Chemical constituents from the stem bark of Pentaclethra macrophylla Benth (Fabaceae)

Abstract

Background: Plants have served as source of lead discovery in drug development; there is the need to look into our ethnomedicinal plants for the purpose of identifying bioactive antibacterial agents to combat the growing antibacterial resistance to current drugs.

Objective: The present study investigated the antibacterial activity of the extract and soluble fractions of Pentaclethra macrophylla against some selected pathogens and isolation of active constituents using chromatography and spectroscopic techniques.

Materials and methods: The pulverized stem bark of P. macrophylla was extracted to exhaustion with 70% methanol and the combined crude methanol extract after removal of solvent was partitioned with ethyl acetate and n-butanol to give ethyl acetate and n-butanol soluble fractions. Antibacterial activity was evaluated on the crude methanol extract,ethylacetate and n-butanol fractions against five pathogenic bacteria using agar diffusion assay method. The active ethylacetate and n-butanol fractions were subjected to Column chromatography and subsequent purification over sephadex LH-20 afforded compounds I, II and III. The structures of these compounds were elucidated using NMR and MS and are reported in this plant for the first time.

Results: Antibacterial activity showed that the extract and fractions at 10mg/mL showed activity against B. subtilis with zones of inhibition of 8.0 ± 2.89, 16.0 ±1.73 and 9.5 ± 1.78 respectively for 70% methanol extract, ethyl acetate, and n-butanol fractions. Ethyl acetate soluble fraction at (10mg/mL) had a good activity against B. subtilis and S. aureus with inhibition zones of 16.0 and13.5mm compared with Levofloxacin (1.5μg/mL) having zones diameter of 21.1 and 16.6mm, however, none of the extract or fractions showed activity against K. pneumonae. Compound I was identified as methyl gallate, compound II: Bergenin and compound III: 11-O-galloylbergenin. The structures of these compounds were elucidated using NMR and ESI-MS and compared with literature.

Conclusion: Finding from this work showed that the ethylacetate fraction is the most active and compounds I, II were isolated from the fraction, while the n-butanol furnished compound III. These compounds are being reported for the first time in this plant and have been known to possess antibacterial activity, thereby given credence to the ethnomedicinal use of this plant against infections.

Keywords: Pentaclethra macrophylla, Antibacterial, Methyl gallate, Bergenin, 11-O-galloylbergenin