Microcalorimetric studies of solvent-induced conformational change of sodium and cesium salts of poly(L-glutamic acid) in aqueous media

Organic solvent-induced coil helix conformational change of poly(sodium) L-glutamate (NaPLG) and poly(cesium L-glutamate) (CsPLG) in solution in aqueous mixed solvents have been studied at 25°C. Heats of dilution of NaPLG in the water–dioxane pair have been measured as a function of polymer concentration and solvent composition. The results indicate that the overall chain conformation in the disordered form is not too different from that in the α-helical form. Heat capacity measurements by flow microcalorimetry have also been done. The apparent monomolar heat capacity at constant pressure of the polymer, Cp, ϕ, decreases with dilution similarly to other strong polyelectrolytes in aqueous media. In the water–dioxane pair, Cp, ϕ increases with the dioxane content due to partial desolvation of ionic species resulting from increasing ionic association. In the case of the water-2-chloroethanol (CE) pair, the transition takes place at low CE content and results show a fast decrease in Cp, ϕ when the α-helical conformation predominates. It is believed carboxylate groups and CE molecules associate themselves into a complex formation responsible for the transition. The size of the cation plays a significant role in the thermodynamic properties of these polyelectrolytes in solution since sodium ions are more strongly bound to the chain than cesium ions.