Fenton-like Inactivation of Tobacco Peroxidase Electrocatalysis at Negative Potentials

The effect of the operational potential on the stability of electrochemical biosensors is particularly relevant in the case of peroxidase biosensors, because these enzymes can catalyze the reduction of hydrogen peroxide via either a high-potential redox cycle [involving Compound I, Compound II, and Fe(III)] or a low-potential redox cycle [involving Fe(III) and Fe(II)]. Herein, it is shown that recombinant tobacco peroxidase immobilized on a graphite surface displays two well-separated electrocatalytic waves, associated with each of these two catalytic cycles. While continuous scanning in the high potential region does not alter significantly the electrocatalytic current, it is shown that just modest incursions into the low-potential region cause an irreversible loss of the electrocatalytic response. A quantitative analysis of the extent of inactivation as a function of time, potential, and hydrogen peroxide concentration is shown to be consistent with a fast inactivation caused by hydroxyl radicals generated by a Fenton-like mechanism. Accordingly, the inactivation process is shown to slow via the addition of radical scavengers to the solution. Preliminary results indicate that the same inactivation process may also be present in horseradish peroxidase-modified electrodes.