Antioxidant and antiradical activities of flavonoids.

The relationship between the structure of 42 flavonoids and their antioxidant and antiradical activities was elucidated by heat-induced oxidation in a beta-carotene and linoleic acid system and by the 1,1-diphenyl-2-picrylhydrazyl decoloration test. From seven structurally divergent groups of flavonoids, only flavonols with a free hydroxyl group at the C-3 position of the flavonoid skeleton showed high inhibitory activity to beta-carotene oxidation. Antiradical activity depended on the presence of a flavonol structure or free hydroxyl group at the C-4' position. The effect of the 4'-hydroxyl was strongly modified by other structural features, such as the presence of free hydroxyls at C-3 and/or C-3' and a C2-C3 double bond.