Uniformity of Monomer Composition and Material Properties of Medium-Chain-Length Polyhydroxyalkanoates Biosynthesized from Pure and Crude Fatty Acids

Medium-chain-length (MCL) polyhydroxyalkanoate (PHA) homopolymers consist exclusively of 3-hydroxyalkanoate (3HA) monomers with 6–14 carbon atoms per repeating unit and comprise new types of flexible and transparent polyesters. In this study, MCL-PHA homopolymers were biosynthesized from pure fatty acids employing metabolically engineered Escherichia coli LSBJ. By increasing metal components in the culture medium, the yields of poly(3-hydroxydecanoate) [P(3HD)] and poly(3-hydroxydodecanoate) [P(3HDD)] were increased to 1.47 g/L and 0.42 g/L, respectively. The homopolymers were purified and processed into solvent-cast films for characterization. Drawing these films 3–4 times increased their tensile strength up to 45 MPa because of an enhancement of the degree of crystalline orientation. Additionally, these films exhibited high transparency regardless of the drawing treatment. To develop a more practical production process, a fatty acid mixture, palm kernel fatty acid distillate (PKFAD), was utilized as a polymer feedstock. Using PKFAD, we were able to synthesize 3HDD-rich PHAs containing 74–85 mol % 3HDD, with mean material properties between those of P(3HD) and P(3HDD) homopolymers. This study has advanced our understanding of the relationship between the uniformity of monomer composition and material properties of MCL-PHAs.