Comparing the catalytic efficiency of some mediators of laccase

The mechanism of oxidation of non-phenolic substrates by laccase/mediators systems has been investigated. Oxidation of 4-methoxybenzyl alcohol (1), taken as a benchmark reaction, enabled us to compare and to rank the relative ability of twelve mediators: TEMPO proved most effective, and a ionic mechanism is suggested for its action. Data on intermolecular selectivity of substrate oxidation are in favour of an electron transfer (ET) mechanism in the case of ABTS-mediated oxidations, and of a radical mechanism in HBT- and HPI-mediated reactions. Investigation by cyclic voltammetry (CV) of some of the mediators revealed that an important role in determining the mechanism of substrate oxidation may be played by the stability of the oxidised form of the mediator, as well as by its redox potential.