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

Abstract

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 −305.5kJ/mol, confirming also the formation of this investigated complex.