Methyl syringate: An efficient phenolic mediator for bacterial and fungal laccases

► The mechanism for phenolics oxidation by bacterial laccases is proposed for the first time. ► The efficiency of laccase-mediator system using phenolics is independent of the enzymes properties. ► Reactivity vs. stability of phenoxy radicals is a key issue in the laccase-mediator systems. ► C–O coupling products were identified from a newly putative radical coupling route. ► A catalytic cycle of laccase-mediator systems with an interplay of competitive routes is proposed. The aim of the present work is to provide insight into the mechanism of laccase reactions using syringyl-type mediators. We studied the pH dependence and the kinetics of oxidation of syringyl-type phenolics using the low CotA and the high redox potential TvL laccases. Additionally, the efficiency of these compounds as redox mediators for the oxidation of non-phenolic lignin units was tested at different pH values and increasing mediator/non-phenolic ratios. Finally, the intermediates and products of reactions were identified by LC–MS and 1H NMR. These approaches allow concluding on the (1) mechanism involved in the oxidation of phenolics by bacterial laccases, (2) importance of the chemical nature and properties of phenolic mediators, (3) apparent independence of the enzyme’s properties on the yields of non-phenolics conversion, (4) competitive routes involved in the catalytic cycle of the laccase-mediator system with several new C–O coupling type structures being proposed.