Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2017
Publication Date: 2/26/2018
Citation: Ashby, R.D., Solaiman, D., Nunez, A., Strahan, G.D., Johnston, D. 2018. Burkholderia sacchari DSM 17165: A source of compositionally-tunable block-copolymeric short-chain poly(hydroxyalkanoates) from xylose and levulinic acid. Bioresource Technology. 253:333-342.
Interpretive Summary: Environmental concerns are driving the search for plastic substitutes that can easily break down in the environment without greenhouse gas emissions. Unfortunately, while these materials exist, they have historically been too expensive to produce and apply in large quantities. In this study we document our work using two readily-available, inexpensive carbon source materials to produce unique bacterially-derived polyesters that may be more economically sound. Xylose, a common sugar found in plant biomass, and levulinic acid, which is easily produced from the sugars in plant biomass, were used as fermentation feedstocks with the harmless bacterial strain known as Burkholderia sacchari to produce biodegradable polyesters containing two distinct components (3-hydroxybutyric acid, 3HB and 3-hydroxyvaleric acid, 3HV). Interestingly, most of the time the components of these polyesters are randomly organized throughout the molecule but in these newly made polyesters there are regions rich in 3HB and 3HV which provides unique opportunities for mechanical property control. The results of this study demonstrate the possibility to produce unique ‘eco-friendly’ polyesters from cheap starting materials and may help to alleviate the economic concerns associated with these biopolyesters.
Technical Abstract: Burkholderia sacchari DSM 17165 was used as a biocatalyst for the production of poly-3-hydroxybutyrate-co-3-hydroxyvalerate block copolymers (Poly-3HB-block-3HV) from xylose and levulinic acid. Among the carbon source mixtures, levulinic acid was preferred and was consumed early in the fermentations resulting in 3-hydroxyvalerate (3HV) contents as high as 95 mol% in the polymers derived from 2% xylose and 0.8% levulinic acid at 24 hours. The composition ratios between 3HB and 3HV were controlled based on the amount of levulinic acid present and the length of the fermentations. Thermal analysis revealed that at higher levulinic acid concentrations and longer durations, two glass transition temperatures (Tg) were present each approximating the Tg values associated with poly-3HB and poly-3HV suggesting the presence of polymeric sequences rich in 3HB and 3HV monomers. The carbonyl regions within the 13C-NMR analyses confirmed the presence of high concentrations of 3HB-3HB and 3HV-3HV homopolymeric dyads in the mature polymers, further suggesting block sequences. Mass spectrometry of the base-catalyzed partial hydrolysis products did not conform to Bernoullian statistics for randomness which confirmed the presence of non-random sequences. Conversely, the MS/MS analysis of specific oligomeric fractions showed the sequential loss of 86 amu (corresponding to a single 3HB unit) and 100 amu (corresponding to a single 3HV unit) proving the presence of some degree of randomness within the polymers. The results of this study verify the potential for producing Poly-3HB-block-3HV copolymers from readily-available, inexpensive feedstocks without the need for sequential addition of unrelated carbon sources.