ANTIOXIDANT ACTIVITY OF FLAVONOIDS - EFFICIENCY OF SINGLET OXYGEN (1(DELTA)G) QUENCHING

Flavonoids, polyphenolic pigments widely present in plants, have been reported to act as scavengers of various oxidizing species. However, m ost often an overall antioxidant effect was measured. In this paper we report the results of a systematic study of the reactivity of 13 sele cted flavonoids (from the flavonol, flavone, flavanone and flavane fam ilies) with singlet oxygen (O-1(2)(1DELTA(g))) in order to establish a structure-activity relationship. The rate constants of the chemical r eaction of these flavonoids with O-1(2)(k(r)) and their rate constants Of O-1(2) physical quenching (k(q)) have been determined by kinetic m easurements and neaT-IR singlet oxygen luminescence. The efficiency of the physical quenching is mainly controlled by the presence of a cate chol moiety on Ting B, whereas the structure of ring C (particularly t he presence of a hydroxyl group activating the double bond) is the mai n factor determining the efficiency of the chemical reactivity of thes e compounds with O-1(2). The total reactivity scale is dominated by k( q), which is in general higher than k(r). (+)-Catechin is the most eff icient quencher of the series.