Interaction between palladium(II) ions and asparagine in aqueous solutions
The interaction between Pd(II) and asparagines (Asn) in aqueous solutions with the physiological concentration of chloride ions (0.15 mol/L KCl) has been studied by election spectroscopy and pH potentiometry depending on the ratio of Pd(II):Asn and pH. The formation of equimolar and bisligand complexes has been found where the Asn molecule is coordinated to the central metal ion in a bidentate fashion either by the amine group nitrogen atom and the carboxyl group oxygen atom or by the nitrogen atoms of the amine group and the deprotonated amide group. In the case of bidentate coordination of Asn in complexes with the ratio of metal : ligand = 1 : 1, the Pd(II) coordination sphere is complemented by two chloride ions. The composition of the first coordination sphere of Pd(II) complexes has been characterized on the basis of the position of the absorption band maximum in the electronic spectra of solutions. By the PSEQUAD programme the formation constants of Pd(II) asparaginate complexes: lgβ [PdLCl2]- = 19.36 (2); lgβ [PdLH-1Cl2]2- = 13.63 (1); lgβ [PdL2] = 25,99 (2); lgβ [PdL2H-1]- = 19.97 (4); lgβ [PdL2H-2]2- = 12.78 (4) have been calculated taking into account the concentration of chloride ions. Based on the equilibrium concentration distribution diagrams constructed for the complexes it has been shown that at the physiological values of pH and the chloride ion concentration the complexes [PdLH-1Cl2]2- in the ratio of metal : ligand = 1 : 1 and [PdL2H-1]-, [PdL2H-2]2- in the ratio of metal : ligand = 1 : 2 predominate in solutions.
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