Phytochemical investigation of Alstonia Boonei for Antimicrobial activities

Abstract

Alstonia boonei was investigated with the aim of identifying bioactive compounds present in its leaves. The plant was chosen because it is one of the most widely used plants in traditional herbalimedicine. The leaves of Alstonia boonei were collected from Nakawuka village, Wakiso District, Central Uganda, in November 2017. Extraction of the crude was done by soaking the dry powdered leaves in a mixture of dichloromethane and methanol (1:1, iv/v) followed by preliminary phytochemical screening. Isolation and purification of compounds was carried out using Column Chromatography over silica gel and the extent of separation was monitored using Thin Layer Chromatography(TLC). Bioactivity of ithe crude extract, as well as one isolated compound, was determined against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Salmonella typhi Preliminary phytochemical analysis revealed the presence of tannins, flavonoids, saponins, terpenoids, cardiac glycosides and alkaloids. The isolated compounds were identified with the help of a combination of both 1D and 2D NMR spectroscopic techniques. Two compounds were isolated and identified; viz: an amide derivative, trans fagaramide (1) and a triterpene, lupeol i(2). Compound 1 is hereby reported for the first time from the genus Alstonia. The crude extract exhibited strong antibacterial activity against E. coli in the disc diffusion assay with a diameter zone of inhibition of 20 mm and moderately active against P. aeruginosa and S. aureus (diameter zone of inhibition of 10 and 9 mm, respectively), but it was inactive against S. typhi. Trans-fagaramide showed moderate antibacterial activities in the disc diffusion assay with zones of inhibition (diameter) of 13, 12, 10 and 10 imm against S. typhi, P. aeruginosa, S. aureus and E. coli, respectively. The minimum inhibitory concentration of trans-fagaramide was 125 mg/mL against P. Aeruginosa and 250 mg/mL against the rest of the test bacterial strains. The results in this study justify the use of Alstonia boonei in the management of infectious diseases by traditional healers.