Poly(N-isopropylacrylamide) Networks Conjugated with Gly–Gly–His via a Merrifield Solid-Phase Peptide Synthesis Technique for Metal-Ion Recognition

The temperature-dependent swelling behavior of poly(N-isopropylacrylamide) and tripeptide Gly–Gly–His (GGH)/poly(NIPAAm) conjugate hydrogel coatings are investigated using a quartz crystal microbalance with dissipation (QCM-D) while in contact with NaCl, ZnCl2, NiCl2, and CuCl2 solutions. To fabricate the tripeptide conjugated gels, precursor gels of poly(NIPAAm-co-3-aminopropylmethacrylamide[3.5 mol%]) are synthesized via free-radical polymerization. The metal-binding tripeptide, GGH, is subsequently synthesized in the gel via a Merrifield solid-phase peptide synthesis (SPPS) technique, in which the amino group of the copolymer gel provides a functional site to support peptide synthesis. It is found that the logarithm of the transition temperature of the tripeptide GGH/poly(NIPAAm) conjugate hydrogel is proportional to the ionic strength, showing two distinct regions at low and high ionic strengths for the divalent ions. In the low-ionic-strength regime, the salting out constants are 0.08, 0.07, and 0.06 M−1 for Cu2+, Ni2+, and Zn2+, respectively, which follows the known trend for binding of ions to GGH. In the high-ionic-strength region, when the metal-ion binding sites in the tripeptide conjugate hydrogel are saturated, the salting out constants are similar to the salting out constants associated with pure poly(NIPAAm).