Vibrational sum-frequency generation spectroscopy at the water/lipid interface: molecular dynamics simulation study.

The sum-frequency generation (SFG) spectrum from the water/[1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine] (DMPC) interface in the OH stretching mode region of water is simulated and shows three spectral peaks which are assigned to different environments. The weak 3590 cm(-1) peak is attributed to a few water molecules coupled to the glycerol backbone of DMCP. The 3470 cm(-1) feature comes from the top water layer adjacent to the hydrophilic headgroup of DMPC. The 3290 cm(-1) peak arises from the near-bulk water nonadjacent to DMPC. The stretching mode corresponding to the 3290 cm(-1) peak is strongly coupled to the neighboring water molecules. In contrast, the 3470 cm(-1) mode is decoupled from the surrounding water molecules, and the orientation of water is governed by DMPC. This decoupling explains the slow relaxation dynamics of water measured in the time-resolved SFG experiment. Despite the similarity of the SFG spectra, the peak origins of water/lipid and water/vapor interfaces are different.