Ferryl radical

A hydroxyl ion is now cleaved from this intermediate. Uptake of a proton gives rise to H2O and the reactive form of oxygen mentioned above. In this ferryl radical, the iron is formally tetravalent. 

Treatment of DNA with iron chelators such as EDTA, DETAP AC or desferrioxamine protects it from degradation. Thus, the destruction of DNA by bleomycin is iron-dependent. The breakdown of bleomycin may be initiated by a ferryl radical species or a hydroxyl... 

It has long been known that ascorbate is oxidized during the reaction, but not stoichiometricaUy with hydroxylation of proline and decarboxylation of 2-oxoglutarate. The purified enzyme is active in the absence of ascorbate but, after 5 to 10 seconds (about 15 to 30 cycles of enzyme action), the rate of reaction begins to fall. The loss of activity is from a side reaction of the highly reactive ferryl radical in which the iron is oxidized to Fe +, which is catalyticaUy inactive - so-called uncoupled decarboxylation of 2-oxoglutarate. Activity is only restored by ascorbate, which reduces the iron back to Fe + (Kivirikko and Pihlajaniemi, 1998). 

The classical Fentons reagent,hydroxylates alkanes by producing hydroxyl radicals in solution. This reagent is relatively unselective and inefficient since much of the peroxide reagent is wasted by catalytic decomposition to O2. Ferryl radicals, FeO, have sometimes been invoked as intermediates by analogy with P-450 chemistry, but conclusive evidence is still lacking. Metal peroxide complexes are also known to hydroxylate alkanes. ... 

The ferryl radical cation-substrate complex (I) has two unpaired electrons in the iron d orbitals and 2 porphyrin orbital. This may give rise to either a quartet state if all spins are unpaired or, if the... 

Fig. 4.9 The two-state reaction manifold as formulated by Shade and coworkers [68,69]. The ferryl radical cation of compound I (7) has two unpaired electrons in iron d orbitals and one in the Si2u porphyrin orbital. This electron configuration can give rise to either a quartet state ( Aju) if all spins are unpaired or a doublet if the spin of the electron in the aju orbital is inverted. A hydrogen atom is abstracted fi om the substrate in the first step of the reaction and an electron is transferred to either the iron, producing the ferrous state (as shown), or to the porphyrin, neutralizing the radical cation. A quartet or doublet state is...

Whether lipid oxidation in muscle foods is catalysed by the iron redox cycle or by formation of the ferryl ions is not clear. However, ferrous ions react with lipid hydroperoxides much faster than with hydrogen peroxide. As shown above, if the reaction of metmyoglobin with hydroperoxides produces ferryl radicals capable of initiating lipid oxidation, it is necessary to prevent the formation of metmyoglobin or methemoglobin. At acidic pH, ferric myoglobin can initiate lipid oxidation in the presence of lipid hydroperoxides. 

In order to understand the potential for haem proteins to mediate the oxidative modification of LDLs, the interaction between ruptured erythrocytes (Paganga et al., 1992) and ruptured myocytes (Bourne etal., 1994) with LDL has been explored. Previous studies from this group have demonstrated that ferryl myoglobin radicals and ruptured cardiac myocytes, which generate ferryl myoglobin species on activation (Turner et al., 1990,... 

Compound 1 (Fig. 18.18) reversibly forms an analogous ferric-superoxo/Cu adduct at 60 °C, as demonstrated by resonance Raman spectroscopy. However, warming the sample to 40 °C results in a rapid four-electron reduction of the bound O2 ligand, generating a ferryl/Cu /phenoxyl radical derivative (Fig. 18.18) [Collman et al., 2007a]. 

Then the ferryl ion either reacts with water to form the hydroxyl radical, or oxidizes another Fe2+ ion [14,16],... 

Thus, two routes of transformation are possible for the Fe2+(H202) complex one-electron transfer to form the hydroxyl radical and two-electron transfer to form the ferryl ion. It is difficult to prove experimentally the formation of the ferryl ions because they are very reactive, so that this route of interaction of H202 with Fe2+ remains hypothetical to a great extent. Another change in the mechanism of H202 decomposition with increasing pH is related to the acidic dissociation of H02 (pKa = 4.4)... 

This mode of superoxide-dependent free radical-mediated damaging activity remains an important one although the nature of the generated reactive species (free hydroxyl radicals or perferryl, or ferryl ions) is still obscure. However, after the discovery of the fact that many cells produce nitric oxide in relatively large amounts (see below), it became clear that there is another and possibly a more portent mechanism of superoxide-induced free radical damage, namely, the formation of highly reactive peroxynitrite. 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

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