Synthesis of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) with high 4HB composition and PHA content using 1,4-butanediol and 1,6-hexanediol for medical application

Abstract

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] with high 4HB monomer harbors enhanced biocompatibility and mechanical properties, which are useful as implantable and absorbable biomaterial in medical and pharmaceutical fields. Transformant Cupriavidus sp. USMAA1020 with an additional PHA synthase gene, phaC was found to produce P(3HB-co-4HB) with 86 mol% of 4HB monomer composition and high PHA content of 69 wt% in shake flask cultivation using mixed substrate of 1,6-hexanediol and 1,4-butanediol. Single-stage cultivation in 3 L fermentation has confirmed the ability of this strain to produce high 4HB monomer composition of 95 mol% in large scale fermentation with 75 wt% PHA content and high PHA concentration of 18.7 g/L. Interestingly, this strain was capable of surviving higher carbon concentration (1.05 wt% C) than the wild-type strain (0.69 wt% C). The present study results in P(3HB-co-4HB) with 1.3 and 2.3-fold PHA content and concentration respectively, with the ability to accumulate 2.1-fold 4HB monomer composition higher compared to the wild-type strain. A higher specific growth rate of 0.123 h−1 accompanied by high product yield, Yp/x of 3.8 times more than the wild-type strain were also obtained. Image from TEM showed cells with PHA content of 75 wt%, whichwere occupied with significant PHA granule. This copolymer possesses an average molecular weight (M w) and a polydispersity index of 156 kDa and 3.5 respectively with a tensile strength, elongation at break and Young’s modulus of 22.9 MPa, 463.2% and 187.3 MPa respectively. This polymer has a glass transition temperature (T g) and melting temperature (T m) of −48.9 °C and 61.9 °C respectively.