Porphyrins cation radicals

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

Oxoiron(V) porphyrins—red in color—an isoelectronic form of oxoiron(IV) porphyrin cation radicals—green—have been proposed (references 55a-c), although the density functional theory calculations have indicated that there are no true oxoiron(V) porphyrins (references 55d-f). Further spectroscopic studies are necessary to confirm the existence of this intermediate. 

Compound I is a two-electron oxidized enzyme intermediate containing a oxyferryl iron and a porphyrin cation radical while compound II is an one-electron oxidized intermediate (13), With lignin pa oxidase, as with other peroxidases, the substrate oxidation products are fir radicals which undergo nonenzymatic disproportionation reactions to give rise to the final products. 

Recent structural studies of single crystals of meso tetraaryl porphyrin cation radicals reveal that their Zn(II), Cu(li), Fe(III)Cl and Mg(II) complexes are all saddle shaped (17-20) Since the unoxidized species are either planar or slightly domed, oxidation to the radicals results in a major conformational change An example of this effect is shown in Fig 6 which presents the displacements of the atoms that comprise the skeleton of the cation radical of Mg... 

Figure 1. Block diagram of EPR apparatus used to measure the lifetime of photoexcited porphyrin cation radicals.

Ru (0)2(por " )] (for por = TMP and OEP) have been generated by oxidation of [Ru (0)2-(por)] with phenoxathiin hexachloroantimonate. The products show a broad Q band and a less intense and blue-shifted Soret band, consistent with the formation of the porphyrin cation radical. 

Oxidation of 2-methoxyethanol or 1,2-ethanediol by [0s04], in the presence of a porphyrin, is used to prepare [Osn(P)(CO)(X)] complexes [P is a porphyrinato(2-) ligand] (139). Recently, more convenient preparations of porphyrinato complexes were developed using the reactions of LOs3(CO)i2] with the appropriate porphyrin (Section II,C,4,d) (38, 40, 140,141). Electrochemical studies show that both the Os(III) and Os(III)/porphyrin-cation-radical complexes are moderately stable... 

The neutral, high-spin ferric (Fem) resting state reacts with peroxide to form a FeIV porphyrin-cation radical (compound I), which is subsequently reduced to the native resting state by the reaction with a second peroxide. Additional information on iron catalases can be obtained by consulting recent (post-1991) literature [71],... 

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). 

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. 

Immoos CE, Di Bilio AJ, Cohen MS, van Der Veer W, Gray HB, Farmer PJ. Electron-transfer chemistry of Ru-linker(heme)-modified myoglobin rapid intraprotein reduction of photogenerated porphyrin cation radical. Inorg Chem 2004 43 3593-6. 

Electron Hopping and Porphyrin Cation Radical Mechanisms... 

Nam, W., Goh, Y. M., Lee, Yoon J., Lim, M. H., and Kim,. (1999) Biomimetic alkane hydroxylations by an iron(III) porphyrin complex with H2O2 and by a high-valent iron(IV) oxo porphyrin cation radical complex, Inorg. Chem. 38, 3238-3240. 

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]. 

Tn other studies, reaction of [MnX(TPP)] (X = Nf or OCN ) with iodosylbenzene in hydrocarbon or halocarbon solvents yielded products formulated as yi-oxo dimers, [MnlvX(TPP)]20.678 [Mn,vN3(TPP)]20 has been characterized by X-ray diffraction (see Section 41.5.7.1). Infrared evidence supports678,679 the formulation of the dimers as being metal-centred oxidized products rather than containing porphyrin cation radicals the possibility of ligand-centred versus metal-centred oxidation in higher-valent metallo porphyrins has been discussed by several authors.680-682... 

Cytochrome P-450, which is the most extensively studied of the monooxygenase proteins, has a heme-iron active center with an axial thiol ligand (a cysteine residue). However, most chemical model investigations use simple iron(III) porphyrins without thiolate ligands. As a result, model mechanisms for cytochrome P-450 invoke a reactive intermediate that is formulated to be equivalent to Compound I of horseradish peroxidase, (por+-)Fe =0, with a high-potential porphyrin cation radical. Such a species would be reduced by thiolate, and therefore is an unreasonable formulation for the reactive center of cytochrome P-450. 

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