Protein-like energetics of conformational transitions in a polyampholyte hydrogel

The volume phase transition of a hydrogel of the polyampholyte network N-isopropylacrylamide (6000) - N-(3-aminopropylmethacrylamide) (120) - Acrylic acid (120) - N,N′-Methylenebisacrylamide (40) was investigated by high-sensitivity differential scanning calorimetry in relation to pH, concentrations of NaCl and two oppositely charged drug ligands (ibuprofen and propranolol). The transition temperature, enthalpy, heat capacity increment and width against these thermodynamic variables were determined. The experimental data were used for calculation of the excess free energy functions of the phase transition which are measures of the relative stability of the dense (collapsed) state of the polyampholyte network under different conditions. By analogy with globular proteins, the stability of the network dense state is maximal at the isoelectric point of the polyampholyte and is increased by the kosmotropic salt and specific ligands. We demonstrate that the polyampholyte hydrogel consists of independent cooperative domains. The sizes of these domains in the hydrogel (9–15 kDa) are comparable with those in globular proteins.