Structure of human stratum corneum as a function of temperature and hydration: A wide-angle X-ray diffraction study

Abstract

Wide-angle X-ray diffraction measurements were performed as a function of temperature and hydration on human stratum corneum. Reflections located close to the beam stop ( d > 1 nm) could be explained on the basis of two lamellar structures. These findings confirmed results obtained with small-angle X-ray diffraction (Bouwstra et al., J. Invest. Dermatol., 97 (1991b) 1005–1012), in which the several complicated diffraction peaks could be ascribed to two lamellar structures with repeat distances of 6.4 and 13.4 nm. In the wide-angle region of the diffraction pattern, we found two diffraction lines, that are characteristic of orthorhombic and hexagonal lateral packing of the lipids. Moreover, the strongest reflections of polycrystalline cholesterol were detected. Upon heating stratum corneum, a phase transition from an orthorhombic to a hexagonal phase in the lateral packing of the lipids at 40°C was observed. Between 60 and 75°C disordering of the lamellar structure occurred, while the hexagonal lateral packing was still present. Between 75 and 95°C, the hexagonal lateral packing disappeared. No changes were found in the reflection lines characteristic of protein. After recrystallisation of the lipids from 120 or 95°C, several higher diffraction orders of the lamellar structure with a repeat distance of 13.4 nm were found. Variation in the hydration level between 6 and 40% w/w did not lead to a shift in the position of the reflections characteristic of lateral packing. In addition, in the recent paper cited above, swelling of the bilayers was not observed, indicating that no water is absorbed between the lamellar regions of human stratum corneum.