Anomalous Diffusion of Water Molecules in Hydrated Lipid Bilayers

Abstract

We use all-atom molecular dynamics (MD) simulation to study the structure and dynamics of water molecules in a hydrated lipid membrane. By using a 0.1 microsecond long MD trajectory of a fully solvated DMPC phospholipid bilayer we identify (by means of Voronoi tessellation) four dynamically connected water regions termed: buried, hydration, intermediate and bulk, respectively. Due to their proximity to the polar lipid headgroups, buried and hydration waters have qualitatively different dynamical properties from bulk water. To identify and quantify these differences we investigate the time evolution of the lateral mean square displacement (MSD) of water molecules and the lifetime of hydrogen bonds between water and lipid molecules. We find that before entering the linear diffusion regime (t>10ns), on sub-nanosecond time scale buried and hydration waters undergo anomalous diffusion characterized by well separated sub-diffusive (t<20ps) and super-diffusive (0.1ns<t<1ns) regimes. Apparently the latter has not been reported before, and it appears to be correlated with the out-of-plane fluctuations of the lipid molecules. The anomalous super- and sub-diffusive regimes lead to a self-intermediate scattering function with compressed- and stretched exponential relaxation, respectively. In principle, these predictions should be testable using neutron scattering (NS) experiments. In practice, however, the NS signal will most likely be totally dominated by the contribution from the abundant ordinary bulk water.Computer time was generously provided by the University of Missouri Bioinformatics Consortium.