Thermal dependence of Raman descriptors of ceramides. Part I: effect of double bonds in hydrocarbon chains

The barrier function of the stratum corneum (SC) is directly related to: (1) the nature and the composition of the lipids in the intercellular spaces and (2) the conformational order of the ceramides within this layer. The aim of this work was to determine Raman descriptors for the lateral packing, the conformation, and the structure of ceramides III, IIIA, and IIIB issued from the same phytosphingosine ceramide and only presenting differences in the number of double bonds in the hydrocarbon chains. Temperature was used as a variable parameter in order to access the different states of the conformational order and supramolecular organization of the three ceramides, and Raman spectra were collected at each temperature. By using a high-resolution Raman spectrometer and working on a spectral range going from 400 to 3,200 cm−1, we were able to assess simultaneously the different descriptors of structure and organization, i.e., the methyl rocking bands (840–910 cm−1) for the chain-end conformers, the C–C skeletal stretching (1,060–1,130 cm−1), and the CH stretching region (2,800–3,050 cm−1) for the trans and gauche conformations, the CH2 scissoring bands to follow the changes in the lateral packing, and finally the amide I band to evaluate the state of the H-bonds between the polar and head groups.