Biosynthesis and characterization of novel poly(3-hydroxybutyrate-co-3-hydroxy-2-methylbutyrate): thermal behavior associated with α-carbon methylation

Hydroxy-2-methylbutyrate (3H2MB) has been identified as a minor component of polyhydroxyalkanoates (PHAs) synthesized by bacteria living in activated sludge. In this study, we found that PHA synthase derived from Aeromonas caviae (PhaC(Ac)) polymerizes 3H2MB. By expressing PhaCAc in recombinant Escherichia coli LS5218 and growing cells in the presence of tiglic acid, a PHA copolymer [P(3HB-co-3H2MB)], mainly consisting of 3-hydroxybutyrate (3HB) and up to 37 mol% 3H2MB, was obtained. Analysis of the thermal properties of this novel copolymer indicates that incorporation of 3H2MB into P(3HB) sequence reduced the glass transition temperature (T-g), melting temperature (T-m), and melting enthalpy (Delta H-m). The cold crystallization temperature (T-cc) was also lowered by incorporating 7 or 23 mol% 3H2MB, in contrast to the findings for other PHA copolymers. This result suggests that P(3HB-co-3H2MB) copolymers are easier to crystallize than P(3HB) and other PHAs. Thus, 3H2MB provides promising new opportunities to generate 3HB-based polymers with novel thermal properties.