Radical ferryl porphyrin cation

This is usually isolated from yeast, and has a molecular weight of 53 000 with one heme b. It catalyzes the oxidation of ferrocytochrome c by hydrogen peroxide. It is the first peroxidase for which the structure has been determined. The imidazole axial ligand is His-174, while Arg-48, Trp-51 and His-52 provide distal catalytic groups. The mechanism involves nucleophilic attack of peroxide on the Fe, loss of the ROOH proton to the imidazole of His-52, and transfer of this proton to the leaving RO group. Compound I of cytochrome c peroxidase is red, and differs from HRPI in that the additional oxidizing equivalent is on a protein residue. ESR and ENDOR spectra have been interpreted in terms of a methionine-centred free radical. One possibility is that a ferryl porphyrin cation radical is formed with cytochrome c peroxidase (it is attractive to assume that this would be common to all peroxidases), but that cytochrome c peroxidase has a readily oxidizable substrate which reduces the porphyrin radical. ... 

Figure 1.6. (a) Energy level diagram and reaction coordinate computed by Yoshizawa et al. for the hydroxylation of camphor by a ferryl-porphyrin cation radical (7). Adapted from ref [67], (b) Reaction profile for oxygen rebound computed by Shaik et al. Adapted from refs [60], [62] and unpublished material from S. Shaik. 

PREPARATION AND CHARACTERIZATION OF OXO-FERRYL PORPHYRIN CATION RADICALS BY MODEL SYSTEMS... 

Low DW, Winkler JR, Gray HB. Photoinduced oxidation of microperoxidase-8 Generation of ferryl and cation-radical porphyrins. / Am Chem Soc 1996 118 117-20. 

Hamachi I,Tsukiji S, Shinkai S, Oishi S. Direct observation of the ferric-porphyrin cation radical as an intermediate in the phototriggered oxidation of ferric- to ferryl-heme tethered to Ru(bpy)3 in reconstituted myoglobin. J Am Chem Soc 1999 121 5500-6. 

Fig. 7. Optical spectra of ferryl iron and free radicals in peroxidases. Optical spectra of intermediates in the catalytic cycle of horse-radish peroxidase (HRP) in the Soret (left) and visible (right) regions (A) Soret and visible spectra of native HRP (B) Soret and visible spectra of HRP compound II (C) Soret spectrum of HRP compound I (D) visible spectrum of HRP compound I. Note the unusual low haem absorbance in the Soret for compound I, where the nature of the porphyrin cation radical dominates the spectrum. Reprinted with permission from Dunford, H.B. (1982) Adv. Inorg. Biochem.

All of these compounds are expected to have ferryl iron with no porphyrin cation radical. As with optical spectroscopy the presence of the distant tryptophan radical in cytochrome c peroxidase compound I appears to have no effect on the MCD spectra. This was confirmed by a direct comparison of cytochrome c peroxidase compounds I and II [172] in the visible region. Tryptophan has a distinct MCD spectrum at 280 nm [173]. However, none of the changes in the UV MCD spectrum that occurred upon compound I formation could be attributed to the formation of the tryptophan radical [174]. 

Erman, J. E., Vitello, L. B., Mauro, J. M., and Kraut, J., 1989, Detection of an oxy-ferryl porphyrin JC-cation radical in the reaction between hydrogen peroxide and a mutant yeast cytochrome c peroxidase. Evidence for tryptophan-191 involvement in the radical site of compound I, Biochemistry 28 799297995. 

The kinetics at 430 mn after a laser flash of the photocatalytic system (pH 8.5) in Scheme 11 are biphasic—a fast reaction on a millisecond time-scale because of formation of [(P)Fe ] + was followed by a much slower reaction on a second time-scale because of the conversion of [(P)Fe ] + to compound II, (P)Fe" =0 [168]. In general, ferric porphyrins have ligand-centered one-electron oxidation potentials at which ferric porphyrins are oxidized to ferric porphyrin n radical cations these are higher than oxidation potentials for metal-centered one-electron oxidation to ferryl porphyrins [102, 170]. Despite the smaller (by ca 0.3 eV) driving force for ligand-centered oxidation than for metal-centered oxidation [102, 170], ligand-centered oxidation of HRP occurs before metal-centered oxidation (Scheme 11). This is be-... 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

Liu, M.H. and Y.O. Su (1998). Selective electrocatalysis of alkene oxidations in aqueous media. Electrochemical and spectral characterization of oxo-ferryl porphyrin, oxo-ferryl porphyrin radical cation and their reaction products with alkenes at room temperature. J. Electroanal. Chem. 452, 113-125. 

In addition to the cationic, radical, and non-synchronous concerted mechanisms outlined above, many other proposals have been offered [9, 56], In a recent provocative paper, an organometallic mechanism was postulated for the activation of alkanes by a ferryl porphyrin model species [79]. Less reactive substrates such as H2, D2 and CH4 were observed to inhibit the reaction between the synthetic catalyst and cyclohexane. In the proposed mechanism, a 2 -t- 2 C-H addition across the Fe-O bond is preceded by coordination of the alkane to the metal center to form an intermolecular <7-adduct. Inhibition arises from preferential binding of the smaller substrates to the congested metal site. Attempts to identify a similar effect with sMMO have been unsuccessful the presence of H2 had no effect on the rate of reaction between methane and Q (A. M. Valentine, S. S. Stahl, S. J. Lippard, unpublished results). 

Likewise, HRP [an iron(in)-heme protein with an axial histidine] activates HOOH for the epoxidation of olefins(as does the cyt P-450/O2/RedH2 system,but notMMO) via a porphyrin-cation-radical-ferryl intermediate [(por+OFe V=0, Compound I], ... 

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