Water in Contact with a Cationic Lipid Exhibits Bulk-Like Vibrational Dynamics

Water in contact with lipids is an important aspect of most biological systems, and has been termed ‘biological water’. We used time-resolved infrared spectroscopy to investigate the vibrational dynamics of lipid-bound water molecules, to shed more light on the properties of these important molecules. We studied water in contact with a positively charged lipid monolayer using surface-specific 2-dimensional sum frequency generation vibrational spectroscopy with sub-picosecond time resolution. The dynamics of the O-D stretch vibration was measured both for pure D₂O and isotopically diluted D₂O under a monolayer of 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP). It was found that the lifetime of the stretch vibration depends on the excitation frequency, and that efficient energy transfer occurs between the interfacial water molecules. The spectral diffusion and vibrational relaxation of the stretch vibration was successfully explained with a simple model, taking into account the Förster transfer between stretch vibrations and vibrational relaxation via the bend overtone. These observations are very similar to those made for bulk water, and as such lead us to conclude that water at the positively charged lipid interface behaves similarly to bulk water. This contrasts the behavior of water in contact with negative or zwitterionic lipids, and can be understood by noting that for the cationic lipids, the charge-induced alignment of water molecules results in interfacial water molecules with O-D groups pointing towards the bulk