The diffusional water dynamics in the hydration layer of a
dipalmitoylphosphatidylcholine bilayer is studied using molecular dynamics
simulations. By mapping the perpendicular water motion on the ordinary
diffusion equation, we disentangle free energetic and friction effects and
show that perpendicular diffusion is strongly reduced. The lateral water
motion exhibits anomalous diffusion up to several nanoseconds and is
characterized by even further decreased diffusion coefficients, which by
comparison with coarse grained simulations are explained by the transient
corrugated effective free energy landscape imposed by the lipids. This is in
contrast to homogenous surfaces, where boundary hydrodynamic theory
quantitatively predicts the anisotropy of water diffusion