Structure and orientation of water at charged lipid monolayer/water interfaces probed by heterodyne-detected vibrational sum frequency generation spectroscopy.

Cell membrane/water interfaces provide a unique environment for many biochemical reactions, and associated interfacial water is an integral part of such reactions. A molecular level understanding of the structure and orientation of water at lipid/water interfaces is required to realize the complex chemistry at biointerfaces. Here we report the heterodyne-detected vibrational sum frequency generation (HD-VSFG) studies of lipid monolayer/water interfaces. At charged lipid/water interfaces, the orientation of interfacial water is governed by the net charge on the lipid headgroup; at an anionic lipid/water interface, water is in the hydrogen-up orientation, and at the cationic lipid/water interface, water is in the hydrogen-down orientation. At the cationic and anionic lipid/water interfaces, interfacial water has comparable hydrogen bond strength, and it is analogous to the bulk water.