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 log  K ex ( 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: log  β nb ( 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 −228.3 kJ/mol, confirming the formation of this investigated complex as well.