Antioxidant activity of chalcones: The chemiluminescence determination of the reactivity and the quantum chemical calculation of the energies and structures of reagents and intermediates

Six antioxidants from the class of chalcones (ArOH), compounds from which flavonoids are obtained in nature, were studied. The antiradical activity of chalcones and a number of related compounds was determined by a chemiluminescence method using the scavenging of peroxide radicals ROO^· + ArOH → ROOH + OAr^· (with the rate constant k ₇) in a model reaction of diphenylmethane (RH) oxidation. The structures and energies of the reagents and intermediates were determined by semiempirical quantum chemical (PM3, PM6) calculations. 3,4-Dihydroxychalcone and caffeic acid, which have a catechol structure, that is, two neighboring OH groups in phenyl ring B, exhibited high antioxidant activity (k ₇ ≈ 10⁷ l mol⁻¹ s⁻¹); this is consistent with the lowest bond strengths D(ArO-H) of 79.2 and 76.6 kcal/mol, respectively. The abstraction of a hydrogen atom by the ROO^· radical is the main reaction path of these compounds; however, the low stoichiometric coefficients of inhibition (f = 0.3–0.7) suggest a contribution of secondary and/or side reactions of ArOH and OAr^·. In the other chalcones, the ArO-H bond is stronger (D(ArO-H) = 83–88 kcal/mol) and the antioxidant activity is lower (k ₇ = 10⁴–10⁵ l mol⁻¹ s⁻¹).