- Hydrophobic effect
The hydrophobic effect is the property that non-polar molecules tend to form intermolecular aggregates in an aqueous medium and analogous intramolecular interactions. [GoldBookRef|title=hydrophobic interaction|file=H02907] ["Interfaces and the driving force of hydrophobic assembly" Nature, Volume 437, Issue 7059, pp. 640-647 (2005)DOI|10.1038/nature04162] 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, the hydrophobic effect is an important driving force for biological structures and responsible forprotein folding, protein-protein interactions, formation oflipid bilayer membranes,nucleic acid structures, 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 the higher entropic cost of forming a cavity around nonpolar molecules. [Csaba Horvath et al in J.Chromatogr., 125 (1976) 129-156.] These losses can be minimized by forcing nonpolar molecules together (see
Thermodynamics ).Amphiphiles
Amphiphiles are molecules that have both hydrophobic and hydrophilic domains.Detergent s are composed of amphiphiles that allow hydrophobic molecules to be solubilized in water by formingmicelle s and bilayers (as insoap bubbles ). They are also important tocell membranes composed of amphiphilicphospholipid s 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 understand the structure of proteins that have hydrophobic
amino acid s, such asalanine ,valine ,leucine ,isoleucine ,phenylalanine , andmethionine 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 side chains exposed to water is the principal driving force behind the folding processcite journal |author=Pace C, Shirley B, McNutt M, Gajiwala K |title=Forces contributing to the conformational stability of proteins |journal=FASEB J. |volume=10 |issue=1 |pages=75–83 |year=1996 |url=http://www.fasebj.org/cgi/reprint/10/1/75 |pmid=8566551] , although a recent theory has been proposed which reassesses the contributions made by hydrogen bondingcite journal |author=Rose G, Fleming P, Banavar J, Maritan A |title=A backbone-based theory of protein folding |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |issue=45 |pages=16623–33 |year=2006 |url=http://www.pnas.org/cgi/content/abstract/103/45/16623 |pmid=17075053 |doi=10.1073/pnas.0606843103] .The energetics of
DNA tertiary structure assembly were determined to be primarily driven by the hydrophobic effect, as opposed toWatson-Crick base pairing (which is responsible for sequence selectivity), although there is also a significant contribution from stacking interactions between the aromatic bases.Fact|date=April 2008Thermodynamics
The transfer free energy of non-polar molecule from non-polar solvent to aqueous solvent is often used to quantify the hydrophobic effect. The transfer free energy of hydrophobic molecule, , is positive. The can be decomposed to the enthalpy component and entropy component by the thermodynamic relation . In room temperature, is approximately zero, and 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, , which has a positive value, as contrasted with 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 that there are fewer 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 hydrophobic effect might also be understood as the "the lipophobicity of water".
ee also
*
Hydrophobe
*Hydrophile
*Entropic force References
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