Antioxidants reduction potential

Direct reaction of oxygen with most organic materials to produce radicals (eq. 13) is very slow at moderate temperatures. Hydrogen-donating antioxidants (AH), particularly those with low oxidation—reduction potentials, can react with oxygen (eq. 14), especially at elevated temperatures (6). 

The antioxidant properties of flavonoids are attributable to the ring whose radical has the lower reduction potential. Conjugation between the 2-aryl and the fused benzene rings is very inefficient <96JCS(P2)2497>. 

The electron transfer mechanism for antioxidant activity corresponding to eq. 16.5 makes the standard reduction potentials of interest for evaluation of antioxidative activity. The standard reduction potential of the phenoxyl radical of several flavonoids has been determined and forms the basis for correlation of rate of electron transfer for various oxidants from the flavonoid (Jovanovic etal., 1997 Jorgensen and Skibsted, 1998). The standard reduction potentials have also been used to establish antioxidant hierarchies. 

The standard reduction potentials determined in aqueous solution give hierarchies slightly different from the antioxidant hierarchy established in DMF. For the potential determined by pulse radiolysis the ordering according to tendency of regeneration is (Jovanovic et al, 1994) ... 

For aqueous solutions, ascorbate can be included in the hierarchy, while a-tocopherol has to be replaced by its water-soluble analogue trolox, which is often assumed to have the same standard reduction potential. The ordering of the antioxidants based on the two different determinations of E in water is rather similar, and it should be noted that ascorbate is the antioxidant which will regenerate the other antioxidants, with the ascorbate itself ending up being oxidised. In contrast to what was observed for DMF, the ordering in water predicts that quercetin could regenerate a-tocopherol from its oxidised form. 

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 . 

The results on the cellular protection against N()2 can be interpreted as the N()2 reacting with the three antioxidants to produce their radicals, with ascorbic acid reacting least efficiently, probably due to the lower reduction potential of its radical. Moreover, Arroyo et al. (1992) reported that NO - and N02 -induced mutations in Salmonella typhimurium TA1535 were inhibited efficiently by P-CAR and tocopherols, but not at all by ascorbic acid. 

The studies of this equilibrium as a function of pH enabled the estimation of the absolute one-electron reduction potentials of CAR + in an aqueous micellar environment (Edge et al. 2000, Burke et al. 2001b) (see Table 14.12 for typical results). As can be seen, the potentials of all the dietary carotenoid radical cations are very similar but LYC + has the lowest potential implying that it is the best carotenoid antioxidant against free radicals (of course, this is an oversimplification, see above). 

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

Jovanovic, S.V., Steenken, S., Hara, Y., and Simic, M.G., Reduction potentials of flavonoid and model phenoxyl radicals. Which ring in flavonoids is responsible for antioxidant activity J. Chem. Soc. Perkin Trans. 2, 2497, 1996. 

Table 3.1 Biological Standard Reduction Potentials (E ) for a Series of Antioxidant Compounds and Flavonoids and Biological Standard Free Energy Variation (AG" ) for the Reactions between These Compounds and Oxygen Free Radicals... 

The reactions between free radicals and flavonoids (or polyphenols) are assumed to form aroxyl radicals (PO) (reaction 8). The stability of these secondary radical species is an important element to be considered in their antioxidant actions. Flavonoids with similar reduction potentials can originate radicals with very different reactivity toward other molecules present in biological systems. While a stable and relatively nonreactive PO is also nonreactive to propagate the chain reaction, a high reactive PO would propagate rather than interrupt a chain reaction. 

Land EJ, Ebert M (1967) Pulse radiolysis studies of aqueous phenol. Water elimination from dihy-droxycyclohexadienyl radicals to form phenoxyl. Trans Faraday Soc 63 1181-1190 Lind J, Shen X, Eriksen TE, Merenyi G (1990) The one-electron reduction potential of 4-substituted phenoxyl radicals in water. J Am Chem Soc 112 479-482 Loft S, Poulsen HE (1999) Markers of oxidative damage to DNA antioxidants and molecular damage. Methods Enzymol 300 166-184... 

Antioxidant, regulation of intracellular oxidation-reduction potentials, hydroxylation reactions that require copper or iron... 

The antioxidants studied can be classified into two broad types phenolic antioxidants and non-phenolic antioxidants. Phenolic antioxidants have been found to be more promising as they are obtained from dietary sources.Vitamin E (a-tocopherol), the first known chainbreaking antioxidant, is also an o-methoxy phenol. Pulse radiolysis studies of vitamin E and its water-soluble analogue, trolox C, have been reported several years ago. a-tocopherol reacts with almost all the oxidizing free radicals, and the phenoxyl radicals produced during oxidation reactions absorb at -460 nm (Fig. 1). The regeneration reaction of a-tocopherol phenoxyl radicals back to a-tocopherol by water-soluble antioxidant ascorbic acid was also first reported by pulse radiolysis method. The one-electron reduction potential of vitamin E is -0.48 V vs. NHE. Both a-tocopherol and trolox C are used as standards for evaluating the antioxidant ability of new compounds. 

Radiation chemical methods and pulse radiolysis technique in particular, have been proved to be extremely useful in the selective generation of ROS and RNS and the direct monitoring of their reactions. Using these methods, a number of natural and synthetic products have been evaluated as new antioxidant molecules. Estimation of rate constants and one-electron reduction potentials in most of the promising cases confirmed the role of electron transfer... 

This relationship was then used to calculate the BDE for a-Toc, giving a value of 77.2 kcalmor, in excellent agreement with the experimental value of 77.3 kcalmoP. This empirical method does depend on the electronic effects of groups (methyls and para-ether) around the phenyl ring and provides some confirmation of the role these play in weakening the O—H bond and thus raising the antioxidant activity. In a similar manner, Jovanovic and coworkers obtained a correlation between the measured reduction potentials and the a+ constants for twenty-one substituted phenols at pH 7 (equation 59) and pH 0 (equation 60) ". ... 

Several newer techniques, such as cyclic voltammetry (CV) are now used to identify a proper choice of an antioxidant. CV is an electrolytic method that uses microelectrodes and an unstirred solution, so that the measured current is limited by analyte diffusion at the electrode surface. The electrode potential is ramped linearly to a more negative potential, and then ramped in reverse back to the starting voltage. The forward scan produces a current peak for any analyte that can be reduced through the range of the potential scan. The current will increase as the potential reaches the reduction potential of the analyte, but then falls off as the concentration of the analyte is depleted close to the electrode surface. As the applied potential is reversed, it wiU reach a potential that will reoxidize the product formed in the first reduction reaction, and produce a current of reverse polarity from the forward scan. This oxidation peak will usually have a similar shape to the reduction peak. The peak current, ip, is described by the Randles-Sevcik equation ... 

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