Ferryl like intermediates

Isotope labeling experiments have shown chlorite to be the sole source of the oxygen atoms in the O2 product.30 EPR and UV-Vis studies also suggest that Cld forms a compound I-like intermediate upon reaction with CIO2. However, the hypochlorite (CIO ) by-product is believed to not diffuse freely into solution, but instead remains protein bound for further reaction with the ferryl to afford 02. The mechanism illustrated in Figure 3.8 has been put forth for this unique heme enzyme. 

Interestingly, we have shown that also the reactions of ferrylMb and ferrylHb with NO proceed via the rapid formation of an intermediate [19]. Because of the radical-like character of the oxo-ligand in the ferryl forms of the proteins [32], it is reasonable to assume that the first step of the reactions is rapid radical-radical recombination (Scheme 2.6.2), which leads to the O-nitrito intermediates MbFemONO and HbFemONO. As shown in Figure 2.6.3, the absorbance maxima in the spectrum of the Mb-intermediate are found at 504 nm (c504 = 8.7 mM-1 cm-1) and at 631 nm (e63i = 5.1 mM-1 cm-1), and are thus consistent with... 

The combination of photo-Fenton and ozonation results in an important enhancement of the destruction efficiency of organic compounds like phenol [96], 2,4-D [97], aniline or 2,4-chlorophenol ([33] and references therein). As mentioned in Sect. 2.5.1, metal ions catalyze ozone decomposition. In the dark, Fe(II) catalyzes O3 degradation giving the ferryl intermediate (Fe02+, see Sect. 2.6.9), which can directly oxidize the organic pollutant or evolve to a hydroxyl radical ... 

Although several studies indicate that HO is formed in Fenton systems according to Eq. 37 and it is responsible for the efficiency of degradative reactions, it is presently believed that other Fe(IV) or Fe(V) species like Fe03+ and ferryl complexes, are also active agents in the processes [53— 55,58, 112]. For example, Kremer [112] identified a mixed valence binuclear species, FeOFe 5+, and proposed a new mechanism for the Fenton reaction, in which Fe02+ acts as the key intermediate. 

Mechanisms for cleavage of the intermediate diol that involve further oxidation at C-22 are excluded by the fact that the 22(5) hydrogen is retained in the 4-methyIpentanal produced as a result of C-C bond cleavage. The most likely mechanism is thus one in which one of the hydroxyl moieties is activated in some fashion, followed by decomposition with C-C bond cleavage. The nature of this activation has led to a number of mechanistic proposals. First, a hydrogen may be abstracted from one of the alcohols by the ferryl species to form an alkoxy radical, which... 

Figure 6.48. Mechanistic proposals for the P450-catalyzed C-C bond cleavage during the biosynthesis of aflatoxin Bj. One possibility involves a Baeyer-Villiger-like reaction of the ferric peroxo species with the keto tautomer of the phenolic substrate while the other proceeds via the epoxide intermediate typical of ferryl catalyzed aromatic oxidations.

The likely mechanism of the CA[Mn]-catalyzed epoxidation of olefins is similar to that proposed by Burgess for free manganese involving peroxycarbonate as the key intermediate [41, 43 5], CA[Mn] forms a stable manganese-bicarbonate complex in the active-site [60], Hydrogen peroxide may add to carbonyl of this bicarbonate complex and displace water thereby forming a peroxycarbonate, which then may epoxidize a bound olefin. In contrast, heme peroxidases catalyze epoxidations either by a radical mechanism or by a ferryl-oxygen transfer mechanism [55]. 

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