The kinetics of the reaction of superoxide radical with Fe(III) complexes of EDTA, DETAPAC and HEDTA.

To gain an understanding of the mechanism by which the hydroxyl free radical can arise in superoxide generating systems and learn how different chelaters of iron can inhibit this reaction, a pulse radiolysis kinetic study of the reaction of O;- with Fe(III)EDTA, Fe(III)HEDTA and Fe(III)DETAPAC (or DTPA)was undertaken. Superoxide reacts readily with Fe(III)EDTA and Fe(III)HEDTA with a pH-dependent second-order rate constant having values of 1.9 x lo6 M-l. s-’ and 7.6 x 16 M-‘. s-’ at pH 7,respectively. However, the rate constant for the reaction of O;- with Fe(III)DETAPAC was found to be much slower, the upper limit for the rate constant being 104 M-l. s-l. These results in conjunction with spin-trapping experiments with Fe(II)EDTA, Fe(II)HEDTA, Fe(II)DETAPAC and Hz02 suggests that DETAPAC inhibits the formation of 'OH by slowing the reduction of Fe(II1) to Fe(I1) and not by inhibiting the Fenton reaction.