Catalytic efficiency of some mediators in laccase-catalyzed alcohol oxidation

The enzyme laccase oxidises phenolic groups of lignin but not the non-phenolic ones. Redox mediators activate laccase towards the non-phenolic groups, particularly the benzyl alcohols. The oxidation step is performed by the oxidised form of the mediator, generated on its interaction with laccase. The oxidised mediator can follow an electron transfer, a radical hydrogen atom transfer or an ionic mechanism in the oxidation of the non-phenolic subunits. Support for these conclusions is provided by (i) investigating the product pattern with suitable probe substrates, (ii) measuring the intramolecular kinetic isotope effect. Determination of electrochemical properties and bond dissociation energies via semiempirical calculations enabled us to rationalise the origin of the different mechanistic behaviour of the mediators. Finally, a comparison of different laccase-mediator-systems (LMS), when applied to the delignification of wood pulp, indicates violuric acid as the most efficient mediator, in an oxidation that is selectively directed towards lignin only.