Synthesis and Metal Ion Adsorption Properties of a Dense Triazole Polymer Carrying Cysteine Residues

Copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) is promising as a reaction to synthesize functional polymers consisting of 1,2,3-triazole units. Recently, we reported a series of dense 1,2,3-triazole polymers of 4-azido-5-hexynoic acid derivatives. Herein, we designed a new water-soluble dense 1,2,3-triazole polymer carrying amino acid residues in the side chains, that is, poly(N-(4-azido-5-hexynoyl)cysteine) (poly(AC)), bearing L-cysteine as a ligand for metal ions. CuAAC polymerization of a protected monomer followed by deprotection and neutralization yielded the sodium salt of poly(AC) (poly(AC)Na). The adsorption capacity of poly(AC)Na with Cd²⁺ was investigated with a colorimetric assay. Furthermore, the interactions for poly(AC) with group 12 metal ions (Zn²⁺, Cd²⁺, and Hg²⁺) were investigated using isothermal titration calorimetry (ITC) and transmittance measurements. Utilizing the colorimetric assay data, the Cd²⁺ adsorption capacity per unit weight of poly(AC)Na was evaluated to be 4–5 mmol g⁻¹, comparable to those of Cd²⁺ adsorbents reported previously. The ITC data demonstrated that the interactions of poly(AC)Na with Zn²⁺, Cd²⁺, and Hg²⁺ were predominantly exothermic. Based on the ITC data, apparent dissociation constants of interactions were roughly estimated to be in the order of 10⁻⁵–10⁻⁴ M, which are comparable to those for metal-binding biomolecules. The transmittance data indicated that the poly(AC)Na/CdCl₂ system underwent phase separation.