HELIX-COIL TRANSITION OF A TITRATING POLYELECTROLYTE ANALYZED WITHIN THE POISSON-BOLTZMANN CELL MODEL - EFFECTS OF PH AND COUNTERION VALENCY

The influence of counterion valency and concentration of salt on the pH-induced helix-coil transition of aqueous poly(glutamic acid) has been studied experimentally and theoretically. The conformational equilibrium is not influenced by the addition of divalent salt (2:1 electrolyte) despite both conformations being charged. Addition of 1:1 electrolyte to mixed systems, 1:1 and 2:1 electrolyte, produced a weak decrease in the helical content. These features were analyzed by the theoretical model based on the Poisson-Boltzmann cell model taking into account both the electrostatic free energy and the free energy of binding due to the protonation of carboxylate groups.