Weakly Hydrogen-Bonded Water Inside Charged Lipid Monolayer Observed with Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

Many biological processes such as photosynthesis and signal transduction proceed at lipid/water interfaces. Although water molecules inside lipid membranes are considered to have a critical role in these processes, it has been difficult to experimentally observe those water molecules with high selectivity. Here, we measure the Im χ(2) (imaginary part of second-order nonlinear optical susceptibility) spectra of water at anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol, DPPG) and cationic lipid (1,2-dipalmitoyl-3-trimethylammonium-propane, DPTAP) interfaces with single-channel heterodyne-detected sum frequency generation spectroscopy and investigate the structure of water molecules at the lipid interfaces. The presence of weakly hydrogen-bonded water molecules located above the lipid head group is clearly identified. The Im χ(2) spectra indicate that water molecules above the head group are oriented oppositely to those below the head group and they donate hydrogen bonds to the carbonyl and glycer...