Binding of Cu(II)-poly(N-isopropylacrylamide/vinylimidazole) copolymer to histidine-tagged protein:: A surface plasmon resonance study

The thermoresponsive copolymer of N-isopropylacrylamide (NIPAM) with 1-vinylimidazole (VI), poly(NIPAM-VI), synthesized by radical polymerization has been used to purify the histidine-tagged green flourescent protein (His-tag GFP) from recombinant E. coli by metal-chelate affinity precipitation. The purified protein was immobilized on the BIAcore sensor chip by carbodiimide coupling. Affinity binding of the Cu(II)-loaded copolymer, poly(NIPAM-VI), to the His-tag GFP-immobilized surface was monitored by surface-plasmon-resonance (SPR) measurements. Complete recovery of the metal copolymer from the surface was achieved with either using the monomer displacer, 200 mM imidazole buffer, or the polymeric displacer, copolymer of poly(NIPAM-VI) (26 mol % VI). The conformation of the copolymer was a critical factor for the metal interactions and hence displacement of the metal copolymer. With the proposed conformation of protein-like copolymers (Wahlund, P.-O.; Galaev, I. Yu.; Kazakov, S. A.; Lozinsky, V. I.; Mattiasson, B. Macromol. Biosci. 2002,2,33.), the SPR study confirmed the prediction of exposed imidazole groups in the poly(NIPAM-VI). The complete elution of the affinity-bound metal copolymer was achieved with protein-like copolymer (imidazole groups exposed to the outer solution), and no recovery was obtained with IMAC nonbound copolymer fraction (imidazole groups unexposed). The SPR measurement showed a sharp phase transition of affinity adsorbed thermoresponsive Cu(II)-poly(NIPAM-VI) copolymer at 32 degreesC, thus proposing a sensitive way to determine lower critical solution temperature.