Rate constants for the reaction of peroxyl free radical with flavonoids and related compounds as determined by the kinetic chemiluminescence method

Abstract

The kinetic chemiluminescence method was applied to determine rate constants (k_s) for the reaction of the peroxyl radical from diphenylmethane, ROO˙, with a number of flavonoids, catechol derivatives as well as two ‘standard’ phenolic antioxidants at 50 °C in chlorobenzene. It was found that the kinetics of the interaction of ROO˙ with the majority of flavonoids and catechols is complicated by a slow process which could be optimally simulated by a mono-molecular transformation of the respective semiquinone radicals. The rate constants for this transformation (k₁₇), indirectly derived from the kinetics of chemiluminescence are listed in parentheses. The following rate constants k₅(in dm^–3 mol^–1 s^–1) and ^k₁₆(in s^–1) have been obtained: quercetin, 2.1 × 10⁷(2.0); dihydroquercetin, 1.9 × 10⁷(0.92); luteolin, 2.2 × 10⁷(0.8); catechin, 6.6 × 10⁶(7.5); fisetin, 1.2 × 10⁷(1.1); naringenin, 3.4 × 10³(0.2); kaempferol, 1.0 × 10⁶(0.8); caffeic acid, 1.5 × 10⁷(1.9); 3,5-di-tert-butylcatechol, 1.9 × 10⁷(0.65); nordihydroguaiaretic acid, 1.0 × 10⁷(∼ 0); α-tocopherol, 8.5 × 10⁶(0); 2,6-di-tert-butyl-4-methylphenol (BHT), 3.6 × 10⁴(0).