The molecular mechanisms underlying optical isomer arbutin permeating the skin: The molecular interaction between arbutin and skin components

Arbutin, a typical optical isomer, has garnered widespread acclaim in the whitening cosmetics for its favorable efficacy and safety. However, the molecular mechanisms underlying α-arbutin and β-arbutin permeating across the skin have not elucidated clearly yet. Herein we aimed to unveil how α-arbutin and β-arbutin interacted with keratin or SC lipids, further demonstrating their relationship with their drug permeability. We found that α-arbutin displayed significantly higher drug accumulation into the porcine skin than β-arbutin within 24 h through in vitro permeation test. Moreover, α-arbutin predominantly induced the alternations of secondary structure of amide II during the drug permeation, which was favorable for α-arbutin permeation. On the contrary, β-arbutin exhibited an observable effect on the stretching vibration of SC lipids, possessing a significantly stronger mixing energy, binding energy and compatibility with ceramide (Cer) than that of α-arbutin, which ultimately restricted its permeation. Interestingly, free fatty acids and ceramides of the SC lipids specifically utilized its oxygen atom of carboxyl group to dock the arbutin molecules, enhancing their affinity with β-arbutin, as confirmed by molecular simulation and 13Carbon Nuclear Magnetic Resonance. Nevertheless, a favorable compatibility between α-arbutin and keratin was observed. It was emphasized that the distinct spatial configuration and opposite optical rotation of arbutin was the leading factor impacting the intermolecular force between arbutin and the SC, and resulted in a diverse drug permeation. In cellular and in vivo skin pharmacokinetic studies, α-arbutin also possessed a higher cellular uptake and topical bioavailability than β-arbutin. This study revealed the transdermal permeation mechanisms of optical isomer arbutin at the molecular levels, providing methodological reference for the investigations of permeation behaviors of other isomers with similar spatial configuration.