Slow Relaxations of Chemically Confined Hydration Layers near Lipid Bilayers: Dynamical Heterogeneities above Supercooling

Abstract: A hydrated 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) lipid membrane is investigated using an all atom molecular dynamics simulation at 308K to find out the physical sources of universal slow relaxation of hydration layers. Continuously residing interface water (IW) hydrogen bonded to each other and concertedly to different moieties of lipid heads are identified. The non-gaussian parameter of all IW show a crossover from cage vibration to translational diffusion. A significant non-gaussianity is observed for the IW prevailing large length correlations in translational van Hove functions. Two time-scales for the ballisitic motions and hopping transitions are obtained from the self intermediate scattering functions of the IW with an additional long relaxation which disappears for the BW. This is attributed to the coupled dynamics of IW cages hydrogen bonded to lipid heads. Our calculations reveal that the water near membranes are slowed down due to dynamical heterogeneities above room temperature and have implications to bioprotection mechanism at freezing conditions.