Isolation and characterization of phytochemicals from the stem barks of lepisanthes rubiginosa (Roxb.) and its antibacterial activity / Siti Zafirah Zulkifli

Lepisanthes rubiginosa, known as ‘Pokok Mertajam’ among Malaysian, is a plant traditionally used to treat various ailments and possesses a broad spectrum of antimicrobial activities. This study was purposely conducted to discover the phytochemicals of the stem barks of L. rubiginosa using a dereplication strategy for early chemical profile analysis prior to isolation. The stem barks of L. rubiginosa were extracted using hexane, dichloromethane, and methanol. Each extract was screened for its antibacterial activity against gram-positive (Staphylococcus haemolyticus and Methicillin-resistant Staphylococcus aureus) and gram-negative (Klebsiella pneumonia, Pseudomonas aeruginosa, Salmonella typhimurium, Shigella sonnei, and Escherichia coli) bacteria. The hexane extract exhibited significant activity towards S. haemolyticus. Further examined using Gas-Chromatography Mass Spectrometry (GCMS), and has successfully identified palmitic acid as a major constituent with percent of abundance of 45%. Mass-based dereplication strategy using an online mass database revealed the presence of saponins, terpenoids, phenolic compounds, flavanols, butanolide, and amino acids in the dichloromethane and methanol extracts. Isolation and purification of methanol extract of L. rubiginosa led to the characterization of five saponins and one farnesyl glycoside. Four out of six are new compounds, namely 3-O-{β-D-glucopyranosyl-(1→2)-[α-D-xylopyranosyl(1→3)]-β-D glucopyranosyl}oleanolic acid (lepiginoside A) (128), 3-O-{β-D-glucopyranosyl- (1→2)-[4-O-acetyl-α-D-xylopyranosyl(1→3)]-β-D-glucopyranosyl}oleanolic acid (lep iginoside B) (129), 3-O-{β-D-glucopyranosyl-(1→2)-[4-O-acetyl-β-D-xylopyranosyl (1→3)]-β-D-glucopyranosyl}-oleanolic acid (lepiginoside C) (130), and 1-O-{α-L arabinopyranosyl(1→6)-[β-D-glucopyranosyl(1→3)-α-L-rhamnopyranosyl-(1→2)]-α-L -arabinopyranosyl(1→3)]-β-D-glucopyranosyl}-all-trans-farnes-1-ol (lepiginoside D) (131), whilst lepisantheside A (19) and gleditsoside C (132) are known compounds. All structures were elucidated using 1D- and 2D-NMR spectroscopy, supported by MS, UV and IR data. The isolated compounds were tested against the same bacteria strains previously used to screen the crude extracts.