Antioxidant and structure-activity relationships of five tetraoxygenated xanthones from Swertia minor (Griscb.) Knobl.

Antioxidant activity-guided fractionations of chloroform extract of Swertia minor yielded five known tetraoxygenated xanthones (1-5), of which compound 3 was isolated for the first time from plant sources. Compounds 1-5 were identified with the aid of extensive NMR spectroscopic studies. A relationship between the structural features of 1-5 and their antioxidant activity was also determined. Among these bioactive isolates (1-5), compound 4 exhibited strong scavenging effect in DPPH (IC(50) = 10.31 µg mL(-1)), FRAP (8536.32 ± 34.1 Mmol Fe (II) g(-1)) and metal chelating (17.2 ± 0.88 mg EDTA g(-1)) assays. Compound 5 was potently active in secondary antioxidant assay (i.e. binding to metal ions strongly); however, its primary antioxidant capacity was found to be feeble.