BIOSYNTHESIS AND CHARACTERIZATION OF HYDROXYBUTYRATE-HYDROXYCAPROATE COPOLYMERS

Most polyhydroxyalkanoates (PHAs) reported to date fall into one of two broad classes: either hydroxybutyrate-hydroxyvalerate copolymers (typified by the PHA produced by Alcaligenes eutrophus), or hydroxyoctanoate-rich heteropolymers (typified by the PHA produced by Pseudomonas oleovorans). Few reports of copolymers rich in hydroxybutyrate (HB), but containing a minor proportion of a co-monomer with a higher carbon number than valerate, have appeared. Here we report on the biosynthesis and characterization of HE-rich polymers containing 2-4 mol% of hydroxycaproate (HC) units, as well as a terpolymer containing HC and hydroxyoctanoate (HO) units. These polymers were produced in good yield by Comomonas testosteroni, Bacillus cereus and an unidentified third organism when grown on caproate or octanoate. The minor co-monomers were found to be rejected from the PHB crystallites by X-ray analysis and by quantitative analysis of the melting point depression. The greatly reduced melting point, coupled with the retention of a high degree of crystallinity, could make these materials attractive as melt-processible thermoplastics.