Capillary affinity electrophoresis and ab initio calculation studies of valinomycin complexation with Na+ ion

In a combined experimental and theoretical approach, the interactions of valinomycin (Val), macrocyclic depsipeptide antibiotic ionophore, with sodium cation Na⁺ have been investigated. The strength of the Val–Na⁺ complex was evaluated experimentally by means of capillary affinity electrophoresis. From the dependence of valinomycin effective electrophoretic mobility on the sodium ion concentration in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, 0–40 mM NaCl), the apparent binding (stability) constant (K_b) of the Val–Na⁺ complex in methanol was evaluated as log K_b = 1.71 ± 0.16. Besides, using quantum mechanical density functional theory (DFT) calculations, the most probable structures of the nonhydrated Val–Na⁺ as well as hydrated Val–Na⁺·H₂O complex species were proposed. Compared to Val–Na⁺, the optimized structure of Val–Na⁺·H₂O complex appears to be more realistic as follows from the substantially higher binding energy (118.4 kcal/mol) of the hydrated complex than that of the nonhydrated complex (102.8 kcal/mol). In the hydrated complex, the central Na⁺ cation is bound by strong bonds to one oxygen atom of the respective water molecule and to four oxygens of the corresponding C=O groups of the parent valinomycin ligand.