Wide-angle X-ray Diffraction of Human Stratum Corneum: Effects of Hydration and Terpene Enhancer Treatment

Abstract

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.

Oxide Terpenes as Human Skin Penetration Enhancers of Haloperidol from Ethanol and Propylene Glycol and Their Modes of Action on Stratum Corneum.

“…6). The same observation was reported earlier with 100% v/v PG 25) and it was also reported that 100% v/v PG did not decrease lipid bilayer reflections of SC in small angle X-ray diffraction 8) and these observations were attributed to the disruption of lipid bilayers whilst leaving overall bilayer structure intact. The thermogram of SC treated with pinene oxide in 100% v/v PG for 48 h showed a decrease in the peak areas of T1 and T2.…”

“…PssϭJ · Ta/Clp (8) Clp is plasma clearance of HP, which is 36.5 l/h. 16) Therapeutic transdermal daily dose (Td) was calculated from the equation 9 where F is the bioavailabilty in percentage after oral administration and Do is the oral daily dose TdϭDo · F/100 (9) The bioavailability of HP is 60% on average 17) and its daily maintenance oral dose is 3-10 mg. 18) Permeated daily doses (Dss) from 16 cm 2 transdermal patch was calculated from equation 10, where t is 24 h…”

Micron-scale assessment of molecular lipid organization in human stratum corneum using microprobe X-ray diffraction

“…The former ring is insensitive to lipid extraction and is therefore attributed to protein component of SC, in particular, the soft keratin ( 4,16 ). This repeat distance is likely related to the molecular arrangement of keratin in the fi lament structure; for example, to the inter-chain distance ( 4,(16)(17)(18). The scattering signal corresponding to the 0.95 nm spacing is noticeably polarized, the intensity in the meridional plane being about 12 nm because of its relatively low intensity (note that the delipidized profi le in Fig.…”

“…The anisotropy can be explained by relative preference of the keratin fi laments to be oriented closer to the plain of corneocytes; i.e., parallel to the SC surface ( 19 ). The second diffuse ring around 0.46 nm is a superposition of the scattering signal from the secondary structure elements of keratin ( 4,16,18 ) and the diffuse contribution from the fl uid-phase lipids characterized by an average inter-chain distance of 0.46 nm.…”

In Vivo Immunomodulatory, Cumulative Skin Irritation, Sensitization and Effect of d-Limonene on Permeation of 6-Mercaptopurine through Transdermal Drug Delivery

“…It has also been reported that terpenes increase the drug percutaneous permeation mainly by disrupting the intercellular packing of stratum corneum lipids. 24,25) Here, the results have proved hydrocarbon d-limonene is a potential chemical enhancer and more effective for 6-MP TDD. The effectiveness of hydrocarbon d-limonene has also been demonstrated for the other lipophilic drugs such as ketoprofen, indomethacin and estradiol.…”