hydrophobic
Summary
Hydrophobic means "water-fearing": a nonpolar molecule or group that cannot hydrogen bond with water and tends to cluster away from it. The key testable point is that this clustering is driven mainly by a gain in the entropy of water, not by a strong attraction between the nonpolar groups.
Detail
Meaning: Hydrophobic groups are nonpolar, such as hydrocarbon chains and the side chains of alanine, valine, leucine, isoleucine and phenylalanine. Water cannot hydrogen bond with them, so water molecules nearby become more ordered. When the nonpolar groups cluster together, that ordered water is released and the entropy of the water increases. This is the hydrophobic effect. It helps explain why proteins fold with nonpolar side chains in the core and polar or charged ones on the surface, and why phospholipids form bilayers and micelles.
Equation: ΔG = ΔH − TΔS, where ΔG is the change in free energy (J/mol), ΔH is the change in enthalpy (J/mol), T is absolute temperature (K) and ΔS is the change in entropy (J/(mol·K)). Releasing ordered water makes ΔS positive, so −TΔS pushes ΔG negative.
On the MCAT: A passage may describe a mutation that swaps a buried leucine for aspartate, and you predict that the fold is destabilized. Other setups include transmembrane helices rich in nonpolar residues, detergents, and nonpolar molecules dissolving poorly in water but well in nonpolar solvents.
Common confusion: Nonpolar groups do not repel water in a forceful way, and they are not held together by a special bond. Their clustering is mainly the result of water's entropy gain. London dispersion forces add only a small stabilizing contribution.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Voet's Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.