Chain-breaking antioxidants polyphenols

There has been some evidence of a higher antioxidant effect when both flavonoids and a-tocopherol are present in systems like LDL, low-density lipoproteins (Jia et al., 1998 Zhu et al, 1999). LDL will incorporate a-tocopherol, while flavonoids will be present on the outside in the aqueous surroundings. A similar distribution is to be expected for oil-in-water emulsion type foods. In the aqueous environment, the rate of the inhibition reaction for the flavonoid is low due to hydrogen bonding and the flavonoid will not behave as a chain-breaking antioxidant. Likewise, in beer, none of the polyphenols present in barley showed any protective effect on radical processes involved in beer staling, which is an oxidative process (Andersen et al, 2000). The polyphenols have, however, been found to act synergistically... 

Other Physicochemical Aspects Associated to the Chain-Breaking Antioxidant Effect The Stability of the Polyphenol-Derived Free Radicals... 

POLYPHENOLS AS ANTIOXIDANTS Polyphenols as Chain-Breaking Antioxidants... 

Polyphenols could act as effective chain-breaking antioxidants (AH = POH) through the one-electron transfer reactions 4 and 5 if they produce a stable and relatively nonreactive antioxidant radical (A = PO) [Jovanovic et al., 1994], Reactions 4 and 5 can be represented by the reaction 8, where L or LOO represents the oxidant free radical. This reaction can be decomposed in two half-reactions one reduction (reaction 9) and one oxidation (reaction 10) ... 

Salah, N., Miller, N.J., Paganga, G., Tijburg, L., BolweU, GR, and Rice-Evans, C. (1995) Polyphenolic Flavanols as Scavengers of Aqueous Phase Radicals and as Chain-Breaking Antioxidants, Arch, Biochem. Biophys. 322,339-346. 

Rice-Evans C. Plant polyphenols free radical scavengers or chain-breaking antioxidants Biochem Soc Symp 1995 61 103-116. 

Salah N, Miller NJ, Paganga G, Tijburg L, Bolwell GP, et al. (1995) Polyphenolic fla-vanols as scavengers of aqueous phase radicals and as chain-breaking antioxidants. Arch Biochem Biophys 322 339-346. 

The analysis of the thermodynamic and kinetic characteristics of these reactions will allow a better understanding of the biological importance of chain breaking in the antioxidant actions of polyphenols. 

Polyphenols can act as antioxidants by a number of potential pathways. The most important is likely to be by free radical scavenging, in which the polyphenol can break the radical chain reaction. Polyphenols are effective antioxidants in a wide range of chemical oxidation systems, being capable of scavenging peroxyl radicals, alkyl peroxyl radicals, superoxide, hydroxyl radicals, nitric oxide and peroxynitrate in aqueous and organic environments [121]. This activity is due to the ability of donating an H atom from an aromatic hydroxyl group to a free radical, and the major ability of an aromatic structure to support an unpaired electron by delocalization around the 7i-electron system. Phenolic acids... 

This chain reaction can be effectively inhibited by antioxidants of many classes, of which the polyphenolic are of primary importance. Further, a simple hydrogen abstraction reaction is the most important mechanism for breaking the reaction chain and inhibiting peroxidation (Uri, 1961). 

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