Why the Solvation Water around Proteins Is More Dense than Bulk Water

The main aim of this work is to propose a rational explanation of the commonly observed phenomenon of increasing water density within solvation shell of proteins. We have observed that the geometry of the water-water hydrogen bond network within solvation layer differs from the one in bulk water, and it is the result of interactions of water molecules with protein surface. Altered geometry of the network reflects changes in the structure of solvation water. Our explanation of the observed changes is based on model proposed by Tanaka (Tanaka, H. J. Chem. Phys. 2000, 112, 799). According to this model, in liquid water exist some special structures formed by water molecules thanks to their unique ability to create the branched network of hydrogen bonds. These structures have two characteristic features: a low potential energy of internal interactions and a large specific volume. We provide some evidence for the supposition that deformation of the geometry of the water-water hydrogen bond network is responsible for destabilization of these structures and therefore for increased local density of water. Our model is constructed on the basis of the analysis of solvation water of some specific protein, the motor head of kinesin. Subsequently, we used it for description of solvation of purely hydrophobic surface. It has been found that in this case an unoccupied space between the hydrophobic surface and neighboring solvation layer exists. It has been found that thickness of this region depends on local geometry of the water-protein interface, and it is a result of maintaining a balance between water-surface interactions and water-water interactions. In our opinion, existence of this space region is one of the main factors that differentiates the hydrophobic hydration from hydration of the native form of kinesin. Its existence also explains why the density is greater for solvation water around the native form of the protein than in the vicinity of the hydrophobic surface.