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

From extraction experiments and T-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq) + 1.Na+(nb) reversible arrow 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 K-ex (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 degrees C: log beta(nb) (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 .(C) 2015 Elsevier B.V. All rights reserved.