Phytochemical profiling and anti-microbial studies of Australian marine and terrestrial organisms: the search for new natural product derived anti-biotic and anti-fungal agents
Creators: name: Brkljaca, Robert
Publisher: name: RMIT University
Year: 2024

description: This thesis describes the identification of sixty five secondary metabolites from thirteen separate organisms (three terrestrial and ten marine specimens). A total of twenty two new and forty two known secondary metabolites, as well as one new natural product derivative were identified by chemical profiling utilising HPLC-NMR and HPLC-MS together with traditional off-line isolation strategies. Of the new compounds identified, fourteen displayed antibiotic activity. For nine of the compounds identified the 2D NMR characterisation is now reported for the first time in addition to five carbon and one proton NMR chemical shift reassignments. The single X-ray crystallographic structures for three of the compounds identified were also reported. Five previously reported compounds are now reported as natural products for the first time. HPLC-NMR was vital in the identification of an unstable meroditerpenoid that could not be isolated by off-line approaches. This PhD study also resulted in the first report of the acquisition of a gHMBCAD NMR spectrum obtained in the stop-flow HPLC-NMR mode, without pre-concentration, microcoils or cryogenically cooled probes.<br><br>Chapters 1-3 provide an introduction to natural products, isolation techniques as applied to natural products, an explanation and examples of dereplication strategies, an outline of the common NMR experiments used for natural product identification and a review of the recent advancements made in HPLC-NMR as well as its application to natural product identification.<br><br>Chapter 4 details the limit of detection determined for five key NMR experiments in HPLC-NMR, which were found to be within the 700 ng to 1 mg range, typical of that observed in natural product crude extracts. HPLC-NMR was subsequently successfully applied to a natural product crude extract for which complete 2D NMR data could be obtained for the major component.<br><br>Chapter 5 outlines the constituents identified from the flowers and bulbs of the Australian plant, Macropidia fuliginosa. The flowers and bulbs produced very different chemistry, and a total of sixteen compounds were identified, including six new and ten previously reported compounds. The single X-ray crystal structure for three compounds is also presented.<br><br>Chapter 6 describes the identification of eleven compounds including two new phenylphenalenones and two new chromenes from the bulbs of the Australian plant Haemodorum spicatum.<br><br>Chapter 7 summarises the identification of three fatty acids from the yellow fungus Leucocoprinus birnbaumii. The positions of the double bonds were secured by ozonolysis methods.<br><br>Chapter 8 outlines the dereplication of compounds from seven marine algae. Compounds were dereplicated using HPLC-NMR and HPLC-MS data in conjunction with the MarinLit database. Sixteen compounds were dereplicated including four new compounds.<br><br>Chapter 9 highlights the identification of four new and nine known meroditerpenoids from the brown alga Sargassum paradoxum. HPLC-NMR was vital for the identification of an unstable meroditerpenoid that oxidised during off-line isolation.<br><br>Chapter 10 describes the dereplication and identification of phloroglucinols from two Cystophora species, including four new and eight known phloroglucinols. HPLC-NMR and HPLC-NMR was able to rapidly establish that both marine algae contained the same structure class of compounds.<br><br>Chapter 11 summaries the relative configuration of elatenyne which was proposed on the basis of 1D nOe NMR and chemical derivatisation methods.<br><br>