Structural and thermodynamic aspects of the hydrophobic effect

Abstract

From analyses of the solvation structure around HS solutes for HS solvents and various water models by the RISM integral equation method, the following conclusions are drawn: 1) Water has more small cavities than HS solvents, which makes it easier for water to accommodate small solutes with a radius less than about 1A. 2) With increasing solute radius from 0 to 3A, the average orientation of hydrating water varies from an inward to an outward orientation, which shows that some reorganization of water occurs in response to the change in solute size. 3) The hydration structure is formed as a network structure due to H-bonding interactions between water molecules being supported by the cooperation of repulsive forces between solute and water. Repulsive interactions of not only O atoms but also H atoms with solute are essential to the formation of such hydration structure. With regard to the physical mechanism of the hydrophobic effect, the following is concluded from an analysis of the physical meaning of the basic formula for the free energy of cavity formation: 4) It is predicted from the scaled particle theory that the solvent exclusion effect caused by the introduction of solute into solvent is an important factor of the hydrophobic effect. 5) The large negative transfer entropy at room temperature characteristic of the hydrophobic hydration results primarily from the decrease in the configuration entropy of water due to the solvent exclusion effect. 6) The structuralization of hydrating water results in exactly compensating changes in enthalpy and entropy, and a large positive change in heat capacity. As a result, the hydrophobic effect is dominated by the entropy effect at room temperature, while it is driven by enthalpy at temperatures higher than 110 degrees C. 7) Hydrating water is energetically similar to bulk water, and the term "highly structured" is not appropriate to describe it. The following descriptions can be made on estimating the free energy of transfer of biomolecules from gas or organic-liquid phase to water: 8) Derivation of the basic formula for the transfer free energy of solute with variable conformation was presented and physical meanings of the contributions to it were explained. 9) Applying the formula to the transfer of 40 organic molecules from gas phase to water, the two best models for diving all the constituent atoms into several hydration-thermodynamically independent groups and the atomic hydration parameters of respective groups were determined.(ABSTRACT TRUNCATED AT 400 WORDS)