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

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq) + NaL+ (nb) <-> NH4L+(nb) + Na+ (aq) taking place in the two-phase water-nitrobenzene system (L = beauvericin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (NH4+, NaL+) = 1.5 ± 0.1. Further, the stability constant of the NH4L+ complex in water-saturated nitrobenzene was calculated for a temperature of 25 °C as log βnb (NH4L+) = 4.6 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the NH4L+ cationic complex species was derived. In this complex having C3 symmetry, the ammonium cation NH4+ is bound by three strong linear hydrogen bonds to the three corresponding oxygen atoms of the parent beauvericin ligand L. The interaction energy of the resulting complex NH4L+ was found to be -828.8 kJ/mol, confirming the formation of the considered complex NH4L+.