Extraction and ab initio calculation studies on the complexation of Ca²⁺ with valinomycin

From extraction experiments and γ-activity measurements, the exchange extraction constant corresponding to the equilibrium Ca²⁺(aq)+1 x Sr²⁺(nb) ↔ 1 x Ca²⁺ (nb) + Sr²⁺(aq) taking place in the two-phase water-nitrobenzene system (1 = valinomycin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K_ex (Ca²⁺, 1 x Sr²⁺) 2.4±0.1. Further, the stability constant of the valinomycin-calcium complex (abbrev. 1 x Ca²⁺) in nitrobenzene saturated with water was calculated for a temperature of 25 deg C: logβ_nb (1 x Ca²⁺) 8.3±0.1. By using quantum mechanical DFT calculations, the most probable structure of the 1 x Ca²⁺ x 2H₂O complex species was predicted. In this complex, the 'central' Ca²⁺ cation is bound by strong bonds to two oxygen atoms of the respective water molecules and to four ester carbonyl oxygens of the parent valinomycin ligand 1. Finally, the calculated binding energy of the considered complex 1 x Ca²⁺ x 2H₂O is -319.2 kcal/mol, which confirms the relatively high stability of this cationic complex species. (author)