Graduate Thesis Or Dissertation
 

Natural products studies of the marine cyanobacterium Lyngbya majuscula

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  • The marine cyanobacterium Lyngbya majuscula has proven to be extraordinarily rich in bioactive secondary metabolites. This dissertation describes the chemistry of L. majuscula collected from Grenada, Fiji and Papua New Guinea, and the biosyntheses of two L. majuscula metabolites, curacin A and barbamide. The chemical studies with a Grenada collection of L. majuscula revealed three new metabolites, grenadadiene, debromogrenadadiene and grenadamide. These three compounds are the only reported cyclopropyl-containing fatty acids from Lyngbya species. The chemistry of a mixed assemblage ofL. majuscula/Schizothrix species from Fiji was investigated and shown to contain two novel depsipeptides, yanucamides A and B. Both compounds possessed the unique 2,2-dimethyl-3-hydroxy-7-octynoic acid, a unit that has only been described in the structures ofkulolide-1 and kulokainalide-1, metabolites from the marine mollusk Philinopsis speciosa. Chemical investigation of the highly brine shrimp toxic extract of Papua New Guinea collection of L. majuscula led to the isolation of the previously described cytotoxins, curacins A and D. Upon further investigation ofthe same extract, two new depsipeptides, clairamide and carliamide, were discovered. Clairamide contains 3-amino2-methyl-pentanoic acid, a component unique to cyanobacterial metabolites, while carliamide possesses the 3-amino-2-methyl-7-octynoic acid, a unit that has only been found in the structure of onchidin A, a metabolite from the marine mollusk Onchidium sp. Biosynthetic investigations of barbamide, a unique trichloromethyl-containing metabolite, were carried with the cultured L. majuscula. Results from the feeding experiments have established the biosynthetic units ofthe compound. In addition, isotope-incorporation studies also revealed that barbamide biosynthesis involves chlorination that exclusively occurs at the unactivated pro-S methyl group of leucine. The biosynthesis of curacin A was also examined. Stable isotope feeding experiments have illustrated the cysteine-initiated (or a thiazoline acyl CoA-initiated) polyketide chain assembly of curacin A with C 17 and the OCH₃ arising from methionine. Moreover, the labeling pattern of acetate at C 18-C22 of curacin A is consistent with the five-carbon unit deriving from a branched triketide-derived precursor or isopentyl diphosphate (IPP)/dimethylallyl diphosphate (DMAPP).
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