Ferric-peroxo complexes

One-electron reduction of this complex to form the Fe +-00 , ferric-peroxo complex. 

The binding of O2 will yield an Fe(II)-O2 species that is a very poor oxidizing agent, except toward particularly reactive substrates. The next step in the catalytic activation of the substrate by the P450s or NOS involves a one-electron reduction of the Fe(II)-02 species and the formation of several intermediates that are less well characterized [249]. The first product is a ferric peroxo complex of the type Fe(II)-0—0 that can be protonated to yield Fe(lII)-0—OH. The last intermediate in the cycle is obtained via a heterolytic... 

Fig. 6.10 Description of known monomeric ferric-peroxo complexes.

S.3 Cytochrome P450 Model Compounds Functional. Ferric-peroxo species are part of the cytochrome P450 catalytic cycle as discussed previously in Section 7.4.4. For instance, these ferric-peroxo moieties are known to act as nucleophiles attacking aldehydic carbon atoms in oxidative deformylations to produce aromatic species.An example of this work, establishing the nucleophilic nature of [(porphyrin)Fe (02)] complexes, was achieved for alkene epoxidation reactions by J. S. Valentine and co-workers. The electron-deficient compound menadione (see Figure 7.18) yielded menadione epoxide when reacted with a [(porphyrin)Fe X02)] complex. 

The current understanding of oxygen activation by P-450 is summarized as follows (Scheme II) (Ic, 5a, b) (i) incorporation of a substrate to the resting ferric state (1) of the active site of the enzyme to afford the ES complex (2) (//) one-electron reduction of the heme from NAD(P)H via an associated reductase enzyme (Hi) reaction of the reduced heme (3) with O2 to form an oxy complex (4a, 4b) (iv) one-electron reduction of the oxy complex to yield a peroxo complex (5) (v) protonation (or possibly acylation) of the peroxo oxygen (vi) the formation of active species, the so-called oxenoid [FeO + (7)], by heterolytic 0-0 bond cleavage of (6) (v/7) oxygen transfer to the substrate. Thus, the overall stoichiometry can be expressed as in Eq. (2), where R is orR C(0) ... 

McCandlish et al. have isolated a peroxo-iron(III) complex (9) (Fig. 4) in the reaction of Fe(III)TPP(Cl) and KO2 according to Eq. (4) (37). The Soret band of 9 appears at 437 nm with unusually red-shifted a- and -b-bands (565 and 609 nm in DMSO). The EPR spectmm of 9 at 77K showed a relatively narrow, sharp resonance at g = 4.2 and weak resonances at g = 2 and g = 8, typical of rhombic high-spin ferric complexes such as Fe" EDTA (JS). Such a spectrum is not typical of high-spin ferric porphyrin complexes, which usually show resonance at g = 2 and 6, indicative of axial symmetry. An IR band at 806 cm" was observed to shift to 759 cm when K 02 rather than K 02 was used to prepare solutions of 9 these observations suggested the side-on bonding formulation illustrated in Fig. 4. Extended X-ray absorption fine structure (EXAFS) studies of 9 also... 

Protonation of the distal oxygen of this peroxo anion to generate the Fe- -OOH , ferric-hydroperoxo complex. 

Figure 6.40. Conversion of enolized analogue of the mtural aromatase substrate to the corresponding aromatized compound is catalyzed by a model peroxo ferric porphyrin complex.

Rates of Fe binding/ oxidation by recombinant H and L ferritins differ over a 1000-fold. The L type of ferritin protein forms polynuclear complexes, as soon as the iron is oxidized, that are indistinguishable from the mineral (B. H. Huynh and E. C. Theil, unpublished results), whereas the H type ferritin proteins form a series of ferric intermediates that include a diferric-peroxo as the first product. The di-ferric peroxo species is similar to complexes that form in methane monoxygenase and ribonucleotide reductase (see Chapter 16). Thus, the Fe - - O2 inorganic chemistry... 

In this case, a ferric porphyrin peroxo complex plays the role of the analogue of the reactive intermediate implicated in the third oxidative step of aromatase, the enzyme responsible for the conversion of androgens to estrogens (Scheme XI. 16). 

The synthesis of a ferric-peroxo porphyrin eomplex, [(Porp)Fe (02)], was first reported by Valentine and co-workers. The ferrie-peroxo speeies was prepared by reacting Fe(TPP)Cl or Fe(OEP)Cl with two equivalents of superoxide in the presenee of erown ether in aprotie solvents sueh as aeetonitrile or toluene. The first equivalent of superoxide reduees the ferric porphyrin to the ferrous porphyrin, and the second superoxide eonverts the ferrous porphyrin to the ferrie-r/ -peroxo porphyrin complex (Equation (14)). The EPR speetrum of [(0EP)Fe (02)] eomplex... 

In 1980, Valentine et al. examined reactions of KO2 with Fe (TPP)Cl and Fe (OEP)Cl (OEP octaethylporphyrin) in aprotic solvents [29]. The first equivalent reduces the complexes to the ferrous state, and the second equivalent affords a complex of superoxide-Fe(II) porphyrin adducts. Examination of the complexes including NMR [30], EPR [29, 31], UV [29], IR [29, 31], and EXAFS [32] supports the structure of the complexes to be ferric-peroxo intermediate with side-on structure (Fig. 2). The same intermediates could... 

As described above, Li-peroxo iron(III) complexes are only stable at a low temperature and its crystallographic characterization is extremely difficult to achieve. To date, few X-ray structures of iron complexes containing a peroxo moiety have been reported. One of the examples is tetranuclear-ferric fi -peroxo complex, whose molecular view is presented in Fig. 2 [73]. 

Figure 2. The crystal structure of tetranuclear ferric I -peroxo complex [73].

Jameson GNL, Weili J, Krebs C, Pereira AS, Tavares P, Liu XF, Theil EC, Huynh BH. 2002. Stoichiometric production of hydrogen peroxide and parallel formation of ferric multimers through decay of the diferric-peroxo complex, the first detectable intermediate in ferritin mineralization. Biochemistry 41 13435-13443. 

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