Abstract

Background and
Purpose: Increased
incidence of resistance of microorganisms to existing synthetic antibiotics and
the harmful residual effects of these antibiotics have necessitated a
continuous search for novel antimicrobial principles from natural sources. This
study was, therefore, carried out to investigate the aerial parts of the
medicinal plant Antigonon leptopus
for potential antimicrobial properties, isolate and determine the structures of
the constituent compounds, and determine the antimicrobial efficacy of each of
the isolated compounds.

Experimental approach:

The
tannin-free ethanol extract was subjected to preparative High Performance Liquid
Chromatography (HPLC) analysis. Gradient elution was carried out with 5%
acetonitrile (MeCN) in 95% water/trifluoroacetic acid (TFA-0.05%), flow rate of
20 mL min^_1, for 0-40 min. A total of 79 fractions were collected.
The eluted fractions were detected by their absorbance between 200 and 400 nm.
Fractions with similar elution profiles were combined and concentrated under reduced
pressure on rotary evaporator to give 15 pooled fractions, from which compounds
1-4 were isolated and characterised by 1- and 2- dimensional NMR and mass
spectrometry.

Using the disk
diffusion method, compounds 1-4 were screened for antimicrobial activity against
two gram-positive bacteria, Staphylococcus
aureus (ATCC 29213) and Bacillus
subtilis (ATCC 6051), and one fungus, namely Candida albicans (ATCC 10231). The antibiotic agents, gentamycin
(25µg) and filipin (25µg), were used as positive controls in the antibacterial and
antifungal assays, respectively. Antimicrobial tests were conducted in
duplicates and zones of inhibition (mm) were expressed as the mean of the
replicate values.

Key results: Antigonon
leptopus
ethanol extract yielded four novel anthraquinones, named:
1,8-dihydroxy-6-(hydroxymethyl)-3-methoxy-2-pyrrolidinium anthraquinone (1),
1,8-dihydroxy-6-(methyl)-3-methoxy-2-pyrrolidinium anthraquinone (2), 1,8-dihydroxy-6-(hydroxymethyl)-3-methoxy-2-piperidinium
anthraquinone (3) and 1,8-dihydroxy-6-(methyl)-3-methoxy-2-piperidinium
anthraquinone (4). Compounds 1 & 3 (250µg/mL) were
active against B. subtilis and S. aureus, while no compound was active
against C. albicans.

Conclusion and Implications:

The aerial parts of A. leptopus
afforded four new anthraquinone derivatives. Two of these compounds, with more
hydroxyl groups, had significant antibacterial activity and could, therefore, serve
as chemical leads for the development of future antibiotics against infections
caused by S. aureus.