0 _ _ affinity column chromatography (GE Healthcare UK Ltd., HiTrap Chelating HP) 6 1 0.5 M Imidazole 1 20 mM Tris-HCl, 50 mM Imidazole, 0.5 M NaCl, pH 7.5 crib220u270rib220u49432i Column chromatography