STUDY ON THE INHIBITORY EFFECT OF TANNINS AND FLAVONOIDS AGAINST THE 1,1-DIPHENYL-2-PICRYLHYDRAZYL RADICAL

Fifty-one tannins and forty-one flavonoids isolated from Oriental medi cinal herbs were evaluated for their antioxidant ability with a 1,1-di phenyl-2-picrylhydrazyl (DPPH) radical-generating system. The results showed that tannins and certain flavonoids are potential free-radical scavengers, and that their activity against the DPPH radical is closel y associated with their chemical structure. A comparison of the two cl asses of compounds showed that tannins have more potential than flavon oids because almost all the tannins demonstrated significant scavengin g action within a low concentration range, whereas the activity of fla vonoids varied distinctively among the different compounds. An increas e of galloyl groups, molecular weight, and ortho-hydroxyl structure en hanced the activity of tannins, whereas the number and position of hyd roxyl groups were important features for the scavenging of free radica ls by flavonoids, Moreover, it appeared that when the free hydroxyl gr oup was methoxylated or glycosylated, the inhibitory activity was obvi ously decreased or even abolished. (C) 1998 Elsevier Science Inc.