Raman spectra of some saturated, unsaturated and deuterated C18 fatty acids in the HCH-deformation and CH-stretching regions.

We have analyzed the Raman spectra of stearic acid and two deuterated derivatives thereof, 2,2-dideutero stearic acid and 18,18,18-trideutero stearic acid, as well as of four unsaturated, C18 acids, (cis,-9; cis, cis-9,12, cis, cis, cis-9,12,15; and trans, trans-9,12) in their solid and liquid states. The different spectra of stearic acid and the two deuterated derivatives exhibit distinctive HCH-deformation and CH-stretching features for CH2 residues adjacent to CO2H or CH3 residues. Unsaturated hydrocarbon chains reveal additional features attributed to CH2 residues adjacent to one or two HC=CH groups. The spectra of the various fatty acids in their solic or liquid states show that both the HCH-deformation and CH-stretching regions are very sensitive to chain architecture, the CH-stretching region most dramatically so. Evaluation of the spectral features in the two regions indicates that the detailed structure of the CH-stretching region depends strongly upon interaction, enhanced by Fermiresonance, between CH-stretching fundamentals and HCH-deformation overtones. The data further suggest that the differences between HCH-deformation, in the solid and liquid states, and the resulting altered interactions between CH-stretching fundamentals and HCH-deformation overtones, produce the large modifications of the CH-stretching region that accompany solid-liquid transitions. The results thus provide a basis for the changes in the CH-stretching region, that accompany order-disorder transitions in model and biological membranes.