Application of capillary affinity electrophoresis and density functional theory to the investigation of valinomycin–lithium complex

Capillary affinity electrophoresis (CAE) and quantum mechanical density functional theory (DFT) have been applied to the investigation of interactions of valinomycin (Val), a macrocyclic dodecadepsipeptide antibiotic ionophore, with lithium cation Li+. Firstly, from the dependence of effective electrophoretic mobility of Val on the Li+ ion concentration in the background electrolyte (BGE) (methanolic solution of 50 mM chloroacetic acid, 25 mM Tris, pHMeOH 7.8, 0–40 mM LiCl), the apparent binding (stability) constant (Kb) of Val–Li+ complex in methanol was evaluated as log Kb = 1.50 ± 0.24. The employed CAE method include correction of the effective mobilities measured at ambient temperature, at different input power (Joule heating) and at variable ionic strength of the BGEs to the mobilities related to the reference temperature 25 °C and to the constant ionic strength 25 mM. Secondly, using DFT calculations, the most probable structures of the non-hydrated Val–Li+ and hydrated Val–Li+·3H2O complex species were predicted.