Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 25, 2002
Publication Date: October 1, 2002
Citation: Ashby, R.D., Solaiman, D., Foglia, T.A. 2002. Poly(ethylene glycerol)-mediated molar mass control of short-chain- and medium-chain-lenght poly(hydroxyalkanoates) from pseudomonas oleovorans. Applied Microbiology and Biotechnology. 60:154-159. Interpretive Summary: The increase in environmental concern has driven the need for the development of more "environmentally benign" materials as substitutes for petrochemical-based products. Poly(hydroxyalkanoates) (PHA) are a family of biodegradable polyesters that can be produced from numerous agricultural feedstocks (including animal fats and vegetable oils) by many bacteria under stressful growth conditions. PHA polymers have industrial potential provided their properties can be controlled. Presently, PHA polymers are classified as either rigid or elastomeric, depending on their origin and chemical structure. The problem is that all potential applications of PHA require specific polymer properties that are dictated by the application itself. The most obvious way of controlling the properties of PHA is to find ways of varying the molecular composition of the polymers. However, additional property control can be realized by reducing the size (molar mass) of the polymers. Poly(ethylene glycol) (PEG) is a water-soluble polymer that is relatively nontoxic to bacterial cells. In this study, we determined that the addition of PEG to bacterial growth media reduced the molar mass of the PHA polymers regardless of whether the PHA was rigid or elastomeric in nature. We found that the extent of the decrease in PHA molar mass could be controlled by the size and amount of PEG added to the bacterial growth media. The control of PHA molar mass provides an additional mechanism by which we can control the PHA properties. This will ultimately broaden the application potential of these renewable polymers.
Technical Abstract: Three strains of Pseudomonas oleovorans, a well known poly(hydroxyalkanoate) (PHA) producer, were tested for the ability to control PHA molar mass and end group structure by addition of poly(ethylene glycol) (PEG) to the fermentation media. Each strain of P. oleovorans -- NRRL B-14682 (B-14682), NRRL B-14683 (B-14683), and NRRL B-778 (B-778)-- synthesized a different type of PHA from oleic acid when cultured under identical growth conditions. Strain B-14682 produced poly(3-hydroxybutyrate) (PHB), while B-14683 synthesized a medium-chain-length PHA (mcl-PHA) with a repeat unit composition ranging from C4 to C14 and some mono-unsaturation in the C14 alkyl side chains. Strain B-778 synthesized a mixture of PHB (95 mol%) and mcl-PHA (5 mol%). The addition of 0.5% (v/v) PEG (Mn = 200 g/mol, PEG-200) to the fermentation broth of strains B-14682 and B-778 resulted in chain termination through esterification at the carboxyl terminus of the PHB with PEG chain segments, thus reducing the molar mass by 54% and 23%, respectively. The molar mass of the mcl-PHA produced by strains B-14683 and B-778 also showed a 34% and 47% reduction in the presence of PEG-200, respectively, but no evidence of esterification was present. PEG (Mn = 400 g/mol, PEG-400) had a reduced effect on PHA molar mass. In fact, the molar masses of the mcl-PHA derived from strain B-14683 and both the PHB and mcl-PHA from B-778 were unchanged by PEG-400. In contrast, the PHB produced by B-14682 showed a 35% reduction in molar mass in the presence of PEG-400.