N-cation radicals

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. 

The [Fe =0(TMP+ )]+ complex exhibited a characteristic bright green color and corresponding visible absorbance in its UV-vis spectrum. In its NMR spectrum, the meta-proton doublet of the porphyrin mesityl groups were shifted more than 70 ppm downfield from tetramethylsilane (TMS) because they were in the presence of the cation radical, while the methyl protons shift between 10 and 20ppm downfield. In Mossbauer spectroscopy, the isomer shift, 5 of 0.06 mm/s, and A q value of 1.62mm/s were similar to those for other known Fe(IV) complexes. Electron paramagnetic resonance (EPR), resonance Raman (RR), and EXAFS spectroscopies provided additional indications of an Fe =0 n-cation radical intermediate. For instance,... 

The first step of the reaction path involves the addition of H2O2 to the Fe " resting state to form an iron-oxo derivative known as Compound I, which is formally two oxidation equivalents above the Fe state (Fig. 2). The well studied Compound I contains a Fe" = 0 structure and a n cation radical. In the second step. Compound I is reduced to Compound II with a Fe =0 structure. The reduction of the n cation radical by a phenol or enol is accompanied by an electron transfer to Compound I and a proton transfer to a distal basic group (B), probably His 42 (Fig. 3, step 1). The native state is regenerated on one-electron reduction of Compound II by a phenol or an enol. In this process, electron and proton transfers occur to the ferryl group with simultaneous reduction of Fe" to Fe (Fig. 3, steps 2-3) and formation of water as the leaving group (Fig. 3, step 4). 

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. 

Where MPe represents the phthalocyanine in an excited triplet state. The CBr3 species is expected to dimerize rapidly following the reaction. The quantum yields of the resulting phthalocyanine n cation radical are shown in Table H. The EPR spectra obtained at 79 K for the ZnPc and RuPc n C U ion radicals give isotropic g values (Table II), which are very close to the free electron value of 2.0023 and are characteristic of the n caJtion radic ds of phthalocyanines (10,15). 

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

Several reports appear in the more recent literature of syntheses using electrochemical or PIET oxidation of compounds containing >C=N— bonds. These fall into three categories based upon a mechanism or presumed mechanism Cycloadditions, nucleophilic attack on >C=N— + cation radicals and radical annulations. The latter will not be reviewed here179 as none of the annulations appears to involve >C=N— + cation radicals. It should be pointed out that it is by no means certain that the electronic structure of >C=N— + is that of a 7r-cation radical rather than of an iminium cation radical (Figure 5). As will be seen below, reactivity appears sometimes in one guise and sometimes in the other. 

Another recent report of ring closings involving C=N cation radicals, generated by anodic oxidation, appears to involve intramolecular nucleophilic attack (Scheme 81)184. 

Back electron-transfer processes of n-anion and n-cation radicals with reversible electron donors or acceptors (e.g. aquated Fe3+, [Fe(CN)6]3, quinones) are fast reactions realized in nano- or picosecond time scale. In cases when irreversible redox partners are used (e.g. S20, CBr4, CC14, EDTA) tetrapyrrole ring ring localized radicals dimerize [193], decompose [212], undergo disproportionation [215] or other stabilization reactions. Photoformation of stable products will be discussed later. 

This mechanism accounts for the observed end products, metHb and nitrate, but does not consider the fate of peroxides in the presence of metHb, which results in ferryl n cation radical formation [see Refs. (189, 190) for reviews]. The tight isosbestic points (170) do not support formation of stable species other than metHb. An alternative, hypothetical consideration is the reduction by Fen02 or HNO to H2NO the resulting Fera02 species would dissociate rapidly ... 

The first reduced intermediate, Compound II, is formed when the n cation radical is reduced and a proton is transferred to the distal base (His). Compound II is then reduced to the Feln resting state with the simultaneous formation of water [70]. This second electron transfer step is one to two orders of magnitude slower than Compound I formation and is usually rate-limiting [72]. For HRP, the rate constant for Compound I formation (kfj is 2.0 x 107 M 1s 1 [73], while the rate-limiting step in phenol oxidation by HRP has a rate constant k 3.0 x 105 M 1s 1 [74, 75]. 

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... 

Remarkably, a synthetic polymer is able to stabilize porphyrins against oxidation. If Mg-octaethylporphin (10) in solid poly(l-vinylimidazole-co-styrene) with more than 20% imidazole units is irradiated in the presence of 02 the n-cation radical of the prophyrin is formed quantitatively48. With less than 0.1% imidazole units the porphine is oxidized to formylbiliverdin. The results are compared with chlorophyll which is perhaps stabilized in the natural protein against 02 in the same manner. 

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) ... 

Deprotonation of /neso-hydroxyoctaethylporphyrinato-iron(III) (OEPOH)Fe, discussed in Section 6.1.5 below, and simultaneous coordination of two pyridine ligands in pyridine-ds, produces a species whose optical and NMR spectra suggested that it should be characterized as an Fe oxophlorin n -cation radical. This would be another example of valence tautomerism, that is of (OEPQ-)Fe (Py)2 " ... 

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