Structure and Dynamics of Water at Model Human Lung Surfactant interfaces

We investigated the structure and dynamics of water in contact with a monolayer of artificial lung surfactant (LS), composed of four types of lipids and one protein. The interfacial water is investigated with frequency-domain and time-domain surface sum-frequency generation spectroscopy, in which the vibrational relaxation of specifically interfacial water molecules can be followed. We compare the response of water interfacing with three systems: a monolayer of the pure lipid that is dominant in the LS mixture, a monolayer of the four lipids, and a monolayer of the four lipids including the lung surfactant protein. We find remarkable differences in the vibrational energy relaxation mechanisms between the pure DPPC/water system and the mixtures which essentially reveals the underlying differences in the associated interfacial water structure.