Binding of Cetyl- and Dodecylpyridinium Cations by Poly (L – Glutamic Acid)†

Andrej Godec* and Ksenija Kogej
Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology,
University of Ljubljana, Aškerčeva 5, P. O. Box 537, SI-1000 Ljubljana, Slovenia

The cooperative binding of cetyl-(CPC) and dodecylpyridinium (DPC) chloride by poly (L – glutamic) acid (PGA) and their effect on the conformation of PGA in aqueous solution without added simple salts was studied by the potentiometric and circular dichroism (CD) measurements. The degree of neutralization of the polyelectrolyte solution was α = 0.25, 0.50, 0.75 and 1.0. The concentration of the polyelectrolyte solutions was 5.0 × 10–4 monomol/kg and the measurements were carried out at 25.0 °C. Binding isotherms have shown that while the binding of DPC to PGA is negligible, the opposite is true for CPC. The cooperative binding of CPC to the polyion starts at a total surfactant concentration below 5 × 10–4 mol/kg, which is almost two orders of magnitude lower than its cmc. This result is independent of the degree of neutralization of the polyelectrolyte. It was also found that the amount of binding β depends almost linearly on α. In the case of fully ionized polyelectrolyte, β is approximately 1. By extrapolating β to α = 0 it was shown that there is some cooperative binding of CPC also by the unionized polyacid. CD measurements showed that both surfactants induce partial helical structure of the polyelectrolyte with DPC being somewhat more effective. This finding is attributed to a smaller size of the DPC micelle.

Keywords: Poly (L-glutamic acid), degree of neutralization, cetyl- and dodecylpyridinium cations, potentiometry, binding isotherms, circular dichroism, conformational change.