FTIR spectroscopic analyses of the temperature and pH influences on stratum corneum lipid phase behaviors and interactions

A thermotropic investigation of different lipid dispersions containing ceramide 3 (CER3) or sphingomyelin (SPM), perdeuterated palmitic acid (PA-d31) and cholesterol (CH) or cholesterol sulfate (CS) at pH 5.2 and 7.4 used as model membranes, has been carried out by Fourier transform infrared (FTIR) spectroscopy in order to estimate the importance of these lipids and of the temperature and pH for the maintenance of the structural organization of the stratum corneum (SC) lipid lamellae. The results obtained for the CER3 and SPM mixtures at pH 5.2 and 7.4 indicated that the little size of the polar headgroup of CER3 compared with that of SPM could permit a more closely packing in the CER3 acyl chains. Moreover, the CH and CS induced an increase of the order in the CER3 acyl chains over the physiological temperatures while a disordering was seen above 60 °C. In addition, the thermal phase behaviors observed for the CER3/PA-d31 dispersion at pH 5.2 and 7.4, suggested a phase separation between the CER3 and PA-d31 molecules in this mixture. Nevertheless, the miscibility between the CER3 and PA-d31 was raised in the presence of CH or CS at pH 5.2. In particular, the incorporation of these sterols into the CER3/PA-d31 dispersion at pH 5.2 appeared to result in an increase of the order in the acyl chains of CER3 and PA-d31 at about 37 °C. In contrast, a phase separation was observed between the CER3 and PA-d31 in the CER3/PAd31/CH and CER3/PA-d31/CS dispersions at pH 7.4. Interestingly, the pH change from 5.2 to 7.4 in these tertiary dispersions was also accompanied by a substantial deprotonation of the PA-d31 molecules which seemed more pronounced in the presence of CH as compared with CS. Altogether, the results suggested that the ceramides, fatty acids and sterols could play an important structural role in the SC cohesion.