Raman scattering in bilayers of saturated phosphatidylcholines and cholesterol. Experiment and theory

Raman spectroscopy has been applied to a model biomembrane structure in order to obtain information about the effect of cholesterol upon phospholipid hydrocarbon chain ordering. The intensity of the 1130-cm-1 Raman line obtained from a dipalmitoylphosphatidylcholine (DPPC) coarse aqueous dispersion was measured as a function of temperature for two concentrations, c, of cholesterol: c=0.15 and c = 0.35. The contribution of cholesterol to this line was deduced. Intensities of all lines were taken as peak areas. By use of a theory for assigning raman intensities to chain conformations as well as a model of lipid bilayers containing cholesterol, the temperature and concentration dependence of the 1130-cm-1 line was calculated. Good agreement with DPPC experimental data was obtained, and predictions are made for dimyristoylphosphatidylcholine. The experimental results are interpreted in terms of a DPPC-cholesterol phase diagram and the average number of gauche bonds per DPPC molecule.