Tangential flow filtration for continuous processing of crystallized proteins

Protein crystallization has the potential to provide nearly pure protein products in a single processing step. Most previous studies of protein crystallization have used centrifugation for dewatering and washing the crystallized solids, but this approach would be difficult to adopt in continuous bioprocessing operations. This study provides the first demonstration that crystallized proteins can be successfully filtered using hollow fiber tangential flow filtration (TFF) modules. Experiments were performed with human serum albumin (HSA) crystallized by the addition of trivalent CeCl₃, both with and without polyethylene glycol (PEG). Greater than 85 % protein yield was achieved using 4.5 mM CeCl₃ in the presence of 3 wt% PEG with the crystallization occurring in less than 20 s, which is ideal for continuous processing. The addition of PEG had minimal effect on the crystallization yield, but it did lead to the formation of larger and denser crystals and a corresponding increase in the critical flux, which is the maximum value of the flux that causes minimal increase in the transmembrane pressure (i.e., minimal fouling). Filtration experiments performed at optimal conditions were successfully run for 24 h with the transmembrane pressure remaining below 5 kPa, demonstrating the feasibility of using continuous crystallization with tangential flow filtration for the purification of high value protein products.