ABSTRACT

This research work was carried out to identify the bioactive constituents of the leaves, stems and roots of Lactuca (Launaea) taraxacifolia, a medicinally important plant of the Asteraceae family. The plant was selected for this study due to its widespread use in traditional medicine and on the basis that the plant is almost going into extinct in Nigeria. The present study deals with the extraction, isolation and characterization of the bioactive compounds of the chloroform extracts of the leaves, stems and roots of L. taraxacifolia. The extraction was done by subjecting the air dried pulverized samples into the chloroform solvent for a period of some days, followed by filtration and concentration. Separation and purification of the various constituents of the crude extracts were carried out using thin layer chromatography and column chromatography. The TLC and column chromatography were carried out using different mobile phases and the solvent blends used were: normal hexane, chloroform, ethyl acetate and methanol. Qualitative TLC on precoated aluminium silicate strips was used to monitor the column fractions to ascertain the degree of purity of the eluents. The structural elucidation of the isolated compounds was deduced using MS, FTIR, NMR (¹H and ¹³C) and 2D COSY NMR Spectra. This chemical investigation resulted in the isolation of two novel compounds: SB2 (Octadecahydro-4,4-dimethoxy-12b, 14b-dimethyl-10-(napthalen-2-yl) Picen-3-(4H, 6bH, 14bH)-one and SB3 (Octadecahydro-10-(1, 2, 3, 4-tetrahydronaphtalen-3-yl)-4,4-dimethoxy-6b, 12b, 14b-trimethyl Picen-3-(4H, 6bH, 14bH)-one. Compound SB2 was isolated using 90% chloroform and 10% ethyl acetate at an Rf value of 0.821, while compound SB3 was isolated using 50% hexane and 50% chloroform at an Rf value of 0.692. The MS spectrum of compound SB2 gave a molecular ion peak at m/z 527.75, which corresponds to a molecular formular of C₃₆H₄₇O_3.The MS spectrum of compound SB3 gave a molecular ion peak at m/z 545.81, which corresponds to a molecular formular of C₃₇H₅₃O₃. These terpenoid derivatives characterized by the presence of carbonyl and methoxy groups accounted for the antimicrobial, anti-tumor and anti-oxidant properties of the plant.The antimicrobial activities of the crude extracts were carried out using the disc diffusion technique. Fifteen micro-organisms (five-Gram positive, five Gram-negative and five fungi) were selected for the antimicrobial screening of the solvent extracts of the leaves, stems and roots of L. taraxacifolia. The zone diameters of inhibition of the extracts ranged from 10.1 mm to 17.2 mm. In all cases, the phytochemicals in the chloroform extracts had greater broad spectrum effects when compared to other extracts used in this work, thus inhibiting all the test bacteria used in this work, though with varying diameters of inhibition, while the phytochemicals in methanol extract had the least effect, inhibiting six out of the ten bacteria used in the study. Generally, the extracts had negligible inhibition effects on the selected fungi. Phytochemicals in n-hexane and ethyl acetate were only effective on Candida albicans at zone diameters of inhibition of 17.2 mm and 15.2 mm respectively. A major set-back associated with the intake of herbal medicine is the problem of overdose, which may lead to the destruction of tissues and cells. Hence the minimum inhibitory, bactericidal and fungicidal concentrations of the extracts were carried out. The results showed that the minimum inhibitory concentrations (MIC) of the extracts ranged from 200 mg/ml to greater than 250 mg/ml. Some of the extracts had minimum bactericidal concentration (MBC) of 250 mg/ml, which is the concentration of most conventional drugs.