Molecular Ordering and Hydration in Model Lipid Rafts Studied by Vibrational Sum Frequency Spectroscopy

Vibrational sum frequency spectroscopy (VSFS) has been employed to study the molecular order, orientation, and hydration in model lipid rafts consisting of monolayers on water of deuterated DSPC (d-DSPC), cholesterol, and the sphingolipid GlcCer(d18:1/18:0), compounds that are believed to be enriched in lipid rafts. Experiments have been carried out at surface pressures of 4, 30, and 55 mN/m for GlcCer and at 30 mN/m for cholesterol, d-DSPC, the 50/50 binary mixture of GlcCer/d-DSPC, and the ternary mixture, and the orientation of the terminating methyl group of d-DSPC has been calculated. At all surface pressures and in all mixtures, the hydrocarbon chains exhibited a well-ordered molecular structure with few gauche defects, indicating that mixtures of these constituents have the ability to form ordered structures, which lipid rafts are believed to form. In addition, the orientation of the methyl group in d-DSPC remained fairly constant in all mixtures. The water -OH stretching region (∼3000-3800 cm^-1) was dominated by the effect of d-DSPC being the only charged molecule and hence with a large ability to orient surface water molecules.