Phenothiazine Drugs as Redox Mediators in Horseradish Peroxidase Bioelectrocatalysis

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Abstract
The homogeneous bioelectrocatalytic response between the enzyme Horseradish Peroxidase (HRP) and two structurally related phenothiazine drugs, promazine (PMZ), and promethazine (PMTZ), has been studied in acetate buffer pH 4.5, in the presence of hydrogen peroxide. Experiments were realized by cyclic voltammetry (CV) and chronoamperometry (CA) at a solid paraffin based carbon paste electrode. Distinct CV and CA patterns were observed for those structurally related isomers. The molecule PMZ behaved classically, as most phenothiazine drugs, with formation of the corresponding sulfoxide via an intermediate, relatively stable and electrochemically reversible, PMZ cation radical. Catalytic currents were obtained during the electroreduction of the latter due to the presence of the HRP/H2O2 system in solution which regenerated the cation radical. PMTZ electro- and enzymatic oxidation gave no corresponding stable cation radical but the molecule suffered a lateral chain breakdown with liberation of the phenothiazine nucleus (PHZ). The latter behaved reversibly at the electrode and catalytic currents were observed during the phenothiazine cation radical electroreduction. The overall rate constant between the phenothiazine drugs and HRP/H2O2 was determined by CA giving 3.8 × 105 and 2.9 × 105 M−1s−1 for PMZ and PMTZ(PHZ), respectively.

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