STRUCTURE-ACTIVITY-RELATIONSHIPS FOR ANTIOXIDANT ACTIVITIES OF A SERIES OF FLAVONOIDS IN A LIPOSOMAL SYSTEM

Structurally diverse plant phenolics were examined for their abilities to inhibit lipid peroxidation induced either by Fe(II) and Fe(III) me tal ions or by ate-derived peroxyl radicals in a liposomal membrane sy stem. The antioxidant abilities of flavonoids were compared with those of coumarin and tert-butylhydroquinone (TBHQ). The antioxidant effica cies of these compounds were evaluated on the basis of their abilities to inhibit the fluorescence intensity decay of an extrinsic probe, 3- (p-(6-phenyl)-1,3,5-hexatrienyl)phenylpropionic acid (DPH-PA), caused by the free radicals generated during lipid peroxidation. All the flav onoids tested exhibited higher antioxidant efficacies against metal-io n-induced peroxidations than peroxyl-radical-induced peroxidation, sug gesting that metal chelation may play a larger role in determining the antioxidant activities of these compounds than has previously been be lieved. Distinct structure-activity relationships were also revealed f or the antioxidant abilities of the flavonoids. Presence of hydroxyl s ubstituents on the flavonoid nucleus enhanced activity, whereas substi tution by methoxy groups diminished antioxidant activity. Substitution patterns on the B-ring especially affected antioxidant potencies of t he flavonoids. In cases where the B-ring could not contribute to the a ntioxidant activities of flavonoids, hydroxyl substituents in an catec hol structure on the A-ring were able to compensate and become a large r determinant of flavonoid antioxidant activity. (C) 1998 Elsevier Sci ence Inc.