The question of protein hydration, i.e., the interaction of protein molecules with water, and of the involvement of water molecules in protein reactions has been a focus of attention for the good part of a century. Yet the number of water molecules that interact with one protein molecule, W_H (waters of hydration), has been an elusive quantity, notwithstanding measurements by transport (1,2) and NMR (3) techniques. The reason for this lies in the essence of the physical phenomenon of protein hydration. The fact is that there is no rigid shell of water around a protein molecule, but rather there is a fluctuating cloud of water molecules that are thermodynamically affected more or less strongly by the protein molecule. Nevertheless, the description of the overall effect in terms of an effective number of whole water molecules, W_H, has been a useful concept, analogous to the electrostatic double layer representation of the ionic distribution in the cloud around a central ion in the Debye-Huckel theory (4), or the surface of shear used in the analysis of transport phenomena (sedimentation, electrophoresis, diffusion) (5).
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