Solution and solid state study of copper(II) ternary complexes containing amino acids of interest for brain biochemistry – 1: Aspartic or glutamic acids with methionine or cysteine

Four new ternary complexes of Cu(II) containing aspartic or glutamic acids and methionine or cysteine as ligands were studied both in solution and in solid state. The solution study was performed using potentiometry, UV–Vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and electrochemistry. The stability constants of the ternary complexes were determined by potentiometry. The order of the values for the stability constants was CuGluMet (complex 4) < CuAspMet (complex 2) < CuGluCys (complex 3) < CuAspCys (complex 1), with complex 1 ≈ complex 3 ≫ complex 2 ≈ complex 4. This behavior was confirmed by the electronic spectra, EPR parameters and cyclic voltammetry. The compounds were synthesized in the solid state and characterized by thermogravimetry as well as vibrational and EPR spectroscopy. The results found in solid state are in agreement with the data found in solution studies where methionine, aspartic and glutamic acids coordinate to the copper(II) ion using the nitrogen atom of the amino group and one oxygen atom of the carboxylate group and cysteine coordinates through the nitrogen and sulfur atoms. The EPR data suggest that the complexes are square planar and that the complexes with cysteine as one of the ligands have some distortion.