Interfacial Fresnel Coefficients and Molecular Structures of Model Cell Membranes: From a Lipid Mono layer to a Lipid Bilayer

During the model membrane formation process from a lipid monolayer of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1?-rac-glycerol) (sodium salt) (DPPG) in air to a lipid bilayer of DPPG/deuterated-DPPG (dDPPG) on water, the intensity of sum frequency generation (SFG) vibrational signals from the DPPG molecules increased by similar to 34 times. The increased signal intensity could be caused by inherently different molecular ordering of lipid molecules in the monolayer/bilayer or by optical effects induced by different contacting mediums (prism in air and prism contacting water) (see Figure 2). We resolve the two possibilities by analyzing tilt angles of the methyl groups at DPPG hydrophobic ends for the monolayer/bilayer which reflect the molecular ordering information and by evaluating the Fresnel coefficients which reflect the contacting medium-induced optical effect. DPPG molecules were more ordered after transformation from a lipid monolayer in air to a lipid bilayer on water, which induced a slight signal increase. The augmentation of the Fresnel coefficients was well-correlated to the enhancement of the SFG resonant signal. Therefore, this case study is applicable to similar interfacial systems probed by the SFG spectroscopy to deduce the relative contributions of interfacial molecular ordering and interfacial Fresnel coefficients to the detected SFG signal.