Effect of bilayer curvature on vibrational Raman spectroscopic behavior of phospholipid-water assemblies

In order to clarify the effect of bilayer curvature upon phospholipid conformation, vibrational Raman spectra were recorded for dipalmitoyl and dimyristroyl phosphatidylcholine in the gel state for both multilayer and single-wall vesicle assemblies. An intensity comparison, based upon a nonperturbing internal standard, between the two classes of bilayrer systems reflected a decrease in peak height intensity for the observed hydrocarbon chain transitions in the single shell vesicle form. No intensity change between bilayer form was detected, however, for the two observed head group modes. Trends in the peak height intensity rations for the 1100 cm-1 carbon-carbon stretching vibrations indicated an increase in hydrocarbon chain transgauche isomerization for the vesicle in comparison to the multilayer arrangements. The sensitivity of the methylene carbon-hydrogen stretching modes to interchain interactions was demonstrated by comparisons of the intensity patterns in the 2900 cm-1 region to the intensity characteristics of the carbon-carbon stretching region for polycrystalline, multilayer and vesicle materials. Examination of various carbon-carbon stretching mode intensity ratios for cholesterol doped dipalmitoyl phosphatidylcholine bilayers indicated that while 25 mol% cholesterol increased the transgauche acyl chain isomerization in multilayers, no comparable effect was observed for the vesicle forms. In contrast, the methylene twisting/methylene deformation intensity ratios for the cholesterol containing systems suggested that some further type of interchain perturbation occurs in the vesicle aggregations.