WIDE-ANGLE X-RAY-DIFFRACTION OF HUMAN STRATUM-CORNEUM - EFFECTS OF HYDRATION AND TERPENE ENHANCER TREATMENT

Wide-angle X-ray-diffraction experiments were used to investigate the molecular organization of barrier components of human stratum corneum. Diffraction lines related to the side-by-side lipid packing arrangeme nts in the intercellular bilayers were identified as were patterns ari sing from secondary protein structures in intracellular keratin. Refle ctions were also identified which may be produced by proteins in the c orneocyte envelopes. The effects of hydration on stratum corneum struc ture were monitored using 0, 20-40, 40-60, 60-80 and approximately 300 % hydrated samples. The packing arrangements in the intercellular lipi d bilayers remained the same over the entire hydration range, as did k eratin structures. A new diffraction ring, attributable to liquid wate r, was produced by 300% hydrated samples with a repeat spacing of 0.35 to 0.30-0.29 nm. The effects of three terpene enhancers, (+)-limonene , nerolidol and 1,8-cineole, on stratum corneum structure were monitor ed, Treatment with each of the terpenes produced additional reflection s which were attributed to the presence of the respective liquid enhan cers within the stratum corneum, (+)-Limonene produced an additional r eflection at 0.503-0.489 nm, nerolidol, an additional reflection at 0. 486-0.471nm and 1,8-cineole, an intense reflection at 0.583-0.578 nm. Reflections characteristic of gel-phase lipids and crystalline lipids also remained after all terpene treatments. These results provide no c lear evidence of lipid bilayer disruption by the terpenes and suggest that areas of liquid terpene exist within the stratum corneum. The mec hanisms underlying propylene glycol synergy with terpene enhancers wer e investigated. Treatment of stratum corneum with each terpene mixed w ith propylene glycol gave rise to two additional reflections. One refl ection, always positioned at 0.452-0.448 nm, had been observed in cont rol studies following propylene glycol treatment and may have been ass ociated with bilayer structures disrupted by propylene glycol or alter ed keratin structures. The second reflection was developed by the resp ective terpene enhancer. For example, treatment with a 1,8-cineole/pro pylene glycol mixture produced reflections at 0.457-0.451nm (propylene glycol-disrupted lipids or altered keratin) and 0.591-0.578 nm (liqui d 1,8-cineole). Since the reflection at 0.452-0.448 nm was unaffected by co-application of propylene glycol with terpene enhancers, this stu dy offers no evidence to support the theory that propylene glycol syne rgy with the terpenes occurs through enhanced lipid disruption.