Experimental and Theoretical Study on the Complexation of Beauvericin with the Ammonium Cation

From extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq) + NaL+ (nb) double left right arrow 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 K-ex (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 degrees C as log beta(nb) (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 C-3 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+.