Kinetics of laccase-catalysed TEMPO oxidation

The oxidation of TEMPO (2,2,6,6-tetramethyl-piperidine-1-oxyl radical) has been studied in the presence of recombinant laccases (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) from Polyporus pinsitus (rPpL), Myceliophthora thermophila (rMtL), Coprinus cinereus (rCcL) and Rhizoctonia solani (rRsL) in buffer solution pH 4.5–7.3 and at 25 °C. At pH 5.5 the oxidation constant calculated from the initial rate of TEMPO oxidation was 1.7 × 10⁴, 1.4 × 10³, 7.8 × 10² and 5.2 × 10² M^?1 s^?1 for rPpL, rRsL, rCcL and rMtL, respectively. The maximal activity of rPpL-catalysed TEMPO oxidation was at pH 5.0. The pK_a obtained in neutral pH range was 6.2. The reactivity of laccases is in a good agreement with laccases copper type I redox potential.

TEMPO oxidation rate increased 541 times in the presence of 10-(3-propylsulfonate) phenoxazine (PSPX). The model of synergistic TEMPO and PSPX oxidation was proposed. Experimentally obtained rate constants for rPpL-catalysed PSPX oxidation were in a good agreement with those calculated from the synergistic model, therefore confirming the feasibility of the model. The acceleration of TEMPO oxidation with high reactive laccase substrates opens new possibilities for TEMPO application as a mediator.