HYDROGEN ION TITRATION AND SODIUM ION ACTIVITY OF TROPOMYOSIN SOLUTIONS

Potentiometric titrations and Na+ ion activity measurements in tropomyosin solutions were carried out over a wide range of pH by using an improved hydrogen electrode and a Na+-sensitive glass electrode. The experimental results were analyzed by the theories of rodlike polyelectrolyte, since both tropomyosin monomer and its fibrous polymers, whose transformation is reversibly controlled by salt concentration, are highly charged and rodlike. The titration data indicated the normal titration behavior of the main ionizable groups in high and low salt concentrations. The agreement of the experimental and theoretical values of Linderstrøm-Lang parameter w was satisfactory in acidic and neutral pH's. A further analysis of wexptl in the polymer state supported side-overlapping polymerization. The Henderson-Hasselbach relation was useful for the analysis of carboxyl regions, and these plots showed abnormal pH changes in the isoelectric precipitation regions. The small values of wexptl at high pH were interpreted to be due to the medium penetration into protein resulting from unfolding. The additivity rule for counterion activity was perfectly established at high and neutral pH's, and a thermodynamic relationship between pH and counterion activity derived from the additivity rule was also experimentally confirmed.