Antioxidants hierarchies

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) ... 

MORTENSEN A, SKIBSTED L H (1997) Relative stability of carotenoid radical cations and homologue tocopheroxyl radicals. A real time kinetic study of antioxidant hierarchy. FFBS Letters, 417, 261-6. 

Jorgensen, L.V., Madsen, H.L., Thomsen, M.K., Dragsted, L.O., and Skibsted, L.H. 1999. Regeneration of phenolic antioxidants from phenoxyl radicals an ESR and electrochemical study of antioxidant hierarchy. Free radical research, 30(3), 207-20. 

The reaction of eq. 16.9 will regenerate the antioxidant Arj-OH at the expense of the antioxidant At2-OH. Despite the fact that such regeneration reactions are not simple electron transfer reactions, the rate of reactions like that of eq. 16.9 has been correlated with the E values for the respective Ar-0. Thermodynamic and kinetic effects have not been clearly separated for such hierarchies, but for a number of flavonoids the following pecking order was established in dimethyl formamid (DMF) by a combination of electrolysis for generating the a-tocopherol and the flavonoid phenoxyl radicals and electron spin resonance (ESR) spectroscopy for detection of these radicals (Jorgensen et al, 1999) ... 

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. 

The hierarchy of antioxidant effectiveness in plasma has been defined from in vitro studies (Frei etoL, 1988). This, of course, must be interpreted in relation to the amount of each antioxidant as well as the rate of its reaction with the specific radical substrate and the relevance of that substrate in human plasma. The hierarchy is ... 

APIPAT. This is the patent database produced by the American Petroleum Institute and covers patents from 1964 of interest to the petrochemical industry, including petroleum refining, pollution control, uses of petrochemicals, and catalysts. Enhanced indexing includes terms applied from a hierarchical thesaurus with automatic posting to the broader terms in the hierarchy. Fragments called chemical aspects are linked to describe each compound, and the compounds are further linked to roles (eg, reactant or product) and use (eg, antioxidant or lubricant). ORBIT provides access to a merged APIPAT/WPI file, which allows searchers to draw on the strengths of both databases without the need to search them separately (95). 

Concerning these, and those previously reported, there results a hierarchy of antioxidant activity that has been proposed in which flavanone 4 -hydroxylated keeps the lowest activity and flavonol with catechol structure in the B-ring the highest one, Fig. (6). Table 2 shows the antioxidant activity of some flavonoid families obtained by different researchers. 

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