A STUDY OF POLYMER-SMALL MOLECULE INTERACTIONS FOR SOLID-LIQUID TWO PHASE PARTITIONING BIOREACTORS WITH EMPHASIS ON THE BIOPRODUCTION OF 2-PHENYLETHANOL
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Biphasic systems have been studied for in situ product removal (ISPR), and have shown improvements in bioreactor performance. With immiscible solvents, concerns associated with solvent biocompatibility, bioavailability and operation have been identified. One alternative is a solid-liquid system in which polymer beads are used, absorbing and removing target compounds from the aqueous phase while maintaining equilibrium conditions. In such systems, the capability of a polymer to absorb the compound of interest is an important parameter. This work has identified polymer properties that may be important to the interaction between polymers and target compounds for selected biotransformation molecules including 2-phenylethanol, cis-1,3-indandiol, iso-butanol, succinic acid and 3-hydroxybutyrolactone. Furthermore, the biotransformation from L-phenylalanine to 2-phenyethanol, an important aroma compound in industry, was examined in detail. It was found that relatively hydrophobic compounds tend to be absorbed by polymers better than hydrophilic ones based on partition coefficient tests. Since all of the biotransformation molecules tested have polar functional groups such as alcohol, acid and lactone, polar polymers such as Hytrel® performed better than non-polar polymers such as Kraton® possibly due to the hydrogen-bonding interaction between the polymer and the solute. Crystallinity and intermolecular hydrogen-bonding were also found to be important polymer properties. Hytrel® 8206 was identified as the best working polymer to absorb 2-phenylethanol. A solid-liquid batch mode two phase partitioning bioreactor (TPPB) with 500 g Hytrel® generated an overall 2-PE concentration of 13.7 g/L, the highest reported in the current literature. This was based on a polymer phase concentration of 88.74 g/L and aqueous phase concentration of 1.2 g/L. Better results were achieved via contact with more polymers with the aqueous phase applying a semi-continuous reactor configuration. In this system, a final 2-PE concentration (overall) of 20.4 g/L was achieved. The overall productivities of these two reactor systems were 0.38 g/(L-h) and 0.43 g/(L-h), respectively. This experiment successfully demonstrated that with the appropriate selection of polymer, solid-liquid TPPB systems were able to greatly enhance bioproductions associated with end product inhibition in terms of final product concentration and productivity. The ease of operation is also attractive compared to two liquid phase systems.