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

Authors: Ashby, Richard - Rick; Solaiman, Daniel; Strahan, Gary; LEVINE, A. C. - State University Of New York (SUNY); NOMURA, C. T. - State University Of New York (SUNY)

Submitted to: International Journal of Biological Macromolecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2014
Publication Date: 12/24/2014
Publication URL: http://handle.nal.usda.gov/10113/60183
Citation: Ashby, R.D., Solaiman, D., Strahan, G.D., Levine, A., Nomura, C. 2014. Methanol-induced chain termination in poly(3-hydroxybutyrate) biopolymers: molecular weight control. International Journal of Biological Macromolecules. 74:195-201. DOI: 10.1016/j.ijbiomac.2014.12.026.

Interpretive Summary: Biodiesel is a renewable alternative to petroleum-based fuels; however, production costs impede its widespread use. The chemical reactions involved in biodiesel production typically result in a low-value crude co-product that is composed primarily of glycerine, a common chemical used in many household products, but it may also contain varying concentrations of alcohol (typically methanol; required for the biodiesel synthetic process). The amount of methanol present in the crude glycerine stream is dependent on whether the biodiesel producers efficiently recover the methanol for reuse. In order to improve the value of the crude glycerine co-product and thus help the profitability of the biodiesel production process, we have demonstrated that crude glycerine can be used as a chemical feedstock for the synthesis of poly(3-hydroxybutyrate) (PHB), an ‘environmentally benign’ bacterial polyester with properties resembling polyethylene and polypropylene. In the present paper, we used varying concentrations of methanol (0 – 0.85%) in multiple glycerine-based fermentations to prove that the amount of methanol present in a crude glycerine stream and the duration of the fermentations affect the physical properties of the PHB chains. Methanol acts as a chain-terminating agent causing a reduction in the molecular weights of the PHB polymer chains. If the crude glycerine stream is to be used as a fermentation feedstock, the impact of each of the components in that stream must be understood to control the process and produce materials with the desired properties. In this paper, we have shown that the methanol present in the crude glycerine stream acts to reduce the molecular size of the PHB polymers. By controlling the amount of methanol present in the fermentation, it is possible to actively control PHB properties and tailor the PHB polymers to various applications.

Technical Abstract: 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.