Short CommunicationExperimental and theoretical study on complexation of the ammonium cation with enniatin B

- Stability of the enniatin B-NH4+ complex was determined.
- Quantum mechanical DFT calculations were applied.
- Structure of the resulting complex was predicted.

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq) + 1·Na+(nb) ⇆ 1·NH4+(nb) + Na+(aq) taking place in the two-phase water-nitrobenzene system (1 = enniatin B; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (NH4+, 1·Na+) = 1.9 ± 0.1. Further, the stability constant of the 1·NH4+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1·NH4+) = 6.4 ± 0.2. Finally, applying quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1·NH4+ was derived. In the resulting 1·NH4+ complex, the “central” cation NH4+ is bound by three relatively strong hydrogen bonds to the corresponding three carbonyl oxygens of the parent enniatin B ligand. The interaction energy, E(int), of the considered complex 1·NH4+ was found to be − 305.5 kJ/mol, confirming also the formation of this investigated complex.