Raman studies of conformational changes in model membrane systems

Laser Raman spectra of concentrated samples of phosphatidyl choline and phosphatidyl ethanolamine were taken at approximately 10° intervals over a temperature range of 90°–19°C. The spectral region from 30 to 3300 cm^?1 was investigated. Several new spectral features were discovered which are correlated to phospholipid liquid crystalline structure. It is shown that 1) frequency shifts occur in the $\text{PO}{}_2{}^{\mathrm{?}}$ symmetric stretch band which suggest a change in exposure of the PO₂ group to the solvent upon melting, 2) the frequency of the translational hydrocarbon mode around 150 cm^?1 appears to indicate the degree to which the hydrocarbon chain is extended, 3) the methyl and methylene stretch bands at 2890 and 2850 cm^?1 very clearly demonstrate hydrocarbon chain melting, and 4) the 720 cm^?1 band, previously assigned to the symmetric OPO diester stretch, appears to be due instead to the symmetric CN stretch of choline.