Properties of Absorbent Polymer Extractants for the Selective Removal of Target Molecules from Fermentation Systems
Polymer , Absorption , Partitioning , Bioreactors , Selectivity , Biocatalysis
This thesis investigated polymer properties for their application as extractants in two-phase partitioning bioreactors (TPPBs), which are intended to remove inhibitory fermentation products as they are produced. Three applications of polymer TPPB extractants were studied, followed by an investigation into poly(ether)-based polymers’ affinity toward representative target molecules, to identify properties which confer improved extraction performance. The first investigation aimed to replace a liquid extractant (silicone oil) using a block copolymer, Hytrel® 8206, in the biotransformation of indene to cis-(1S,2R)-indandiol, a chiral pharmaceutical intermediate, by Pseudomonas putida ATCC55687. The polymer simultaneously delivered substrate and removed the product and by-products to alleviate inhibition, improving operability and productivity relative to silicone oil, which could only deliver substrate. Subsequently, soft segment composition and proportion were varied in different block copolymers to selectively extract product or by-product(s) from the same biotransformation, altering the cells’ production profile. This demonstrated selective polymer extraction to help direct substrate utilization toward the product rather than by-product(s) in complex biotransformations. The next study was on absorptive extraction of a hydrophilic target molecule, 4-valerolactone, produced by recombinant Pseudomonas putida KT2440, featuring an equilibrium-limited final step. The aim was to demonstrate the first application of equilibrium-pulling using selective product absorption, improving production by 30%. Furthermore, this study showed that limited polymer water absorption is helpful to aid in extraction of hydrophilic target molecules, but high polymer water content compromises selectivity, diminishing the equilibrium-pulling effect. Finally, the effects of soft block proportion, molecular weight, and chain-end composition on affinity toward representative target molecules, carveol and carvone, were studied using commercial block copolymers and their representative homopolymer components. Target molecule affinity improved at low molecular weights in the absence of polar homopolymer end-groups. End-group polarity had an effect whose direction depended on the polarity of the target molecule, improving affinity toward a third, polar target molecule, 4-valerolactone, thereby providing a means to tailor selectivity. Crystallinity and hard segment proportion were both found to reduce uptake. This work has provided insights into the selection of polymeric TPPB absorbents by identifying polymer properties which improve affinity and selectivity toward different fermentation target molecules, especially relatively hydrophilic ones. The future design of purpose-built polymer extractants will benefit from considering these findings.