pH and salt effects on chiral separations using affinity ultrafiltration

Affinity ultrafiltration can provide stereoselective separations using a large macroligand to selectively bind and retain one of the enantiomers. The detailed binding interactions can be strongly influenced by the solution pH and salt concentration, although there is currently little understanding of the impact of these phenomena on the overall performance of the affinity ultrafiltration process. Experimental studies were performed using a model system with bovine serum albumin as the stereoselective macroligand for the separation of D- and L-tryptophan. Binding data were obtained over a range of pH and ionic strength, with the results used to calculate the yield and purification factor during a diafiltration process using a recently developed model. L-tryptophan binding was greatest between pH 7 and 10, leading to a significant increase in purification within this pH range. Overall system performance, including the trade-offs between purification factor and yield, was examined by constructing purification factor — yield diagrams for the affinity ultrafiltration process.

0.5 1.5 2.5 Number of Diavolumes, N D Fig. 4. Yield and purification factor for L-tryptophan during a constant volume affinity diafiltration process as a function of the number of diavolumes. Solid curves are model calculations using CTDo = CTLo = 0.2 mM and CBSA = 0.6 mM at pH ranging from 4.8 to 11.4. Filled symbols are experimental data obtained during an actual diafiltration process at pH 7.71.