Rate constants for reactions of horseradish peroxidase compounds I and II with 4-substituted arylboronic acids

Abstract

The rate constants for the reactions of horseradish peroxidase compound I (k₁) and compound II (k₂) with three 4-substituted arylboronic acids, which enhance chemiluminescence in the horseradish peroxidase catalyzed oxidation of luminol by hydrogen peroxide, were determined at pH 8.6, total ionic strength 0.11 M, using stopped-flow kinetic measurements. For comparison, the rate constants of the reactions of 4-iodophenol with compounds I and II were also determined under the same experimental conditions. The three arylboronic acid derivatives and their rate constants are: 4-biphenylboronic acid, k₁ = (1.21 ± 0.08) × 10⁶ M⁻¹ s⁻¹, k₂ = (4.6 ± 0.2) × 10⁵ M⁻¹ s⁻¹; 4-bromophenylboronic acid, k₁ = (5.5 ± 0.2) × 10⁴ M⁻¹ s⁻¹, k₂ = (3.6 ± 0.2) × 10⁴ M⁻¹ s⁻¹; and 4-iodophenylboronic acid, k₁ = (1.1 ± 0.2) × 10⁵ M⁻¹ s⁻¹, k₂ = (1.3 ± 0.1) × 10⁴ M⁻¹ s⁻¹. 4-Biphenylboronic acid, which shows comparable luminescent enhancement to 4-iodophenol, has the highest reactivity in the reduction of both compounds I and II among the three arylboronic acid derivatives tested.