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

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 Kex (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 βnb (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.