Marine biodiversity: exploring bioactive chemical space

Abstract

This thesis is focused on exploration of Australian and Antarctica marine plants and invertebrates for the discovery of new and unusual natural products. A marine extract library (Capon) of over 2500 Australian and Antarctica marine plants and invertebrates was the platform for this research. High-throughput chemical profiling was executed by UHPLC-DAD to rapidly de-replicate, prioritise and fast track the chemical analysis of 2626 marine extracts. The acquired data was compiled as a chromatography-based library, establishing one central platform equipped with chromatography knowledge of the entire marine collection. Concurrently, assessing the biological potentials of the extract library was carried out in order to rapidly prioritise extracts. Employed biological assays included Bacillus Calmette-Guerin (BCG) screening and a5b3g2-GABAA modulatory screening. Bioassay-guided fractionation was employed to guide the discovery of bioactive natural products. The prioritised hits were subjected to extraction, isolation and purification employing various techniques such as trituration, solvent-solvent partitioning, SPE fractionation, size exclusion chromatography, and analytical/semi-preparative and preparative HPLCs. Spectroscopic methods (UV-Vis, [a]D, HRMS, and 1H, 13C, 1H-13C HSQC, 1H-1H COSY, 1H-13C HMBC, 1H-1H NOESY and 1H-15N HSQC NMR) were utilized for structural elucidations of small molecules. Additionally, QTOF-MS/MS, along with 1D and 2D NMR analysis, and C3 Marfeyrs analysis was utilized to solve small peptide structures. Furthermore, chemical derivatisation methods including total and partial acid hydrolysis, and C3 Marfeyrs method was utilized to develop an innovative 2D technique for assigning regiochemistry of cyclic peptides. To a lesser extend, additional support for characterisation of structures was also accessed through synthetic methodologies. The isolated natural products were assessed for anti-microbial and cytotoxic activities along with GABAA modulatory activity. Chapter 1 A literature review on isolation of marine sponges-derived new natural products from Australia and the Antarctica during the period of 2000 to 2016. The trend in publications resulting from discoveries of Australian sponge-derived marine natural products is also highlighted from 1976 to 2016. Chapter 2 This chapter describes the chemical and biological profiling conducted on a collection of over 2500 marine invertebrates and plants sourced out from locations across Southern Australia and Antarctica over a period of 20+ years. It further outlines the strategy involved in prioritisation of extracts and discusses the isolation and characterisation of natural products discovered through bioassay-guided fractionation of prioritised bioactive extracts. Chapter 3 This chapter illustrates the function of the in-house UHPLC-DAD based database, which is a chromatographic-based record of the chemical constituents of the marine organisms in Capon marine collection, thus allowing us to search the entire library and detect compounds with unique chromophore present in one or more organisms. We used this database to search for organisms producing dragmacidin D in our collection consisting of over 2500 samples. Consequently, we reported that an Australian marine sponge RJC-98-305 contains dragmacidin D as a racemic mixture, while sponge RJC-91-011 contains (+)-dragmacidin D as enantiomeric excess. Additionally, the isolation and characterisation of two new dragmacidin analogues, dragmacidins I and J, together with the known dragmacidin E are described. Chapter 4 In this chapter, we report the isolation, characterisation and structure elucidation of leucettazoles AnD from a southern Australian marine sponge Leucetta sp. (CMB-01047), and provide commentary on likely biosynthetic relationships and chemical stability. Leucettazoles AnC are particularly noteworthy in that they feature dual N-fused 2-aminoimidazoles, and a biphenyl ether moiety, and can be viewed as a new sub-class of 2-aminoimidazoles, where leucettazole D can be viewed as a biosynthetic monomeric precursor to leucettazoles AnC. Chapter 5 This chapter describes an investigation into the chemistry of two Australian Irciniidae (CMB-01018, Psammocinia sp. and CMB-01012, Sarcotragus sp.) sponges. These investigations afforded four new sesterterpene glycinyl-lactams, two new sesterterpene phenylethylene lactams, a new sesterterpene lactone, four known variabilin isomers, glycinyl lactam ircinialactams A and C and a C-21 furanoterpene acid. Chapter 6 This chapter accounts for a comprehensive exploration of the structural class of the CspB inhibitor (oceanapiate A) from a marine sponge Oceanapia sp. (CMB-02017). Structural characterisation portrays in-depth analysis of LC-QTOF-MS along with utilisation of synthesis revealing a new class of natural product. Chapter 7 This chapter describes method development and verification of 2D C3 Marfeyrs method using a known cyclic hexapeptide, desotamide A. The 2D C3 Marfeyrs method is an analytical approach for determining the regiochemistry of enantiomeric amino acid residues in natural products. Furthermore, we describe its application in structure elucidation of a new heptapeptide, talarolide A, accomplished by NMR data and extensive MS/MS interpretation.