Activity-guided isolation, identification and quantification of biologically active isomeric compounds from folk medicinal plant Desmodium adscendens using high performance liquid chromatography with diode array detector, mass spectrometry and multidimentional nuclear magnetic resonance spectroscopy

Abstract

The antioxidant activity of the crude extract (60% ethanol) from the leaves of Desmodium adscendens (Sw.) DC. (Fabaceae) was observed in DPPH, xanthine/xanthine oxidase, lipid peroxydation and neutrophils burst tests. Further activity-guided fractionation on C18 column (water, 20% methanol, 50% methanol and 100% methanol) resulted in the separation of the fraction (50% methanol) with the highest antioxidant capacity. HPLC-DAD analysis of biologically active fraction revealed the presence of two pairs of flavonoid isomers as the dominant constituents. Those compounds were isolated and purified by multi-step liquid column chromatography (Sephadex LH20). Their structures were elucidated by various spectroscopic techniques, including NMR, UV and MS. Based on 1D and 2D NMR spectra as well as ion fragmentation, flavonoids were identified as: isovitexin 2″-O-xyloside (1), vitexin 2″-O-xyloside (2), vitexin (3) and isovitexin (4). The hybrid HSQC-DEPT technique provided very fast determination of the glycosylation positions in aglycone and the type of glycosidic bond in the flavonoid isomers. This study provides novel information concerning identity of the major compounds present in the leaves of D. adscendens cultivated in Ghana, which broadens the knowledge about anti-inflammatory, antiallergic and antioxidant properties of their extracts.