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

On the basis of extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium Ca2+(aq) + 1.Sr2+ (nb) reversible arrow 1.Ca2+ (nb) + sr(2+)(aq) occurring in the two-phase water-nitrobenzene system (1= [Gly(6)]-antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log K-ex, (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 degrees C: log beta(nb) (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 -1221.2 kJ/mol, confirming the formation of this cationic species as well. (C) 2017 Elsevier B.V. All rights reserved.