Interaction of sodium and calcium ions with poly(alkylene phosphate)—a synthetic analogue of natural teichoic acids

Abstract

The competitive binding of sodium and calcium ions to poly(alkylene phosphate)s, which can be considered as synthetic models of bacterial teichoic acids, and to poly(styrenesulfonate)s was studied experimentally by potentiometry with ion-selective electrodes. The fractions of apparently free counterions, f_fi, were determined and compared with theoretical predictions obtained from the Poisson–Boltzmann theory applied to the cylindrical cell model of polyelectrolyte solutions containing a mixture of counterions differing in charge. For the studied systems, the dependence of f_fNa and f_fCa on the composition of the counterion mixture and on a charge density parameter of the polyion was found to be fairly well described by theory. For poly(alkylene phosphate)s the calculated values of f_fi noticeably exceeded the measured fractions of apparently free ions. Much better agreement between theory and experiment was obtained if the effective value of the charge density parameter was used.