Thesis

Phytochemical and antibacterial studies on Arctium lappa, Tussilago farfara and Verbascum thapsus

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Awarding institution
  • University of Strathclyde
Date of award
  • 2014
Thesis identifier
  • T13782
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Abstract
  • This thesis described the isolation and structure elucidation of secondary metabolites from three medicinal plants selected on the basis of their traditional use in the treatment of infectious diseases. The work also focused on the evaluation of the plant extracts and some of the isolated compounds for activity in vitro against Mycobacterium tuberculosis. Compounds obtained in suffient yield were further tested for activity in vitro against Methicillin-resistant Staphylococcus aureus. A total of 27 pure compounds and two mixtures were isolated from the three plants investigated: Arctium lappa, Tussilago farfara and Verbascum thapsus. Phytochemical investigation of the aerial parts of A. lappa led to the isolation of four terpenoids (taraxasterol, taraxasterol acetate, isololiolide and melitensin), two steroids (sitosterol/stigmasterol mixture and daucosterol), three flavonoids (quercetin, kaempferol and kaempferol-3-O-glucoside), two phenolic acids or derivatives (caffeic acid and 1, 3-dicaffeoylquinic acid) and one alkane (n-nonacosane). Isololiolide, melitensin, kaempferol-3-O-glucoside and n-nonacosane are reported for the first time from this species, and daucosterol and kaempferol are first reported from the aerial parts of this plant. Phytochemical investigation of T. farfara aerial parts led to the isolation of a monoterpene lactone (loliolide), two steroids (sitosterol/stigmasterol mixture and daucosterol), three flavonoids (quercetin, kaempferol and kaempferol-3-O-glucoside), and six phenolic acids or derivatives (p-coumaric acid, p-coumaric acid/4-hydroxybenzoic acid mixture, caffeic acid, 3, 4-dicaffeoylquinic acid, 4, 5-dicaffeoylquinic acid and methylcaffeate). Among them, loliolide is reported for the first time from this species. The aerial parts of V. thapsus afforded two pheophorbides (pheophorbide A and pheophorbide A ethyl ester), two pheophytins (pheophytin A and pheophytin B), one steroid (α-spinasterol), one known flavonoid (luteolin), one phenylethanoid glycoside (verbascoside), three simple pheonolic acids (trans-cinnamic acid, 4-hydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid) and one fatty acid (1-monoacylglycerol). All compounds, except for α-spinasterol, luteolin and verbascoside, are reported for the first time from this species. α-Spinasterol is first reported from the aerial parts of this plant. When screened for activity against M. tuberculosis in the SPOTi assay, A. lappa n-hexane extract and dichloromethane phase of methanol extact, and T. farfara n-hexane and ethyl actate extracts were active at MICs of 62.5 μg/mL; A. lappa ethyl acetate extract and T. farfara methanol extract were active at MICs of 125 μg/mL; V. thapsus ethyl acetate extract was active at the concentration of 250 μg/mL. Among the tested compounds isolated from active extracts, p-coumaric acid displayed the highest activity (MIC=31.3 μg/mL, 190.7 μM); p-coumaric acid/4-hydroxybenzoic acid mixture showed good activity (MIC=62.5 μg/mL); sitosterol/stigmasterol mixture exhibited moderate activity (MIC=125 μg/mL); loliolide, caffeic acid and trans-cinnamic acid revealed weak activity (MICs=250 μg/mL, or 1273.9, 1387.6 and 1687.4 μM, respectively). This is the first time that the antitubercular acitivity of A. lappa, T. farfara and V. thapsus has been investigated. The anti-TB activity of all tested compounds is also first reported in the SPOTi assay. When initially screened for activity against M. tuberculosis in the MABA assay at the highest concentrations of 25 or 50 μg/mL, all plant extracts and tested compounds were identified as inactive at such concentrations. This is the first report of the screening of A. lappa, T. farfara and V. thapsus extracts and of all tested compounds in the MABA assay. Among the compounds screened for activity against Methicillin-resistant S. aureus, luteolin exhibited good activity with an MIC value of 62.5 μg/mL (218.3 μM), and α-spinasterol had an MIC of 500 μg/mL. No other compound was active at the highest concentration (500 μg/mL) used in this assay. This is the first report of the investigation of the anti-MRSA activity of kaempferol, α-spinasterol, 1, 3-dicaffeoylquinic acid, 3, 4-dicaffeoylquinic acid, 4, 5-dicaffeoylquinic acid, 1-monoacylglycerol, pheophorbide A ethyl ester, pheophytin A, pheophytin B and verbascoside.
Resource Type
DOI
Date Created
  • 2014
Former identifier
  • 1036402

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