RATE CONSTANTS FOR REACTIONS OF HORSERADISH-PEROXIDASE COMPOUND-I AND COMPOUND-II WITH 4-SUBSTITUTED ARYLBORONIC ACIDS

The rate constants for the reactions of horseradish peroxidase compound I (k(1)) and compound II (k(2)) 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, k(1) = (1.21 +/- 0.08) x 10(6) M(-1) s(-1), k(2) = (4.6 +/- 0.2) x 10(5) M(-1) s(-1) 4-bromophenylboronic acid, k(1) = (5.5 +/- 0.2) x 10(4) M(-1) s(-1), k(2) = (3.6 +/- 0.2) x 10(4) M(-1) s(-1); and Ciodophenylboronic acid, k(1) = (1.1 +/- 0.2) x 10(5) M(-1) s(-1), k(2) = (1.3 +/- 0.1) x 10(4) 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.