Substituent Effect on the Oxidation of Phenols and Aromatic Amines by Horseradish Peroxidase Compound I

Abstract

A stopped-flow kinetic study shows that the reduction rate of horseradish peroxidase compound I by phenols and aromatic amines is greatly dependent upon the substituent effect on the benzene ring. Moreover it has been possible to relate the reduction rate constants with the ionization constants of monosubstitutcd substrates by a linear free-energy relationship (Hammett equation). The correlation of log (rate constants) with σ values (Hammett equation) and the absence of correlation with σ⁺ values (Okatnoto-Brown equation) can be explained by a mechanism of aromatic substrate oxidations, in which the substrate gives an electron to the enzyme compound I and simultaneously loses a proton. The analogy which has been made with oxidation potentials of phenols or anilines strengthens the view that the reaction is only dependent on the relative ease of oxidation of the substrate. The rate constant obtained for p-aminophenol indicates that a value of 2.3 × 10⁸ M⁻¹ s⁻¹probably approaches the diffusion-controlled limit for a bimolecular reaction involving compound I and an aromatic substrate.