Membrane systems for biological separations: Exploiting complex affinity interactions

Affinity interactions can provide the very high selectivity needed for the purification of important biomolecules, but the throughput and economics in existing affinity systems have often been inadequate for large-scale separations. Although previous studies have demonstrated the feasibility of using membrane filtration with a large stereospecific affinity ligand in free solution to bind and retain a specific chiral isomer, there is still no clear understanding of the factors governing the separation performance of these systems. The overall objective of this thesis was to develop an appropriate framework for describing the performance of ultrafiltration systems for affinity separations, including the effects of different binding equilibria, the use of multiple stages, and the impact of solution properties on the overall separation characteristics.; Experiments were performed using a model system of bovine serum albumin as a stereospecific binding agent for the amino acid tryptophan. The equilibrium binding characteristics for the BSA-tryptophan system were evaluated using a simple ultrafiltration technique over a range of conditions. Actual separations were performed using a constant volume diafiltration system to wash the less strongly bound isomer through the membrane. A single stage process could produce L-tryptophan at an enantiomeric purity of 97%.; Experimental and theoretical studies demonstrated that multi-stage affinity ultrafiltration processes can provide even higher purification factors and yields for enantiomeric separations. The overall optimization of the 2-stage cascade system is quite complex, involving trade-offs between yield and purification and the appropriate choice of the concentration of the affinity ligand and the ratio of the stage volumes.; Affinity membranes were prepared using immobilized BSA that was attached to the surface of a polyethersulfone UF membrane via glutaraldehyde chemical linkages. Data were obtained for the solution flux, the sieving coefficients of both enantiomers, and the stereoselectivity for membranes formed using different immobilization procedures. These affinity membranes were more permeable to D-Trp due to the preferential binding of L-Trp by the immobilized BSA. The experimental and theoretical results presented in this thesis provide new insights into the performance and design of affinity membrane systems for the purification of biomolecules.