Structure and dynamics of interfacial water in model lung surfactants

The last decade has seen a transformation in understanding of the role of membrane-bound interfacial water. Whereas until recently water was treated principally as a continuum (primarily screening charges of lipids and proteins), it has become apparent recently that consideration of water's molecular-level properties is critical in understanding a variety of biochemical and biophysical processes. Here we investigate the structure and dynamics of water in contact with a monolayer of artificial lung surfactant, composed of four types of lipids and one protein. We probe this water using frequency-domain sum-frequency generation (SFG) spectroscopy, and a newly developed time-domain, three-pulse technique, in which the vibrational relaxation of interfacial water molecules is followed in real time. We characterize interfacial water in three systems: a monolayer of the pure lipid that is the majority of the lung surfactant mixture, a monolayer of the four lipids constituting the mixture, and a monolayer of the four lipids and the protein. We find subtle differences in the water structure and dynamics that depend on the mixture density and composition. In particular, frequency-domain measurements suggest that in the lipid mixture and the lipid mixture + protein, the relatively bulky lipids (those that have either three or unsaturated hydrocarbon tails) tend to be squeezed out at higher pressure. Measurements using the time-domain, three-pulse technique make clear that structural relaxation of interfacial water is significantly slowed down upon adding small amounts of protein to the lipids. Both results are consistent with prior measurements using other techniques in which more fluid lipids were shown to be 'squeezed out' of lung surfactant at high compression and the role of protein in the mixture was demonstrated to be a catalyst for the formation of multilayers under compression that are subsequently reintegrated into the monolayer on expansion.