STRUCTURE OF STRATUM-CORNEUM LIPIDS CHARACTERIZED BY FT-RAMAN SPECTROSCOPY AND DSC .3. MIXTURES OF CERAMIDES AND CHOLESTEROL

The thermotropic and lyotropic phase behaviour of mixtures of ceramide s type IV and cholesterol was investigated using Fourier transform (FT ) Raman spectroscopy and differential scanning calorimetry (DSC). In t he dry and in the fully hydrated state of these mixtures the DSC-curve s exhibit an eutectic melting followed by the melting of the residual solid component. The Raman spectrum of the mixtures is complex, nevert heless, the appearance of the conformationally dependent bands indicat es the ordered structure of the hydrocarbon chains. The temperature de pendence of the conformationally sensitive bands in the CH, stretching region (2800-2975 cm(-1)) and in the chain C-C stretching region (105 0-1150 cm(-1)) was used to estimate the degree of order in terms of th e relative population of trans and gauche conformers. The spectrum of the pure cholesterol shows only a weak temperature dependence in the C H, stretching region and, therefore, the decrease of the intensity of the asymmetric CH, stretching mode at 2880 cm(-1) can be attributed to the melting of the alkyl chains of ceramides. The temperature and wid th of the phase transition, derived from Raman data, are similar to th ose of the DSC study. (C) 1997 Elsevier Science Ireland Ltd.