Characterization of poly(allylamine) as a polymeric ligand for ion-exchange protein chromatography

- Poly(allylamine)-grafted Sepharose gels were characterized for protein chromatography.
- Both capacity and uptake rate started to increase at ionic capacity of 373 mmol/L.
- Drastic increases of capacity and uptake rate were obtained at ionic capacity of 618 mmol/L.
- Protein adsorption on the resins exhibited salt tolerance till 500 mmol/L NaCl.
- A mild condition of pH 5.0 offered efficient elution of bound proteins.

This work reports poly(allylamine) (PAA), as a polymeric ion-exchange ligand for protein chromatography. Sepharose FF was modified with PAA, and six anion exchangers with ionic capacities (ICs) from 165 to 618 mmol/L were prepared. Inverse size exclusion chromatography, adsorption equilibrium, uptake kinetics and column elution were performed. It was found that both the adsorption capacity and effective diffusivity maintained low values in the IC range of 165-373 mmol/L, but they started to increase beyond 373 mmol/L, and increased by 80% and 23 times, respectively, when the IC reached 618 mmol/L. Interestingly, a drastic decrease of pore size was observed around the IC of 373 mmol/L. The results suggest that the PAA chains played an important role in protein adsorption by altering the inner pore structure of the gels. It is considered that, PAA chains turn from inextensible states with multipoint-grafting on the pore surface at low coupling densities (IC<373 mmol/L) to closer, extended and flexible grafting states with less coupling points at higher coupling densities (IC>373 mmol/L). These characters of the grafted chains at higher IC values benefit in protein adsorption by three-dimensional binding and encouraged the happening of “chain delivery” of bound proteins on the chains. Besides, the ion exchangers showed favorable adsorption and uptake properties in a wide ionic strength range, 0-500 mmol/L NaCl, indicating much better salt tolerance feature than the so-far reported ion exchangers. Moreover, a mild condition of pH 5.0 offered effective recovery of bound proteins in elution chromatography. The results indicate that the PAA-based anion exchanger of a high IC value is promising for high-capacity protein chromatography dealing with feedstock of a wide range of ionic strengths.