Extraction and DFT study on interaction of the cesium cation with enniatin B

Abstract

By using extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs+(aq)+A−(aq)+1(nb)⇆1⋅Cs+(nb)+A−(nb) taking place in the two-phase water–nitrobenzene system (A−=picrate, 1=enniatin B; aq=aqueous phase, nb=nitrobenzene phase) was evaluated as logKex (1⋅Cs+, A−)=2.3±0.1. Further, the stability constant of the 1⋅Cs+ complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: logβnb (1⋅Cs+)=4.2±0.1. Finally, applying quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1⋅Cs+ was derived. In the resulting 1⋅Cs+ complex, which is most energetically favored, the “central” cation Cs+ is bound by nine bonding interactions to the corresponding nine oxygen atoms of the parent enniatin B ligand. The interaction energy of the considered complex 1⋅Cs+ was found to be −228.3kJ/mol, confirming the formation of this investigated complex as well.