The kinetics and mechanism of redox reactions of the iron(III) complex of mesoporphyrin(IX)

Abstract

The iron(III) complex of mesoporphyrin(IX), mesoferriheme, MFH, has been studied as a peroxidase enzyme model. Investigations show that mesoferriheme reacts rapidly with hypochlorite ion (OCl$\sp{-1}$) to form an "intermediate state", which is a spectroscopically distinct oxidized heme species viewed as an analogue of peroxidase compound I. The lack of pH dependence of these spectra implies that only one form of intermediate is produced. Stoichiometric and kinetic studies of these reactions indicate that two moles of mesoferriheme are stoichiometrically equivalent to one mole of two-electron oxidant. This is consistent with previous observations in the reaction of deuteroferriheme(DFH) with oxygen-donor oxidizing agents. It was found that the rate constant for the reaction of monomeric heme with hypochlorite ion, k$\sb{\rm m}$, is independent of pH in the range 6.92 to 8.88. Its value, k$\sb{\rm m}$ = 3.72 x 10$\sp6$ M$\sp{-1}$ s$\sp{-1}$ at pH 6.92, is comparable to that obtained in the DFH-OCl$\sp-$ system (3.18 x 10$\sp6$ M$\sp{-1}$ s$\sp{-1}$ at pH 6.97). This implies that the ethyl substituents, which are located in the 2,4-positions of the porphyrin ring and which have been found to influence the degree of heme dimerization, do not play a major role in the catalytic activity of the ferriheme toward the oxygen donor. The mesoferriheme derived intermediate (MFH-I) decomposes faster than that of DFH in the absence of added reductants. Unlike the DFH system, the spontaneous regeneration of MFH was typically accompanied by 10-12% heme destruction via porphyrin ring degradation. A destruction mechanism is proposed which involves an initially formed oxidation product of heme (Fe$\sp{\rm v}$O) as the susceptible species toward degradation. As in the DFH system, biphasic kinetics were observed for the spontaneous regeneration of MFH from MFH-I. Data are compatible with two mechanistic models, namely, parallel and series first-order reactions. Kinetic analysis alone does not allow a distinction between these models. A temperature-dependence study of the DFH system was carried out to examine the regeneration mechanism. The spectrum of the DFH oxidation product was found to be independent of temperature. Rate constants for the DFH oxidation (activation energy = 10 kcal/mole), however, do change with temperature. A single form of "intermediate state" is suggested which indicates regeneration to proceed via the series model. The MFH-I exhibits strong peroxidatic activity in its reaction with reductants such as ascorbic acid, hydroquinone and phenol. In particular, the oxidation of phenol by MFH-I was studied through stopped-flow spectrophotometry. The reaction is biphasic and second-order rate constants for the phenol reduction of MFH-I were found to be k$\sb{\rm 1ph}$ = 1.35 $\times$ 10$\sp4$ M$\sp{-1}$ s$\sp{-1}$ and k$\sb{\rm 2ph}$ = 2.67 $\times$ 10$\sp3$ M$\sp{-1}$ s$\sp{-1}$ at pH 6.88, t = 25$\sp\circ$C.