Molecular Interaction between Intercellular Lipids in the Stratum Corneum and l-Menthol, as Analyzed by Synchrotron X-Ray Diffraction

l-Menthol increases drug partitioning on the surface of skin, diffusion of drugs in the skin, and lipid fluidity in the stratum corneum and alters the rigidly arranged lipid structure of intercellular lipids. However, l-menthol is a solid at room temperature, and it is difficult to determine the effects of l-menthol alone. In this study, we vaporized l-menthol in order to avoid the effects of solvents. The vaporized l-menthol was applied to the stratum corneum or lipid models comprising composed of ceramides (CER) [EOS], the longest lipid acyl chain of the ceramides in the stratum corneum lipids that is associated with the barrier function of the skin; CER [NS], the shorter lipid acyl chain of the ceramides, and the most components in the stratum corneum of the intercellular lipids that is associated with water retention in the intercellular lipid structure of the stratum corneum; cholesterol; and palmitic acid. Synchrotron X-ray diffraction, differential scanning calorimetry, and attenuated total reflection Fourier transform infrared spectroscopy analyses revealed that the lipid models were composed of hexagonal packing and orthorhombic packing structures of different lamellar periods. Taken together, our results revealed that l-menthol strongly affected the lipid model composed of CER [EOS]. Therefore, l-menthol facilitated the permeation of drugs through the skin by liquid crystallization of the longer lamellar structure. Importantly, these simple lipid models are useful for investigating microstructure of the intercellular lipids in the stratum corneum.