Kinetic Study of Free-Radical-Scavenging Action of Flavonoids in Homogeneous and Aqueous Triton X-100 Micellar Solutions

Abstract

Kinetic study of free-radical-scavenging action of six kinds of flavonoids (flavone, chrysin, flavonol, apigenin, rutin, and quercetin) has been performed. The second-order rate constants for the reaction of flavonoids with aroxyl (ArO•) (ks) and 5,7-diisopropyltocopheroxyl (Toc•) radical (kr) have been measured in ethanol, 2-propanol/water (5:1, v/v), and aqueous Triton X-100 micellar solution (5.0 wt %). The rate constants (ks and kr) observed for flavone, chrysin, and flavonol are very slow, indicating that the reactivities of 5- and 7-OH groups at A-ring and 3-OH group at C-ring are very weak and almost negligible. Rutin and quercetin with 3‘- and 4‘-OH groups at B-ring showed high reactivity, indicating that the o-dihydroxyl (catechol) structure in the B-ring is the obvious radical target site for flavonoids. The rate constants (ks and kr) obtained in micellar solution showed notable pH dependence. For instance, both the ks and kr values of rutin increased with increasing pH value from 4 to 11. Rutin is a tetrabasic acid and can exist in five different molecular forms, depending on the pH value. By comparing the ks values with the mole fraction (f) of each molecular form of rutin, the reaction rate ks1 for undissociated form (RuH4), ks2 for monoanion (RuH3-), ks3 for dianion (RuH22-), and ks4 for trianion (RuH3-) were determined; the values are 9.5 × 10 M-1 s-1, 4.0 × 102 M-1 s-1, 3.8 × 103 M-1 s-1, and 4.0 × 103 M-1 s-1, respectively. The reaction rates (ksi) increase remarkably with increasing the anionic character of rutin, that is, the electron-donating capacity of rutin. It was found that quercetin and rutin have high activity in vitamin E regeneration.