hydrophobic effect
Summary
The hydrophobic effect is the tendency of nonpolar groups to cluster together in water. It is driven mainly by the entropy gain of the surrounding water, not by strong attraction between the nonpolar groups. It is a major driving force for protein folding, which buries nonpolar side chains in the core.
Detail
Meaning: Water molecules hydrogen bond with one another. A nonpolar group cannot join in, so water around it becomes more ordered, forming a cage-like shell. When nonpolar groups cluster, they expose less surface, and the ordered water is released into the bulk. The entropy of the water rises, which makes the overall ΔG negative. This drives protein folding (nonpolar side chains in the core, polar and charged ones on the surface), micelle formation and lipid bilayer assembly.
Equation: ΔG = ΔH − TΔS, where ΔG is the free energy change (J/mol), ΔH is the enthalpy change (J/mol), T is the absolute temperature (K) and ΔS is the entropy change of the system including the water (J/(mol·K)). Clustering is favorable because ΔS is positive, so −TΔS is negative.
On the MCAT: Expect passages on protein folding or denaturation, amphipathic molecules such as phospholipids and detergents, or membrane proteins. Predict that a mutation replacing a buried nonpolar residue with a polar or charged one destabilizes the fold. Membrane-spanning regions typically have nonpolar residues facing the lipid tails.
Common confusion: Students say nonpolar groups attract each other strongly. Their London dispersion forces are weak. The main driver is the increased entropy of water when it is freed from the ordered shell. The entropy that increases belongs to the water, not the nonpolar molecules.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Biochemistry (Voet and Voet)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.