Orientational order of the water molecules across a fully hydrated DMPC bilayer: A Monte Carlo simulation study

The orientational order of water molecules located in different regions of a fully hydrated dimyristoylphos-phatidylcholine (DMPC) membrane is analyze d and compared to that in pure water on the basis of an all-atom Monte Carl o simulation. The preferential orientation of the water molecules relative to the membrane as well as the relative orientation and hydrogen-bonding st ructure of neighboring molecules is discussed in detail. Due to the distrib ution of the charged groups of the Lipid molecules, the water molecules in the interfacial. region of the membrane are turning preferentially toward t he membrane interior with their dipole moments, whereas in the hydrocarbon region the water dipoles are pointing toward the aqueous phase. The density of the water molecules in the hydrocarbon phase is found to be rather inho mogeneous; 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 th em and also the hydrogen-bonded neighbors to be arranged around each other in a coplanar way. It is found that the relative importance of the intersti tial molecules, which left the hydrogen-bonded network of the other molecul es and are located in its cavities, increases considerably upon the approac h of the molecules to the middle of the membrane. It is also found that the geometry of the hydrogen bond around the bonding H atom does not change no ticeably across the bilayer, whereas when the molecules approach the membra ne interior, the arrangement of the hydrogen-bonded neighbors around each o ther becomes less and less tetrahedral, until this preference for tetrahedr al arrangement disappears completely.