Structure of stratum corneum lipids characterized by FT-Raman spectroscopy and DSC. III. Mixtures of ceramides and cholesterol.

The thermotropic and lyotropic phase behaviour of mixtures of ceramides type IV and cholesterol was investigated using Fourier transform (FT) Raman spectroscopy and differential scanning calorimetry (DSC). In the dry and in the fully hydrated state of these mixtures the DSC-curves exhibit an eutectic melting followed by the melting of the residual solid component. The Raman spectrum of the mixtures is complex, nevertheless, the appearance of the conformationally dependent bands indicates the ordered structure of the hydrocarbon chains. The temperature dependence of the conformationally sensitive bands in the CH2 stretching region (2800-2975 cm-1) and in the chain C-C stretching region (1050-1150 cm-1) was used to estimate the degree of order in terms of the relative population of trans and gauche conformers. The spectrum of the pure cholesterol shows only a weak temperature dependence in the CH2 stretching region and, therefore, the decrease of the intensity of the asymmetric CH2 stretching mode at 2880 cm-1 can be attributed to the melting of the alkyl chains of ceramides. The temperature and width of the phase transition, derived from Raman data, are similar to those of the DSC study.