Theoretical Analysis on Hyper-mobile Water Around a Solute

Abstract

We calculate physically insightful components of the rotational entropy of hydration of a solute using the angle-dependent integral equation theory combined with the multipolar model for water. It is shown that when a sufficiently large nonpolar solute is inserted into water, the rotational freedom (RF) of water molecules near the solute is significantly restricted due to the water structuring. When the solute has a moderate surface charge density (SCD), in the region next to the solute and in the region within which the solute-water surface separations are close to the molecular diameter of water, the RF of water molecules becomes higher than in the bulk. As the SCD increases, these regions shift slightly more outside with further enhancement of the RF, but the water molecules in contact with the solute turn largely restrained. It is shown that the appearance of water molecules with anomalously high RF is the most remarkable for a very large solute with high SCD like F-actin.