Antioxidant and free radical-scavenging properties of three flavonoids isolated from the leaves of Rhamnus alaternus L. (Rhamnaceae) : A structure-activity relationship study

Fractionation of the methanolic and total oligomer flavonoid enriched extracts from Rhamnus alaternus leaves resulted in the isolation of three flavonoids: kaempferol 3-O-isorhamninoside (1), rhamnocitrin-3-O-isorhai-nninoside (2) and rtiamnetin-3-O-isorhamninoside (3), along with apigenin, kaempferol and quercetin. The structures were determined using data obtained from FAB-MS, H-1 and C-13 NMR spectra, as well as by various correlation experiments (COSY, HMQC and HMBC). The antioxidant activities of the isolated compounds were evaluated by measuring their ability to scavenge the DPPH radical and superoxide anions, to inhibit H2O2-induced lipid peroxidation in human K562 cells, and to inhibit xanthine oxidase activity. Compound 3 was a strong scavenger of DPPH center dot and superoxide anion radicals, and a potent inhibitor of H2O2-induced lipid peroxidation, with respective IC50 values of 1.5, 35 and 106 mu g/ml, whereas compound I showed the better activity in the inhibition of xanthine oxidase activity with an IC50 value of 18 mu g/ml, showing some structure-activity relationships, (C) 2009 Elsevier Ltd. All rights reserved.