Physical properties of microbial polythioesters: Characterization of poly(3-mercaptoalkanoates) synthesized by engineered Escherichia coli

Physical properties of chiral poly(thioesters), PTEs, prepared by engineered Escherichia coli, were examined by GPC, C-13 CP/MAS solid-state NMR, X-ray diffraction, and thermal analysis. Microbial homopolymers of PTEs, poly(3-mercaptopropionate), PMP, and poly(3-mercaptovalerate), PMV, showed different solubility characteristics compared to poly(hydroxyalkanoates), PHAs. Generally, PTEs required higher temperatures for dissolution. Poly(3-mercaptobutyrate), PMB, and PMV dissolve in chloroform, and the molecular weight values were revealed by GPC as 176 000 and 165 000, respectively. The density values for PMP and PMB were 1.42 and 1.27 g/cm(3), respectively. These values are similar to those for oxygen analogues. The NMR spectra for PTEs showed that carbonyl carbons are greatly shifted downfield by the sulfur atoms in the chain backbone compared to the PHA family. X-ray powder diffraction data indicated that PTEs are crystalline materials, but they do not crystallize as well as in the PHA family. The melting point, T-m, for PMP was 170 degreesC, which is about 100 degreesC higher than the equivalent oxygen analogue, poly(3-hydroxypropionate), PHP, and almost the same as that of bacterial poly (3-hydroxybutyrate), PHB. According to thermal analysis, only the PMP sample had enhanced heat stability, e.g., the decomposition temperature for PMP was 277 degreesC at 5% weight loss, whereas the values for PHP and PHB were 233 and 260 degreesC at the same weight loss, respectively.