Three known iridoid glucosides (gentiournoside A, gentiournoside E and depressoside) were isolated from the flowers of Gentiana urnula Harry Sm. through activity-guided fractionations with a 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. All three compounds exhibited excellent DPPH radical scavenging activities (IC50: 10-20 μmol L(-1)) comparable to that of ascorbic acid and Trolox. However, examination of the NMR data revealed that the reported chemical structure of depressoside, previously isolated from the leaves of G. depressa, needed correcting due to incorrect elucidation around C-7 of the iridane skeleton, and was corrected to 6-β-(2,3-dihydroxyphenyl)-D-glucosyl 7-O-(2,3-dihydroxybenzoyl)-loganate. Depressoside exhibited a much higher scavenging activity against superoxide radicals (IC50: 45.5 μmol L(-1)) than the other two extracted compounds (IC50: more than 900 μmol L(-1)) due to the crucial presence of a pyrogallyl unit.
Keywords: 1,1-diphenyl-2-picrylhydrazyl (DPPH); Gang-ga Chung; Gentiana; antioxidant; depressoside; iridoid.