Copper(II), nickel(II) and zinc(II) complexes of amino acids containing bis(imidazol-2-yl)methyl residues

• Published in: Inorganica Chimica Acta Vol. 339; pp. 373 - 382
• Main Authors: Ősz, Katalin, Várnagy, Katalin, Süli-Vargha, Helga, Sanna, Daniele, Micera, Giovanni, Sóvágó, Imre
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.11.2002
• Subjects: Bis(imidazolyl); EPR; Histidine; Potentiometry; Stability constants; Transition metal ion; Transition metal ion; Histidine; Stability constants; Potentiometry; EPR; Bis(imidazolyl)
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(02)00954-4

Copper(II), nickel(II) and zinc(II) complexes of Phe-BIMA and His-BIMA were studied by potentiometric, UV–Vis and EPR spectroscopic methods. The nitrogen donor atoms of the bis(imidazol-2-yl)methyl residues were described as the primary metal binding sites in all systems studied. Deprotonation and coordination of the terminal amino and amide nitrogen atoms took place in the copper(II) complexes of both ligands and resulted in the formation of dinuclear complexes ([Cu2H−2L2]2+) containing [NH2, N−, N(Im)] tridentate ligands and imidazole bridging. The presence of the histidyl side chain provides a great versatility in the complex formation reactions of His-BIMA. The existence of the species [Cu2L2]4+ was detected in slightly acidic solution and its structure was described as a mixture of three isomeric forms. Deprotonation of the imidazole-N(1)H donor functions was detected under slightly alkaline conditions with pK values of 8.13 and 8.93. An excess of copper(II) ions shifted this reactions even into the slightly acidic pH range and resulted in the formation of a trinuclear complex ([Cu3H−4L2]2+). The formation of imidazole-bridged dinuclear complexes ([Cu2H−2L2]2+) is characteristic in equimolar solutions of copper(II) with Phe-BIMA and His-BIMA at physiological pH. Deprotonation of imidazole-N(1)H groups of His-BIMA takes place under slightly alkaline conditions and it results in the formation of a trinuclear complex ([Cu3H−4L2]2+) in the presence of excess metal ions.