Three kinds of ampholites, i.e., 3-aminopropionic acid (NH 2C2H4COOH), (2-aminoethyl)-phosphonic acid (NH2C2H4PO3H2), and 2-aminoethane-1-sulfonic acid (NH2C2H4SO 3H), were introduced into an epoxy group-containing polymer brush grafted onto a porous hollow-fiber membrane with a porosity of 70% and pore size of 0.36 μm. The amphoteric group density of the hollow-fiber ranged from 0.50 to 0.72 mmol/g. Three kinds of proteins, i.e., lactoferrin (Lf), cytochrome c (Cyt c), and lysozyme (Ly), were captured by the amphoteric polymer brush during the permeation of the protein solution across the ampholite-immobilized porous hollow-fiber membrane. Multilayer binding of the protein to the amphoteric polymer brush, with a degree of multilayer binding of 3.3, 8.6, and 15 for Lf, Cyt c, and Ly, respectively, with the (2-aminoethyl)-phosphonic acid-immobilized porous hollow-fiber membrane, was demonstrated with a negligible diffusional mass-transfer resistance of the protein to the ampholite immobilized. The 2-amino-ethane-1-sulfonic acid-immobilized porous hollow-fiber membrane exhibited the lowest initial flux of the protein solution, 0.41 m/h at a transmembrane pressure of 0.1 MPa and 298 K, and the highest equilibrium binding capacity of the protein, e.g., 130 mg/g for lysozyme. Extension and shrinkage of the amphoteric polymer brushes were observed during the binding and elution of the proteins.
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