New aspects of polyhydroxyalkanoates: chain folding and precursor self assembly; polythioesters

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Poly(3-hydroxyalkanoates), PHA, are a family of high molecular weight biodegradable polyesters which are carbon reserve material for bacteria. Because of their stereoregularity and high melting point, poly(3-hydroxybutyrate), PHB, form well-defined lath-shape single crystals which are used as model biodegradable crystalline plastics. This is the biotech plastic that has come to the rescue as world oil supplies dwindle. Its origin is biomass fermentation, like making beer. The factory is a biorefinery with corn as the raw material. R & D in this field focuses on a new understanding of the crystal growth and biodegradation process which is generated from studies using depolymerase enzymes as physical texture probes. A new crystal growth strategy from dilute PHB solution involving quenching, whiskers and self-assembly has been proposed. The new PHB morphologies obtained suggest that biodegradation of this sustainable plastic is a reversal of the crystal growth process. Because PHA polyesters are considered biocompatible, they are being considered for biomedical applications such as bone cement, sutures and drug delivery. Like PHB, polythioesters (PTEs) accumulate as granules in engineered bacterial cells with the addition of mercaptoalkyl substrates. The physical properties vary substantially in comparison with microbial oxyesters. The role of sulfur-sulfur interactions will be studied to seek an explanation for greater thermal stability of some PTEs.