HYDROPHOBIC EFFECT

The 'hydrophobic effect' is the property that nonpolar molecules tend to form intermolecular aggregates in an aqueous medium and analogous intramolecular interactions.^[1][2] The name arises from the combination of water in Attic Greek ''hydro-'' and for fear ''phobos'', which describes the apparent repulsion between water and hydrocarbons. At the macroscopic level the hydrophobic effect is apparent when oil and water are mixed together and form separate layers or the beading of water on hydrophobic surfaces such as waxy leafs. At the molecular level hydrophobic effect is important driving force for biological structures and responsible for protein folding, protein-protein interactions, formation of lipid bilayer membranes, nucleic acid structure, and protein-small molecule interactions.
According to the solvophobic theory of Reversed Phase Chromatography (RPC) the hydrophobic effect is driven by the loss of hydrogen bonding and higher entropic cost of forming a cavity around a nonpolar molecules.^[3] These losses can be minimized by forcing nonpolar molecules together (see Thermodynamics).

Contents

Amphiphiles

Biological folding

Thermodynamics

See also

References

Amphiphiles

Amphiphiles are molecules that have both hydrophobic and hydrophylic domains. Detergents are composed of amphiphiles that allow hydrophic molecules to be solubilized in water by forming micelles and bilayers (as in soap bubbles). They are also important to cell membranes composed of ampiphic phospholipids that prevent the internal aqueous environment of a cell from mixing with external water.

Biological folding

In the case of protein folding, the hydrophobic effect is important to understanding the structure of proteins that have hydrophobic amino acids, such as alanine, valine, leucine, isoleucine, phenylalanine, and methionine grouped together with the protein. Most folded proteins have a hydrophobic core in which side chain packing stabilizes the folded state, and charged or polar side chains on the solvent-exposed surface where they interact with surrounding water molecules. It is generally accepted that minimizing the number of hydrophobic sidechains exposed to water is the principal driving force behind the folding process^[4], although a recent theory has been proposed which reassesses the contributions made by hydrogen bonding^[5].
The energetics of DNA tertiary structure assembly were determined to be primaraly driven by the hydrophobic effect, as opposed to Watson-crick base pairing which are responsible for sequence selectivity. Although there is also a significant contribution from stacking interactions between the aromatic bases.

Thermodynamics

The hydrophobic effect can be nullified to a certain extent by lowering the temperature of the solution to near zero degrees Celsius; at such temperatures, water tends toward an ordered structure and the order generated by hydrophobic patches is no longer as energetically unfavorable. This is neatly demonstrated by the increased solubility of benzene in water at temperatures lower than room temperature.
The transfer free energy of nonpolar molecule from nonpolar solvent to aqueous solvent is often used to quantify the hydrophobic effect. The transfer free energy of hydrophobic molecule, $Delta\; G\_\{$
m t}, is positive. The $Delta\; G\_\{$
m t} can be decomposed to the enthalpy component $Delta\; H\_\{$
m t} and entropy component $-TDelta\; S\_\{$
m t} by the thermodynamic relation $G\; =\; H\; -\; TS$. In room temperature, $Delta\; H\_\{$
m t} is approximately zero, and $Delta\; S\_\{$
m t} is negative. In other words, the hydrophobic effect is entropy driven at room temperature. The other characteristic thermodynamic quantity of the hydrophobic effect is heat capacity change in transfer, $Delta\; C\_\{p,\; \{$
m t}}, which has a positive value as contrasted to a negative value in the transfer of a hydrophilic molecule.
Another way of understanding the hydrophobic effect is the example of a hydrophobic substance in water. Pure water molecules adopt a structure which maximizes entropy (S). A hydrophobic molecule will disrupt this structure and decrease entropy, and creates a 'cavity' as it is unable to interact electrostatically with the water molecules. When more than one 'cavity' is present, the surface area of disruptions is high, meaning there are less free water molecules. To counter this, the water molecules push the hydrophobic molecules together and form a 'cage' structure around them which will have a smaller surface area than the total surface area of the cavities. This maximizes the amount of free water and thus the entropy. Therefore the hydorphobic effect might also be understood as the "the lipophobicity of water."

See also

★ Hydrophobe

★ Hydrophile

★ Entropic force

References

1.
2. ''Interfaces and the driving force of hydrophobic assembly'' Nature, Volume 437, Issue 7059, pp. 640-647 (2005)
3. Csaba Horvath et al in J.Chromatogr., 125 (1976) 129-156.
4. Forces contributing to the conformational stability of proteins, Pace C, Shirley B, McNutt M, Gajiwala K, , , FASEB J., 1996
5. A backbone-based theory of protein folding, Rose G, Fleming P, Banavar J, Maritan A, , , Proc. Natl. Acad. Sci. U.S.A., 2006