Interfacial Water Structure Associated with Phospholipid Membranes Studied by Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy

Phase-sensitive vibrational sum frequency generation is employed to investigate the water structure at phospholipid/water interfaces. Interfacial water molecules are oriented preferentially by the electrostatic potential imposed by the phospholipids and have, on average, their dipole pointing toward the phospholipid tails for all phospholipids studied, dipalmitoyl phosphocholine (DPPC), dipalmitoyl phosphoethanolamine (DPPE), dipalmitoyl phosphate (DPPA), dipalmitoyl phosphoglycerol (DPPG), and dipalmitoyl phospho-l-serine (DPPS). Zwitterionic DPPC and DPPE reveal weaker water orienting capability relative to net negative DPPA, DPPG, and DPPS. Binding of calcium cations to the lipid phosphate group reduces ordering of the water molecules.