PHASE-BEHAVIOR OF ISOLATED SKIN LIPIDS

Ceramides were isolated from the pig stratum corneum (SC) and mixed in varying molar ratios with either cholesterol or with cholesterol and free fatty acids. The phase behavior of the mixtures was studied by sm all-(SAXD) and wide-angle (WAXD) X-ray diffraction. Ceramides alone di d not exhibit a long range ordering. Upon addition of cholesterol to c eramides, lamellar phases were formed and a hexagonal lateral packing was detected similar to that seen in intact SC. At a cholesterol/ceram ide molar ratio of 0.1, only one reflection at 5.9 nm was observed. At a cholesterol/ceramide molar ratio of 0.2, three reflections correspo nding to 12.3, 5.56, and 4.26 nm appeared. The reflections were based on two phases. Increasing the cholesterol/ceramide ratio to 0.4, the p eak positions were slightly shifted. The diffraction pattern revealed the presence of two lamellar phases with periodicities of 12.2 and 5.2 nm, respectively. The positions of the peaks remained unchanged when the cholesterol/ceramide ratio was increased up to 1.0. At a cholester ol/ceramide molar ratio of 2.0, the intensity of various peaks based o n the 12.2 nm phase decreased in intensity. The phase behavior of the cholesterol/ceramide mixtures in a ratio between 0.4 and 1.0 was very similar to that found in intact pig SC in which two lamellar phases wi th periodicities of 6.0 and 13.2 nm are present. Our data further indi cate that the formation of the 5.2 nm lamellar phase requires a higher cholesterol content than the formation of the 12.2 nm lamellar phase. Furthermore, when the relative amount of cholesterol is very high, th e 5.2 nm phase is the most pronounced one. Addition of free fatty acid s increased the solubility of cholesterol, indicating the role free fa tty acids may play for the skin barrier function. The phase behavior o f cholesterol/ceramide/fatty acid mixtures was found to be dependent o n the chain length of fatty acids used. Namely, addition of short-chai n free fatty acids (C14-C18) did not change the periodicity of the 12. 2 and 5.2 nm phases, but induced the formation of an additional 4.2 nm phase. In the presence of long-chain free fatty acids (C16-C26), the periodicity of the lamellar phases was slightly increased (to 13.0 and 5.3 nm, respectively) but no additional 4.2 nm phase was formed. Thes e results indicate that the lipid phase behavior of the cholesterol/ce ramide/free fatty acid mixtures closely mimics that of the intact stra tum corneum only in the presence of long-chain free fatty acids.-Bouws tra, J. A., G. S. Gooris, K. Cheng, A. Weerheim, W. Bras, and M. Ponec , Phase behavior of isolated skin lipids.