Production and characterization of poly-β-hydroxyalkanoate copolymers from Burkholderia cepacia utilizing xylose and levulinic acid

Poly(beta-hydroxybutyrate-co-o-hydroxyvalerate) (P(3HB-co-3HV)) copolymers were prepared via shake-flask fermentations of Burkholderia cepacia (formerly Pseudomonas cepacia) containing 2.2% (w/v) xylose and concentrations of levulinic acid ranging from 0.07% to 0.67% (w/v). Periodic harvest of shake-flask cultures from 48 to 92 h post-inoculation yielded 4.4-5.3 g/L of dry cell biomass, containing 42-56% (w/w) P(3HB-co-3HV), with optimal product yield occurring between 66 and 74 h. Growth and PHA accumulation enhancement were observed with concentrations of levulinic acid from 0.07 to 0.52% (w/v), producing dry cell biomass and P(3HB-co-3HV) yields of 9.5 and 4.2 g/L, respectively, at the 0.52% (w/v) concentration of levulinic acid. Representative samples were subjected to compositional analysis by 300 MHz H-1 and 150 MHz C-13 NMR, indicating that these random copolymers contained between 0.8 and 61 mol % 3-hydroxyvalerate (3HV). Solvent-cast film samples were characterized by differential scanning calorimetry, which demonstrated melting temperatures (T) to decrease in a pseudoeutectic fashion from 174.3 degreesC (0.8 mol % 3HV) to a minimum of 154.2 degreesC (25 mol % 3HV) and the glass transition temperatures (T-g) to decrease linearly from 2.1 to -11.9 degreesC as a function of increasing mol % 3HV. Thermogravimetric analysis of the copolymer series showed the temperature for onset of thermal decomposition (T-decomp) to vary as a function of mol % 3HV from 273.4 to 225.5 degreesC. Intrinsic viscosities (eta) varied from 3.2 to 5.4 dL/g, as determined by dilute solution viscometry. Viscosity average molecular weights (M-v) of the copolymers were determined to range from 469 to 919 kDa, indicating that these P(3HB-co-3HV) copolymers are of sufficient molecular mass for commercial application.