Experimental and theoretical study on complexation of the ammonium cation with enniatin B.

From extraction experiments and γ -activity measurements, the extraction constant corresponding to the equilibrium NH 4 + (aq) + 1 ·Na + (nb) ⇆ 1 ·NH 4 + (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 K ex (NH 4 + , 1 ·Na + ) = 1.9 ± 0.1. Further, the stability constant of the 1 ·NH 4 + complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β nb ( 1 ·NH 4 + ) = 6.4 ± 0.2. Finally, applying quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1 ·NH 4 + was derived. In the resulting 1 ·NH 4 + complex, the “central” cation NH 4 + 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 ·NH 4 + was found to be − 305.5 kJ/mol, confirming also the formation of this investigated complex. [ABSTRACT FROM AUTHOR]