Fenton cycle

In the presence of PhSeSePh and excess hydrocarbon substrate the Fenton process [Fe(PA)2/HOOH] produces carbon radicals (RO, which are trapped by PhSeSePh to give PhSe-R products (Table 5-2 and Scheme 5-1). The distribution of PhSe-R isomers appears to reflect the isomer abundance for the R- radicals from the Fenton cycle [Eqs. (5-10) and (5-11)]. For n-hexane and 2-Me-butane the R-SePh isomer distribution (Table 5-2) indicates that the relative reaction probabilities of HO- with a C-H bond in — CH3, CH2, and CH groups are 0.074, 0.44, and 1.00 (the respective C-H bond energies are 100, 95, and 93 kcal), which are in accord with the relative values for aqueous HO- (0.10/0.48/1.00).i3 Thus, PhSeSePh provides the means to trap first-formed carbon radicals and thereby give insight to the mechanism of their generation. 

Finally, hydrogen peroxide is decomposed by Fe " ions, which oxidize to Fe closing the photo-Fenton cycle [95]. Thus, when the degradation of organic compounds ends solubilized iron, ions turn back to the catalyst surface. 

Liang J, Komarov S, Hayashi N, Kasai E (2007) Recent trends in the decomposition of chlorinated aromatic hydrocarbons by ultrasound irradiation and Fenton s reagent. J Mat Cycles Waste Manage 9(1) 47—55... 

Iron chelators can also be used to selectively bind iron in areas where oxidative stress is observed, thereby preventing the iron from taking part in Fenton reactions without interfering with normal iron homeostasis. Charkoudian et al. have developed boronic acid and boronic ester masked prochelators, which do not bind metals unless exposed to hydrogen peroxide (237,238). The binding of these chelators to iron(III) prevents redox cycling. Similar studies of these systems have been performed by a separate group (239,240). 

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

In this classical Haber-Weiss cycle iron is being reduced by superoxide anion radical (02T), ascorbic acid or glutathione and subsequently decomposes hydrogen peroxide - formed by spontaneous dismutation of 02T - in the Fenton reaction to produce 0H. This iron-driven 0H formation has a stringent requirement for an available iron coordination site, a sine qua non met not only by hexaaquoiron(III) but by most iron chelates (28). Thus, Fe-EDTA, -EGTA, and -ATP retain a reactive coordination site and catalyze the Haber-Weiss cycle. Phytic acid, however, occupies all available iron coordination sites and consequently fails to support 0H generation (Figure 6). 

Such generated OH radieals eould then reaet with organie matter present in water. Although reaction shown by equation (1) is often referred as Fenton reaction [43], and presents the key step in the Fenton proeess, other important reactions also occur. The following set of reaetions deseribes the oeeurrenee of Fenton eatalytie cycle ... 

From the point of view of the redox properties of the formed complex, Merkofer et al. [2006] have studied the Fenton reaction 18 catalyzed by different iron chelators and concluded that a Fe-complex can participate in cell redox cycling only when two conditions are fulfilled (i) the oxidized complex (ligand-M(" + 1 +) can be reduced by a physiologically relevant compound [e.g., NAD(P)H] [Pierre et al., 2002], then the 0 (Ligand M( +i)+/Ligand M +) is higher... 

A number of other transition metal ions can also participate in Fenton-type cycles to produce hydroxyl radical. Examples include Cu+, V02+, Ti3+, Cr2+, and Co2+ [4], although other reducing metals are also active in the formation of hydroxyl radical from peroxide. 

Fenton reagent is simply due to the catalytic cycle of H2O2 decomposition, a process that generates HO radicals. 

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

Fig. 5.14 Schematic representation of the catalytic cycle in the photo-Fenton reaction using ferrioxalate (following Hislop and Bolton, 1999).

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