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Methanol-induced chain termination in poly(3-hydroxybutyrate) biopolymers: molecular weight control

Richard D. Ashby, Daniel K. Y. Solaiman, Gary D. Strahan, Alex C. Levine, Christopher T. Nomura
International journal of biological macromolecules 2015 v.74 pp. 195-201
Pseudomonas oleovorans, biodiesel, carbon, diffusivity, feedstocks, fermentation, hydrodynamics, manganese, methanol, molecular weight, nuclear magnetic resonance spectroscopy, polyhydroxyalkanoates, synthesis, weight control
A systematic study was performed to demonstrate the impact of methanol (MeOH) on poly(3-hydroxybutyrate) (PHB) synthesis and molecular weight (MW) control. Glycerine (init. conc. = 1.0%; w/v), was used as the primary carbon source in batch-culture fermentations with varying concentrations (0 to 0.85%, w/v) of MeOH. Methanol retarded but did not completely inhibit growth and PHB production in Pseudomonas oleovorans. Proton-NMR analysis revealed that the PHB polymers were end-capped with methoxy chemical groups causing MW reductions. The magnitude of the MW decreases was contingent upon the initial MeOH media concentration. The largest impact occurred at an initial MeOH concentration of 0.10% (w/v) where the number average molecular weights (Mn) decreased by 39%, 55%, and 72% in the 48, 72, and 96 hour cultures, respectively. Diffusion ordered NMR spectroscopy revealed a MeOH media concentration-based diffusivity (D) increase in the PHB polymers. At 72 hours post-inoculation, the D values of the PHB polymers ranged from 0.12 to 0.66 (x 10-10 m2/s). These diffusivity increases indicated a reduction in hydrodynamic radii consistent with shorter chain-lengths, and were another sign of reduced MWs. Crude glycerine from the biodiesel production process continues to be used as an inexpensive fermentation feedstock for polyhydroxyalkanoate (PHA) synthesis, but it is never compositionally identical from one production facility to another. Therefore, the information contained in this paper will be vital to tailor PHA properties to specific applications.