Oxoferryl porphyrin

Proniewicz LM, Paeng IR, Nakamoto K. 1991. Resonance raman spectra of two isomeric dioxygen adducts of iron(II) porphyrins and rr-cation radical and nonradical oxoferryl porphyrins produced in dioxygen matrixes Simultaneous observation of more than seven oxygen isotope sensitive bands J Am Chem Soc 113 3294. 

Taurog et al. [216] showed that contrary to previous suggestions, both iodination and coupling are catalyzed by the oxoferryl porphyrin Tr-cation radical of TPO Compound I and not the oxoferryl protein radical. HRP catalyzed the oxidation of bisulfite to sulfate with the intermediate formation of sulfur trioxide radical anion S03 [217] HPO, MPO, LPO, chloroperoxidase, NADH peroxidase, and methemoglobin oxidized cyanide to cyanyl radical [218],... 

Scheme 39 Allylic oxidation of olefins with an oxoferryl porphyrin complex.

Oxoiron(IV) tefraphenylchlorin complexes have been prepared as the first models of a reaction intermediate in the catalytic cycle of cytochrome d Optical absorption spectra show a characteristic red-shified band at 630 nm as observed in the oxoferryl intermediate of cytochrome d, and the proton NMR spectra of the N-Melm complex exhibit very small hyperfine shifts of the pyrrole protons, as is true for oxoferryl porphyrin complexes. The pyrroline protons of the saturated pyrrole ring show unusual splitting into upheld and downfield resonances. The N-Melm complex also shows normal Fe =0 stretching frequencies as compared to the corresponding oxoferryl porphyrin complexes. And finally, for iron porphycenes, both peroxo and ferryl intermediates have been detected by H NMR spectroscopy during the oxygenation of the Fe complexes. ... 

To clarify the mechanism of reaction of P-450, it is crucial to characterize the reactive intermediates in the rate-determining step. Definitive evidence for an electron-transfer mechanism (C in Scheme 2) for the 7V-demethylation of N,N-dimethylanilines has been obtained by direct observation of the reduction of the high-valent species responsible for P-450 catalysis [96]. For peroxidase, an oxoferryl porphyrin 7r-radical cation, compound I ([(P)Fe =0] "), has been well characterized as the species equivalent to the proposed active intermediate of P-450 [97-103]. Compound I of horseradish peroxidase (HRP) can be readily generated by chemical oxidation of HRP [100-103]. The involvement of the electron-transfer process of compound I in the oxidation of several amines catalyzed by HRP was... 

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. 

Watanabe, Y. (2001). Alternatives to the oxoferryl porphyrin cation radical as the proposed reaetive intermediate of cytochrome P450 Two-electron oxidized Fe(III) porphyrin derivatives. J. Biol. Inorg. Chem. 6, 846-856. 

Recently, the active sites of non cnantiosclcctivc iron salcn catalyzed oxidation of sulfides by PhIO were proposed to be [O-Fc (salen)) species (46J however, the characterization was doubtful. Our spectroscopic observations evidence in favor of other active species. First of all, oxoferryl x-cation radicals arc expected to have a typical S=3/2 spectra with resonances at g -4 and [131-133]. However, treatment of complexes 14, 15 with PhIO and m-CPBA cation radicals [77,131-133]) did not lead to formation of S=3/2-typc spectra, only a sharp peak at g"=4.2 attributable to some unidentified S=5/2 species being observed. The latter species, however, could not contribute to the catalytic cycle, as swm as it is stable for hours even in the presence of the substrate, and its concentration estimated by HPR docs not exceed 10% of total Fc concentration. Thus, it must be interpreted as a minor inactive high-spin Fe " admixture. 

Iron (IV, V) porphyrins were studied by resonance Raman spectroscopy. These compounds have biological significance, because oxoferryl posphyrins 0=Fe (TV)(por) are involved in enzymatic reactions of cytochrome P450, horseradish peroxidase (HRP). and related heme proteins. The mechanism of formation of the oxoferryl porphyrin Ti-cation radical was determined from resonance Raman measurements. The former radical is considered to be a model compound of HRP-I. 

Egawa T, Proshlyakov DA, Miki H, Makino R, OguraT, Kitagawa T, IshimuraY (2001) Effects of a thiolate axial ligand on the pi -> pi electronic states of oxoferryl porphyrins a study of the optical and resonance Raman spectra of compounds I and II of chloroperoxidase. J Biol Inorg Chem 6 46-54... 

The first step in the catalytic cycle is the reaction between H2O2 and the Fe(ni) resting state of the enzyme to generate compound I, a high-oxidation-state intermediate comprising an Fe(IV) oxoferryl center and a porphyrin-based cation radical... 

Fig. 8 AMI semiempirical molecular orbital calculations of esfenvalerate (9a) and 7t-cation radical of oxoferryl(IV) porphyrin...

Peroxidases are haem proteins that are activated from the ferric state to one-electron oxidants by H202. They play a significant role in the generation of radicals from xenobiotics. The compound I state contains one oxidising equivalent as an oxoferryl-haem entity and the second as a porphyrin -radical cation. Upon the oxidation of a substrate the porphyrin radical is repaired, giving the compound II. Reduction of the oxoferryl haem back to the ferric state by a second substrate molecule completes the enzyme cycle. In addition to the classical peroxidases, several other haem proteins display pseudo-peroxidase activity. The plant enzyme horseradish peroxidase (HRP) is often employed in model systems. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Thus, HRP-I (green) and HRP-II (red) have oxidation states higher than the native Fe(III) state by two and one, respectively. It has been found that both intermediates are oxoferryl (Fe(IV)) porphyrins and that HRP-II is low spin Fe(IV), whereas HRP-I is its jr-cation radical, which is one electron deficient in the porphyrin jr-orbital of HRP-II. 

It was later discovered that the reactive intermediates in the iron porphyrin model systems were high-valent oxoiron porphyrin complexes. A green oxoiron(IV) porphyrin cation radical species (13) has been well characterized by various spectroscopic techniques, including visible spectroscopy, NMR, EPR, Mossbauer, and EXAFS (Figure 1.11) . It has recently been shown by Nam and Que that the oxygen-atom transfer from certain iodosylarenes is reversible with some iron porphyrins. For the case of 1,2-difluoro-4-iodobenzene both an oxoferryl species and an iodosyl-ferric species were observed to be in equilibrium . 

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