Mixed-ligand complexes of copper(II) ions with L-glutamic acid in the systems with triamines and non-covalent interaction between bioligands in aqueous solution

Abstract

Abstract The interactions between L-glutamic acid (Glu) and 1,7-diamino-4-azaheptan (3,3-tri) and 1,8-diamino-4-azaoktan (Spd) in metal-free systems and in the appropriate Cu(II) ions complexes were studied by potentiometric and spectral methods. The composition and overall stability constants of complexes forming in the systems studied were determined and the reactions centres were identified. In the complex formation conditions, the ligands have positive and negative reaction centres, which are also the potential sites of metal coordination. Below pH 7 in metal-free systems, the terminal amine groups from both triamines and the oxygen atoms from –C(5)OO- group as well as the amine group from Glu do not take part in the weak noncovalent interactions between the ligands, but at higher pH all available active centres of the ligands are involved in the interactions in the adducts formed and the inversion effect is observed. In the Cu(II)/Glu/triamine systems, in the species MLHL’, both 3,3-tri and Spd were found to coordinate Cu(II) ions in the same way, while only in the Cu(Glu)H(Spd) complex the oxygen atoms from –C(5)OO- group of the amino acid do not take part in metallation. In the species MLL’ and MLL’OH with 3,3-tri, the chromophore formed was of different type than that formed in the corresponding species with Spd. This is related to the fact that the nitrogen atom from the amine group of Glu is inactive in the coordination. Thus, not only the length of the polyamine carbon chain but also the length of the amino acid carbon chain influence the interactions between the bioligands. The introduction of metal ions to Glu-Spd metal-free system excludes the interaction between the bioligands, and the ligands centres that do not take part in the weak interactions in the adduct (Glu)H(Spd) are involved in the metallation, but the centres that took part in the weak interactions are not involved in the coordination in a heteroligand complex.