Molecular dynamics simulation evidence of anomalous diffusion of protein hydration water

The mean square displacement of protein hydration water has been found to increase nonlinearly in time, as observed for long times, by molecular dynamics simulations at low hydration levels and in proximity of the protein surface at full hydration. While such an anomalous diffusion is traced back to the more general properties of disordered media, some caution in the use of the self-diffusion coefficient $D$ to characterize the water dynamics in these systems is suggested.