Vinyl Polymers Based on L-Histidine Residues. Part 1. The Thermodynamics of Poly(ampholyte)s in the Free and in the Cross-Linked Gel Form

Amphiphilic vinyl polymers (in the free and cross-linked forms), carrying carboxyl and imidazole groups, were prepared by a radical polymerization of the purposely synthesized N-acryloyl-L-histidine. The protonation thermodynamic studies (at 25 °C in 0.15 M NaCl) showed high polyelectrolyte character of the soluble polymer. Unlike the linear decreasing trend of the basicity constant, over the whole range of α (degree of protonation), the enthalpy changes for the protonation of the imidazole nitrogen in the polymer showed a decreasing pattern only at α > 0.5. This was ascribed to the formation of hydrogen bonds between protonated and free neighboring monomer units. Viscometric data revealed a minimum hydrodynamic volume of the polymer at its isoelectric point (pH 5), whereas at higher or lower pHs, the macromolecule expanded greatly as a consequence of the charged sites formation. This produced a preferential solvation of the protonated imidazole and carboxylate ions, the latter being surrounded by more water molecules in the hydration shell. The peculiar hydration behavior was confirmed in the cross-linked polymer. The hydrogel showed an equilibrium degree of swelling (EDS), strongly dependent on pH, in a similar manner as viscometric data of the soluble polymer. A linear relationship between the reduced viscosity and the EDS was found. The polymer was non toxic against the RAW264 cell line. © 2004 American Chemical Society.