Reduction potentials peroxyl radical

Fig. 16.5 Synergistic regeneration of a-tocopherol by quercetin at a lipid-water interphase. a-tocopherol is reacting with a lipid peroxyl radical in a chain-breaking reaction. According to the standard reduction potential, the phenoxyl radical of quercetin can further be regenerated by ascorbate.

MbFe(IV)=0 and with lipid peroxyl radicals (Castellucio et al, 1995). It may accordingly be concluded that the most relevant single parameter for predicting the antioxidative activity of a new plant phenol would be the standard reduction potential, E . 

From Table 14.6 it can be seen that, with the exception of astaxanthin (ASTA), the rate constants for the electron transfer reactions decrease for each carotenoid in the order 9-phenanthryl peroxyl > 1-naphthyl peroxyl > 2-naphthyl peroxyl. This order of reactivity should be related to the reduction potentials of the radicals, with 9-phenanthryl peroxyl having the highest reduction potential. The same order of reactivity for these three arylperoxyl radicals reacting with Trolox was shown by Neta and coworkers (Alfassi et al. 1995). The reactivities of all the carotenoids studied are similar... 

The practical significance of the reaction depicted in Scheme 7.43 consists of the development of novel antioxidants carrying chalcogen atoms. Divalent organochalcogen compounds react readily with many types of oxidants (peroxide, peroxyl radicals, peroxynitrite, singlet oxygen, and ozone). The tetravalent organylchalcogenides formed are reduced by many mild reductants. Therefore, compounds of this sort have the potential to act as catalytic antioxidants. 

In the presence of GSH, 5 -d(T4AT7) and 5 -d(T4A) are formed [reactions (20) and (21)]. In the presence of 02, the primary radical is trapped by 02. In a subsequent step, the C(4 ) peroxyl radical is reduced to the corresponding hydroperoxide (the source of the reduction equivalent is as yet unknown potentially 02 generated in side reactions), and treatment with NH3 increases the yield of the glycolate which is also formed upon the bimolecular decay of the peroxyl radial [reactions (23)-(25)]. 

Fig. 2. Thermodynamics of the reduction of oxygen species Potential diagram at pH 7. Figures are apparent standard redox potentials versus NHE, at pH 7. Figures used for the alkyl peroxide species at the top of the diagram are approximations derived from computations by Koppenol [19]. An upper value for the ROO /ROOH redox couple would be 1.1 V, i.e. the value derived from the trichloromethyl peroxyl radical, the strongest oxidant known among peroxyl radicals. See the discussion in the text. Note (see a) A aq -0.16 V (relative to 1 M 02) is a better reference than —0.33 V for the reversible...

Superoxide (02 ) and the peroxyl radical (H02) have been intensively studied, and a good account of their thermochemistry is presented in Standard Potentials (pp. 60-63). They are related by the pKa of H02, which is 4.8 0.1 (51). The reduction potential of the 02/02 couple has been determined by a variety of methods, including, for example, the equilibria with various quinone-semiquinone systems. The value cited, — 0.33 V, is taken with respect to a standard state of 1 atm 02 pressure. When expressed relative to the 1 Mstandard state of 02, the potential is —0.16 V. Standard NBS data permit calculation of AfG° = 4.4 kJ/mol and 31.8 kJ/mol for H02 and 02, respectively. Some related potentials include 0.12 V for (H+, 02)/H02,1.44 V for (H+, H02)/H202, and 0.75 V for H02/H02 . 

The one-electron reduction potential (E°) also provides redox information to predict the direction of free radical processes since the change, AE°, indicates the position of the equilibrium . For example, if one considers hydrogen atom abstraction from polyunsaturated lipids (PUFA-H) by peroxyl radicals (PUFA-OO ), information on the two redox couples can be used, where for PUFA-OO , H+/PUFA-OOH, E° = 1000 mV and for PUFA , H+/PUFA-H, E° = 600 mV, so the reaction in equation 56... 

Aliphatic alkoxyl radicals have reduction potentials of about 1600 mV vs SHE at pH 7 making them better oxidising agents than alkyl peroxyl radicals (E 1000 mV SHE) [130], Phenoxyl radicals usually have even lower reduction potentials, e.g. phenoxyl radical (CsHsO ) with E7 900 mV vj SHE and tocopheroxyl radical with E 500 mV vj SHE [130], and these can also be produced in vivo via the oxidation of phenols, such as the amino acid tyrosine, flavenoids and other phenolic antioxidants (e.g. tocopherols), or via the reduction of quinones. 

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