Surface Deae Groups Facilitate Bound Protein Transport on Polymer Chains in Deae-and Deae-Dextran-Modified Sepharose

To explore the contribution of surface ligands (ion exchange groups) to the mass transport of bound proteins on polymer chains in ion exchangers with both surface  and polymeric ligands, Sepharose FF was modified with diethylaminoethyl-dextran (DEAE-dextran, DexD) and/or DEAE (D) to fabricate three types of ion exchangers, i.e., FF-DexD (grafting-ligand resin), FF-D (surface-ligand resin), and FF-D-DexD (mixed-ligand resin), for protein adsorption equilibria and kinetics studies. It was found that both adsorption capacity and uptake rate (effective diffusivity, D_e) were significantly enhanced by grafting DEAE-dextran. Notably, the D_e/D₀ (D₀, free diffusivity) values in FF-DexD and FF-D-DexD were larger than 1.4, six times greater than those in FF-D (D_e/D₀<0.3). More importantly, the increase of surface-ligand density greatly enhanced uptake kinetics in FF-D-DexD. The results indicate that the surface ligands assisted the transport of bound proteins on polymer chains in the mixed-ligand resins. That is, the surface ligands worked as “transfer stations” between two neighboring chains, resulting in enhanced transport of bound proteins on chains. The research thus disclosed the unique role of surface ligands in facilitating protein uptake kinetics in polymer-grafted porous ion exchangers.