Date of Award
Doctor of Philosophy (PhD)
Biotechnology Science and Engineering
Poly-beta-hydroxyalkanoates., Microbial polymers., Pseudomonas--Biotechnology., Polymer colloids.
Poly(hydroxyalkanoate)s, PHAs, due to their biocompatibility and biodegradability have found many useful applications in biomedicine, prominently in the cardiovascular area. The spectrum of applications can be further expanded for PHAs by post biosynthetic chemical modification of terminal functional side chain groups. This should allow for the development of PHA platforms suited for specialized applications such as coatings, temporary medical implants, wound dressings etc. and for which novel applications can be envisioned. In this dissertation, four specific “click” chemistry tools, copper catalyzed azide-alkyne cycloaddition (CuAAC), strain promoted azide-alkyne cycloaddition (SPAAC), thiol-yne radical “click” reaction and thiol-halogen nucleophilic substitution, have been studied to determine their potential effectiveness as ligation tools for the chemical modification of PHAs. PHAs bearing terminal bromo (PHNUBr) and alkyne (PHNUD groups were biosynthesized using a co-feeding technique. CuAAC was successfully used to attach the small molecules, propargyl benzoate, propargyl acetate, methyl-2-azidoacetate and methyl-4-azidomethylbenzoate to PHA copolymers with brominated repeat units that were chemically converted into azide groups, and alkynyl repeat units, introduced by the fermentation. This was achieved by using different molecular setups and catalytic systems. Molecular architectures and reaction parameters need to be chosen carefully as not all catalytic systems are equally successful for reactions of various polymers and small molecules. The proof-of-concept for “clicking” of BCN-OH to the functionalized azido-PHA via SPAAC shown here, should allow for further exploration and optimization of this technique for preparing various modified PHAs. The advantage of being metal-free makes it an ideal process for preparing chemically modified PHAs for applications in the biomedical industry. The thiol-yne “click” ligation reactions did not yield the expected attachment of the various small molecules tested to the PHA bearing the alkyne functionality despite testing varying experimental conditions. The other thiol-based “click” ligation tool, the thiol-halogen “click” ligation, on the other hand, was useful in the attaching of p-thiocresol, methyl-3-mercaptopropionate and 2- naphthalenethiol to the PHA copolymers bearing brominated repeat units. It seems the reaction is better suited to thiol molecules with electron donating substituents close to the thiolate ion as they increase its nucleophilicity. Lastly, PHNUD was discovered serendipitously as an organo-gelator when reacted with methyl-3-mercaptopropionate and the photoinitiator DMPA in DMF under sunlight. Hitherto there have been no report in literature of PHAs having gelation properties. This discovery therefore opens the door for exploring and developing PHA-organogel platforms for various applications in biomedicine and pharmaceuticals. FTIR and XPS analysis lends strong evidence to the gel network formation via the bis-hydrothiolation of the side chain terminal alkyne followed by the crosslinking at the thioether radical sites.
Nkrumah-Agyeefi, Samuel, "Chemical modification of poly(hydroxyalkanoate)s (PHA)s via "click' chemistry" (2018). Dissertations. 148.