Biological action requires water. For example, the interaction with water triggers the self-assembly of lipids into the membrane bilayer, which separates the inside of the cell from the outside. Moreover, the organization and functioning of proteins – the micromachines of life – that are embedded in membranes, depend on the structure and dynamics of water molecules at the membrane/water interface. Experimental studies of water in these environments are difficult because of the challenge of isolating the signal of interfacial water from that of the bulk.

Here we report a series of surface specific studies of the membrane/water interface using surface-specific vibrational spectroscopies, both in equilibrium and on ultrafast (femtosecond) time scales. Our approach allows us to selectively investigate the one monolayer of water molecules at the at the membrane/water interface. Water is characterized through its O-H stretch vibration. Static vibrational spectra reveal distinct peaks in the O-H stretch region, in contrast to the bulk vibrational spectrum. We can use changes in the amplitudes of these water peaks to detect picomolar concentrations of DNA, by its adsorption to the model membrane surface.

Using femtosecond time-resolved VSFG, we probe the vibrational dynamics of interfacial water molecules. In contrast to water at a variety of other interfaces, membrane-bound water does not rapidly exchange vibrational energy with the underlying bulk. This observation illustrates that membrane-bound water is an inherent part of the membrane: water at the membrane interface does not just terminate the bulk.