Orientational Order of the Water Molecules Across a Fully Hydrated DMPC Bilayer. A Monte Carlo Simulation Abstract: The orientational order of water molecules located in different regions of a fully hydrated dimyristoylphosphatidylcholine (DMPC) membrane is analyzed and compared to that in pure water on the basis of an all-atom Monte Carlo simulation. The preferential orientation of the water molecules relative to the membrane as well as the relative orientation and hydrogen-bonding structure of neighboring molecules are discussed in detail. Due to the distribution of the charged groups of the lipid molecules, the water molecules in the interfacial region of the membrane are turning preferentially towards the membrane interior with their dipole moments, whereas in the hydrocarbon region the water dipoles are pointing towards the aqueous phase. The density of the water molecules in the hydrocarbon phase is found to be rather inhomogeneous, the few water molecules in this region are grouping together and form small hydrogen-bonded clusters. The long, mostly parallel lipid tails are forcing these water molecules to be aligned in planes parallel with them, and also the hydrogen-bonded neighbors to be arranged around each other in a co-planar way. It is found that the relative importance of the interstitial molecules, which left the hydrogen-bonded network of the other molecules and are located in its cavities, increases considerably upon approaching the middle of the membrane. It is also found that the geometry of the hydrogen bond around the bonding H atom does not change noticeably across the bilayer, whereas on approaching the membrane interior the arrangement of the hydrogen-bonded neighbors around each other becomes less and less tetrahedral, until this preference for tetrahedral arrangement disappears completely.