Isolation and characterization of 3,3′-di-O-methyl ellagic acid from the root bark of Afzelia africana and its antimicrobial and antioxidant activities

Abstract

Afzelia africana is a tropical plant with extensive economic and medicinal potential. The plant is reported to have vast secondary metabolites and has been utilized for the treatment of various diseased conditions ethnomedically but no reports of compounds have been isolated from the plant. The study aimed to isolate bioactive compounds from the bark, purify and identify them using column chromatography, spectroscopic and spectrometric methods, and determine their antimicrobial and antioxidant activities. Antimicrobial and antioxidant assays were performed on the identified compound. Column chromatographic fractionation of the methanol extract of the bark of the plant using a solvent system of petroleum ether-ethyl acetate (95:5, 90:10, 85:15, and 70:30) afforded several fractions which were pooled together based on a thin-layer chromatographic profile. The fractions from the solvent system of 85:15 after sub-fractionation using petroleum ether-ethyl acetate (70:30) afforded a 20 mg yellow powder. The isolate was characterized based on 1H, 13C, DEPT-135, HSQC, HMBC, and ESI-MS and unequivocally identified as 3,3′-di-O-methyl ellagic acid. The antimicrobial activity of the compound was tested against Streptococcus mutans, Staphylococcus aureus, Escherichia coli, methicillin-resistant Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumonia, Pseudomonas aeruginosa, Salmonella typhi, and Candida albicans 1 and 2. The compound recorded percentage inhibitions ranging from 40-100% and 0-30% at 6.25-1.56 and 0.78–0.20 mg/mL, respectively. The highest level of inhibition was reported for Escherichia coli (35.0-92.5 %), Salmonella typhi (100%), and Candida albicans 1 (100%) over the concentration range of 6.25-0.20 mg/mL. The compound also reported a broad spectrum of bactericidal activity against all the microbial strains with corresponding MBC/MIC ratios of ≤ 4. The IC50 values for the compound in the ABTS and DPPH scavenging assays were 5.76 ± 0.06 and 9.97 ± 0.27 µg/mL, respectively. This paper reports for the first time the identification of 3,3′-di-O-methyl ellagic acid from the bark of Afzelia africana.