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 arrangements in the intercellular bilayers were identified as were patterns arising from secondary protein structures in intracellular keratin. Reflections were also identified which may be produced by proteins in the corneocyte envelopes. The effects of hydration on stratum corneum structure were monitored using 0, 20-40, 40-60, 60-80 and approximately 300% hydrated samples. The packing arrangements in the intercellular lipid bilayers remained the same over the entire hydration range, as did keratin structures. A new diffraction ring, attributable to liquid water, 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 monitored. Treatment with each of the terpenes produced additional reflections which were attributed to the presence of the respective liquid enhancers within the stratum corneum. (+)-Limonene produced an additional reflection at 0.503-0.489 nm, nerolidol, an additional reflection at 0.486-0.471 nm 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 clear evidence of lipid bilayer disruption by the terpenes and suggest that areas of liquid terpene exist within the stratum corneum. The mechanisms underlying propylene glycol synergy with terpene enhancers were investigated. Treatment of stratum corneum with each terpene mixed with propylene glycol gave rise to two additional reflections. One reflection, always positioned at 0.452-0.448 nm, had been observed in control studies following propylene glycol treatment and may have been associated with bilayer structures disrupted by propylene glycol or altered keratin structures. The second reflection was developed by the respective terpene enhancer. For example, treatment with a 1,8-cineole/propylene glycol mixture produced reflections at 0.457-0.451 nm (propylene glycol-disrupted lipids or altered keratin) and 0.591-0.578 nm (liquid 1,8-cineole). Since the reflection at 0.452-0.448 nm was unaffected by co-application of propylene glycol with terpene enhancers, this study offers no evidence to support the theory that propylene glycol synergy with the terpenes occurs through enhanced lipid disruption.