Characterization and antimicrobial activity of 4-(β-d-glucopyranosyl-1→4-α-l-rhamnopyranosyloxy)-benzyl thiocarboxamide; a novel bioactive compound from Moringa oleifera seed extract

Antimicrobial activity of crude seed extract of Moringa oleifera was investigated by thin layer chromatography bioassay against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Cladosporium cladosporioides, and Penicillium sclerotigenum; most of them were prominently inhibited by an isolate with RF 0.92–0.96. Characterization and identification of the extract revealed the occurrence of three bioactive compounds: 4-(α-l-rhamnopyranosyloxy)benzyl isothiocyanate, methyl N-4-(α-l-rhamnopyranosyloxy) benzyl carbamate (both known compounds), and 4-(β-d-glucopyranosyl-1→4-α-l-rhamnopyranosyloxy)-benzyl thiocarboxamide, existence of which in any Moringa spp. or plant is reported for the first time. The UV spectrum of the novel compound showed maximum absorption at 273 and 225 nm in MeOH while the IR spectrum revealed several characteristic bands at 3100, 2900, 1700, 1500, 1300, 1100 and 1000 cm−1. The 1H-NMR showed signals at 1.2 and 3.77 ppm and the 13C-NMR presented signals at 155, 122, 91.7 and 98.4 ppm. All the compounds at 5 mg/L had very high bactericidal activity against some of test pathogens even at contact period 1–2 h. 4-(β-d-Glucopyranosyl-1→4-α-l-rhamnopyranosyloxy)benzyl thiocarboxamide was the most potent, with 99.2 % inhibition toward Shigella dysenteriae and 100 % toward Bacillus cereus, E. coli and Salmonella typhi within 4 h of contact.