Efficient dimerization of (−)-epigallocatechin gallate using nonthermal plasma as potent melanogenesis inhibitors

(−)-Epigallocatechin gallate (EGCG), which is the most abundant bitter polyphenol in green tea (Camellia sinensis) leaves, is generally unstable under oxidative conditions. In this study, the efficient dimerization of EGCG was successfully achieved without changes in stereochemistry by treatment with nonthermal dielectric barrier discharge (DBD) plasma. The stereochemically pure dimers 2 and 3 connected by a methylene linkage exhibited significantly enhanced melanogenesis inhibitory activities compared to the parent EGCG and the active compounds 2 and 3 suppressed cellular tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 expression in B16F10 melanoma cells, which could be considered one of the mechanisms of action. These results indicate a new method for efficient dimerization of stereochemically active depigment agent induced by DBD plasma treatment.