Water dynamical anomalies evidenced by molecular-dynamics simulations at 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 displacements of water oxygens show a sublinear trend with time. Moreover, the relaxation of hydration layers around the whole protein is found to follow a stretched exponential decay, typical of complex systems, which could as well be ascribed to the non-Gaussian shape of the propagator. In agreement with such findings, the analysis of water translational and reorientational diffusion showed that not only are the solvent molecule motions hindered in the region close to the protein surface, but also the very nature of the particle diffusive processes, both translational and rotational, is affected. The deviations from the bulk water properties, which put into evidence a deep influence exerted by the protein on the solvent molecule motion, are discussed in connection with the presence of spatial (protein surface roughness) and temporal (distribution of water residence times) disorder inherent in the system.