Experimental and DFT Study on the Complexation of Zn2+with Valinomycin

Abstract

AbstractFrom extraction experiments andγ-activity measurements, the exchange extraction constant corresponding to the equilibrium Zn2+(aq) +1 ·Sr2+(nb) ⇌1 ·Zn2+(nb) + Sr2+(aq) taking place in the two–phase water–nitrobenzene system (1= valinomycin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as logKex(Zn2+,1 ·Sr2+) = −0.2±0.1. Further, the stability constant of the valinomycin–zinc complex (abbrev.1 ·Zn2+) in nitrobenzene saturated with water was calculated for a temperature of 25oC: logβnb(1 ·Zn2+) = 6.1±0.1. By using quantum mechanical DFT calculations, the most probable structure of the1 ·Zn2+complex species was predicted. In this complex, the Zn2+cation sits in the center of the coordination cavity formed by six ester carbonyls, which are oriented almost exactly in the “radial directions”. Finally, the calculated binding energy of the resulting complex1 ·Zn2+is −359.5 kcal/mol confirming relatively high stability of the considered cationic complex species.