Capillary electrophoretic and computational study of the complexation of valinomycin with rubidium cation

Affinity capillary electrophoresis and quantum mechanical calculations applied to investigation of [Gly6 ]-antamanide binding with sodium and potassium ions

ACE in a free solution and quantum mechanical density functional theory have been applied to the investigation of interactions of glycine-6-antamanide ([Gly⁶ ]AA), a synthetic derivative of cyclic decapeptide antamanide isolated from the highly poisonous mushroom Amanita phalloides, with sodium or potassium ions in methanol. First, from the dependence of effective electrophoretic mobility of [Gly⁶ ]AA on Na⁺ or K⁺ ions concentration in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, pH_MeOH 7.8, containing 0-50 mM NaCl or 0-40 mM KCl), the apparent binding (stability) constants of [Gly⁶ ]AA-Na⁺ and [Gly⁶ ]AA-K⁺ complexes were evaluated as 26 ± 1 and 14 ± 1 L/mol, respectively. The employed ACE method included correction of the effective mobilities measured at ambient temperature and at variable ionic strength of the BGEs to the mobilities related to the reference temperature 25°C and to the constant ionic strength 10 mM. Second, the interaction energies of the [Gly⁶ ]AA-Na⁺ and [Gly⁶ ]AA-K⁺ complexes (-466.3 and -345.2 kJ/mol, respectively) and the structural details of these complexes, such as position of the Na⁺ and K⁺ ions in the cavity of the [Gly⁶ ]AA molecule and the interatomic distances within these complexes, were determined by the density functional theory calculations.

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

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(+).