Polyhydroxyalkanoate (PHA) synthase 1 (PhaC1 Ps) from Pseudomonas sp. 61-3 is a broad-substrate-specific enzyme. In this study, by employing Ralstonia eutropha PHB-4 recombinant expressing PhaC1 Ps , 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 (T g) increased, demonstrating a distinct thermal behavior as a result of the phenyl side group. Furthermore, PHAs bearing phenylalkyl side groups were synthesized from -phenylalkanoates by Pseudomonas putida KT2440, and their thermal properties were characterized. Based on the measured and predicted T g values, the effect of the acyl chain length in the aromatic side group on T g was investigated. This study demonstrates the role that aromatic side groups play in the thermal properties of PHAs.

Conformational free energies and geometrical parameters derived from ab initio molecular orbital (MO) calculations for monomeric and dimeric model compounds of poly((R)-3-hydroxybutyrate) (PHB) were introduced into the refined rotational isomeric state (RIS) scheme to yield the characteristic ratio, configurational entropy, configurational internal energy, bond conformations and averaged geometrical parameters of PHB. The reliability of the MO calculations was confirmed through comparison with 1 H and 13 C nuclear magnetic resonance experiments for the monomeric model compound. The characteristic ratio (5.60) derived from the refined RIS calculations for the unperturbed PHB chain at 25 1C is in satisfactory agreement with the experimental values (6.1-6.3). The crystalline PHB chain is known to adopt the ttg þ g þ conformation in the repeating unit. A previous study revealed that a trimeric (R)-3-hydroxybutyrate substrate bound on a PHB depolymerase lies in the ttg þ g þ and tg À tt conformations. In our MO calculations, the former conformation is the most stable, and the latter is metastable but further stabilized by intermolecular C ¼ O?H ÀN and C ¼ O?H ÀO hydrogen bonds on the enzyme.

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