WATER DYNAMICAL ANOMALIES EVIDENCED BY MOLECULAR-DYNAMICS SIMULATIONSAT THE SOLVENT-PROTEIN INTERFACE

We present a computer simulation picture of the dynamical behavior, at room temperature, of water in the region close to a protein surface. We analyzed the probability distribution of water molecules diffusing near the surface, and we found that it deviates from a Gaussian, which is predicted for Brownian particles. Consistently, the mean square di splacements of water oxygens show a sublinear trend with time. Moreove r, the relaxation of hydration layers around the whole protein is foun d to follow a stretched exponential decay, typical of complex systems, which could as well be ascribed to the non-Gaussian shape of the prop agator. In agreement with such findings, the analysis of water transla tional and reorientational diffusion showed that not only are the solv ent molecule motions hindered in the region close to the protein surfa ce, but also the very nature of the particle diffusive processes, both translational and rotational, is affected. The deviations from the bu lk water properties, which put into evidence a deep influence exerted by the protein on the solvent molecule motion, are discussed in connec tion with the presence of spatial (protein surface roughness) and temp oral (distribution of water residence times) disorder inherent in the system.