Extraction and DFT study on the complexation of the strontium cation with enniatin B

Abstract By using extraction experiments and γ -activity measurements, the extraction constant corresponding to the equilibrium Sr 2+ (aq) + 2A − (aq) +  1 (nb)  ⇄ 1  · Sr 2+ (nb) + 2A − (nb) occurring 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  · Sr 2+ ,2A − ) = 3.4 ± 0.1. Further, the stability constant of the 1  · Sr 2+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log  β nb ( 1  · Sr 2+ ) = 12.5 ± 0.1. Finally, applying quantum mechanical DFT calculations, the most probable structure of the proved 1  · Sr 2+ cationic complex was derived. In the resulting complex, which is most energetically favored, the “central” cation Sr 2+ is bound by nine bonding interactions to the corresponding nine oxygen atoms of the parent enniatin B ligand. The interaction energy of the considered 1  · Sr 2+ complex was found to be −877.4 kJ/mol, confirming also the formation of this complex.