Protein adsorption to poly(ethylenimine)-modified Sepharose FF: V. Complicated effects of counterions

In the previous studies on protein adsorption to poly(ethylenimine) (PEI)-grafted Sepharose FF resins, a critical ionic capacity (600 mmol/L) of PEI-Sepharose resins was found for the adsorption of bovine serum albumin (BSA), above which both protein capacity and uptake rate increased drastically. In this work, the influence of counterions on the PEI-Sepharose resin with an ionic capacity of 683 mmol/L (FF-PEI-L680) was investigated with sodium salts of SCN−, Cl−, HPO42− and SO42−. Linear gradient elution, batch adsorption and breakthrough experiments showed that counterion preference, effective pore diffusion coefficient (De) and dynamic binding capacity (DBC) values increased in the order of SCN−, Cl−, HPO42− and SO42−, while static adsorption capacity decreased in this order. It is considered that higher counterion preference of the ion exchange groups resulted in lower protein binding strength and adsorption capacity, while the De value increased due to the enhanced “chain delivery” effect (a kind of surface diffusion). Besides, the DBC value was mainly dependent on De value. In particular, SO42− was the most favorable counterion for the PEI-Sepharose resin, which gave rise to the highest De value (De/D0 = 1.17, D0 is protein diffusivity in free solution) and DBC value (118 mg/mL at a residence time of 2 min). Moreover, the effects of counterions on BSA adsorption to DEAE Sepharose FF and Q Sepharose FF, which were non-grafted resins, were also studied for comparisons. It was found that the counterion preferences of the two non-grafted resins were different from each other and also different from that of FF-PEI-L680. The different counterion preferences were attributed to the differences in the ion-exchange ligand chemistries. In addition, the De values for DEAE Sepharose FF and Q Sepharose FF kept unchanged. The low counterion sensitivity of De values could be interpreted as the lack of “chain delivery” effect for the non-grafted resins. The results indicate that protein adsorption and chromatographic performance with PEI-Sepharose can be improved by proper counterions. For the four counterions tested, SO42− was the most favorable for providing the best adsorption and elution outcomes with FF-PEI-L680.