Sub-Diffusion and Super-Diffusion of Hydration Water Molecules at Biological Interfaces

The structure and dynamics of hydration water at the surface of biomolecules (e.g., proteins and lipids in biological membranes) are fundamental for their stability and functioning. Due to the interactions at the surface of a solvated biological membrane, the dynamics of the hydration waters and that of the membrane molecules are to some degree correlated. In spite of previous efforts, little is known about the time and length scale of these correlations. Here, we report on a 0.1 microsecond all-atom molecular dynamics simulation study aimed at investigating the dynamics of the hydration water at the interface of a dimyristoyl-phosphatidylcholine (DMPC) lipid bilayer. We find that, mainly due to hydrogen bonding with the lipid bilayer, the mean-square displacement of the interfacial water has four well defined dynamical regimes, with characteristic power law (t⊥n) time (t) dependence: (1) ballistic, for t < 10 fs, with n=2 ; (2) sub-diffusive, for 0.2 ps < t < 20 ps, with n<1; (3) super-diffusive, for 0.1 ns < t < 1 ns, with 1 10 ns, with n=1. The super-diffusive regime (characterized by a self intermediate scattering function with compressed exponential relaxation) of the hydration water molecules has not been observed before, and possibly determines the length and time scales of the correlation between the dynamics of water and lipid membrane. Furthermore, the water-lipid interactions give rise to an average liquid-like structure of the interfacial water molecules on a length scale corresponding to the average lipid-lipid separation, and the relaxation time of this structure is an order of magnitude larger than what is expected from the self motion of the water. Computer time was generously provided by the University of Missouri Bioinformatics Consortium.