Fenton-type oxidation

A reduction and activation of HjOj by other one-electron donors, like semiquinones, has also to be considered. This follows from a study of the ethylene production from methionine in the presence of pyridoxal phosphate, a reaction characteristic for OH radicals or for Fenton-type oxidants. The ethylene production in the presence of dioxygen, anthraquinone-2-sulfonate, and an NADPH-generating system in phosphate buffer pH 7.6 was inhibited by SOD and by catalase, but stimulated by scavengers of OH radicals, like 0.1 mM mannitol, a-tocopherol, and formiate... 

Oxidation to Phenols. Direct hydroxylation of benzene to phenol can be achieved in a free-radical process with H202 or 02 as oxidants.739-744 Metal ions [Fe(II), Cu(II), Ti(HI)] may be used to catalyze oxidation with H202. Of these reactions, the so-called Fenton-type oxidation is the most widely studied process.742 Oxidation in the presence of iron(II) sulfate was reported in early studies to yield phenol. Since phenol exhibits higher reactivity than benzene, varying amounts of isomeric dihydroxybenzenes were also formed. 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

It was first assumed that the oxidant was H2O2. However, since traces of Fe-cations are present, a Fenton s type oxidation pathway, based on OH radicals, is more likely taking place. 

In agreement with literature results (45,46), independent experiments confirmed that the addition of an aliquot of a CuCl acetonitrile solution to an H202 solution induced the immediate decay of H202 at pH 9.0 (44). Most likely, the OH radical produced is involved in fast oxidation of the H2C-R, SQ-R - or even the Q-R forms of the substrate and the stoichiometry shown in Eq. (54) is not valid anymore. The formation of free radicals was excluded under acidic conditions (36) implying that the Fenton-type decomposition of H202 may gain significance only in alkaline solution. 

Destruction of chromophoric and nonchromophoric pollutants in pulp and paper effluents may be achieved by advanced oxidation methods such as photocatalysis, photo-oxidation using hydrogen peroxide (H202)/UV or ozone (Osj/UV systems, Fenton-type reactions, wet oxidation, and by employing strong oxidants such as ozone. 

The kinetics of oxidation of aldehydes by the Fenton reagent [Fe(II)-H202-0H-] have been studied.89 It has been suggested that different reactivities of PhIO in iron(III)-porphyrin-catalysed alkene epoxidation may be due to the formation of a more reactive iron(IV)-0-IPh complex.90 The iron(m) complex of tetrakis(3,5-disulfonato-mesityl)porphyrin catalyses the oxidative degradation of 2,4,6-trichlorophenol to 2,6-dichloro-l,4-benzoquinone with KHSO5 as the oxygen atom donor a peroxidase-type oxidation is thought to be involved.91... 

Hydroxymethanesulfonic acid (HMSA) is a complex formed from formaldehyde and S(IV). It has been detected in atmospheric liquids (i.e., rain and snow). The complex has high resistance to oxidation by oxygen as well as ferric ions and oxygen. Martin et al. (1989) first studied the oxidation of HMSA. Graedel et al. (1986) proposed that Fenton-type reactions are possible in atmospheric liquid water. 

Since H202 is easier to handle than 02, we will focus on the use of the former. Many metals can be used for this transformation [50]. Among them, iron compounds are of interest as mimics of naturally occurring non-heme catalysts such as methane monooxygenase (MMO) [51a] or the non-heme anti-tumor drug bleomycin [51b]. Epoxidation catalysts should meet several requirements in order to be suitable for this transformation [50]. Most importantly they must activate the oxidant without formation of radicals as this would lead to Fenton-type chemistry and catalyst decomposition. Instead, heterolytic cleavage of the 0—0 bond is desired. In some cases, alkene oxidation furnishes not only epoxides but also diols. The latter transformation will be the topic of the following section. 

Mundy CJ, Colvin ME, Quong AA (2002) Irradiated guanine a Car-Parinello molecular dynamics study of dehydrogenation in the presence of an OH radical. J Phys Chem A 106 10063-10071 Murata-Kamiya N, Kamiya H, Muraoka M, Kaji H, Kasai H (1998) Comparison of oxidation products from DNA components byy-irradiation and Fenton-type reactions. J Radiat Res 38 121-131 Nabben FJ, van der Stroom HA, Loman H (1983) Inactivation of biologically active DNA by isopropanol and formate radicals. Int J Radiat Biol 43 495-504 Nakashima M, Hayon E (1979) Rates of reaction of inorganic phosphate radicals in solution. J Phys Chem 74 3290-3291... 

Besides inducing a Fenton-type (i.e., free-radical) chemistry, H2O2 (at high concentrations) can oxidize nucleobases, and in the case of Ade and its derivatives the formation of the N7-oxide has been reported (Rhaese 1968), and further reactions seem to occur as well. This lesion, now attributed to the N1-oxide [reaction (50)], has been detected by the 32P-postlabelling technique (Mouret et al. 1990) and polyclonal antibodies have been raised to detect this lesion in oxidized DNA (Signorini et al. 1998). 

Although soluble guanylyl cyclase is commonly considered to be the only primary chemical receptor for NO, heme proteins can react with NO in a variety of oxidation states. For example, NO can complex at near diffusion control with hypervalent iron states formed in Fenton-type reactions (63-65). 

Various systems have been developed in recent decades for the generation of acyl radicals under mild conditions, mostly by Minisci and co-workers. For example, the Fenton-type tert-butyl hydroperoxide/Fe(II) system, gives rise l-BuO radical, which is able to abstract the hydrogen from aldehydes, achieving the corresponding acyl radicals (Equations 14.2 and 14.3) [9], Fe(III), formed in Equation 14.2, acts as oxidant on the intermediate, making the process catalytic in Fe(II) (Equation 14.4). 

Edwards JO, Curci R. Fenton type activation and chemistry of hydroxyl radical. In Strukul G, ed. Catalytic Oxidations with Hydrogen Peroxide as Oxidant. Dordrecht, Netherlands Kluwer Academic Publishers, 1992 97-151. 

---