hydrophobic interactions
Summary
Hydrophobic interactions are the tendency of nonpolar groups to cluster together in water, which releases ordered water molecules and increases the entropy of the water. They help drive protein folding, burying nonpolar side chains in the core.
Detail
Meaning: Nonpolar groups cannot hydrogen bond with water, so water forms an ordered shell around them. When the groups cluster, less nonpolar surface is exposed and ordered water is released to the bulk, raising the entropy of the water. The driving force is mainly this entropy gain, not a strong attraction between the nonpolar groups.
Equation: ΔG = ΔH − TΔS, where ΔG is the free energy change (J/mol), ΔH is the enthalpy change (J/mol), T is absolute temperature (K), and ΔS is the entropy change (J/(mol·K)). Clustering is favorable when the entropy gain of the water makes ΔG negative.
On the MCAT: Nonpolar side chains (Val, Leu, Ile, Phe) tend to be buried in the core of a folded protein, while polar and charged ones lie on the surface. A mutation swapping a core Leu for Asp is predicted to destabilize tertiary structure. Amphipathic molecules form micelles and bilayers for the same reason.
Common confusion: Hydrophobic interactions are not covalent bonds, and nonpolar groups do not actively attract each other. A folding protein loses conformational entropy, yet folding is still favorable because the entropy gain of the water outweighs that loss.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.