Abstract
The essential role of water in elasticity of the stratum corneum (SC) has been well acknowledged, while water-holding property of the SC is mainly associated with natural moisturizing factors (NMF) and intercellular lipids. Because available evidence indicates that water itself has no substantial capacity of increasing elasticity in the SC depleted of NMF, it has been speculated that a specific intermolecular interaction between keratin fibers and NMF is required for the complete achievement of elasticity due to association with water molecules. In order to elucidate precise molecular mechanisms underlying SC elasticity, we have measured the molecular dynamics of chemical residues within keratin fibers of human plantar SC under various conditions by cross polarization/magic angle spinning (CP/MAS) 13C-NMR technique in which the elasticity of keratin fibers can be evaluated by comparing the intensities of their NMR spectra.The intensities of NMR spectra responsible for amide bond-carbonyl, Cα methine and sidechain aliphatic carbons in the intact SC were found to decrease markedly with increasing water content up to 30% of dry SC and then to reach a constant value over 30%. Lipid extraction of intact SC with acetone/ether (1/1) did not induce any significant change in the NMR spectrum, whereas an additional treatment with water that releases NMF consisting mainly of amino acids, caused the SC to lose elasticity as revealed by significantly increased intensity of the spectrum even in the presence of excess water. The observed decrease in elasticity of the SC was found to recover after treatment with basic and neutral amino acids, but not with acidic amino acid. Parallel studies for complex modulus on elasticity of SC sheet by the rheovibron also demonstrated that removal of NMF reduces elasticity of the SC whose effect is reversible with the application of basic and neutral amino acids. These findings suggest that structural proteins mainly consisting of keratin acquire their elasticities by reducing intermolecular forces between keratin fibers with the help of hydrated NMF.
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