two species cannot coexist together.

2. Predation. Predation is the interaction between different species where one species is the predator, and it kills and eats the other species, the prey. This interaction benefits the predator since the predator gets the nutrients from the prey in order to survive and reproduce. This interaction brings about the adaptations for both the predator and the prey. Predators adapt by having sharp claws, teeth, fangs, or poison to catch and kill their prey. On the other hand, prey have adapted to prevent themselves from being killed by predators by adapting their speed, color, mimicry, and shell. Some animals have skin colors that camouflage them with the surroundings to hide from predators. Many animals synthesize toxins and poisons that make them dangerous if eaten by a predator. These animals display aposematic coloration, which are bright colors that warn predators that they are poisonous to eat. Furthermore, prey species often display mimicry, which is displaying colorations or traits that resemble other species. In Batesian mimicry, a harmless species looks like a harmful species, which prevents predators from eating them. In Mullerian mimicry, two poisonous species resemble each other in order to teach the predator that they are dangerous to eat. Prey have adapted to hide from predators or warn predators that they are dangerous to eat.

3. Herbivory. Herbivory is the interaction between an organism and a plant. The organism eats parts of the plants to get their nutrients. As a result, since plants do not have the ability to flee from an herbivore, plants have adapted by growing thorns and spikes as well as producing poisonous toxins to warn and prevent herbivores from eating them. In response, some animals and insects have sensors that allow them to distinguish between poisonous and delicious plants. Some herbivores also have specialized cells in their digestive systems that help them to digest the tough cellulose and cell walls of the plants.

4. Symbiosis. Symbiosis is a relationship between two species that directly affect each other. There are three types of symbiosis. The first is parasitism, where one species leeches off another species. The leeching species is the parasite, and it steals the nutrients from the other species, the host. As a result, the host is harmed since it loses part of its nutrients to the parasite. Furthermore, the parasite oftentimes changes the behavior of its host that eventually causes the death of the host. For example, ticks live on moose and suck on the blood of the moose. It also causes hair loss on the moose, which could further cause the death of the moose in the winter due to the cold weather. Another type of symbiosis is mutualism, which is an interaction between two or more species where both species benefit. Both species help each other out to receive more nutrients than they would have received if they were alone. For example, the mutual relationship between fungi and plant roots allows the fungi to receive nutrients and sugars from the plant while it helps the plant absorb additional nutrients from the soil. Another example of mutualism is the relationship between trees and ants. The tree provides food as well as shelter for the ants, while the ants provide protection to the tree. Day and night, the ants patrol the tree and attack potential herbivores. As a result, the ants help the tree survive longer. The third type of symbiosis is commensalisms, where the interaction between two species benefits one species without affecting the second species. These interactions are rare since most interactions will always have an effect on both species. Several examples of commensalisms occur when one species latches onto another species for a “free ride.” Algae oftentimes thrive on shells of turtles. As the turtle swims from one area to another, the algae are able to get to different areas in the ocean.

Trophic Structure

A trophic structure is the structure of the feeding structures between organisms in an ecosystem. A food chain depicts the order of which predators eat prey, but more importantly, food chains show the path that food energy is transferred between organisms and returned to the system in a cycle to recycle nutrients in the ecosystem. Energy from the autotrophs are transferred to herbivores and carnivores as they are eaten. When these animals die, they are decomposed by fungi and bacteria, which returns the nutrients to the ground to be used again by plants. This cycle ensures that the supply of food energy will not be depleted. Many food chains are connected together to form a food web, which depicts the relationships between organisms and show which organisms will eat other organisms. Food webs show the important relationships between each organism, and if one organism were to be removed, it would cause disastrous effects on the rest of the organisms in the community since many other organisms depend on the organism for food.

Many species play an important role in the community. The most highly abundant species is the dominant species. These species affect their community by causing other species to adapt around them. For example, if a certain type of tree is the most abundant in a community, it controls the shade and the amount of nutrients that are available in the soil. If this tree were to be removed, many species that depended on the tree for food may decrease and even become extinct. Another type of species are the keystone species, which are species that have huge influential power on the community. For example, sea otters prey on sea urchins, and sea urchins prey on kelp. When there are lots of sea otter in a section of water, the population of kelp is high, but when the otter are present, the sea urchins will quickly prey on the kelp, and there will be a low amount of kelp in the community. Finally, the ecosystem engineers are the species in the community that change the physical environment. Beavers are a huge example of ecosystem engineers since they use trees and branches to build dams and slow down the flow of water through a community ecosystem. As a result, ponds can form, and different species of organisms can thrive in the pond.

Reference: Reece, Jane B., and Neil A. Campbell. Campbell Biology. Harlow: Pearson Education, 2011. Print. The endosymbiotic theory deals with the origins of mitochondria and chloroplasts, two eukaryotic organelles that have bacteria characteristics. Mitochondria and chloroplasts are believed to have developed from symbiotic bacteria, specifically alpha-proteobacteria and cyanobacteria, respectively. The theory states that a prokaryotic cell was consumed or engulfed by a larger cell. By some unknown reason, the prokaryotic organelle was not consumed. Such lack of consumption would later lead to both cells forming a mutualism, receiving surviving benefits from each other. Over time, the newly combined organelle would develop into the complex eukaryotic cell of today. The endosymbiotic theory has been widely accepted as one of the possibilities of the origins of mitochondria, chloroplasts, and other eukaryotic organelles and cells.

Mitochondria and Chloroplasts

Mitochondria and Chloroplast

File:Chloroplasts WP.jpeg

Chloroplasts are believed to be the result of endosymbiosis.

Mitochondria and chloroplast are two organelles found in eukaryotic cells. Chloroplast is only found in plants while majority of eukaryotic cells have mitochondria. Even though both organelles are found in eukaryotic cells, both mitochondria and chloroplast have characteristics often found in prokaryotic cells.

These prokaryotic cell characteristics include: an enclosed double membrane, circular DNA, and bacteria-like ribosomes. Mitochondria and chloroplast both conduct prokaryotic activities. Mitochondria performs respiration while chloroplast performs photosynthesis.

These observed characteristics of both, mitochondria and chloroplast, are heart of the Endosymbiotic Theory.

A comparison in chloroplasts and Mitochondria: Both chloroplasts and mitochondria generate ATP by chemiosmosis, but they use different sources of energy. Mitochondria transfer chemical energy from food to ATP. Chloroplasts transform light energy into the chemical energy of ATP. Mitochondria can be found in animal cells and Chloroplasts can be found in plant cells. In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis. In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into stroma.

History

Schimper, Mereschcowsky, Wallin and The Symbiogenesis Theory

In 1883, French botanist Andreas Franz Schimper (1856–1901) observed that the division of chloroplasts was similar to that of the free-living cyanobacteria. Schimper would later propose in a footnote that symobiotic union of organisms lead to the evolution of green plants. He was the first to study and describe the potential endosymbiotic nature in these cells.

While conducting research on lichen, Russian biologist and botanist Konstantin Mereschcowsky (1855–1921) formulated the symbiogenesis theory. In 1905, he first suggested the idea of plastids originating as endosymbionts, which argued that symbiosis was the main driving force of evolution. Mereshcowsky published his finding of mitochondria in his 1926 work, Symbiogenesis and the Origin of Species in collaboration with Ivan Wallin. Mereschocowky proposed that smaller and less complex cells formed symbiotic relationships with larger complex cells. Mereshcowsky believed that many large complex cells like chloroplasts evolved through this process.

File:Ivan-wallin.jpg

Ivan Wallin was the first to suggest that eukaryotic cells were composed of microorganisms. Alongside Mereschocowsky, he published his works in "Symbiogenesis and the Origin of Species".

American biologist Ivan Emanuel Wallin (1883–1969) proposed, after studying and working with mitochondria, that species derived from bacteria have origins in endosymbiosis. He was the first to suggest the idea that the eukaryotic cell was composed of microorganisms. This lead to the formation of the Endosymbiotic Hypothesis. Wallin published his findings in his 1926 work, Symbiogenesis and the Origins of Species, alongside Mereschocowsky, where they formulated their ideas of symbiogenesis. Their theories were originally rejected due the assumption that mitochondria and chloroplasts did not contain DNA. However, this was proven false during the 1960s, when Hans Ris revived the theory.

File:Cyano chloro.jpg

Margulis and others hypothesized that chloroplasts (bottom) evolved from cyanobacteria (top).

In 1981, Dr. Lynn Margulis contributed to the endosymbiosis theory with the publication of her work, Symbiosis in Cell Evolution. Her research claimed that the origin of mitochondria were separate organisms that originally entered into a symbiotic relationship with eukaryotic cells through endosymbiosis. This became the primary support for the endosymbiotic theory, causing her to became the leading figure behind the endosymbiotic hypothesis.

Margulis essentially argued against the idea of random mutation, which was accepted as the main source of genetic variation with species. Instead she thought a symbiotic merger played a much larger role in the creation of new genomes and genetic diversity. She believed that instead of mutations, DNA in the cytoplasm of cells originated from the genes of prokaryotes(bacteria) that had become organelles.

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Dr. Lynn Margulis at the University of Massachusetts Amherst.

Dr. Lynn Margulis continued to study the origins of mitochondria and chloroplast in eukaryotic cells during her time at University of Massachusetts Amherst. She discovered that these organelles originated as prokaryotic endosymbionts that later started to show in eukaryotic cells. Margulis showed convincing research evidence that mitochondria evolved from aerobic bacteria called Proteobacteria, and chloroplasts evolved from endosymbiotic cyanobacteria. Her research was published in her work “Symbiosis in Cell Evolution” (1981).

Dr. Margulis proposed that eukaryotic flagella and cilia originated from endosymbiotic spirochetes. Due to lack of DNA and the fact that they do not show any ultrastructural similarities to prokaryotes, there is not enough evidence to support this claim. Even though DNA is not present, peroxisomes are considered to be a consequence from the origin of endosymbiotic. As a matter of fact, the original emdosymbionts were projected by Christian de Dave himself.

Considering Darwin’s idea of evolution, Margulis and Sagan believed, ‘Life did not take over the globe by combated, but by networking,’ for example, by cooperation, interaction, and mutual dependence between living organisms.

Dr. Margulis was awarded the National Medal of Science by President Bill Clinton in 2000, for her amazing work on the endosymbiotic hypothesis. She was also a member of the Russian Academy of Natural Science and the National Academy of Sciences. E.O Wilson has titled her ‘one of the most successful synthetic thinkers in modern biology’. Her work has considerably helped promote the study of endosymbiosis from a hypothesis to theory.

Margulis and The Modern Endosymbiotic Theory

While studying the structure of cells in the late 1960s, American biologist Lynn Margulis (1938- ), noticed, like many before her, that mitochondria have similar characteristics to bacteria. She would argue in her 1966 work, The Origin of Mitosing Eukaryotic Cells, that the major force of cell evolution is symbiosis. Her 1966 work would become a landmark for the modern endosymbiotic theory. The idea of endosymbiotic theory was that multiple prokaryotic organism were engulfed by one another. Both survived and evolved to eukaryotic cells over a period of a millions of years. Margulis would later connect this theory with her previous work in her 1970 book, Origin of Eukaryotic Cells. Margulis’s theory gained support in the 1980s after research showed that the genetic material of mitochondria and chloroplasts differed from that of nuclear DNA. The theory has also been accepted as the theory of evolution for some organelles.

File:1endosymbiosis theory.gif

The Endosymbiotic theory.

The Theory

Endosymbiotic Theory proposes that mitochondria developed from proteobacteria, or Rickettsia, (respiring Bacteria) and that chloroplasts were originally from cyanobacteria.
Anaerobic bacteria engulfed the aerobic bacteria, which was not completely digested. They formed a symbiotic relationship, and both cells benefited from each other mutually. The anaerobic bacteria procured food for the aerobic bacteria and provided it with a safe shelter. The engulfed bacteria was able to provide a new aerobic method for converting oxygen that was toxic to the anaerobic bacteria into ATP. Aerobic bacteria eventually became mitochondria, which is supported by the fact that mitochrondria have their own cellular mechanisms and DNA. In addition to these cells, the heterotrophic host cell also engulfed photosynthetic cyanobacteria, which were able to use photosynthesis to convert sunlight into ATP. This additional trait was beneficial for the newly formed eukaryotic cell. This symbiont cell became the chloroplast. Over numerous years, mitochondria and chloroplast became gradually more specialized to such an extent that they now cannot survive outside of the cell. These cells had a greater advantage over other cells, and, through natural selection, became more prevalent than other types of cells. Eukaryotes with only mitochondria became animal and fungi cells, whereas those with both mitochondria and chloroplasts became plants cells.

Evidence

Modern scientists believe that certain organelles found exclusively in eukaryotic cells may have arisen from various prokaryotic ancestors that initiated endosymbiotic relationships with host cells that consumed them. This belief is supported by evidence such as the discovery of DNA and ribosomes within organelles like mitochondria and chloroplasts. Examining the DNA has led scientists to find similarities in the sequences to those of modern living bacteria. For example, each organelle in eukaryotes has a single, circular DNA that is more similar to prokaryotes than eukaryotes. In addition, transfer RNA, ribosomes, and other molecules involved into transcription and translation processes were examined and compared with prokaryotes; it was discovered that they are similar in term of nucleotide sequence, size and even sensitivity to certain antibiotics. Furthermore, the existence of double membranes over many of these organelles suggests the possibility that the inner membrane may have belonged to the original prokaryote while the outer membrane may have formed from food vacuoles as the host cell devoured the prokaryote. The inner membrane of these organelles contains enzymes and transport systems that are similar to the plasma membrane of prokaryotes. In addition, certain species of modern organisms such as amoebas have been known to live, via endosymbiosis, with aerobic prokaryotes.

Mitochrondria and chloroplasts also replicate by a splitting process similar to binary fission in prokaryotes.

Based on the nucleotide sequences on RNA of organelles such as mitochondria and chloroplasts, systematists found similarities between mitochondria and alpha proteobacteria and chloroplasts and cyanobacteria. They concluded that alpha proteobacteria gave rise to mitochondria and cyanobacteria gave rise to chloroplast in eukaryotes.

General Overview of Engulfing Process

Gene Transfer Between the Mitochondria and the Nucleus

It appears that over time, there has been a transfer of genes from the mitochondrial genome to the nuclear genome. The ancestral bacterial genes that were thought to be part of the mitochondria have been found in the nuclear genome. Evidence for this hypothesis can be seen from orthologous genes. In some species, the orthologous genes can be found in the mitochondria, while it is found within the nuclear genome of other species.

This hypothesis of gene transfer from the mitochondria to the nucleus explains why proteins necessary for mitochondrial DNA (mtDNA) replication are not coded in the mitochondria. In fact, proteins for mtDNA replication, including transcription and translation, are coded for in the nucleus of the cell.

Mitochondrial Retention of Genes

Despite the transfer of genes between the mitochondria and nucleus, the mitochondria has still retained much of its own independent genetic material. Speculation has occurred as to why the mitochondria is still necessary for maintaining complex enzymatic processes, resulting in complicated genomes that may only code for a few genes.

One possible explanation is that it may be difficult to transport hydrophobic proteins across the mitochondrial membrane, and then ensure that they are shipped to the correct location. This explanation suggests that these proteins may be necessary to be produced within the mitochondria. Another possible explanation is that there are differences in codon usage between the nucleus and mitochondria, making it difficult to be able to fully transfer the genes. A third possible explanation is that the mitochondria needs to produce its own genetic material so as to ensure metabolic control in eukaryotic cells. This indicates that mitochondrial DNA may directly have an effect on the respiratory chain and the reduction/oxidation (redox) processes of the mitochondria.

References

1. The Endosymbiotic Theory. 14 Jan. 2002. <http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html>
2. http://learn.genetics.utah.edu/content/begin/cells/organelles/ 3. Campbell Reece. Biology. Seventh edition. 3. Lynn Margulis <http://en.wikipedia.org/wiki/Lynn_Margulis>
4. Endosymbiosis: Lynn Margulis <http://evolution.berkeley.edu/evolibrary/article/history_24>
5. San Jose Science, Technology and Society. <http://www.isepp.org/Pages/San%20Jose%2004-05/MargulisSaganSJ.html>
6. Andreas Franz Wihelm Schimper. 15 Nov. 2010. <http://en.wikipedia.org/wiki/Andreas_Franz_Wilhelm_Schimper>
7. Konstantin Mereschkowski. 15 Nov. 2010. <http://en.wikipedia.org/wiki/Konstantin_Mereschkowski>
8. Biography 30: Ivan Emanuel Wallin (1883–1969). <http://www.dnalc.org/view/16654-Biography-30-Ivan-Emanuel-Wallin-1883-1969-.html>
9. Endosymbiotic Theory. 20 Nov. 2010. <http://en.wikipedia.org/wiki/Endosymbiotic_theory>
10.Symbiogenesis. 19 Nov 2010. < http://en.wikipedia.org/wiki/Symbiogenesis>
11. Falkenberg M, Larsson NG, Gustafsson CM. DNA Replication and Transcription in Mammalian Mitochondria. Annu Rev Biochem 2007;76:679-99. <http://www.annualreviews.org/doi/pdf/10.1146/annurev.biochem.76.060305.152028> 12. Neil A. Campbell, Jane B. Reece "Biology 8th edition"
13. Word Press. 2012. <http://endosymbiotichypothesis.wordpress.com/primary-versus-secondary-endosymbiosis/>
14. Word Press. 2012. <http://endosymbiotichypothesis.wordpress.com/history-the-formation-of-the-endosymbiotic-hypothesis/> Cellular membranes are highly complex biological machines, responsible for regulating the import and export of metabolites and polymers.

Metabolic network of the Arabidopsis thaliana citric acid cycle. Enzymes and metabolites are shown as red squares and the interactions between them as black lines.

The Interdependency of Lipid Membranes and Membrane Proteins

The cell membrane contains various types of proteins, including ion channel proteins, proton pumps, G proteins, and enzymes. These membrane proteins function cooperatively to allow ions to penetrate the lipid bilayer. The interdependency of lipid membranes and membrane proteins suggests that lipid bilayers and membrane proteins co-evolved together with membrane bioenergetics.

Ideas About The Earliest Membranes

Several hypotheses of the origin of cellular membranes exist:

• Evolution subsequently took place in vesicles, which were formed by the accumulation of abiogenically formed amphiphilic molecules. The vesicles then transformed into envelopes, likely reminiscent of viral envelopes.
• Proto-cells evolved from the folding of vesicles, upon which the first life forms existed.

A viral envelope.

From Water-Soluble Proteins to Integral Membrane Proteins

Water-soluble proteins evolved gradually into highly hydrophobic membrane proteins. Primordial membranes initially contained pores, which enabled ions, small molecules, and polymers to be exchanged passively between protocells and their environment. In contrast, modern membrane proteins must be inserted into the membrane by membrane protein complexes. Membrane protein complexes could not have logically existed prior to the existence of membrane proteins. Although a single α-helix is thermodynamically unfavorable, one model explains how increasingly complex membrane proteins were derived from a stand-alone hydrophobic α-helix. Molecular dynamics was used to show that an α-helix spontaneously dimerizes on the membrane surface upon entrance, and then oligomerizes within the membrane.

Top view of the α-helix. Four carbonyl groups are pointing upwards towards the viewer, spaced roughly 100° apart on the circle, corresponding to 3.6 amino-acid residues per turn of the helix.

The Co-Evolution of Lipid Bilayers, Membrane Bioenergetics, and Membrane Proteins

In order to maintain proper bioenergetics, cells meticulously regulate the flow of matter and therefore energy through the cellular membrane. There is an inverse relationship between membrane permeability and the number of enzymatic pathways present in a cell.

The Domains of Life.

LUCA and Early Membranes

The first major split between the domains of life is between archaea and bacteria. Yet, there are essential genes that are still common between the two domains that lead to the concept of the LUCA, or last universal common ancestor. Based on the standard model of evolution, the LUCA was based on DNA and led to the three domains of life and existed approximately 3.5 to 3.8 billion years ago. Archaea and bacteria, however, differ greatly in their biogenesis pathways as well as in the hydrophobic chains contained in the membrane structure. The fact that both archaea and bacteria contain membrane-embedded molecular machines such as ATP synthases strongly indicates that the LUCA contained some derivative of membranes, though perhaps not as complex and structured.

It is seemingly impossible to have the formation of impermeable membranes without membrane proteins and translocases to shuttle essential materials in and out of the cell. Consequently, it is also unlikely that very specialized membrane proteins were able to form without a membrane initially present. It is hypothesized that the development of a permeable, porous membrane eventually integrated evolving proteins, to later evolve into a highly specialized and efficient impermeable membrane.

ATPases as Basis of Membrane/Membrane-Protein Co-evolution

F-type ATPases are found in eukaryotic and bacteria mitochondria and chloroplasts, while A-type ATPases around found in archaea (and some bacteria). V-type ATPases are found in eukaryotic cells, particularly in vacuole membranes.

Na⁺/K⁺ATPase

From ‘Pore’ to ATPase

F- and A/V- type ATPases are membrane-embedded proteins and were feasibly present in the LUCA (last universal common ancestor) due to their omnipresence in modern cellular life. These ATPases function as efficient ATP synthases through the completion of a reaction cycle with a physical rotation of a ‘rotor.’ However a proton gradient is required for this function, and therefore an ion-imperable membrane is necessary.

In prokaryotes, sodium ions as well as proton translocations are found in both F- and V- type ATPases. However the ‘rotor’ base, or c-oligomer, in sodium ion translocating F- and V- type ATPases are found to contain almost identical sets of amino acids which serve as sodium ion binding sites. This shows that the last ancestor containing ATPases also contained a sodium ion binding site.

The c-oligomers in the F- and V- ATPases are homologous in their sub-units of composition, which are all unrelated. However, the ‘rotors’ or stalks of the ATPases are not. Based on this information, it is proposed that the F- and V- ATPases evolved from a protein translocase that is ATP dependent, which initially served as the ‘rotor’ or stalk. Membrane components of these ATPases function as membrane ion translocases. They do not function as channels, as the ion binding sites are not accessible from both sides of the membrane at the same time. The c-oligomers that form the base of the rotating stalk/rotor are ~2-3 nm in internal diameter, large enough to allow for transport of materials in and out of a cell membrane. Combining this diameter, along with the ion transport function and potential acquisition of an ATP dependent protein to form the stalk, lend strongly to the idea that ATPases could have once functioned as pores in the membrane.

Birth of Bioenergetics: Beginning of ATP Synthesis and Sodium Tight Membranes

ATPases potentially evolved from a sodium binding pore/protein combination. During the transition phase from porous to ion-tight membranes, there still could have been a demand for sodium binding.

Inside the cell, there is a higher concentration of proteins and polynucleotides, which are negatively charged. Such components could have potentially led to the beginning of a transmembrane electric potential. The Donnan effect shows that this can equate to an electric potential of up to 50mV. Additionally, because of this negative potential inside the semi-porous cell, positive portions of outside proteins tend to insert themselves positive end-first into the cell membranes.

In the Archean era, sodium ion concentration was already 1M in the ocean compared to approximately .01M inside the cell. With the combination of increased ocean salinity (and subsequent sodium ion gradient) and the transmembrane potential, there was enough drive present to turn the ATP dependent rotor/stalk proteins from hydrolyzing ATP into synthesizing ATP. Proposed here is the first membrane bioenergetics: the coupling of outward sodium ion pumps with ATP synthases and dependence on a sodium ion gradient.

Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2005.

Sodium-Based Bioenergetics

As the salinity of the ocean continues to increase, evolving cells must constantly remain in ionic-homeostasis. Therefore, sodium-tight membranes are required to prevent solute from entering or exiting the modern cell freely. Additionally, the evolution of membrane pumps was integral for the active expunction of excess Na+ out of the cell.

Proton-Based Bioenergetics

The cellular regulation of proton movement through lipid bilayers requires a significantly more advanced control mechanism than the regulation of sodium, and thus requires more advanced cellular machinery. The necessity of more stringent regulation arises from the chemical fact that proton transfer can be coupled to oxidation-reduction (redox) reactions. Furthermore, lipid bilayers are more conductive to protons than to sodium ions, and protons can integrate into hydrocarbon chains by dissolving in trapped water molecules.

The transition to proton-tight membranes is particularly more complicated because of the fact that lipid bilayers are much more conductive to protons than to sodium ions. Protons can easily enter into bunches of water molecules that are nestled in between chains of lipid hydrocarbons, while sodium ions cannot. The rate-limiting step of trans-membrane conduction of protons is the jumping from one bunch of water molecules to the next. This method of proton transfer can be stopped in two ways: increasing the density of the hydrocarbons, (limiting the bunches of water molecules), or by lowering the mobility of the lipids.

Different organisms have various solutions to the problem of passive proton transfer through the cell membrane:

• Archea form single C40 membrane-spanning lipid molecules by the fusing of two diether lipids.
• Bacteria incorporate additional steric hindrance by attaching bulky compounds such as cyclohexane and cycloheptane to the terminus of membrane fatty acids.

The steric effect makes reactions difficult.

The Hypothesized Co-Evolutionary Scheme of Membranes, Membrane Proteins, and Bioenergetics

Primordial cellular membranes were undoubtedly non-existent in pre-cellular life forms. The earliest cellular life forms contained leaky and inefficient cellular membranes, amphiphilic membrane proteins, and characteristically simple bioenergetics. In contrast, modern cells are carefully regulated by proton-tight membranes, highly hydrophobic membrane proteins, and complex bioenergetics.

References

1. Mulkidjanian A, et al. Co-evolution of primordial membranes and membrane proteins. Trends Biochem Sci. 2009 Apr;34(4):206-15. Epub 2009 Mar 18.
2. Eisenberg D (2003). "The discovery of the alpha-helix and beta-sheet, the principal structural features of proteins". Proc. Natl. Acad. Sci. U.S.A. 100 (20): 11207–10. doi:10.1073/pnas.2034522100. PMC 208735. PMID 12966187. {{cite journal}}: Unknown parameter |month= ignored (help)

Overview

Darwin’s theory of natural selection can be described as a competition between species in a certain area containing a variety of consistent effects. However, when looking at a simplistic, molecular level, the theory of natural selection could apply as well within the boundaries of a cell. At the molecular level, consistent effects that influence selection of certain molecules or atoms include reaction thermodynamics and reaction kinetics. Reaction thermodynamics are defined by equilibrium constants, which are explained even further through physical properties of reaction products such as solubility and stability. However, reaction kinetics is defined on how exactly equilibrium of a system or reaction is approached. In biochemistry, reaction kinetics helps to explain why certain biological pathways are selected for more than others. It is certain consistent effects that select for these certain pathways such as concentrations of reactants and products and amount of competitors as well as physical constraints within a cell such as membranes and fluid flow acting as transport systems. These are what are known as kinetic barriers, and these are what help to describe the selections and non-selections for atoms such as aluminum and silicon.

Selection of Aluminum

Aluminum is the most plentiful metal on the planet and the third most abundant element in the Earth’s crust. However, in biochemistry, aluminum has a very limited, small role. This can be explained through two reasons: its selection out of biological systems due to its physical and biochemical properties or its nonexistent participation in the selection of essential elements in biochemistry. Both reasons have been explained through the properties of aluminum. For example, the lack of selection of aluminum in biological systems can be explained through its slow ligand exchange rate, which makes aluminum a very poor metal co-factor, a non-protein chemical compound bound to a protein in order for it to function, for enzymes. However, it is aluminum’s nonexistent participation in the selection of essential elements in biochemistry that explains this phenomenon the best.

Aluminum has proven to be a very reactive element, but yet, it has ultimately been selected out of biochemical pathways. Aluminum is the main instigator for the occurrence of acid rain due to its high affinity to bind to oxygen-based functional groups. An example of its high affinity to bind to oxygen-based functional groups is that it out-competes Mg (II) by a large factor for the complex molecule, ATP. Al (III) is a redox inactive cation, but it is a very strong pro-oxidant that helps catalyze certain reactions such as iron driven redox reactions. Aluminum has also been shown to be a good immunogen due to its role as an immunological agent to modify other agents. Certain physical and biochemical properties have shown that aluminum contains the potential to appear in certain biochemical pathways such as its substitution for Mg (II) in metal-nucleotide complexes. Although many of these observations prove that aluminum is a relatively abundant, reactive atom, one observation has supported the theory of the evolution in the absence of biologically available aluminum. Christopher Exley performed a study on the acute toxicity of aluminum in the Atlantic salmon, and he discovered the aqueous form of silicon, silicic acid, protected these fish from the toxicity of aluminum. Based on this evidence, the non-selection of aluminum can be explained through silicic acid’s ability to reduce the biological activity of aluminum, making it inactive in nature.

Selection of Silicon

Silicon is the second most abundant element in the lithosphere and is known to be an essential element. However, silicon has been studied and many scientists have determined that silicon contains no biochemical activity or function. One reason for this can be described for its lack of bioorganic diversity. For example, no Si-C bonds or Si-O-C bonds are known to exist in nature, and because of this non-selection, silicon has not been known to be present in organisms. The fact that silicon bonds require much more energy to break also explains why organisms contain carbon within their biochemical pathways instead of silicon. Silicic acid is the most common form of silicon known to exist in nature. Silicic acid is weak acid as it loses its first proton when the pH reaches 10. Because the pH of systems usually range from 7 to 8, the fact that silicic acid cannot lose a proton until the pH reaches 10 indicates that silicic acid is rather inert and not reactive. In addition, only three significant reactions involve the element silicon. These reactions are the autocondensation to yield amorphous hydrated silica, its reaction with aluminum hydroxide to yield hydroxyaluminosilicates, and its reaction with excess molybdate to yield Keggin-like molybdosilicic acid complex. Aside from these reactions, no known bioorganic reactions have been known to take place with silicon in them. As a result of this, the selection for silicon in biochemical pathways have been absent and selected against.

References

Exley, C., 2009, Darwin, natural selection and the biological essentiality of aluminum and silicon, Vol. 34 No. 12, 20 September 2009. Speciation is a term which aims to describe the manner in which different species come about. This is very important because it explains how the original common ancestor has diverged into the multitude of species that are living today.

Major Theories on Speciation

Biological Species Concept

The Biological Species concept aims to explain speciation by looking at the likelihood of species to interbreed and produce viable and fertile offspring. In other words, if species are able to mate and bear healthy, fertile offspring, this will not lead to speciation, but if there is anything that barriers this interbreeding, a new species could arise.

Reproductive isolation is an example of a case where due to the lack of interbreeding within a species, a new species could arise. Factors which lead to reproductive isolation could be mechanical differences meaning the two species could not physically mate successfully. Behavioral differences could also lead to a lack of mating between species.

Morphospecies Concept

The Morphospecies concept tries to explain speciation by looking at similarities in structures from species to next.

The similar bone structures between a human and a bat

One example of this is mammalian forelimbs. A human, a cat, a whale and a bat all have similar forelimbs.

It is through these similarities in morphological structure that scientists can deduce some sort of common ancestor which contained these structures, and from there hypothesize on where the speciation occurred. Of course, it is much harder to deduce this for something such as a bat and a human, but there are many cases which this concept can be more directly applied. For example, the different species of finches due to there different beak sizes and shapes.

Phylogenic Species Concept

The Phylogenic Species concept aims to explain speciation by looking at genetic history of species. In this method physical characteristics and molecular sequences of organisms are compared to find the set of organisms that share a unique genetic history. As a result, the groups of the individuals that are sufficiently different are separate species. This analysis has some advantage; for instance, it can be applied to all types of organisms - asexual organisms, such as bacteria. In addition, Sibling Species - Species that are very similar that they cannot be distinguished by morphological analysis - can be recognized. Phylogenic Species analysis also has some disadvantages. For example, good phylogenies are not available for most organisms and it is not clear that how much genetic divergence represents long-term isolation and thus speciation.

Different types of Speciation

To recognize speciation, it is significant to have a definition of a species. A species is a group of populations through which genes can flow and whose offspring have a fitness equal to the parents. As evolution progresses, gene flow is ultimately stopped between one population and the species as a whole. As a population undergoes changes in gene frequencies it could diverge entirely until it is reproductively distinct from other populations of the species from which it diverged. This is called phyletic evolution or anagenesis. Alternatively the population could evolve into two distinct species that exist simultaneously. This is called true speciation or cladogenesis.

Cladogenesis occurs because reproductive-isolating mechanisms prevents two sub-populations from interbreeding. This reproductive barrier could be the result of

1) the isolation of one portion of the population by some physical barrier

2) a sub-population becoming established in a new ecological niche not previously occupied by the species

3) a polymorphism becoming established in a population before it moves to a new ecological niche

Speciation involves the genetic transform in a subgroup of a population that renders the new population unavailable of reproducing offspring with the original population. If a reproductive barrier occurs because of isolation by a physical barrier, the isolated population can evolve and develop into a new species. This process is termed allopatric speciation. This has been considered for a long time to be the primary mode of speciation. Speciation can also occur when a subpopulation migrates into a new niche. This is termed parapatric speciation, and this process seems to have been used by some annual plants.

The final form of speciation is called sympatric speciation. This type of speciation occurs when a subpopulation that occupies the same niche as the remainder of the species develops a unique mutation that prevents it from mating with the original population. That new species may have an ecological advantage which permit its establishment as a species in the same niche. A nice example of this method of speciation is the development of the new saltmarsh species Spartina townsendii, that was derived from S. alterniflora and S. maritima, but is reproductively incompatible with either parent. This new species is better adapted to the coastal regions of Holland than either of the parental species and was able to better establish itself in that niche.

Theories behind Evolutionary Speciation

At the current time there are two accepted theories on ways in which the evolution of a species, leading to speciation, can occur. The two theories are gradualism and punctuated equilibrium. The difference between these two theories primarily lie in the time scale between when speciation may occur in a particular organism.

Gradualism is the theory that selection and variation happens in a more gradual state. In other words, over a short period of time, it is difficult to recognize the change that is happening, either phenotype or genetically, and small variations that fit an organism become gradually better to its environment. Over a long period of time, one could recognize a change and can lead to subsequent speciation.

Punctuated Equilibrium speaks of change that comes in spurts. There is a period of very little change and then one or few huge changes occur. This usually occurs through mutations in the genome of an individual organism. Mutations are random changes in the DNA sequence that are not inherited from the previous parental generation. Although some mutations are harmful, some mutations prove to be beneficial to an organism surroundings and are naturally selected. Once selected, speciation follows suit to forever change the organisms to its better adaption to its environment.

Reference

1.Campbell, Neil A. (2005). Biology. Pearson. ISBN 0-8053-7146-0. {{cite book}}: Check |isbn= value: checksum (help); Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

http://www.globalchange.umich.edu/globalchange1/current/lectures/speciation/speciation.html

Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman and, 2006. Print. Gene flow is the movement of genes from one population to another through the transfer of alleles. This can occur within a population as well as across a population. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If genes are carried to a population where those genes previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene for brown coloration moves from one population to another.

The rate of gene flow depends on the capability of individual organisms. For example, a wind-pollinated corn will have a possible gene flow range of about 50 feet whereas fruit flies have migrated from over 15km.

Random Drifts

Random drifts: the idea of allele frequencies in gene flow is that all alleles have an equal opportunity of being transfered. Although some alleles appear to be more favorable, this is accounted by its increased appearance in the population. Random drift is the important mechanism of evolution. One aspect of genetic drift is the random nature of transmitting alleles from one generation to the next, given that only a fraction of all possible zygotes become mature adults. The easiest case to visualize is the one which involves binomial sampling error. If a pair of diploid sexually reproducing parents (such as humans) have only a small number of offspring, then not all of the parent's alleles will be passed on to their progeny due to chance assortment of chromosomes at meiosis. In a large population, this will not have much effect in each generation because the random nature of the process will tend to average out. But in a small population the effect could be rapid and significant.

Gene Flow in Humans

Gene flow in humans can be primarily accounted by migration. For example, the Vietnam War caused the mating of US soldiers with Vietnamese women, thus leading to a differing gene pool frequency. The loss or gain of people can also affect the gene flow. For example, if red-haired Scottish were to move out of the country, then the next generations of Scottish would consist of fewer red-heads. Population genetic studies are mainly based on the description of genetic variability and on interpopulational comparisons using genetic distance measures. The evolutionary dynamics of the populations are inferred from these parameters and accurate estimates of gene flow may be critical. The present study reevaluates the role of gene flow in human populations by different statistical methods from a number of microsatellite and protein polymorphism data. The estimated number of individuals exchanged per generation (Nm) was greater than 1 in all data sets with all statistical methods. The correlation between geographic and genetic distances suggests a pattern of isolation by distance, characteristic of demographic and genetic equilibrium conditions among populations worldwide. Thus the high values of Nm may be interpreted as a reflection of high gene flow between geographically close populations. As expected, gene flow appears to exert a pivotal role in the genetic history of humans.

Gene Flow between Species

Gene flow can occur between species through viruses. The DNA of one cell is extracted by a virus and injected into another. The extent of this type of gene flow between humans is unclear at the moment, although estimates of 40-50% are plausible. However, this can be observed in several species such as insects, fish, reptiles, mammals, and especially microorganisms.

Researchers hypothesize that differential gene flow across species boundaries is a pattern resulting from selection, in which the absence of selection on some traits or alleles between species results in gradual transitions of these traits across a hybrid zone. In contrast, ongoing selection to maintain species at other traits prevents the dispersal of alleles across species boundaries and is expected to produce a pattern of steep transitions of traits across a hybrid zone. In house mice, researchers found that alleles near the center of the X chromosome exhibit steep transitions across the hybrid zone, and these alleles are in a region previously shown to be associated with hybrid sterility. This work demonstrates the utility of hybrid zones to test for the effect of selection on interspecies gene flow in particular regions of the genome.

By continuing to study hybridizing species and hybrid zones, researchers can further understand which sets of genes have permissible gene flow and which are more restricted. Then, by studying genes in the latter category, scientists can learn which genes or gene sets may be important in maintaining species integrity in the face of gene flow.

Allele Frequency in Subpopulations

The average allele frequency can be calculated via this formula: pt = p + (po - p ) (1-m)^t

p is the frequency of alleles in generation t m is the migration rate o is the initial frequency of an allele in a subpopulation t is the number of generations

OMPG is used to provide a balance between drift and gene flow and allow heterozygosity by minimizing the loss of alleles.

Given subpopulation size, number of subpopulations, the migration rate m, generations to evolve, this script shows changes of allele frequencies among subpopulations at single locus, which results from the effect of migration. All subpopulations have equal number of individuals. Migration is depicted in the form of a N*N matrix, which is consisted with identical non-zero values at non-diagonal positions and zero on the diagonal line, where N is the number of subpopulations. Sum of values of elements on row i (i = 1, 2, …, N) of the migration matrix equals to the migration rate, thus it ensures that every individual in subpopulation i has probability m to become a migrant, and such a migrant has equal possibility of entering any other subpopulation. Initialization of allele frequencies among subpopulations is to use a list of numbers, which have counts equal to number of subpopulations and been set evenly distributed between 0 and 1, including 0 and 1. Therefore, the average allele frequency named theoretical value that all subpopulations share after a long term of evolution will be 0.5.

Reference

• http://evolution.berkeley.edu/evosite/evo101/IIIC4Geneflow.shtml
• Gene flow at Wikipedia
• Gene flow at Encyclopædia Britannica
• www.life.illinois.edu%2Fib%2F443%2Flectures%2FEVOLGEN4.PPT

Organismic and Evolutionary Biology Terms

Abdomen - Section of the body of an animal that is furthest from the mouth & usually contains reproductive organs & part of the digestive system.

Adapt - In terms of evolution to undergo natural selection so that members of a population are on average better able to survive & reproduce. In everyday usage to adapt may simply mean to adjust to a situation which does not necessarily imply that evolution has occurred.

Adaptation - A feature produced by natural selection for its current function.

Adaptive radiation - Periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles, or niches, in their communities

Allele - One of the versions of a gene that may exist at a locus. For example the pea color locus may have either the yellow allele or the green allele. Different alleles of the same locus are often symbolized by capital & lowercase letters (e.g. the Y & y alleles).

Allometric growth - When some part of the organism grows at a rate different from the rest of the organism during development. For example the neck vertebrae of fetal giraffes must grow at a faster rate than the rest of the body (in comparison to giraffe's short-necked relatives).

Allopatric speciation - Speciation that depends on an external barrier to gene flow (such as geographic isolation) to begin or complete the process of speciation.

Amino acid - A building block of proteins. There are about 20 amino acids & protein-coding DNA tells the cellular machinery which amino acids to use to build a particular protein.

Analogy/analogous structure - Similar because of convergent evolution & not because of common ancestry. Two characters are analogous if the two lineages evolved them independently. See also homologous homoplasious.

Anthropocentric Centering on humans & considering all other things in relation to humans.

Anthropologist - A scientist who studies humans. This can include studying human evolution.

Apomorphy - The derived or changed character state for a particular clade under consideration. For example within the clade of terrestrial vertebrates (in which "has four legs" is the ancestral or plesiomorphic character state) birds have the apomorphic character state "has two legs & two wings."

Appendage - Any limb that extends from the body. Arms & legs for example are appendages. Arthropods' mouthparts are often small limb-derived extensions of the body & so are considered appendages.

Archipelago - A group of islands.

Arms race - In evolutionary biology a process in which two or more lineages coevolve such that each in turn evolves more & more extreme/efficient defenses & weapons in response to the other parties' evolution.

Arthropod - Any member of the large animal clade Arthropoda. Living lineages include crustaceans arachnids centipedes, millipedes, & insects. Fossil lineages include the extinct trilobites. All arthropods have a hard exoskeleton that is periodically shed during growth a body that is divided into segments & jointed legs.

Artificial selection - A process in which humans consciously select for or against particular features in organisms. For example the human may allow only organisms with the desired feature to reproduce or may provide more resources to the organisms with the desired feature. This process causes evolutionary change in the organism & is analogous to natural selection only with humans not nature doing the selecting.

Bacterium - A microscopic single-celled organism lacking a well-defined nucleus. Neither plants nor animals bacteria are similar to the first life forms on Earth & are widespread today. Although some bacteria cause diseases in humans the vast majority do not harm humans & are essential to the health of other organisms & Earth's ecosystems. (plural = bacteria)

Base - The information coding part of DNA the letters of the genetic code. The sequence of bases on a stretch of DNA (i.e. the sequence of As Ts Gs & Cs) determines what the DNA does — if it codes for a protein turns on a gene or whatever. In protein-coding regions three base pairs code for a single amino acid. For example the base pair sequence ATG codes for the amino acid methionine. In a str& of DNA bases are paired & are lined up across from one another: A pairs with T & G pairs with C.

Bilateral symmetry - A condition in which the right & left sides of an item (a shape or animal) are mirror images of one another. For example since the right side of the human body generally mirrors the left side humans are bilaterally symmetric.

Biochemistry - Set of chemical reactions that occur within or associated with living things.

Biodiversity - The variety & variability among organisms inhabiting a particular region. However the term may be more specifically defined & measured in different ways. For example sometimes biodiversity refers to the number of species in a particular area sometimes to the number of different ecological niches occupied by organisms in a particular area & sometimes to the amount of genetic divergence that organisms in a particular area experienced.

Biodiversity hotspot - Region that provides a home to a unusually high density of different species

Biogeography - The study of where organisms live & how they came to live where they do.

Biomass - Total mass of all living organisms in a particular area. In measures or estimates of biomass often the mass of the water in organisms is not counted towards their total biomass. book lung An organ used by many l&-dwelling arachnids for breathing. It consists of a cavity in the abdomen containing a set of thin overlapping flaps (like the pages of a book). The inside of each flap is filled with blood & the outside is exposed to air allowing oxygen & carbon dioxide to be exchanged through diffusion.

Bottleneck - An event in which a population's size is greatly reduced. When this happens genetic drift may have a substantial effect on the population. In other words when the population size is radically reduced gene frequencies in the population are likely to change just by random chance & many genes may be lost from the population reducing the population's genetic variation.

Brongniart Alexandre(1770-1847) - French geologist & student of Cuvier who along with his mentor was one of the first to identify & cross-reference geologic strata using fossils a methodological innovation credited to William Smith. Brongniart & Cuvier identified the same fossil layers all across the Paris region & showed that the regional fossil fauna had alternated between marine & freshwater forms over geologic time.

Buckland William(1784-1856) - English geologist & teacher of Lyell. Buckl& is known for his attempts to reconcile religion & geology & for being among the first to identify dinosaur fossils. As a natural theologist he believed that new life forms were continually created. He also believed that the Earth had been shaped by a series of catastrophes & tried to find evidence that a worldwide flood — Noah's biblical flood — was the most recent of these.

Burgess Shale - Rich deposit of fossils from the Cambrian Period located in western Canada. This fossil bed is particularly valuable because the rarely fossilized soft parts of many ocean-dwelling organisms were preserved in these rocks along with their hard parts (e.g. the exoskeleton).

Cambrian Period - Geologic time period 543-490 million years ago. The Cambrian is the first period of the Paleozoic era during which all animals & plants lived in the Earth's oceans. Many organisms that we recognize as members of modern animal groups (including the arthropods sponges chordates & molluscs) made their first unmistakable appearance in the fossil record during the Cambrian.

Carnivore - An organism that eats almost exclusively animals (caro = flesh vorare = to swallow up).

Character - A recognizable feature of an organism. Characters may be morphological behavioral physiological or molecular. They are used to reconstruct phylogenies.

Chelicerate - Chelicerates are a group of arthropods distinguished by the following characters: a body divided into a cephalothorax & abdomen * no antennae but two pairs of appendages on the anterior cephalothorax (chelicerae & pedipalps) & four pairs of walking legs chitinHard tough substance that occurs widely in nature particularly in the exoskeletons of arthropods. Chemically chitin is a carbohydrate & is made from sugar molecules.

Chloroplast - In plants & photosynthetic protists a cellular body that uses energy from the sun (sunlight) to create organic compounds from carbon dioxide & water.

Chordate - Any member of the animal clade Chordata a large group of vertebrates & some marine invertebrates. Chordates have a notochord a rod-like cartilaginous structure supporting the nerve cord that they inherited from their common ancestor. Modern chordates include vertebrates tunicates hagfish & lancelets.

Chromosomal inversion - A mutation in which a section of chromosome is reversed 180 degrees. Because inversions in certain chromosomes can be observed with a light microscope they were particularly important in early genetic studies.

Clade - A group of organisms that includes all the descendents of a common ancestor & that ancestor. For example birds dinosaurs crocodiles & their extinct relatives form a clade.

Codon - A three base unit of DNA that specifies an amino acid or the end of a protein

Coevolution - A process in which two or more different species reciprocally effect each other's evolution. For example species A evolves which causes species B to evolve which causes species A to evolve which causes species B to evolve

Common ancestor - An ancestor that they have in common. For example the common ancestors of two biological siblings include their parents & grandparents; the common ancestors of a coyote & a wolf include the first canine & the first mammal.

Constraint - In terms of evolution an aspect of a lineage's genetic makeup that prevents the lineage from reaching a particular potentially advantageous evolutionary outcome (e.g. an organism's developmental process prevents the evolution of a trait that would allow a lineage to invade a new habitat).

Convergent evolution - Process in which two distinct lineages evolve a similar characteristic independently of one another. This often occurs because both lineages face similar environmental challenges & selective pressures.

Coprolite - Fossilized dung.

Crustacean - Crustaceans are a group of arthropods distinguished by the following characters: a body divided into cephalothorax & abdomen * two pairs of antennae & three pairs of mouth appendages.

Deleterious allele - A version of a gene that on average decreases the fitness of the organism carrying it.

Development - Change in an organism over the course of its lifetime; the processes through which a zygote becomes an adult organism & eventually dies.

DeVries Hugo(1848-1935) - Dutch botanist famous for his contributions to genetics. He rediscovered the results first obtained by Mendel & described genetic changes in his plants. Based on his observations DeVries argued that individual mutations had wide-ranging effects & could cause speciation in a single step; however T. H. Morgan later discovered that many mutations seemed to have rather small effects. DeVries had observed changes in chromosome number not minor change in base pair sequence that are typical of mutation.

Diffusion - Process in which the r&om movement of molecules causes different types of molecules to mix moving from regions of higher concentration to regions of lower concentration & eventually becoming evenly distributed.

Directed mutation - The hypothesis that mutations that are useful under particular circumstances are more likely to happen if the organism is actually in those circumstances. In other words the idea that mutation is directed by what the organism needs. There is little evidence to support this hypothesis.

Dispersal - A process in which a species' range changes because some or all individuals move to a new location. Dispersal is usually contrasted with vicariance as a biogeographic mechanism.

Diversity - In biology a measure of the variety of the Earth's animal plant & microbial lineages. Different measures of biological diversity (biodiversity) include number of species number of lineages variation in morphology or variation in genetic characteristics.

DNA - Deoxyribonucleic acid is the molecule that carries genetic information from generation to generation.

Ectoderm - Layer of tissue present in developing animals that will eventually form organs such as the skin & brain. Other tissue layers (the mesoderm & endoderm) will form other parts of the body.

Ectothermic - Term used to describe an organism that relies on the environment & its own behavior (e.g. moving to a sunny spot) to regulate its body temperature (ecto = outside therm = heat). Many lizards for example are ectothermic.

Endemic - Organism native to a particular restricted area & found only in that place.

Endoskeleton - Evolution is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) & large-scale evolution (the descent of different species from a common ancestor over many generations).

Endosymbiosis - A relationship in which one organism lives inside another to the mutual benefit of both. It is generally accepted that early in the history of eukaryotes eukaryote cells engulfed bacteria forming a symbiotic relationship. Over time they became so mutually interdependent that they behaved as a single organism. The bacteria became what we know as mitochondria & chloroplasts.

Endothermic - Term used to describe an organism that regulates its body temperature by generating its own heat internally. Mammals for example are largely endothermic.

Epithelium - A layer of tissue covering an organism's internal or external surfaces.

Eukaryote - An organism with eukaryotic cells — cells with a membrane-enclosed nuclei & membrane-enclosed organelles.

Evolution - Evolution is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) & large-scale evolution (the descent of different species from a common ancestor over many generations).

Exaptation - A feature that performs a function but that did not arise through natural selection for its current use.

Exoskeleton - Support structure located on the outside of the body (exo = outside). Arthropod bodies for example are supported by an armor-like exoskeleton.

Extant - Not extinct existing.

Extinction - An event in which the last members of a lineage or species die. A single species may go extinct when all members of that species die or an entire lineage may go extinct when all the species that make it up go extinct.

Fitness - A genotype's success at reproducing (the more offspring the genotype leaves the higher its fitness). Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to other genotypes. Experiments & observations can allow researchers to estimate a genotype's fitness assigning it a numerical value.

Food chain/food web - All the feeding interactions of predator & prey along with the exchange of nutrients into & out of the soil. These interactions connect the various members of a community & describe how energy passes from one organism to another. Referred to as the "food web."

Fossil - Trace of a living creature (body part of body burrow footprint) preserved over time.

Founder effect - Changes in gene frequencies that accompany starting a new population from a small number of individuals. The newly founded population is likely to have different gene frequencies than the source population because of sampling error (genetic drift). The newly founded population is to have a less genetic variation than the source population

Fourier Joseph (1768-1830) - French physicist & mathematician most famous for creating the mathematical tools to study how heat flows through solids. His studies of heat led him to argue that Earth's history had a direction beginning warm & cooling through time — an idea at odds with Lyell's view of Earth's history as one of constant but directionless change.

Gene - The unit of heredity. Generally it means a region of DNA with a particular phenotypic effect. A stretch of DNA that includes a transcribed & regulatory region.

Gene flow - The movement of genes between populations. This may happen through the migration of organisms or the movement of gametes (such as pollen blown to a new location).

Gene frequency - (also called allele frequency) Proportion of genes/alleles in a population that are of a particular type. For example at a particular locus pea plants may have either a "yellow pea" allele or a "green pea" allele — so a population of pea plants would have some frequency of yellow pea alleles ranging from zero to one (100%).

Gene pool - All of the genes in a population. Any genes that could wind up in the same individual through sexual reproduction are in the same gene pool.

Genetic drift - Random changes in the gene frequencies of a population from generation to generation. This happens as a result of sampling error — some genotypes just happen to reproduce more than other genotypes not because they are "better but just because they got lucky. This process causes gene frequencies in a population to drift around over time. Some genes may even drift out" of a population (just by chance some gene may reach frequency of 0). Genetic drift has the effect of decreasing genetic variation within a population.

Genetic variation - Loosely a measure of the genetic differences there are within populations or species. For example a population with many different alleles at a locus may be said to have a lot of Genetic variation at that locus. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles already in the population.

Genome - All the genetic information an organism carries.genotype. The set of genes an organism has. Sometimes genotype refers to the entire genome of an organism & sometimes it refers to the alleles carried at a particular locus.genus (genera — pl.) The rank above species in Linnaean classification.

Germ line mutation - Mutation that occurs in reproductive cells & ends up being carried by gametes (egg/sperm).

Gill - Organ used for breathing in many water-dwelling animals including most fish & many arthropods. Gills generally have a large surface area & are filled with blood; gas exchange occurs by diffusion across the surface area of the gill as oxygen passes into the blood & carbon dioxide passes out of them.

Habitat - Place & conditions in which an organism normally lives.

Herbivore - An organism that eats almost entirely plants.

Heterochrony - An evolutionary change in the timing of a developmental event. For example relative to the lineage's ancestor the early maturation of sex organs is an example of heterochrony.

Hominid - Humans & their extinct relatives (i.e. organisms on the "human side" of the human/chimpanzee lineage split). However some scientists use the term hominid to refer to a larger group: humans other great apes (chimpanzees, gorillas & orangutans) & their extinct relatives. However you decide to name the groups the important thing is how all these species are related to one another & not what we decide to call each lineage.

Hominin - This clade includes all living & extinct lineages that are more closely related to humans than they are to chimpanzees. It is essentially the human branch of the tree of life.

Homology/homologous structure - Inherited from a common ancestor. Human eyes & mouse eyes are homologous structures because we each inherited them from our common ancestor that also had the same sort of eyes. Contrast this with homoplasious & analogous.

Homoplasious - Similar but not because of inheritance from a common ancestor. Homoplasious characters may be explained by convergent evolution in 2 different organisms or character reversals.

Horizontal transfer - A process which results in the transfer of genetic material between members of different species. Bacteria for example frequently pass copies of particular genes to one another & pick up foreign genetic material from their environment resulting in horizontal transfer.

Host - Organism that serves as a habitat for another organism. A host may provide nutrition to a parasite or simply a place in which to live.

Hox gene - Gene that regulates development & organization of the major body units.

Hutton James (1726-1797) - Scottish farmer & geologist. In his travels around Britain he made observations which suggested to him that the geologic processes that shaped the ancient Earth could be seen operating all the time an idea which would later form the basis of Lyell's uniformitarianism. Hutton used his observations & hypothesis to argue that the Earth must be extremely old.

Hybridization - The production of offspring from different parental forms. For example if two recognizably different species of plant fertilized one another & produced viable fertile offspring the process would be called hybridization.

Hydrostatic skeleton - A fluid-filled cavity that supports the body of an animal because the fluid cannot be compressed into a smaller volume (hydro = liquid statos = standing unchanging).

Hypothesis - A proposed explanation for a narrow set of phenomena. A hypothesis must be testable with evidence from the natural world. If an explanation can't be tested with experimental results observation or some other means then it is not a scientific hypothesis.

Inbreeding - Mating between relatives. Technically this is defined as a pattern of mating in which mates are more closely related than two individuals selected at random from the population.

Incipient species - Group of organisms that is to become separate species from other related individuals.

Insect - Insects are a group of arthropods distinguished by the following characters: a body divided into head thorax & abdomen one pair of antennae three pairs of mouth appendages three pairs of legs on thorax & often one or two pairs of wings

Intelligent design movement - The intelligent design (ID) movement promotes the idea that many aspects of life are too complex to have evolved without the intervention of a supernatural being — the intelligent designer. Because it relies on supernatural explanations ID is not science. To learn more read our brief on the intelligent design movement.

Intermediate form - A partially assembled adaptation. Complex adaptations evolve in a series of smaller steps & these steps along the history of an adaptation's evolution are called intermediate forms.

Iridium - A rare element that is found in relatively high concentrations in asteroids.

Junk DNA - DNA that doesn't code for proteins. The term "junk DNA" is a bit of a misnomer since some of this non-coding DNA performs important functions like helping to turn genes on & off.

Key innovation - An adaptation that allows an organism to exploit a new niche or resource.

Life history - Traits that make up the life cycle of an organism. An organism's life history includes characteristics related to reproduction development & growth (e.g. fecundity types of larval stages passed through size at adulthood & habitats at different points in life cycle).

Lineage - A continuous line of descent; a series of organisms populations cells or genes connected by ancestor/descendent relationships.

Lineage splitting - An event in which a single historical lineage gives rise to two or more descendent lineages. Every node on a phylogeny is a lineage-splitting event.

Linnaean classification - The standard system of classification in which every organism is assigned a kingdom phylum class order family genus & species. This system groups organisms into ever smaller & smaller groups (like a series of boxes within boxes called a nested hierarchy).

Locus - The place in the DNA where a gene is located. For example the pea color locus is the place in a pea plant's DNA that determines what the color of the peas will be. The pea color locus may contain DNA that makes the peas yellow or DNA that makes the peas green — these are called the yellow & green alleles.

Longevity - Long life; long duration of existence.

Lucy - The name given to a particular female hominid (of the species Australopithecus afarensis) who lived in what is now Ethiopia about three million years ago. "Lucy" is famous because she left behind a very complete fossilized skeleton found in 1974.

Macroevolution - Evolution above the species level. The adaptive radiation of a lineage into many different niches is an example of macroevolution. Since evolutionary change above the species level means that populations & species must be evolving macroevolutionary change entails microevolutionary change.

Marsupial mammal - A mammal such as an opossum or kangaroo whose young are suckled & protected inside a maternal pouch.

Mass extinction - Event in which many different lineages go extinct around the same time. Mass extinctions involved higher rates of extinction than the usual rate of background extinction that is going on all the time.

Mitochondrion - An organelle in eukaryotic cells where cellular respiration takes place. Mitochondria contain a short loop of DNA that is distinct from the DNA contained in the cell's nucleus.

Molecular - In evolutionary biology having to do with DNA sequences or the amino acid sequences of proteins.

Molecule - Group of two or more atoms bonded together.

Molting - A process in which an animal sheds all or part of its outer covering which is then regenerated in some way. For example arthropods molt their exoskeletons in order to grow & birds molt their feathers in order to replace worn out feathers or to prepare for a different season or for breeding.

Morphology - The study of the form & structure of organisms. For example comparing the shape of the femur in different grazing mammals is a morphological study.

Mutation - A change in a DNA sequence usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Changes in the composition of a genome due to recombination alone are not considered mutations since recombination alone just changes which genes are united in the same genome but does not alter the sequence of those genes.

Mutualism - A species interaction in which both of the interacting species profit from the interaction.

Myotome - A segment of muscle.

Myriapod - Myriapods (myria = ten thousand pod = foot) are a group of arthropods distinguished by the following characters: a body built from a head & long repeating trunk one pair of antennae (number of other appendages on head varies) many (but not 10000!) limbs on trunk

Natural selection - Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population.

Neutral theory - The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation "drifting" around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation & emphasizes the importance of mutation & genetic drift.

Niche - In ecology the part of the environment occupied by a particular species along with the resources it uses & produces. A species' niche includes factors as energy consumed time of consumption space occupied temperature required mode of reproduction & behavior.

Node - A recognizable feature of an organism. Characters may be morphological behavioral physiological or molecular. They are used to reconstruct phylogenies. A point on a phylogeny where a single ancestral lineage breaks into two or more descendent lineages.

Notochord - A flexible rod running the length of a chordate providing structural support. The notochord is one of the inherited characteristics shared by all chordates.

Nucleotide - The building blocks of DNA. A chain of nucleotides forms DNA. Nucleotides are made of a sugar a phosphate & a base. See also base.

Omnivore - An organism that eats both plants & animals (omni = all vorare = to swallow up).

Onychoporans (also known as velvet worms) - Share certain characters with arthropods but are lacking a hard exoskeleton or jointed legs. Onychophorans are probably closely related to arthropods & branched off the tree just before a fully hardened exoskeleton & jointed legs evolved.

Organism - Any living creature.

Outbreeding - Mating between very distantly related individuals.

Outgroup - A lineage in a phylogenetic analysis that falls outside the clade being studied. All members of the clade being studied will be more closely related to each other than to the outgroup so the outgroup will branch off at the base of that phylogeny.

Owen Richard (1804-1892) - English anatomist & student of Cuvier. Owen reconstructed the skeletons of many extinct animals even working on some of Darwin's specimens. He was nonetheless an early opponent of Darwin arguing that God created new species by modifying a basic anatomical idea of an "archetype." Later he modified his own views to accept a kind of "divine" evolution. Owen is known for overstating the differences between the human brain & those of other apes in his struggle to place humans on a kind of pedestal apart from the rest of the animal kingdom.

Paedomorphosis - Having some features of the ancestral juvenile stage but being an adult (with a mature reproductive system). This word means "child form & a paedomorphic change is any evolutionary change in the development of an organism that generates an adult with a child's form."

Paleontologist - A scientist who studies fossils (paleo = ancient onto = being ology = study of; study of ancient beings).

Parasite - Organism that lives on or within another organism on which it feeds.

Parsimony - A principle stating that the simplest explanation accounting for the observations is the preferred explanation. When reconstructing the evolutionary relationships among lineages the principle of parsimony implies that we should prefer the phylogeny that requires the fewest evolutionary changes.

Phenotype - The physical features of an organism. Phenotype may refer to any aspect of an organism's morphology behavior or physiology. An organism's phenotype is affected by its genotype & by its environment.

Phenotypic Plasticity - Degree to which an organism's phenotype changes depending upon the environment that it is currently in or its past environment. Two organisms with the same genotype (e.g. identical twins) may have different phenotypes (e.g. one may be taller or heavier) if raised in different environments; those differences represent phenotypic plasticity. All organisms exhibit some degree of phenotypic plasticity (e.g. an animal that receives more food will generally be heavier than a genetically identical animal that receives less food) but sometimes phenotypic plasticity can be extreme (e.g. some fish become either male or female depending upon the temperatures they were exposed to as an egg).

Phylogenetic classification - A system of classification that names groups of organisms according to their evolutionary history. Like Linnaean classification phylogenetic classification produces a nested hierarchy where an organism is assigned a series of names that more & more specifically locate it within the hierarchy. However unlike Linnaean classification phylogenetic classification only names clades & does not assign ranks to hierarchical levels.

Phylogeny - The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered. Many of the phylogenies you encounter are the "family trees" of groups of closely related species but we can also use a phylogeny to depict the relationships between all life forms.

Pigment - substance that absorbs light. Pigments absorb light of particular wavelengths which gives the pigment a characteristic color.

Placenta - In placental mammals the organ that connects a fetus to the wall of its mother's uterus. Nutrients & oxygen pass through the placenta from the mother to the developing embryo & waste products pass back through it into the mother's bloodstream.

Placental mammal - A mammal such as a human whose young completes its embryonic development in the uterus joined to the mother by a placenta.

Plate tectonics - a broad theory that uses movements of continental plates to explain many geographic geologic seismic & even biological observations. The idea is that the Earth's crust & upper mantle are made up of many differently sized & irregularly shaped plates that "slide around" on the lower mantle. The plates may crash into one another slide under one another & change shape as they are broken down & reformed.

Plesiomorphy - The ancestral character state for a particular clade. This character state may change depending on the clade under consideration. For example "has four legs" is plesiomorphic for the clade of terrestrial vertebrates but "has two legs & two wings" is plesiomorphic for the clade of owls.

Ploidy - The number of copies of each chromosome an organism carries. For example humans are diploid (i.e. we have a ploidy of two) because we carry two copies of each chromosome.

Polytomy A node on a phylogeny where more than two lineages descend from a single ancestral lineage. A polytomy may indicate either that we don't know how the descendent lineages are related or that we think that the descendent lineages speciated simultaneously.

Population - Generally a group of organisms living close to one another that interbreed with one another & do not breed with other similar groups; a gene pool. Depending on the organism populations may occupy greater or smaller geographic regions.

Predator - An organism that hunts & eats other organisms. Predators may eat plants or meat. prey Organism killed for food by a predator.

Proboscis - Elongated organ associated with the mouth. For example in elephants the trunk is the proboscis while in butterflies the long coiled feeding tube is the proboscis.

Protein A molecule made of a string of amino acids. Proteins are coded for by DNA & are essential molecules for life.

Radial symmetry - A property of an item (e.g. a shape or an animal) that can be divided into two matching halves by many different lines which all intersect one another at a single point in the center. For example pies snowflakes & starfish are radially symmetric because they have many different lines of symmetry (dividing them into matching halves) & the lines cross one another at the center.

Radiometric dating - A method of determining the date at which an igneous rock solidified based upon the rate of decay of radioactive atoms within the rock.

Random - Unpredictable in some way. Mutations are "r&om" in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. However this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause. Indeed some regions of the genome are more likely to sustain mutations than others & various physical causes (e.g. radiation) are known to cause particular types of mutations.

Recombination - A process in which pairs of chromosomes swap DNA with one another. This happens during gamete formation. A single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process of forming these daughter cells recombination happens so that the chromosomes the daughter cells have are "mosaic" composed of different pieces of the parent cells' chromosomes. Recombination is important for evolution because it brings new combinations of genes together — a source of variation for natural selection to act upon.

Regulatory gene - A gene that controls when protein-coding genes are turned on or off.

RNA - Ribonucleic acid a molecule similar to DNA involved in carrying information & producing proteins in cells. Some viruses carry RNA as their genetic material instead of DNA.

Sedgewick Adam(1785-1873) - English geologist who studied the fossils in different geologic strata & helped give the strata (& corresponding time periods) the names we use today — Cambrian Devonian etc. Although he accepted naturalistic explanations for geologic events & studied them using the biostratigraphic methods of William Smith Sedgwick rejected Darwin's naturalistic explanation for the origin of species & argued that God created new forms of life at the beginning each geologic period.

Segregation - The process in which pairs of chromosomes separate & are shuttled off to different gametic daughter cells. When gametes are formed a single parent cell (containing two sets of chromosomes) will form four daughter cells (with one complete set of chromosomes each). In the process the paired chromosomes of the parent cell separate into different daughter cells. This process is segregation.

Sexual selection - Selection acting on an organism's ability to obtain or successfully copulate with a mate. This process may produce traits that seem to decrease an organism's chance of survival while increasing its chances of mating.

Shocked quartz Crystals with a pattern of fracturing that can be caused by the intense pressure & heat of events such as asteroid impacts.

Sickle cell anemia - A genetically caused disease that generally results in the death of the person with it unless medical interventions are available. Sickle cell anemia is a popular topic for biology courses because it is one the few well-worked out examples of heterozygote advantage that we have. People carrying two copies of the sickle cell allele have the disease people with no copies of the sickle cell allele are normal but people carrying just one copy of the sickle cell allele are resistant to malaria (though they may occasionally have symptoms of sickle cell). So if you live in a region where malaria is common you are at an advantage if you are a heterozygote (i.e. if you carry one sickle cell allele & one normal allele).

Single-celled - Refers to an organism consisting of one cell such as bacteria protozoa & some algae fungi & yeasts.

Sister groups - (sometimes called sister taxa) Clades that are each other's closest relatives. On a phylogeny sister groups occur anytime a single ancestral lineage gives rise to two daughter lineages: the daughter lineages are sister groups & since they arose from the same ancestor at the same time sister groups are always the same age. Sister groups may differ widely in diversity level: one clade may be comprised of a single species while its sister group may be comprised of 100 species.

Somatic mutation - Mutations occurring in cells that do not form gametes mutations that do not end up being carried by eggs or sperm. For example mutations in your skin muscle or liver tissue are somatic mutations.

Speciation - The process by which species form. This involves reproductive isolation of different parts of an ancestral species so that they form distinct descendent species.

Species- Members of populations that actually or potentially interbreed. In this sense a species is the largest gene pool possible under natural conditions.

Subspecies - A grouping of organisms less inclusive than a species. The term is usually applied to groups within a species that have distinct forms & live in a restricted area.

Symbiosis - A relationship between two different organisms that live in close contact with each other. The relationship may be beneficial to both organisms (mutualism) beneficial to just one (commensalism) or harmful to one (parasitism).

Symplesiomorphy - An ancestral character state (i.e. a plesiomorphy) shared by two or more lineages in a particular clade. For example within the clade of terrestrial vertebrates (in which the ancestral character state is "has four legs") both elephants & salam&ers have four legs — & so having four legs is a symplesiomorphy for those 2 lineages.

Synapomorphy - A derived or changed character state (i.e. an apomorphy) shared by two or more lineages in a particular clade. Synapomorphies are indicators of common ancestry. For example within the clade of terrestrial vertebrates the ancestral or plesiomorphic character state is "has four legs." However both owls & parrots have the synapomorphic character state "has two legs & two wings " indicating that owls & parrots are closely related.

Taxon - (taxa — pl.) Any named group of organisms (e.g. the reptiles Felidae beetles Homo sapiens) whether or not it forms a clade.

Tetrapod - The animal clade containing vertebrates with sturdy legs (as opposed to fins).

Theory - A broad explanation for a wide range of phenomena. Theories are concise coherent systematic predictive & broadly applicable. They usually integrate many individual hypotheses. A scientific theory must be testable with evidence from the natural world. If a theory can't be tested with experimental results observation or some other means then it is not a scientific theory.

Thorax - In animals with 3 body regions the middle body region between the head & abdomen.

Trachea - An internal tube that carries air into the body of an animal for breathing. For example in humans a trachea carries air to the lungs in insects a network of tracheae carries air directly to tissues throughout the body.

Transcription - The process of building an RNA molecule using DNA as a template. In this process complimentary RNA bases are matched to their DNA counterparts so that the str& of RNA that is produced carries the "imprint" of one str& of the DNA molecule.

Transitional forms - Fossils or organisms that show the transformation from an ancestral form to descendant species' form. For example there is a well-documented fossil record of transitional forms for the evolution of whales from their amphibious ancestor.

Translation Part of the process of decoding an RNA molecule composed of nucleotide bases into a protein composed of amino acids.

Trilobite - Trilobites are an extinct group of arthropods distinguished by the following characters: a body built from a cephalon thorax & pygidium a body divided into three lobes running from head to tail one pair of antennae

Vertebrate - Any member of the animal clade Vertebrata. All vertebrates have a backbone that surrounds & protects the nerve cord a character that they all inherited from their common ancestor. Vertebrates are a subgroup of the chordates. Modern vertebrates include fish sharks mammals & amphibians.

Vestigial structure - A feature that an organism inherited from its ancestor but that is now less elaborate & functional than in the ancestor. Usually vestigial structures are formed when a lineage experiences a different set of selective pressures than its ancestors & selection to maintain the elaboration & function of the feature ends or is greatly reduced.

Vicariance - Vicariance is a process in which a species' range is divided even though the species has remained in place. This might happen through tectonic action geologic activity (like the rise of a mountain range or shift in the course of a river) or other processes. Vicariance is usually contrasted with dispersal as a biogeographic mechanism.

References

A graph showing the application of the Hardy-Weinberg equation based on two alleles and three genotypes.

The Hardy-Weinberg Principle attempts to describe populations that are in equilibrium. It states that allele and genotype frequencies in a population remains relatively the same unless experiencing disruptive factors in the system. These factors include mutations, selection, genetic drift, and nonrandom mating; most of which contribute to the genetic variation and microevolution in human beings. However, later we shall see that this principle has some helpful applications.
The equation is as follows:

$p^{2}+2pq+q^{2}=1$

where $p^{2}$ represents the homozygous dominant genotype frequency
$2pq$ represents the "heterozygous genotype" frequency
$q^{2}$ represents the "homozygous recessive" genotype frequency
Another helpful derivation of the equation is:
$p+q=1$

History

The name of the principle probably suggests that it was discovered by two people (Jeff Curry and Steph Curry) who were working together; the truth is otherwise. Wilhelm Weinberg and G. H. Hardy both worked on the mathematical and theoretical part of this principle separately, however published their work at relatively the same time. The problem was Wilhelm Weinberg wrote in German and G. H. Hardy wrote in English. At this period of time (around early 1900s), the study of genetics was most prominent in the english-speaking world, and thus Weinberg's contribution to the principle remained unappreciated until a considerable number of years later. Today, we better understand the existence of changes in a population because of both their studies and work in this field of genetics.

Assumptions and Applications

There are five assumptions to the Hardy-Weinberg principle:
1. No mutations
2. No selection
3. No migration
4. Large population
5. Random Mating
The application of this principle comes when defining change in a population that ultimately leads to microevolution. When the evaluating a population and wondering if it proves selection or any of the five assumptions mentioned above, one only needs to use the equation and perform a chi-squared test to see the impact of disruptive factors on the given population; a indispensable tool in the study of genetic variation in a population and evolution.

References

Curington, John. "Evolution Lecture" BICD 100 Lecture. University of California, San Diego, La Jolla. 15 Nov. 2012. Lecture.