Direct heterogeneous electron transfer reactions of fungal laccases at bare and thiol-modified gold electrodes

Mediatorless (direct) electron transfer between bare and thiol-modified gold electrodes and fungal laccases from different sources has been demonstrated. The electrochemical activity of the enzymes from basidiomycetes Trametes hirsuta, Trametes ochracea, and Cerrena maxima under aerobic and anaerobic conditions can clearly be observed using cyclic voltammetry and spectroelectrochemistry. Bioelectroreduction of oxygen by T. hirsuta laccase immobilized on amino-thiophenol-modified gold electrodes, starting at +625 mV vs. NHE, is demonstrated and differences in bioelectrocatalysis of the enzyme immobilized on bare and thiol-modified electrodes are shown. It was found that hydrogen peroxide was one of the products of oxygen electroreduction on gold electrodes modified with fungal laccases, whereas no significant peroxide formation was observed for T. hirsuta laccase immobilised on thiol-modified gold electrodes. Thus, a hypothesis about two different mechanisms of oxygen electroreduction by fungal laccases adsorbed on bare and thiol-modified electrodes is proposed.