The effect of vicinal polar and charged groups on hydrophobic hydration.

The use of a linear relationship between free energy of hydrophobic hydration and solvent-accessible apolar surface area has been helpful in interpreting the thermodynamics of biological macromolecules. However, a recent study (Y.-K. Cheng, P. J. Rossky, Nature 1998, Vol. 392, pp. 696-699) has established a substantial enthalpic dependence on biomolecular surface topography, originating from solvent hydrogen-bonding loss in a restrictive geometry. In this study, we use molecular dynamics simulations of 2-Zn insulin in water solvent to explore the further effect of vicinal polar or charged groups on hydrophobic hydration at a biomolecular surface. In contrast to the case for solvent more isolated from such polar solute influences, the binding energies of the water that is proximal to the hydrophobic dimeric interface of insulin and vicinal to polar and charged groups are comparable to the bulk solvent value, a result of compensating interaction primarily with the solute counterions. The results suggest a special importance for such polar/charged groups in biological processes involving hydrophobic surface regions of restricted geometry and also suggest a general route for tuning the hydrophobicity of interfaces.