Fenton activity

Silicates causing pneumoconiosis function as Fenton catalysts to generate hydroxyl radicals when incubated with hydrogen peroxide and a reducing substance (Kennedy et aL 1989). In contrast, silicates, which do not cause pneumoconioses demonstrate no Fenton activity. Catalytic activity is decreased by pre-treatment of silicates with the iron chelators, deferoxamine or transferrin. Haemolysis from silicates is decreased by interventions, which remove superoxide anion or hydrogen peroxide from the mediiun, or by pre-treatment of dusts with iron chelators. 

In buffer solutions which were treated to remove Fenton-active metal ions as well as in those not further purified, chromate and reduced glutathione (GSH) induced similar numbers of single-strand breaks in isolated PM2 DNA (Kortenkamp et al. 1996). Molecular oxygen was found to be essential for the formation of single-strand breaks, but Cr(V) species arising from chromate/GSH, unless activated by oxygen, appeared to be unreactive toward DNA. 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

As discussed earlier, is one of the main reactants for membrane chemical degradation. When the membrane is contaminated by metal ions, these ions can catalyze decomposition and generate radicals. It is important to evaluate which metal ions can effectively convert to radicals, i.e., which are Fenton-active. 

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. 

Smith PW, Sollis SL, Howes PD, Cherry PC, Starkey ID, Cobley KN, Weston H, Scicinski J, Merritt A, Whittington A, Wyatt P, Taylor N, Green D, BetheU R, Madar S, Fenton RJ, Motley PJ, Pateman T, Beresford A (1998) Dihydropyrancarboxamides related to zanamivir a new series of inhibitors of influenza virus sialidases. 1. Discovery, synthesis, biological activity, and structure-activity relationships of 4-guanidino- and 4-amino H-pyran-6-carboxamides. J Med Chem 41 787-797... 

The discussion above refers to the classical dark conditions where the chemical activation is achieved thermally. Fenton requires a moderate thermal activation, resulting in a reaction temperature ranging from 25 to 90 °C. The oxidizing capacity of the Fenton reaction can be increased by UV or UV-vis Hght irradiation [160, 161]. The increase is interpreted by means of the photoreduction ability of Fe ... 

By minimizing the Fe concentration (i.e., avoiding extensive Fe-exchange), zeolites, or mesoporous compounds can be detemplated at low temperatures without the need for high-temperature calcination. This third concept refers to the low-temperature Fenton detemplation. Strictly speaking, Fenton requires thermal activation but always below 100 °C. We refer here to quasi room temperature as compared to the high temperatures usually applied for calcination. 

Fig. 16.4 Interaction between quercetin (Quer) and iron and the balance between pro-oxidative and antioxidative effects. Quercetin may reduce Fe(H20) to yield Fe(H20) active in the Fenton region forming hydroxyl radicals ( OFI) or alkoxyl radicals ( OR), in effect being pro-oxidative. In contrast, quercetin may form a complex with iron(II), inactive in reducing FI2O2 to OFI, but rather oxidised in the quercetin ligand, in effect being antioxidative. Quer (-H) is the phenoxyl radical.

The antiulcer agent rebamipide ((2-(4-chlorobenzoy-lamino)-3-[2(lH)-quinolinon-4-yl]propionic acid) dose-dependently decreased hydroxyl radical signal generated by the Fenton reaction in an e.s.r. study. Rebamipide is active as a hydroxyl radical scavenger and inhibitor of superoxide production by neutrophils (Yoshikawa etal., 1993). 

As a part of their activity on magnetic and electrochemical studies on dinuclear copper(II) complexes, Fenton and co-workers reported complex (332) (Cu-Cu 2.957 A 2J- 493 cm ).296... 

The equivalent reaction is not observed with iron. Cu(I) catalyses the Fenton reaction with hydrogen peroxide, just as Fe(II) does. The Cu(I) state exhibits the ability to bind and activate dioxygen via Cu2(p-ri2 ri2-02) and Cu2(p-0)2 species. 

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