Biosynthesis and thermal characterization of polyhydroxyalkanoates bearing phenyl and phenylalkyl side groups

Polyhydroxyalkanoate (PHA) synthase 1 (PhaC1Ps) from Pseudomonas sp. 61-3 is a broad-substrate-specific enzyme. In this study, by employing Ralstonia eutropha PHB-4 recombinant expressing PhaC1Ps, a novel type of 3-hydroxybutyrate (3HB)-based PHA copolymer, poly(3HB-co-3-hydroxy-3-phenylpropionate) [P(3HB-co-3H3PhP)], bearing phenyl side groups, was synthesized. The 3H3PhP fraction was increased up to 8.9 mol% by feeding 3H3PhP precursors. As the phenyl side group was introduced into P(3HB), the melting temperature and the enthalpy of fusion decreased, while the glass transition temperature (Tg) increased, demonstrating a distinct thermal behavior as a result of the phenyl side group. Furthermore, PHAs bearing phenylalkyl side groups were synthesized from ω-phenylalkanoates by Pseudomonas putida KT2440, and their thermal properties were characterized. Based on the measured and predicted Tg values, the effect of the alkyl chain length in the aromatic side group on Tg was investigated. This study demonstrates the role that aromatic side groups play in the thermal properties of PHAs.