Options
Studies on the secondary metabolites of marine cyanobacteria from Singapore
Author
Tripathi Ashootosh
Supervisor
Tan, Lik Tong
Abstract
The search for new and useful natural products from local strains of marine cyanobacteria has been the main focus of this PhD research. The extensive collections of marine cyanobacteria resulted from continual expeditions to nearby islands of Singapore, including, Pulau Hantu Besar, Pulau Hantu Kechil, St. Johns island, and Pulau Subar Laut. As such, the various chapters outlined in this thesis encompasses isolation, structure elucidation (both planar and stereochemical determinations), biological characterization of new secondary metabolites, antifouling assessment, and mode of action studies of marine cyanobacterial compounds.
This thesis begins with a brief background on the marine natural products currently in clinical or preclinical trials as general introduction in chapter 1. The general introduction is followed by a field survey of marine cyanobacteria from Singapore waters in chapter 2, which indicated wide occurrence of benthic filamentous marine cyanobacteria, belonging to the Lyngbya genus. The organic extract made from Lyngbya majuscula (TLT/PHB/002) showed potent biological activity and was further purified yielding two new linear lipopeptides, besarhanamides A and B. The planar structures of these natural products were established using an extensive array of 1D and 2D NMR as well as chemical manipulation involving the advanced Marfey’s method.
Chapter 3 details the isolation of three new cytotoxic metabolites given the trivial names, hantupeptins A-C, isolated from the marine cyanobacterium, L. majuscula (TLT/PHB/002). Their gross structures were elucidated by interpretation of extensive 1D and 2D NMR data as well as some chemical manipulations. Data on their biological activities will also be discussed.
Subsequent recollection of L. majuscula (TLT/PHB/002) from Pulau Hantu and chemical workup yielded three new molecules, namely lagunamides A-B and hantupeptolide. These molecules were discussed in chapter four (lagunamides) and chapter five (hantupeptolide). These secondary metabolites contained unique structural features, including an α-hydroxy acid and a polyketide-derived moiety. Structurally, all three molecules differ only in their polyketide moiety but possess equally potent cytotoxicity against a panel of cancer cell lines. In addition to Marfey’s method and 3JH-H coupling constant values, a modified method based on Mosher’s reagents and analysis using LC-MS was deployed for the determination of the absolute configuration.
In chapter 6, a total of 12 secondary metabolites, isolated from the marine cyanobacterium, L. majuscula (TLT/PHB/002), were tested in vitro to determine if they show activity against barnacle larval settlement. In addition, field testing conducted over a period of 28 days based (using the modified PhytagelTM method) of the cyanobacterial compound, dolastatin 16, showed significantly reduced barnacle settlement as compared to controls at all concentrations tested. The results of this study highlight the importance of marine cyanobacteria as an underexplored source of potential environmentally friendly antifoulants.
Preliminary biological investigations on the mode of action of lagunamide A were carried out and are presented in chapter 7. In this initial study, the compound exhibited nanomolar range of growth inhibition and cytotoxicity against a panel of cancer cell lines. Morphological studies showed blebbing at the surface of cells; as well as cell shrinkage accompanied by loss of contacts with the substratum and with their neighboring cells. Biochemical studies done on HCT8 and MCF7 cells suggest that lagunamide A might act by inducing mitochondrial mediated apoptosis as the mechanism for killing of cancer cells. The thesis ends with a concluding chapter, summarizing the results presented in the preceding chapters.
This thesis begins with a brief background on the marine natural products currently in clinical or preclinical trials as general introduction in chapter 1. The general introduction is followed by a field survey of marine cyanobacteria from Singapore waters in chapter 2, which indicated wide occurrence of benthic filamentous marine cyanobacteria, belonging to the Lyngbya genus. The organic extract made from Lyngbya majuscula (TLT/PHB/002) showed potent biological activity and was further purified yielding two new linear lipopeptides, besarhanamides A and B. The planar structures of these natural products were established using an extensive array of 1D and 2D NMR as well as chemical manipulation involving the advanced Marfey’s method.
Chapter 3 details the isolation of three new cytotoxic metabolites given the trivial names, hantupeptins A-C, isolated from the marine cyanobacterium, L. majuscula (TLT/PHB/002). Their gross structures were elucidated by interpretation of extensive 1D and 2D NMR data as well as some chemical manipulations. Data on their biological activities will also be discussed.
Subsequent recollection of L. majuscula (TLT/PHB/002) from Pulau Hantu and chemical workup yielded three new molecules, namely lagunamides A-B and hantupeptolide. These molecules were discussed in chapter four (lagunamides) and chapter five (hantupeptolide). These secondary metabolites contained unique structural features, including an α-hydroxy acid and a polyketide-derived moiety. Structurally, all three molecules differ only in their polyketide moiety but possess equally potent cytotoxicity against a panel of cancer cell lines. In addition to Marfey’s method and 3JH-H coupling constant values, a modified method based on Mosher’s reagents and analysis using LC-MS was deployed for the determination of the absolute configuration.
In chapter 6, a total of 12 secondary metabolites, isolated from the marine cyanobacterium, L. majuscula (TLT/PHB/002), were tested in vitro to determine if they show activity against barnacle larval settlement. In addition, field testing conducted over a period of 28 days based (using the modified PhytagelTM method) of the cyanobacterial compound, dolastatin 16, showed significantly reduced barnacle settlement as compared to controls at all concentrations tested. The results of this study highlight the importance of marine cyanobacteria as an underexplored source of potential environmentally friendly antifoulants.
Preliminary biological investigations on the mode of action of lagunamide A were carried out and are presented in chapter 7. In this initial study, the compound exhibited nanomolar range of growth inhibition and cytotoxicity against a panel of cancer cell lines. Morphological studies showed blebbing at the surface of cells; as well as cell shrinkage accompanied by loss of contacts with the substratum and with their neighboring cells. Biochemical studies done on HCT8 and MCF7 cells suggest that lagunamide A might act by inducing mitochondrial mediated apoptosis as the mechanism for killing of cancer cells. The thesis ends with a concluding chapter, summarizing the results presented in the preceding chapters.
Date Issued
2012
Call Number
RS160.7 Tri
Date Submitted
2012