Dynamic coupling of a hydration layer to a fluid phospholipid membrane: intermittency and multiple time-scale relaxations.

Understanding the coupling of a hydration layer and a lipid membrane is crucial to gaining access to membrane dynamics and understanding its functionality towards various biological processes. To find out how significant the mutual influence of the hydration layer and bilayer dynamics is, a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) lipid bilayer is simulated atomistically in the presence of the TIP4P/2005 water model at 308 K. Interface water (IW) molecules are classified based on their continuous physical proximity or ability to form hydrogen bonds with different moieties of lipid heads. A gradient in retardation of translational mean square displacements is found to operate coherently for both IW and lipid components across the bilayer normal. Deviations from Gaussianity in van Hove correlation functions increase for the lipids and decrease for the IW from the tails to the heads. The IW molecules exhibit Fickian but intermittent dynamics due to coupled vibrations in the local cage formed by the hydrogen bonds with the lipid heads followed by decoupled translational jumps. Importantly, the differences in regional dynamics of lipid heads are clearly reflected in the dynamics of spatially resolved IW molecules physically close to the lipid heads, but not to the dynamics of the hydrogen bonded IW molecules far from the lipid heads. These analyses imply that spatially resolved interface water dynamics can act as a sensitive reflector of regional membrane dynamics occurring at sub ps to hundreds of ps time-scales for several important biological functions at physiological temperature in the future.