In the present study, stability and structure of single and binary mixed monolayers of an unsaturated phospholipid, DOPC, and a saturated phospholipid, DPPC-d75, on the water surface, were explored using the π-A isotherm, atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy in various environments. Our results demonstrated that DOPC in the monolayers becomes unstable after the exposure to a low concentration of ozone (20 ± 10 ppb) or even to ambient laboratory air, which has a similar ozone level, but is stable in nitrogen or oxygen. DOPC can be selectively oxidized by a trace amount of ozone in the ambient environment but can be partially inhibited by the presence of DPPC in the monolayer. The present study provides useful information for understanding the physicochemical properties of the cell membranes.

Broadband phase-sensitive vibrational sum frequency generation (SFG) spectroscopy was utilized to study the molecular orientation of molecules adsorbed on dielectric solid substrates. A gold thin film was employed to generate a SFG signal as a local oscillator (LO). To simplify the phase measurement, a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) was used as a standard sample for phase correction of the phase-sensitive SFG measurements on the solid/air interface. It was demonstrated that the absolute orientation of molecules in the LB films on a fused quartz surface can be clearly distinguished by phase-sensitive SFG measurement. In addition, the observation on the SAM of d35-OTS reveals that the two C-H stretching modes for α-CH2 group are in opposite phase. Furthermore, by using the present phase-sensitive SFG setup, the orientation flipping of water molecules on positively and negatively charged solid/liquid interface can be distinguished.

Ambient ozone is a common pollutant in the atmosphere that has an extremely high oxidative ability, can dramatically change the structure and functionality of biomolecules, and is harmful to public health. However, the knowledge about the influence of low-level ozone is still very limited at a molecular level. In the present study, the monolayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC, 16:0-18:1 PC) as well as its binary mixed monolayer with 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC, 16:0 PC), which are widely found in many biological systems, have been systematically investigated in a low-level ozone environment (20 ± 10 ppb), by π-A isotherm, sum frequency generation (SFG) vibrational spectroscopy, and atomic force microscopy (AFM). Our results demonstrate that the POPC monolayer is unstable and the C═C moieties in the oleyl chain are selectively oxidized by the low-level ozone. The oxidized lipids from POPC initially remain and reorientate the hydrophilic portion to the water surface and gradually dissolve into the aqueous solution. One should take great caution when using unsaturated lipid molecules to avoid their possible oxidation in the ambient environment. The present study expands and deepens our insights into the oxidation mechanism of unsaturated lipids at a molecular level.

Sum frequency generation (SFG) spectroscopy is widely used to observe molecular orientation at interfaces through a combination of various types of polarization. The present work thoroughly examines the relation between the polarization dependence of SFG signals and the molecular orientation, by comparing SFG measurements and molecular dynamics (MD) simulations of acetonitrile/water solutions. The present SFG experiment and MD simulations yield quite consistent results on the ratios of χ elements, supporting the reliability of both means. However, the subsequent polarization analysis tends to derive more upright tilt angles of acetonitrile than the direct MD calculations. The reasons for discrepancy are examined in terms of three issues; (i) anisotropy of the Raman tensor, (ii) cross-correlation, and (iii) orientational distribution. The analysis revealed that the issues (i) and (iii) are the main causes of errors in the conventional polarization analysis of SFG spectra. In methyl CH stretching, the anisotropy of Raman tensor cannot be estimated from the simple bond polarizability model. The neglect of the orientational distribution is shown to systematically underestimate the tilt angle of acetonitrile. Further refined use of polarization analysis in collaboration with MD simulations should be proposed.