Oxidative coupling reaction of arbutin and gentisate catalyzed by horseradish peroxidase

Horseradish peroxidase catalyzed an oxidative coupling reaction of 4′-hydroxyphenyl β-glucoside (arbutin) and 2,5-dihydroxybenzoic acid sodium salt (gentisate) using H2O2 as an electron acceptor to yield a precipitating yellow compound. An approximate arbutin/gentisate ratio of 1:2 was effective for the synthesis. The addition of 100–300 mM H2O2 to the mixtures of 100 mM arbutin and 200 mM gentisate attained optimized yields of 50–60% in the initial arbutin. Mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance data revealed that the precipitated compound was a novel glycoside bound between the C3′-position of the hydroxyphenyl moiety of arbutin and the C6-position of gentisate, followed by the intramolecular esterification between the phenolic hydroxyl group of arbutin moiety and the carboxyl group of gentisate moiety. The product exerted anti-oxidation and competitive inhibition against mushroom tyrosinase higher than arbutin, while it inhibited mouse melanoma tyrosinase lower than arbutin.