Orientations of Water Molecules near Globular Protein and Lipid Membrane

In computer models of aqueous solutions of globular protein and lipid bilayer membrane, the orientations of water molecules relative to the directions to organic surfaces at different distances were studied. Distributions of distances between molecular surfaces and angles between the internal vectors of water molecules (dipole moment, normal vector, HH-vector) and these directions to organics allowed us to identify molecular layers with fundamentally different orientations. The most probable orientations of the molecules in the nearest layers have been determined and described in detail. It is shown that water molecules in the first layer around the protein most often adopt an orientation that allows them to form donor and acceptor hydrogen bonds with the surface atoms of the protein. Near the membrane, molecules with donor orientations are also most abundant, while acceptor orientations are absent. Instead, orientations describing the interaction of water molecules with hydrophobic regions of the lipid surface are present. Moving to the second and subsequent layers each time radically changes the distribution of orientations, but in all layers except the first, the orientations around the protein and membrane do not differ. The presented method of analysis is not exclusively for water molecules and can be used to study the orientations of any rigid molecules.