Protein adsorption to poly(ethylenimine)-modified Sepharose FF: I. A critical ionic capacity for drastically enhanced capacity and uptake kinetics

To explore the details of protein uptake to polymer-grafted ion exchangers, Sepharose FF was modified with poly(ethylenimine) (PEI) to prepare anion exchanger of 10 different ionic capacities (ICs, 100-1220 mmol/L). Adsorption equilibria and kinetics of bovine serum albumin (BSA) were then studied. It is found that ionic capacity, i.e., the coupling density of PEI, had significant effect on both adsorption capacity (qm) and effective protein diffusivity (De). With increasing ionic capacity, the qm value increased rapidly at IC < 260 mmol/L and then increased slowly till reaching a plateau at IC = 600 mmol/L. In the IC range of 100-600 mmol/L, however, the De values kept at a low level (De/D0 < 0.07); it first decreased from 0.05 ± 0.01 at IC = 100 mmol/L to 0.01 ± 0.01 at IC = 260 mmol/L and then increased to 0.06 ± 0.01 at IC = 600 mmol/L. Thereafter, sharp increases of the qm and De values [36% (from 201 to 273 mg/mL) and 670% (from 0.06 ± 0.01 to 0.49 ± 0.04), respectively] were observed in the narrow range of IC from 600 to 740 mmol/L. Finally, at IC > 740 mmol/L, the qm value decreased significantly while the De value increased moderately with increasing the IC. The results indicate that PEI chains played an important role in protein adsorption and transport. In brief, there was a critical IC (cIC) or PEI chain density, above which protein adsorption and transport behaviors changed drastically. The cIC was identified to be about 600 mmol/L. Estimation of PEI grafting-layer thickness suggests that PEI chains formed an extended three-dimensional grafting-layer at IC > cIC, which provided high flexibility as well as accessibility of the chains for protein binding. Therefore, at IC > cIC, the adjacent PEI chains became close and flexible enough, leading to facilitated transport of adsorbed protein molecules by the interactions of neighboring chains mediated by the bound molecules. It is regarded as “chain delivery” effect. At the same time, improved accessibility of binding sites led the significant increase of binding capacity. The decrease of qm value at IC > 740 mmol/L is considered due to the decrease of effective porosity. The research has thus provided new insight into protein adsorption and transport in polymer-grafted ion-exchange media.