Fenton mechanism

Shi, X. and N.S. Dalai. 1993. Vanadate-mediated hydroxyl radical generation from superoxide radical in the presence of NADH Haber-Weiss vs. Fenton mechanism. Arch. Biochem. Biophys. 307 336-341. 

H202 produced during the Ti02-mediated photocatalysis may diffuse across the cell membrane and produce intracellular OH radicals according to the known Fenton mechanism (17.15). 

At many hazardous waste sites, codisposal of contaminants has occurred that require both reductive and oxidative transformations. Under this set of conditions, neither reductive nor oxidative transformations alone are sufficient to achieve the treatment objective. Rather than employing a complex sequence of oxidative and reductive technologies, or vice versa, the Fenton mechanism, if optimized, may be capable of addressing both treatment needs. 

Peroxynitrite may be an important oxidant produced in acetaminophen-induced MPT. As discussed above, acetaminophen-induced MPT occurred with increased oxidation of the redox-sensitive dye DCFH2. This dye is readily oxidized by peroxynitrite but not by superoxide, hydrogen peroxide, or hypochlorous acid however, it may be oxidized by peroxide plus a peroxidase or a Fenton mechanism (ferrous ions plus peroxide) (Crow 1997 Myhre et al. 2003). Peroxynitrite is known to rapidly react with thiols such as N-acetylcyteine (Crow 2000), and N-acetylcysteine prevented acetaminophen-induced MPT and DCFH2 oxidation (Reid et al. 2005). The finding that nitration was predominantly in mitochondria of acetaminophen-treated mice supports the hypothesis that peroxynitrite formation occurred in that organelle (Cover et al. 2005). As pointed out above, the NOS isoform was probably not iNOS, which suggests involvement of another NOS species such as mitochondrial nitric oxide synthase (mtNOS) (Ghafourifar and Cadenas 2005). 

Godinger, D., Chevion, M., and Czapski, G., On the cytotoxicity of vitamin C and metal ions. A site-specific Fenton mechanism, Eur. J. Biochem., 137,119, 1983. 

Figure 7. The proposed photo-Fenton mechanism with competition of O2 and Fe(III)Ox for C204. Ox is oxalate.

The Me" transition metal ion can be oxidized further, for example, Fe(II), Cu(I), or Ti(III), and can activate oxidative cleavage of hydrogen peroxide, as in the Fenton mechanism. It was suggested that the hydroperoxyl radical, HOO, is the species that attacks the membrane. As will be clearly seen below, some of the transition metal ions may react directly with the sulfonic groups of the membrane, thus providing an additional degradation pathway. 

The alternative is to use Fe(II) or Cu(II) ions to promote the hydroxyl radical formation (Fenton mechanism) ... 

In simplified terms, the difference between WHPCO and WACO mechanisms can be explained as follows. In WHPCO, the rate of reaction depends on the redox reaction of H2O2 with iron (or other redox metals) to form the active radical species. In WACO, the rate depends on the electrophilic character of the catalyst, e.g. its rate of generation of surface radical species. Although this property also depends on the presence of redox sites, the Fenton mechanism is much more effective to close the cycle. 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Much work has been done on the mechanism of the reaction with Fenton s reagent, and it is known that free aryl radicals (formed by a process such as HO- + ArH AR- + H2O) are not intermediates. The mechanism is essentially that outlined on page 898, with HO- as the attacking species, formed by... 

The reverse reaction (that is, the oxidation of a vinyl radical by Fe to the corresponding vinyl cation) may be involved in the reaction of the dimethyl ester of acetylenedicarboxyUc acid 261 with Fenton s reagent [Fe —H2O2, (217)] (216). When 261 was treated with Fe —H2O2 and the reaction mixture was extracted with ether, a small amount of furan 262 was isolated. A possible mechanism (216) for its formation may be addition of hydroxyl radical to the triple bond of 261, followed by addition of the intermediate vinyl radical to a second molecule of 261 and oxidation of the resulting radical with Fe to the corresponding vinyl cation, followed by cyclization to 262, as shown in Scheme XX. 

Scheme XX. Possible Mechanism of Reaction of Fenton s Reagent with Dimethyl Acetyledicarboxylate.

Reduction of vinyl radicals by to the corresponding anion also has been observed (216). When purified acetylene is bubbled through Fenton s reagent, acetaldehyde is formed as a product, presumably via the following mechanism ... 

Luo T, Ai Z, Zhang L (2008) Fe Fe203 core-shell nanowires as iron reagent. 4. Sono-Fenton degradation of pentachlorophenol and the mechanism analysis. J Phys Chem C 112(23) 8675-8681... 

Devi LG, Kumar SG, Reddy KM, Munikrishnappa C (2009) Photodegradation of methyl orange an azo dye by advanced Fenton process using zero valent metallic iron Influence of various reaction parameters and its degradation mechanism. J Hazard Mater 164 459 167... 

Additional applications of BSOCOES and sulfo-BSOCOES include investigations of the cellular and subcellular distribution of the type II vasopressin receptor (Fenton et al., 2007), TNF-alpha (Grinberg et al., 2005), and studying mechanisms in the control of plasmid replication (Das et al., 2005). 

In the preparation of D-glucosone by the direct oxidation of D-glucose, D-fructose, or D-mannose by such reagents as that of Fenton,37 cupric acetate,16- 46- 46 selenious acid,16-61 etc., the degree of oxidation must be carefully controlled if the osone, which is the first product, is to be the main product of the reaction. The nature and mechanism of formation of the products of further oxidation of D-glucosone are discussed on p. 68. 

Another possible pathway of accelerating the in vivo Fenton reaction has been proposed previously [20]. It was suggested that the level of catalytically active ferrous ions may be enhanced as a result of the interaction of superoxide with the [4Fe-4S] clusters of dehydratases such as aconitases. In accord with this mechanism, superoxide reacts with aconitase to oxidize ferrous ion inside of the [4Fe—4S] cluster. In the next step, the remaining ferrous ion is released from the cluster and is capable of participating in Reaction (2) ... 

The formation of hydroxyl or hydroxyl-like radicals in the reaction of ferrous ions with hydrogen peroxide (the Fenton reaction) is usually considered as a main mechanism of free radical damage. However, Qian and Buettner [172] have recently proposed that at high [02]/ [H202] ratios the formation of reactive oxygen species such as perferryl ion at the oxidation of ferrous ions by dioxygen (Reaction 46) may compete with the Fenton reaction (2) ... 

MF effects on FA relatives and healthy donors. (Fanconi anemia is an autosomal recessive disease associated with the overproduction of free radicals, Chapter 31.) It has been shown earlier [215] that FA leukocytes produce the enhanced amount of hydroxyl or hydroxyl-like free radicals, which are probably formed by the Fenton reaction. It was suggested that MF would be able to accelerate hydroxyl radical production by FA leukocytes. Indeed, we found that MF significantly enhanced luminol-amplified CL produced by non-stimulated and PMA-stimulated FA leukocytes but did not affect at all oxygen radical production by leukocytes from FA relatives and healthy donors (Table 21.3). It is interesting that MF did not also affect the calcium ionophore A23187-stimulated CL by FA leukocytes, indicating the absence of the calcium-mediated mechanism of MF activity, at least for FA leukocytes. 

If the mechanism of superoxide production in microsomes by NADPH-cytochrome P-450 reductase, NADH-cytochrome b5 reductase, and cytochrome P-450 is well documented, it cannot be said about microsomal hydroxyl radical production. There are numerous studies, which suggest the formation of hydroxyl radicals in various mitochondrial preparations and by isolated microsomal enzymes. It has been shown that the addition of iron complexes to microsomes stimulated the formation of hydroxyl radicals supposedly via the Fenton... 

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