Rates of reaction of indoleacetic acids with horseradish peroxidase compound I and their dependence on the redox potentials.

The rates of reaction of seven indole-3-acetic acid derivatives with horseradish peroxidase compound 1 at pH 5 were measured by stopped flow, and the reduction potentials and pKa of their radical cations were determined by pulse radiolysis. Reasonable correlation of these properties with Hammett substituent parameters was found, but not with Brown-Okamoto (theta +) parameters. The rates of reaction with compound I correlate well with the reduction potentials under the same conditions, with rates of reaction that increase by ca. 2.5 orders of magnitude with a 100 mV decrease in the reduction potential. This relationship is in agreement with that previously estimated for the reaction of compound I with phenols and anilines, suggesting that the rate of reaction depends solely on the reduction potential of the substrate radical, even for compounds of dissimilar structure.