(650h) Protein Adsorption Equilibria and Uptake Kinetics to Poly(ethylenimine)-Based Ion Exchangers

Sepharose FF was modified with poly(ethylenimine) (PEI) to prepare anion exchangers of 10 different ionic capacities (ICs, 100–1220 mmol/L). Adsorption equilibria and kinetics of bovine serum albumin (BSA) were systematically studied. We found a critical IC (cIC, 600 mmol/L), above which both protein capacity and uptake rate increased drastically. Then, five PEI-Sepharose FF resins of typical ICs were selected to explore the effect of ionic strength (IS) on the adsorption equilibria and uptake kinetics of BSA. Commercially available DEAE (IC=160 mmol/L) and Q Sepharose FF (IC=269 mmo/L) resins were used for comparisons. It is found that at similar ionic capacities, protein adsorption capacities on both the PEI-Sepharose FF resins and the commercial resins decreased with increasing IS, but on the capacity sensitivity to salt concentration, the former was lower than the latter. In addition, the effective diffusivities (D_e) of the former were smaller than the latter in the entire IS range studied. The low IS sensitivity of adsorption capacity of the PEI-Sepharose FF resins could be interpreted by the increase of pore accessibility with increasing IS; the smaller D_e values in the PEI-Sepharose FF resins were considered due to the lack of surface diffusion in the PEI-Sepharose FF resins of low PEI densities. For the PEI-Sepharose FF resins of high ICs (520, 740 and 1220 mmol/L), both protein capacity and D_evalues increased first and then decreased with increasing IS. The increasing trend of protein capacity in the low IS range was considered due to the increase of accessible pores for BSA. The rise-fall trend of D_e was attributed to the dependencies of the “chain delivery” effect on protein capacity and binding strength, both of which are related to IS. Moreover, the IS sensitivity of the D_e for the resins of ICs>cIC (740 and 1220 mmol/L) was much higher than those of ICs<cIC, further proving that the “chain delivery” effect in PEI layer did contribute significantly to the overall mass transfer at IC>cIC. Furthermore, the two PEI-Sepharose FF resins of ICs>cIC kept high adsorption capacities and D_e values up to 200-300 mmol/L NaCl. Therefore, the operating IS ranges for these two PEI-Sepharose FF resins can be much broader than traditional ion-exchange media.