Efficacy of mediators for enhancing the laccase-catalyzed oxidation of aqueous phenol

Laccase from Trametes versicolor was characterized in terms of its catalytic stability and reactivity towards phenol as a model substrate in the absence or presence of four common redox mediators. Laccase was inactivated when incubated in the presence of mediators including 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), violuric acid (VA), 1-hydroxybenzotriazole (HBT) and 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO) at pH 6 and 25 °C. Monophasic and biphasic exponential decay expressions were successfully used to model laccase inactivation under these conditions. Phenol transformation in the absence of mediators was optimum at pH 6. The addition of HBT or VA moderately enhanced phenol transformation between pH 3 and 5. TEMPO and ABTS significantly enhanced both the extent and rates of phenol transformation, particularly at pH 6. Over the course of a reaction, some mediator species gradually transformed into compounds that were no longer active as mediators. In general, the use of mediators enhanced phenol transformation; however, in certain instances their practical application may be limited since they contribute to laccase inactivation and considerable quantities are required.