Acylperoxo radical

A very similar acylperoxo (or alkylperoxo) intermediate has also been postulated in the peroxidase reactions (48). For example, horseradish peroxidase (HRP) is oxidized by hydrogen peroxide to give Compound I, which has been characterized as an oxo porphyrin ir-cation radical, Fe (=0)Pf [Eq. (7)J (49). The major difference between HRP and P-450 is the fifth ligand, the former having imidazole and the latter a thiol from cysteine. 

However, several questions remain to be answered (1) Is the postulated formation of an acylperoxo-iron(III) complex correct (2) If the answer is yes, how can it be proved (3) Does the peroxo-iron(III) porphyrin complex afford an oxoferryl porphyrin cation radical Recently such an acylperoxo-iron(III) porphyrin complex (16) in solution at low temperature has been directly observed and characterized (78). It was found that 16 did indeed react to yield the oxoferryl porphyrin cation radical 14. 

The effect of acid on the formation of 14 was examined by varying the initial amount of peroxy acid [n in Eq. (10)]. The rate of the formation of 14 (A obs) was found to correlate with [H + ]. Furthermore, for a series of peroxybenzoic acids, the substituent effect for the decomposition of 16 showed a good correlation between obs and Hammett or (p = 0.5) (79). In summary, the reaction of Fe(III)TMP with peroxy acids has been shown to give an oxoferryl porphyrin cation radical, 14, via 0-0 bond cleavage of an acylperoxo-iron(III) precursor,... 

P-450 have also been discussed, especially homolysis and heterolysis of the acylperoxo-iron(lll) complex. In both cases, we consider the active species to be oxoferryl porphyrin cation radicals however, many different reactivities exist between peroxidases and P-450. Ortiz de Montellano et al. have proposed that the position of substrates in the active site might depend on the spatial characteristics of the individual enzymes and influence the detailed course of the reaction (139). These propositions should be carefully examined. Scheme XXV illustrates all of the intermediates that have been observed and/or proposed in the oxygen activation mechanism by P-450 and that have been prepared by the model systems. 

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