PHASE PROPERTIES OF MIXTURES OF CERAMIDES

Ceramides have been proposed to have a central role in the function of the stratum corneum. Ceramides also influence the phase properties of model skin lipid mixtures, but the relevance of this to the stratum c orneum function is controversial. Because the stratum corneum contains several classes of ceramides, the type of ceramides used in model mix tures of stratum corneum lipid lamellae may be important. Thus, the pr oper ties of alpha-hydroxy fatty acid containing (HFAC) and nonhydroxy fatty acid containing (NFAC) ceramides and their mixtures have been i nvestigated. Ceramides were obtained by the conversion of purified bov ine brain cerebrosides. Isolated, anhydrous HFAC underwent an endother mic solid to liquid transition at 92 degrees C. With hydration, an end othermic transition at 71.8 degrees C was observed which was accompani ed by a reduction in the birefringence. The enthalpy increased from 66 to 89 j/g with a 20-d storage time. These thermal properties are very similar to those observed with hydroxy fatty acid containing cerebros ides. In contrast, anhydrous nonhydroxy fatty acid containing ceramide s underwent a broad endothermic transition over the temperature range of 50-90 degrees C. When hydrated, the initial endothermic transition was interrupted by an exothermic transition that was followed immediat ely by a second endothermic transition. During these thermal changes, there was a loss of birefringence, and with completion of the second e ndothermic transition, a nonbirefringent liquid was observed. NFAC sam ples, stored at 70 degrees C for 5 min, cooled, and then rescanned, di splayed only one endotherm at 75 degrees C. The thermal behavior of mi xtures of HFAC and NFAC was relatively simple, with a progressive decr ease and broadening in the temperature of the phase transition as the proportion of NFAC increased up to weight fractions of NFAC of 0.7. At lower weight fractions, a plateau in the transition temperature as a function of weight fraction was observed. Even at a weight traction of 0.1 HFAC, no transition to a nonbirefringent liquid state was observe d. The high enthalpic changes observed with mixtures of HFAC and NFAC are consistent with the proposed central role that ceramides have in t he mechanical and permeability properties of the skin. Moreover, the m arked difference in the properties of these two similar lipids may hel p to explain some of the properties of the stratum corneum.