Structural Biochemistry/Chemical Bonding/Hydrophobic interaction

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