Reactions between Flavonoids and α-Hydroxyl Ethyl Peroxyl Radicals: a Pulse Radiolysis Study

Lipid peroxidation (LPO) plays an important role in many pathological processes (such as hepatitis, hepatic sclerosis, atherosclerosis, cerebral hemorrhage and so on), and flavonoids are considered to be effective LPO-inhibitors. Thus we investigated the relationship between the chemical structure of flavonoids and the LPO activity and the antioxidant mechanism of flavonoids. In this work, a-hydroxyl ethyl peroxyl radicals were produced from radiolysis of aerated ethanol to model lipid peroxyl radicals. By detecting the decay of a-hydroxyl ethyl peroxyl radicals in the presence of different concentrations of flavonoids using pulse radiolysis, the reaction rate constants of a-hydroxyl ethyl peroxyl radicals with quercetin, rutin, catechin, and baicalin are determined for the first time. The antioxidant activity of these flavonoids decreases in the order: rutin>quercetin>baicalin>catechin. Flavone and pyrocatechol were used as model compounds for the different components in flavonoids and their reaction rate constants towards a-hydroxyl ethyl peroxyl radicals were (1.7 +/- 0.1) x 10(6) and (2.9 +/- 0.1) x 10(5) mol(-1).dm(3).s(-1), respectively. The effect of chemical structure on the scavenging activity towards a-hydroxyl ethyl peroxyl radicals was investigated. The coexistence of the C-5-hydroxyl group in the A ring with the C-2=C-3 in the C ring or the conjugated double bond of the B-C ring and the catechol group in the B ring provides the best antioxidant activity. In addition, the C-2=C-3 in the C ring or the conjugated double bond of the B-C ring is more effective than the catechol group in the B ring, while the C-3-rutinose in the C ring has no obvious effect. Therefore, we conclude that the addition reaction between double bonds with peroxyl radicals plays an important role in the antioxidant activity of flavonoids in LPO.