Experimental and theoretical study on complexation of the calcium cation with [Gly6]-antamanide

Abstract

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Ca2+(aq)+1∙Sr2+(nb)⇄1∙Ca2+ (nb)+Sr2+(aq) occurring in the two–phase water–nitrobenzene system (1=[Gly6]-antamanide; aq=aqueous phase, nb=nitrobenzene phase) was determined as log Kex (Ca2+,1·Sr2+)=0.6±0.1. Further, the stability constant of the 1·Ca2+ complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: log βnb (1∙Ca2+)=10.2±0.2. Finally, by using quantum chemical DFT calculations, the most probable structure of the cationic complex species 1∙Ca2+ was derived. In the resulting complex, the “central” cation Ca2+ is bound by six bonding interactions to the corresponding six carbonyl oxygen atoms of the parent ligand 1. The interaction energy of the considered 1∙Ca2+ complex was found to be −1221.2kJ/mol, confirming the formation of this cationic species as well.