Hydration sphere structure of proteins: A theoretical study

Hydration is essential for the proper biological activity of biomolecules. We studied the water network around insulin (as a model protein) in aqueous NaCl solutions using molecular dynamics simulations and statistical analysis of the topological properties (hydrogen bond neighbor number and the interaction energy between hydrogen-bonded water molecules) of the water network. We propose a simple method to define the hydration layers around proteins. Water molecules in the first and second layers form significantly less, but stronger hydrogen bonds with each other than in the bulk phase. Furthermore, water molecules over the hydrophilic and hydrophobic surface of the protein possess slightly different H-bonding properties, supporting the hypothesis of structural and dynamical heterogeneity of the water molecules over protein surface. The protein molecule perturbs the solvent structure at least up to the fourth-fifth hydration layer. Our data suggest the peculiar role of the second hydration shell.