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-isorhamninoside (2) and rhamnetin-3-O-isorhamninoside (3), along with apigenin, kaempferol and quercetin. The structures were determined using data obtained from FAB–MS, 1H and 13C 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 and superoxide anion radicals, and a potent inhibitor of H2O2-induced lipid peroxidation, with respective IC50 values of 1.5, 35 and 106 μg/ml, whereas compound 1 showed the better activity in the inhibition of xanthine oxidase activity with an IC50 value of 18 μg/ml, showing some structure–activity relationships.