A statistical-mechanical analysis on the hypermobile water around a large solute with high surface charge density

In connection with the experimental observation that the hypermobile water is induced around F-actin, 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 adjacent 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 considerably higher than in the bulk. As the SCD increases, these regions shift slightly more outside with further enhancement of the RF. For sufficiently high SCD, 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. We argue that the theoretical results are in qualitatively good accord with the experimental observations for the rotational mobility of water molecules near nonpolar side chains of amino acids and F-actin with the domains which are rich in negative charges.