Cholesterol Sulfate and Ca2+ Modulate the Mixing Properties of Lipids in Stratum Corneum Model Mixtures

The influence of cholesterol sulfate (CS) and calcium on the phase behavior of lipid mixtures mimicking the stratum corneum (SC) lipids was examined using vibrational spectroscopy. Raman microspectrocopy showed that equimolar mixtures of ceramide, palmitic acid, and cholesterol underwent a phase transition in which, at low temperatures, lipids formed mainly a mosaic of microcrystalline phase-separated domains, and above 45°C, a more fluid and disordered phase in which the three lipid species were more miscible. In the presence of Ca2+, there was the formation of fatty acid-Ca2+ complexes that led to domains stable on heating. Consequently, these lipid mixtures remained heterogeneous, and the fatty acid molecules were not extensively involved in the formation of the fluid lipid phase, which included mainly ceramide and cholesterol. However, the presence of CS displaced the association site of Ca2+ ions and inhibited the formation of domains formed by the fatty acid molecules complexed with Ca2+ ions. This work reveals that CS and Ca2+ modulate the lipid mixing properties and the lipid order in SC lipid models. The balance in the equilibria involving Ca2+, CS, and fatty acids is proposed to have an impact on the organization and the function of the epidermis.