Dynamic hydration shell restores Kauzmann's 1959 explanation of how the hydrophobic factor drives protein folding

Abstract

Kauzmann's explanation of how the hydrophobic factor drives protein folding is reexamined. His explanation said that hydrocarbon hydration shells are formed, possibly of clathrate water, and they explain why hydrocarbons have uniquely low solubilities in water. His explanation was not universally accepted because of skepticism about the clathrate hydration shell. A revised version is given here in which a dynamic hydration shell is formed by van der Waals (vdw) attraction, as proposed in 1985 by Jorgensen et al. [Jorgensen WL, Gao J, Ravimohan C (1985) J Phys Chem 89:3470-3473]. The vdw hydration shell is implicit in theories of hydrophobicity that contain the vdw interaction between hydrocarbon C and water O atoms. To test the vdw shell model against the known hydration energetics of alkanes, the energetics should be based on the Ben-Naim standard state (solute transfer between fixed positions in the gas and liquid phases). Then the energetics are proportional to n, the number of water molecules correlated with an alkane by vdw attraction, given by the simulations of Jorgensen et al. The energetics show that the decrease in entropy upon hydration is the root cause of hydrophobicity; it probably results from extensive ordering of water molecules in the vdw shell. The puzzle of how hydrophobic free energy can be proportional to nonpolar surface area when the free energy is unfavorable and the only known interaction (the vdw attraction) is favorable, is resolved by finding that the unfavorable free energy is produced by the vdw shell.

Keywords: cavity work; hydrophobic hydration; protein stability.

Fig. 1.

Hydration energetics of the alkanes (ΔG^o, ΔH^o, −TΔS^o) plotted against n, the number of water molecules correlated with an alkane by vdw attraction (ref. 13). All data are from Table 1, which lists the original sources.

Fig. 2.

Plot of n (taken as the number of water molecules in the vdw hydration shell) against ASA for the alkanes discussed here; the value for methane (which is an outlier) is omitted. All data are from Table 1, which also gives the ASA value for methane.