Biophysical Chemistry 105 (2003) 195–209 0301-4622/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-4622Ž03.00087-5 Analysis of thermal hysteresis protein hydration using the random network model Kelly Ryan Gallagher, Kim A. Sharp* E.R. Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, 3700 Hamilton Walk, Philadelphia, PA 19104-6059, USA Received 24 June 2002; received in revised form 22 August 2002; accepted 22 August 2002 Abstract The hydration of polar and apolar groups can be explained quantitatively, via the random network model of water, in terms of differential distortions in first hydration shell water–water hydrogen bonding angle. This method of analyzing solute induced structural distortions of water is applied to study the ice-binding type III thermal hysteresis protein. The analysis reveals subtle but significant differences in solvent structuring of the ice-binding surface, compared to non-ice binding protein surface. The major differences in hydration in the ice-binding region are (i) polar groups have a very apolar-like hydration. (ii) there is more uniform hydration structure. Overall, this surface strongly enhances the tetrahedral, or ice-like, hydration within the primary hydration shell. It is concluded that these two specific features of the hydration structure are important for this surface to recognize, and preferentially interact with nascent ice crystals forming in liquid water.  2003 Elsevier Science B.V. All rights reserved. Keywords: Hydration; Thermal hysteresis protein; Liquid water; Random network mode 1. Introduction It is well established that the unique properties of water dictate much of a protein’s structure and function. One central theme that has emerged is that hydrophobic, or solvophobic, interactions drive or influence many molecular ‘transactions’ w1–6x. In the past, descriptions of this effect have focused on the entropic penalty for ordering water around an apolar solute w7x. Yet the hydration of polar solutes is also usually accompanied by a *Corresponding author. Tel.: q1-215-573-3506; fax: q1- 215-898-4217. E-mail address: sharpk@mail.med.upenn.edu (K.A. Sharp). decrease in entropy. In contrast, the hydration heat capacities of these two types of hydration have different signs w8–12x and thus are a more reveal- ing indicator of the difference between apolar and polar hydration. We have previously directed our efforts towards developing a deeper understanding of heat capacity changes using as a starting point the random network model (rnm) of Henn and Kauzmann w13x. A combination of explicit water simulations and this rnm yields quantitative agree- ment between measured and calculated hydration heat capacities w2,14,15x, as well as a revealing description of the change in water structure induced by apolar and polar solutes w16x. The main