Kinetic simulation studies on the transient formation of the oxo-iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)-porphyrin with hydrogen peroxide in aqueous solution.

Abstract

High-valent oxo-iron(IV) species are commonly proposed as the key intermediates in the catalytic mechanisms of iron enzymes. Water-soluble iron(III) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin (Fe(III)TMPyP) has been used as a model of heme-enzyme to catalyse the hydrogen peroxide (H(2)O(2)) oxidation of various organic compounds. However, the mechanism of the reaction of Fe(III)TMPyP with H(2)O(2) has not been fully established. In this study, we have explored the kinetic simulation of the reaction of Fe(III)TMPyP with H(2)O(2) and of the catalytic reactivity of FeTMPyP in the luminescent peroxidation of luminol. According to the mechanism that has been established in this work, Fe(III)TMPyP is oxidized by H(2)O(2) to produce (TMPyP)(*+)Fe(IV)[double bond]O (k1 = 4.5 x 10(4)/mol/L/s) as a precursor of TMPyPFe(IV)[double bond]O. The intermediate, (TMPyP)(*+)Fe(IV)[double bond]O, represented nearly 2% of Fe(III)TMPyP but it does not accumulate in sufficient concentration to be detected because its decay rate is too fast. Kinetic simulations showed that the proposed scheme is capable of reproducing the observed time courses of FeTMPyP in various oxidation states and the decay profiles of the luminol chemiluminescence. It also shows that (TMPyP)(*+)Fe(IV)[double bond]O is 100 times more reactive than TMPyPFe(IV)[double bond]O in most of the reactions. These two species are responsible for the initial sharp and the sustained luminol emissions, respectively.