  Water hydrating phospholipid membranes determine their stability and
function, as well as their interaction with other molecules. In this article we
study, using all-atom molecular dynamics simulations, the rotational and
translational dynamical properties of water molecules confined in stacked
phospholipid membranes at different levels of hydration, from poorly hydrated
to a completely hydrated membrane. We find that both the translational and the
reorientation dynamics of water are dramatically slowed down as the hydration
is reduced. Analyzing in details the structure and dynamics of the hydrogen
bond at the interface, we show that both those among water molecules and those
between water and lipids slow down by decreasing the hydration, however the
latter are always slower than the former. By increasing hydration, water
saturates all the possible hydrogen bonds with the lipids and, by further
increase of hydration, the hydrogen bonds among waters becomes the majority.
However, the dynamics of the water-lipids hydrogen bonds becomes up to one
order of magnitude slower than that of the water-water hydrogen bonds, inducing
a large slowing down of the dynamics of the entire system even at large
hydration level.
