Chronopotentiometric study of laccase-catalysed oxidation of quinhydrone microcrystals immobilised on a gold electrode surface and of the oxidation of a phenol-derivatised graphite electrode surface

Solid microcrystalline quinhydrone immobilised on the surface of a gold electrode was oxidised under the catalytic action of the enzyme laccase. In a buffered medium, this enzymatic reaction was followed by chronopotentiometry to monitor the conversion of hydroquinone to quinone. From the potential time dependences, the degree of conversion versus time was calculated. The reaction exhibits accelerating kinetics. A variation of the laccase concentration (activity) shows that there is a maximum rate at an activity of 0.023 U ml⁻¹. Further increase in activity leads to a pronounced decrease in reaction rate. When a graphite electrode is first oxidised by boiling nitric acid, then electrochemically reduced in an aqueous potassium nitrate solution, it contains phenolic surface groups. These surface groups can be oxidised aerobically under the influence of laccase and the reaction rate versus laccase activity curve resembles that for quinhydrone oxidation. These experiments indicate that laccase can interact with a solid surface directly.