Porphyrin n-cation radical

In a second type of experiment, oxidative quenching is achieved by use of [Co(NH3)5C1]2+ as the quencher. In the one example reported the ethyl-phenyl derivative of the substrate was used, and the Rum so generated oxidized the heme with k = 6x 103 s l. Prom spectroscopic studies it is believed that the heme is oxidized to a porphyrin n-cation radical and has an axial water ligand. One might anticipate the generation of other oxidized states with the use of other substrate derivatives. 

Once activated, MV-CCP reacts with 1 equiv of H2O2 in a bimolecu-lar reaction, presumably to form compound 0. In YCCP and HRP this species is referred to as compound ES or compound I, respectively, and contains oxyferryl heme and either a porphyrin n -cation radical (HRP) or an amino acid radical (YCCP). However, the presence of an extra reducing equivalent on the second heme in CCP suggests that such an oxidizing radical species close to the active site heme will be very shortlived and readily form compound I (Fig. 10), which is formally Fe(HI) Fe(IV)=0. The bimolecular rate constant for compound I formation is reported to be very close to the diffusion limit (84). 

The two oxidizing equivalents in compoimd I are next utilized to oxidize two substrate molecules. In the second step in the scheme, the first reducing substrate (S) delivers one electron to compoimd I, which reduces the porphyrin n cation radical, thereby generating compound II. A second substrate molecule reduces compoimd II back to the resting state. 

The free radical X could be a protein free radical or a porphyrin n-cation radical... 

Although there has not yet been a report of the preparation and full characterization of a bona fide Fe porphyrin n-cation radical, the [0ETPPFe(4-CNPy)2]C104 complex has... 

Col, which has been described as an oxyferryl porphyrin n cation radical [27], accepts one electron and one proton from a reducing substrate (RH) to yield the corresponding free radical (R ) and the oxyferryl heme intermediate known as compound II (Coll) ... 

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 oxoferryl)porphyrin n-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. 

An Fe oxidation state, apart from those of catalase and peroxidase compounds 1 and II, is attained only in the carbene complexes, RR C—Fe (Por), nitrene complexes,R N—N=Fe (Por), or the dimeric compounds, Fe (Por)=C=Fe (Por), Fe (Por)=N—Fe or) and [Fe (Por)—O—Fe HPor)]". Both carbene and nitrene complexes are diamagnetic, and the former appear to coordinate RNHj, py, Im, ROH, and RS . They lose the axial ligand in the presence of an excess of pyridine, and form Fe"(Por)(py)2. Though the iron oxidation state in these formally Fe (Por) complexes is still ambiguous, their reactivity implies an iron(III) oxidation state. Attempted synthesis of Fe porphyrins by a one-electron oxidation of Fe (TPP)Cl resulted in formation of the corresponding porphyrin n cation radical Fe "(TPP) . The high oxidation state iron porphyrins are of particular interest in relation to the cytochromes P-450, peroxidases and catalases, and Fe" 0(Por)L (L = 1-MeIm, py, Pip) have been spectroscopically characterized (Scheme 18). - "... 

As briefly summarized in Section 1.2.2.1, extensive evidence has been reported indicating that horseradish peroxidase Compound I is an oxo-ferryl [Fe =0] porphyrin n-cation radical and that Compound II is an oxo-ferryl porphyrin. Groves and co-workers have reported an inorganic model complex for Compound I [53, 54] and Balch and co-workers have described a Compound II model [55, 56]. These models each appear to have the expected compositions for the respective enzyme states that they are designed to mimic. 

Dolphin D, Felton RH (1974) The biological significance of porphyrin n-cation radicals. Acc Chem Res 7 26-32... 

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