  An accurate description of the structure and dynamics of interfacial water is
essential for phospholipid membranes, since it determines their function and
their interaction with other molecules. Here we consider water confined in
stacked membranes with hydration from poor to complete, as observed in a number
of biological systems. Experiments show that the dynamics of water slows down
dramatically when the hydration level is reduced. All-atom molecular dynamics
simulations identify three (inner, hydration and outer) regions, within a
distance of approximately 1 nm from the membrane, where water molecules exhibit
different degrees of slowing down in the dynamics. The slow-down is a
consequence of the robustness of the hydrogen bonds between water and lipids
and the long lifetime of the hydrogen bonds between water molecules near the
membrane. The interaction with the interface, therefore, induces a structural
change in the water that can be emphasized by calculating its intermediate
range order. Surprisingly, at distances as far as ~ 2.5 nm from the interface,
although the bulk-like dynamics is recovered, the intermediate range order of
water is still slightly higher than that in the bulk at the same thermodynamic
conditions. Therefore, the water-membrane interface has a structural effect at
ambient conditions that propagates further than the often-invoked 1 nm length
scale. Membrane fluctuations smear out this effect macroscopically, but an
analysis performed by considering local distances and instantaneous
configurations is able to reveal it, possibly contributing to our understanding
of the role of water at biomembrane interfaces.
