Antioxidant activity of flavonoids: Efficiency of singlet oxygen ( 1Δ g) quenching

Abstract

Flavonoids, polyphenolic pigments widely present in plants, have been reported to act as scavengers of various oxidizing species. However, most often an overall antioxidant effect was measured. In this paper we report the results of a systematic study of the reactivity of 13 selected flavonoids (from the flavonol, flavone, flavanone and flavane families) with singlet oxygen ( 1O 2( 1Δ g)) in order to establish a structure—activity relationship. The rate constants of the chemical reaction of these flavonoids with 1O 2( k r) and their rate constants of 1O 2 physical quenching ( k q) have been determined by kinetic measurements and near-IR singlet oxygen luminescence. The efficiency of the physical quenching is mainly controlled by the presence of a catechol moiety on ring B, whereas the structure of ring C (particularly the presence of a hydroxyl group activating the double bond) is the main factor determining the efficiency of the chemical reactivity of these compounds with 1O 2. The total reactivity scale is dominated by k q, which is in general higher than k r. (+)-Catechin is the most efficient quencher of the series.