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

Abstract

From extraction experiments and γ-activity measurements, the exchange extraction constant corresponding to the equilibrium Ca2+ (aq)+1·Sr2+ (nb) ⇆ 1·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 Kex (Ca2+, 1·Sr2+) = 2.4±0.1. Further, the stability constant of the valinomycin-calcium complex (abbrev. 1·Ca2+) in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1·Ca2+) = 8.3±0.1. By using quantum mechanical DFT calculations, the most probable structure of the 1·Ca2+·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·Ca2+·2H2O is −319.2 kcal/mol, which confirms the relatively high stability of this cationic complex species.