Chemical profiling, isolation, and characterization of bioactive compounds from Rhynchosia minima var prostrata and Phragmites mauritianus

Abstract

Indigenous medicine is a common form of healthcare in the rural parts of Africa, South Africa not excluded. Traditional healers have ethnomedicinal reports documented or passed from one generation to another by word of mouth. This knowledge is used for treating diverse ailments in their communities. Some of the medicinal plants used by South African rural dwellers are Rhynchosia minima (L.) DC. var prostrata (Fabaceae) and Phragmites mauritianus Kunth (Poaceae). This study sought to identify, isolate and characterize potential bioactive compounds from the leaves of R. minima and the culm of P. mauritianus provided by a traditional healer. The leaves of Rhynchosia minima were extracted sequentially with hexane, dichloromethane, ethyl acetate and methanol. Phytochemical study of the dichloromethane, ethyl acetate and methanol extracts led to the isolation of eight compounds, including six flavonoids, a norisoprenoid and a cyclitol. These were vitexin, isovitexin, isoorientin, pinitol, loliolide, tectorigenin, ayanin and quercetin. The structures of the compounds were elucidated on the basis of their 1D and 2D NMR and HRESIMS data. The observed data were also compared to values reported in literature. Seven of the eight isolated compounds were reported for the first time in this plant. The extracts obtained had activities ranging from weak to moderate antifungal activity. The hexane extract with an MIC of 312.50 µg/mL had the best activity against C. albicans while the hexane, dichloromethane and methanol extracts had similar moderate activities against C. neoformans with an MIC value of 625.00 µg/mL. The antifungal assay revealed that the minimum concentration of pinitol, loliolide, ayanin and quercetin to inhibit the growth of Candida albicans were 125.00, 6.25, 62.50, and 6.25 µg/mL, respectively. Loliolide and quercetin had similar activities to the positive control, amphotericin B. Against v Cryptococcus neoformans, loliolide and quercetin with minimum inhibitory concentration (MIC) of 6.25 µg/mL had more potent activities than amphotericin B (MIC = 12.50 µg/mL). Pinitol and ayanin (MIC = 125.00 µg/mL) exhibited weak activities against C. neoformans. The dichloromethane and ethyl acetate extracts of R. minima were chemically profiled with UPLC-QTOF-MS. This led to the identification of thirty-three compounds, including flavonoids, glycosides, norisoprenoids, fatty acids and polyols, in the positive and negative electrospray ionization modes. The chemical constituents were identified by their exact masses, fragmentation patterns and comparison using online natural product databases. The phytochemistry of Phragmites mauritianus was investigated for the first time, and this yielded six phenolic compounds, trans-p-coumaric acid, vanillic acid, p-hydroxybenzaldehyde, syringaldehyde, cis-p-coumaric acid, vanillin and a lignan, 4-ketopinoresinol. Four of the isolated compounds have never been isolated from the Phragmites genus. The isolated compounds could not be screened biologically due to the quantity isolated or rapid decomposition. More compounds need to be isolated from this plant and screened for bioactivity. The biological activities displayed by the isolated compounds, alongside those reported in literature, warrant further investigation into their development as bioactive agents.