Antioxidants oxidation-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). 

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

In the case of food, antioxidants are substances with the ability to delay or prevent the development of rancidity and deterioration of sensory attributes related to flavors and aroma and also function as oxidation inhibitors or retarders. The effectiveness of these additives depends on a number of factors, like intrinsic factors, such as the composition (lipids, carbohydrates and proteins), pH, water activity and oxide reduction potential extrinsic factors, such as temperature, storage time, and humidity and atmospheric conditions processing factors and microbial factors, such as the type and quantity of microorganisms, resilience microorganisms and cellular composition (Davidson and Taylor 2007). 

In thermodynamic terms, however, thiols are not the most effective cellular antioxidants. The reduction potentials of key hydrophilic antioxidants such as thiols, and uric and ascorbic acids vary with pH because of ionization of the ground states, but at pH values around 7 the reduction potential of the glu-tathionyl radical, E(GS H /GSH) is ca. 0.8-0.9 V, whereas the corresponding values of the radicals from oxidation of urate and ascorbate are ca. 0.6 V and 0.3 V, respectively [11, 12]. These values imply that thiols are more difficult to oxidize than urate or ascorbate at physiological pH values, although as we shall see... 

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. 

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. 

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... 

Many antioxidants quoted as potential protective agents against free-radical-induced DNA damage have more than one phenolic group. Their chemistry is, therefore, also of some interest in the present context. The semiquinone radicals, derived from hydroquinone by one-electron oxidation or from 1,4-benzoqui-none by one-electron reduction, are in equilibrium with their parents (Roginsky et al. 1999), and these equilibria play a role in the autoxidation of hydroquinone (Eyer 1991 Roginsky and Barsukova 2000). Superoxide radials are intermediates in these reactions. 

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. 

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 ... 

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. 

In this review, the concept of polyphenols as a global antioxidant is considered not appropriate to understand the potential health benefits of such molecules, as supported by recent updates of the concept of oxidative stress. Rather, polyphenols may modulate discreet redox pathways. In addition, I discuss the potential and acute biological impact of polyphenols (flavonoids and phenolic acids) as governed by their reduction potential and lipophilic properties, in connection with two notions ... 

Thus, in addition to a suitable reduction potential to annihilate oxidizing radicals, a safe decay of the polyphenol-derived aroxyl radical, which strongly depends on their bimolecular disproportionation reaction and electron delocalization, is a characteristic that affords an adequate antioxidant activity. 

There has been considerable interest in recent years in the cytoprotective and neuroprotective effeets of flavonoids, especially in the context of their modes of action as antioxidants. The eleetron-donating properties of flavonoids are well defined to explain their antioxidant properties in vitro [10-14]. Structurally important features defining the reduction potential of flavonoids are the hydroxy-lation pattern, a 3, 4 -dihydroxy eatechol structure in the B-ring, the planarity of the molecule, and the presenee of 2,3 unsaturation in conjugation with a 4-oxo-function in the C-ring. Many studies have described the antioxidant efficacy of flavonoids and demonstrated that these polyphenols can inhibit the oxidation of lipids [12,15,16] and other biomolecules such as proteins and DNA [17-19] in... 

Although the stepwise oxidation concept for antioxidant pecking order [12] is consistent with the direct observation [13] that thiyl radicals are repaired by ascorbate, the reactivity of thiyl radicals towards other cellular, hydrophilic antioxidants of intermediate radical reduction potential has not been considered here. One possible candidate, now being explored in the author s laboratory, is the reactivity of urate. This antioxidant has a reduction potential of the radical/reductant couple close to midway between the potentials of thiyl and ascorbyl [11, 12]. Some studies of the antioxidant reactions of urate have already been described [122], but further work is desirable in view of the rather high concentrations of urate in biological fluids and tissues and its depletion during... 

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