Bioassay and NMR-Guided Isolation of Natural Products from Pacific Marine Invertebrates
In the present study, bioassay and NMR approaches were used in combination to guide isolation of two new and three known compounds. An extract library of New Zealand and Tongan marine invertebrates was screened for activity inhibiting the growth of Saccharomyces cerevisiae. From this, ‘hits’ were identified and validated and three New Zealand sponges and a Tongan ascidian were chosen for bulk extraction. These invertebrates were extracted in methanol and purified using normal and reversed-phase chromatography to isolate the compounds of interest. The isolation of compounds was guided by either inhibitory activity towards S. cerevisiae, 1H NMR spectroscopy or a combination of the two. The known trihydroxylated steroid (14) was isolated from the calcareous sponge Leucosolenia sp. A fatty acid and mixture of phthalate esters were isolated from an unidentified Tongan ascidian. The fatty acid was proposed to be cis-vaccenic acid (13) on the basis of 1H and 13C NMR data. The phthalate ester mixture was not purified further. Work on these two compounds was discontinued due to a lack of activity observed in the S. cerevisiae assay and the well-known nature of fatty acids and phthalate esters. Work was also done on an assay targeting the large mechanosensitive channel (MscL) of bacteria. Some parameters were optimised for this assay, however some work remains to be done. The screening of the extract library of New Zealand and Tongan marine invertebrates using this assay was unsuccessful. More studies to better understand the behaviour of this assay are required, some of which are proposed herein. The extraction of the New Zealand marine sponge Haliclona sp. yielded one known and two new 3-alkyl pyridinium alkaloid (3-APA) monomers. The structures of these 3-APA monomers—dehydrohaliclocyclin C (58), dehydrohaliclocyclin F (59) and the known haliclocyclin C (20)—were elucidated using a combination of NMR spectroscopy, mass spectrometry (MS) and chemical degradation. This is the first report of a 3-APA cyclic monomer with unsaturation in the alkyl chain being isolated. 3-APAs were identified as the source of inhibitory activity in the S. cerevisiae assay, however, no inhibitory activity was found for 58, 59 and 20 against the clinically relevant fungus C. albicans.