Immobilization of Carboxymethylated Polyethylenimine–Metal-Ion Complexes in Porous Membranes to Selectively Capture His-Tagged Protein

Membrane adsorbers rapidly capture tagged proteins because flow through membrane pores efficiently conveys proteins to binding sites. Effective adsorbers, however, require membrane pores coated with thin films that bind multilayers of proteins. This work employs adsorption of polyelectrolytes that chelate metal ions to create functionalized membranes that selectively capture polyhistidine-tagged (His-tagged) proteins with binding capacities equal to those of high-binding commercial beads. Adsorption of functional polyelectrolytes is simpler than previous membrane-modification strategies such as growth of polymer brushes or derivatization of adsorbed layers with chelating moieties. Sequential adsorption of protonated poly­(allylamine) (PAH) and carboxymethylated branched polyethylenimine (CMPEI) leads to membranes that bind Ni²⁺ and capture ∼60 mg of His-tagged ubiquitin per mL of membrane. Moreover, these membranes enable isolation of His-tagged protein from cell lysates in <15 min. The backbone amine groups in CMPEI likely increase swelling in water to double protein binding compared to films composed of PAH and the chelating polymer poly­[(<i>N</i>,<i>N</i>-dicarboxymethyl)­allylamine] (PDCMAA), which has a hydrocarbon backbone. Metal leaching from PAH/CMPEI- and PAH/PDCMAA-modified membranes is similar to that from GE Hitrap FF columns. Eluates with 0.5 M imidazole contain <10 ppm of Ni<sup>2+