Conformation-dependent affinity of protein-like copolymers for small ligands. Poly(NIPAM-co-sodium styrene sulfonate) – Polyamines systems

Abstract The thermoresponsive copolymers of N-isopropylacrylamide (NIPAM) with sodium styrene sulfonate (SSS) exhibit a protein-like transition behavior in aqueous medium. They undergo a coil-to-globule conformational transition without phase separation. In the present work we investigated a conformation-dependent binding of polyfunctional ligands to the protein-like NIPAM- b -SSS copolymers modelling their functional behavior by analogy with the enzyme-substrate interaction. Natural polyamines of various charge (putrescine, spermidine and spermine) were used as the polyfunctional ligands. The polyamine-copolymer binding was evaluated by high-sensitivity differential scanning calorimetry. The dependences of the transition parameters (transition temperature, enthalpy, heat capacity increment and thermodynamic width) for two NIPAM- b -SSS copolymers of low and high charge density on the polyamine concentrations were obtained. In general, they pointed to a preferential binding of polyamines to the copolymers in globular conformation, that is particularly pronounced for the copolymer of low charge density. The excess transition enthalpy was introduced as an apparent measure of the ligand binding to the copolymer. The apparent binding constants increased in the series putrescine