Isolation and characterisation of novel antioxidant constituents of Croton zambesicus leaf extract

Abstract

A 1,1-diphenyl-2-picrylhydrazyl (DPPH)-activity-directed fractionation was used to target antioxidant constituents of the ethyl acetate fraction obtained from a 20% aqueous methanol crude extract of Croton zambesicus leaf. Repeated column chromatography of the fraction on silica gel and Sephadex LH-20 led to the isolation of a new natural product, identified as quercetin-3-O-β-6″(p-coumaroyl) glucopyranoside-3′-methyl ether, helichrysoside-3′-methyl ether (1), along with kaempferol-3-O-β-6″(p-coumaroyl) glucopyranoside, tiliroside (2) and apigenin-6-C-glucoside, isovitexin (3) as the antioxidant constituents. The structures of the isolated compounds were elucidated using spectroscopic techniques, namely NMR (1D and 2D) and mass spectrometry. Compounds 1 and 2 are reported from this species for the first time. In the qualitative antioxidant assay, the three isolated compounds instantly bleached the DPPH (0.2% MeOH) purple colour indicating antioxidant activity. In the quantitative antioxidant assay, all the isolated compounds demonstrated weak antioxidant activity compared to quercetin and rutin used as positive control antioxidant agents. The compounds displayed little to no cytotoxicity against Vero cells in an in vitro assay. The presence of these antioxidant compounds in the leaf extract of C. zambesicus could provide a rationale for the ethnomedicinal use of the plant in the management of oxidative-stress-related diseases in folk medicine.

Keywords:

• Croton zambesicus
• antioxidant activity
• DPPH
• cytotoxicity
• quercetin-3-O-β-6″ (p-coumaroyl) glucopyranoside-3′-methyl ether, helichrysoside-3′-methyl ether

Acknowledgements

M.A. Aderogba is grateful to the Organisation for the Prohibition of Chemical Weapons (OPCW) for a Research Fellowship to the University of Botswana. L.J. McGaw acknowledges the University of Pretoria for a Research Fellowship.