Vibrational Sum-Frequency Generation Spectroscopy at the Water/Lipid Interface: Molecular Dynamics Simulation Study

摘要

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 environment. The weak 3590cm⁻¹ peak is attributed to a few water molecules coupled to the glycerol backbone of DMCP. The 3470cm⁻¹ feature comes from the top water layer adjacent to the hydrophilic head-group of DMPC. The 3290cm⁻¹ peak arises from the near-bulk water nonadjacent to DMPC. The stretching mode corresponding to the 3290cm−1 peak is strongly coupled with the neighboring water molecules. In contrast, the 3470cm⁻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.