Chemical messengers: Mediated oxidations with the enzyme laccase

The use of low molecular-weight compounds (viz., mediators) in combination with fungal laccase makes the enzyme suitable for the oxidation of ‘non-natural’ non-phenolic substrates. Benzyl alcohols are thus oxidised to carbonylic products by laccase/mediator systems in the presence of oxygen, although laccase cannot oxidise these substrates directly. The reaction is carried out by the oxidised form of the mediator (Medox), generated on its interaction with laccase, and the structure of the Medox species is crucial for the mechanism of the ensuing nonenzymatic oxidation of the substrate. 1-Hydroxybenzotriazole (HBT), N-hydroxyphthalimide (HPI), violuric acid (VLA) and TEMPO have been investigated as mediators, and experimental evidence is provided that enables the radical hydrogen atom transfer route with the laccase/HBT, laccase/HPI and laccase/VLA systems to be assessed unambiguously, although the laccase/TEMPO system follows a different and ionic oxidation route.