Phytochemistry and quorum sensing inhibitory studies of four vernonia species growing in Nigeria.
Abstract This thesis contains the phytochemical analysis of four medicinal Vernonia species growing in Nigeria and used in ethnomedicine to treat a variety of medical conditions. The four Vernonia species studied were Vernonia blumeoides, Vernonia perrottetii, Vernonia ambigua and Vernonia glaberrima. The thesis also contains a comprehensive review of the sesquiterpene lactones from the genus Vernonia, their structural diversity and biosynthetic considerations. For each of the plants, the crude extracts and selected compounds were tested for their antibacterial activity using the traditional disc diffusion and broth microdilution as well as anti-quorum sensing assays. Three of the four plants studied yielded sesquiterpene lactones, Vernonia blumeoides yielded four novel eudesmanolide sesquiterpene lactones (blumeoidolides A-D), Vernonia perrottettii yielded a novel keto-hirsutinolide 13-acetoxy-1(4β),5(6)β-diepoxy-8α-(senecioyloxy)-3-oxo-1,7(11)-germacradiene-12,6-olide B1 and the known keto-hirsutinolide 13-acetoxy-1,4β-epoxy-8α-(senecioyloxy)-3-oxo-1,5,7(11)-germacratriene-12,6-olide B2 and Vernonia ambigua yielded a novel glaucolide sesquiterpene, 5,6-dehydrobrachycalyxolide. The structures of the sesquiterpenes were determined from their 1H, 13C and 2D NMR spectra along with mass spectra. The crystal structure of one of the eudesmanolide sesquiterpenes allowed the configuration of the stereocentres in the molecule to be determined. In addition to the sesquiterpene lactones, some common sterols and flavonoids were isolated from the plants: stigmasterol was isolated from V. blumeoides, lupeol was isolated from V. blumeoides, V. ambigua and V. perrottettii and lupeol acetate from V. ambigua and V. perrottetti. The flavonoid apigenin was found in V. blumeoides, V. perrottetti and V. glaberrima, luteolin in V. blumeoides and V. perrottetti, velutin in V. perrottetti and V. glaberrima and chrysoeriol in V. ambigua and V. glaberrima. Chrysin was found only in V. blumeoides and luteolin 3',4'-dimethyl ether in V. glaberrima. Several of the isolated sesquiterpene lactones showed good anti-quorum sensing inhibition (QSI). QSI ≥80% was obtained for blumeoidolide A at a concentration ≥ 0.071 mg mL-1, blumeoidolide B (≥ 3.6 mg mL-1) and B1 (1.31 mg mL-1), QSI ≥75% for B2 (0.33 mg mL-1) and QSI ≥84% for 5,6-dehydrobrachycalyxolide (2.6 mg mL-1). The sterols, lupeol and lupeol acetate, were also found to have QSI ≥84% at 2.6 mg mL-1. Molecular docking studies carried out on blumeodolides A-D in the binding sites of CviR and CviR' (transcription activator proteins) suggested that these molecules are able to bind to certain domains in the target protein, thus eliciting an effect. The current work adds to the library of sesquiterpene lactones from the genus Vernonia and provides some lead compounds to antibacterial activity via quorum sensing inhibition.
Doctoral Degree. University of KwaZulu-Natal, Durban.