| Abstract
| - The intrinsic symmetry constraints on sum-frequency vibrational spectroscopy (SFVS) have been exploitedto measure the asymmetric distribution of lipid domains in the proximal and distal layers of a planar supportedlipid bilayer (PSLB) in the gel−liquid-crystalline (l.c.) coexistence region of the phase diagram for severallipid systems. Four saturated phospholipids, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-diheptadecanoyl-sn-glycero-3-phosphocholine (DHPC),and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and two equal-molar binary mixtures of DSPC withDMPC, and DSPC with a unsaturated lipid DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) wereinvestigated. The destructive interference of the symmetric stretch (vs) transition moments from the lipidfatty acid methyl groups (CH3) was used to monitor changes in the symmetry of the bilayer structure. Amaximum in the CH3vs intensity is observed at the phase transition temperature (Tm) due to the break in thelocal symmetry of lipid bilayers caused by the dislocation of the gel and l.c. phase domains. The SFVSresults were correlated to phase segregation in the membranes as measured by fluorescence microscopy. TheSFVS measured Tm for DMPC, DPPC, DHPC, and DSPC were 23.4 ± 0.9 °C, 41.0 ± 0.4 °C, 52.4 ± 0.7 °C,and 57.5 ± 0.5 °C, respectively. These values correlate well with the literature values of 23.6 ± 1.5 °C, 41.3± 1.8 °C, 49 ± 3 °C, and 54.5 ± 1.5 °C for DMPC obtained by differential scanning calorimetry (DSC). Inaddition to providing a direct spectroscopic probe of the Tm of PSLBs, these studies also provide evidencefor the delocalization of the gel and l.c. domain structures between the two layers of lipid bilayers.
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