Effect of pH on direct electron transfer between graphite and horseradish peroxidase

Abstract

The effect of pH on the kinetics of the electroreduction of H 2O 2 catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to −50 mV versus SCE (under steady-state conditions and with an RDE system) and at −50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H 3O +] was shown to enhance significantly the current of the bioelectroreduction of H 2O 2 due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages ( E<0.2 V) H 3O + does not affect the rate of the enzymatic reduction of H 2O 2, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k s, changed from a value of 0.54±0.05 s −1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s −1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k s with increasing [H 3O +] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.