Affinity Capillary Electrophoresis Applied to Investigation of Valinomycin Complexes with Ammonium and Alkali Metal Ions

Abstract

This chapter deals with the application of affinity capillary electrophoresis (ACE) to investigation of noncovalent interactions (complexes) of valinomycin, a macrocyclic dodecadepsipeptide antibiotic ionophore, with ammonium and alkali metal ions (lithium, sodium, potassium, rubidium, and cesium). The strength of these interactions was characterized by the apparent binding (stability, association) constants (K _b) of the above valinomycin complexes using the mobility shift assay mode of ACE. The study involved measurements of effective electrophoretic mobility of valinomycin at variable concentrations of ammonium or alkali metal ions in the background electrolyte (BGE). The effective electrophoretic mobilities of valinomycin measured at ambient temperature and variable ionic strength were first corrected to the reference temperature 25 °C and constant ionic strength (10 or 25 mM). Then, from the dependence of the corrected valinomycin effective mobility on the ammonium or alkali metal ion concentration in the BGE, the apparent binding constants of the valinomycin–ammonium or valinomycin–alkali metal ion complexes were determined using a nonlinear regression analysis. Logarithmic form of the binding constants (log K _b) were found to be in the range of 1.50–4.63, decreasing in the order Rb⁺ > K⁺ > Cs⁺ > > Na⁺ > NH₄ ⁺ ~ Li⁺.