Isolation and Chemical Structural Characterization of the Mixture of Two Related Phytosterols from Ricinus communis L. (Euphorbiaceae) Leaves

Aim: Ricinus communis, a member of the family Euphorbiaceace, is widely used in South African traditional medicine for the treatment of wounds, sores and boils. The plant was selected based on a high antibacterial activity demonstrated in a previous In vitro study. There is a need to isolate compounds that are responsible for antibacterial activity of R. communis leaves.

Methodology: The leaves of R. communis were milled to a fine powder and sequentially extracted with n-hexane, dichloromethane, acetone, and methanol using a serial exhaustive extraction method. Thin layer chromatography was used to analyse the phytochemical components of the extracts and bioautography for the presence of antibacterial compounds. A pure compound was isolated using column chromatography coupled with preparative thin layer chromatography and the structure was elucidated by nuclear magnetic resonance.

Results: Hexane extracts had potent antibacterial activity against Escherichia coli and Enterococcus faecalis and low activity against Pseudomonas aeruginosa and Staphylococcus aureus. The average minimum inhibitory concentration value of hexane crude extracts was 0.47 mg/ml, followed by extracts of dichloromethane (0.94 mg/ml), acetone (0.39 mg/ml) and methanol (1.49 mg/ml). The bioassay guided fractionation of the hexane extract led to the successful isolation of compound 1 at the Rf value of 0.40. Compound 1 was identified as a mixture of stigmasterol and β-sitosterol.

Conclusion: Compound 1 lost its antibacterial activity during the purification process. This may lend support to the theory that some compounds present in plant extracts act synergistically to give a particular biological activity. This study represents the first report of the isolation of stigmasterol and β-sitosterol from the leaves of R. communis.