Interaction of the univalent silver cation with [Gly6]-antamanide: Experimental and theoretical study

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Ag⁺(aq) + 1^.Na⁺(nb) ⇄ 1^.Ag⁺ (nb) + Na⁺(aq) occurring in the two-phase water - nitrobenzene system (1 = [Gly⁶]-antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log K_ex (Ag⁺,1·Na⁺) = 1.5 ± 0.1. Further, the stability constant of the 1·Ag⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β_nb (1·Ag⁺) = 4.5 ± 0.2. Finally, by using quantum chemical DFT calculations, the most probable structure of the cationic complex species 1·Ag⁺ was derived. In the resulting complex, the "central" cation Ag⁺ is coordinated by four noncovalent interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1·Ag⁺ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1·Ag⁺ complex was found to be -465.5 kJ/mol, confirming also the formation of this cationic species.