Catalysis of the Haber-Weiss reaction by iron-diethylenetriaminepentaacetate

Abstract

To help settle controversy as to whether the chelating agent diethylenetriaminepentaacetate (DTPA) supports or prevents hydroxyl radical production by superoxide/hydrogen peroxide systems, we have reinvestigated the question by spectroscopic, kinetic, and thermodynamic analyses. Potassium superoxide in DMSO was found to reduce Fe(III)DTPA. The rate constant for autoxidation of Fe(II)DTPA was found (by electron paramagnetic resonance spectroscopy) to be 3.10 M −1 s −1, which leads to a predicted rate constant for reduction of Fe(III)DTPA by superoxide of 5.9 × 10 3 M −1 s −1 in aqueous solution. This reduction is a necessary requirement for catalytic production of hydroxyl radicals via the Fenton reaction and is confirmed by spin-trapping experiments using DMPO. In the presence of Fe(III)DTPA, the xanthine/xanthine oxidase system generates hydroxyl radicals. The reaction is inhibited by both superoxide dismutase and catalase (indicating that both superoxide and hydrogen peroxide are required for generation of HO·). The generation of hydroxyl radicals (rather than oxidation side-products of DMPO and DMPO adducts) is attested to by the trapping of α-hydroxethyl radicals in the presence of 9% ethanol. Generation of HO· upon reaction of H 2O 2 with Fe(II)DTPA (the Fenton reaction) can be inhibited by catalase, but not superoxide dismutase. The data strongly indicate that iron-DTPA can catalyze the Haber-Weiss reaction.