MODELS OF STRATUM-CORNEUM INTERCELLULAR MEMBRANES - H-2 NMR OF MACROSCOPICALLY ORIENTED MULTILAYERS

Deuterium NMR was used to characterize model membrane systems approxim ating the composition of the intercellular lipid lamellae of mammalian stratum corneum (SC). The SC models, equimolar mixtures of ceramide:c holesterol: palmitic acid (CER:CHOL:PA) at pH 5.2, were contrasted wit h the sphingomyelin:CHOL:PA (SPM:CHOL:PA) system, where the SPM differ s from the CER only in the presence of a phosphocholine headgroup. The lipids were prepared both as oriented samples and as multilamellar di spersions, and contained either perdeuterated palmitic acid (PA-d(31)) or [2,2,3,4,6-H-2(5)]CHOL (CHOL-d(5)).SPM:CHOL.PA-d(31) formed liquid -ordered membranes over a wide range of temperatures, with a maximum o rder parameter of approximately 0.4 at 50 degrees C for positions C3-C 10 (the plateau region). The quadrupolar splitting at C2 was significa ntly smaller, suggesting an orientational change at this position, pos sibly because of hydrogen bonding with water and/or other surface comp onents. A comparison of the longitudinal relaxation times obtained at theta = 0 degrees and 90 degrees (where theta is the angle between the normal to the glass plates and the magnetic field) revealed a signifi cant T,, anisotropy for all positions. In contrast to the behavior obs erved with the SPM system, lipid mixtures containing CER exhibited a c omplex polymorphism. Between 20 and 50 degrees C, a significant portio n of the entire membrane (as monitored by both PA-d,, and CHOL-d,) was found to exist as a solid phase, with the remainder either a gel or l iquid-ordered phase. The proportion of solid decreased as the temperat ure was increased and disappeared entirely above 50 degrees C. Between 50 and 70 degrees C, the membrane underwent a liquid-ordered to isotr opic phase transition. These transitions were reversible but displayed considerable hysteresis, especially the conversion from a fluid phase to solid. The order profiles, relaxation behavior, and angular depend ence of these parameters suggest strongly that both the liquid-ordered CER- and SPM-membranes are bilayers. The unusual phase behavior obser ved for the CER-system, particularly the observation of solid-phase li pid at physiological temperatures, may provide insight into the functi oning of the permeability barrier of stratum corneum.