Stabilities and coordination modes of methionine in copper(II) mixed-ligand complexes with ethylenediamine, diethylenetriamine or N, N, N′, N′′, N′′-pentamethyldiethylenetriamine in aqueous solution

Abstract Solution equilibrium studies on the Cu(II)–polyamine–histidine ternary systems (polyamine: ethylenediamine (en), diethylenetriamine (dien), N , N , N ′, N ″, N ″-pentamethyldiethylenetriamine (Me 5 dien)) have been performed by pH-potentiometry, UV–Vis spectrophotometry and EPR methods. The obtained results suggest the formation of the mixed-ligand complexes with [Cu(A)(His)] + stoichiometry in all studied systems. Additionally, in the systems with dien and Me 5 dien protonated [Cu(A)(H–His)] 2+ species also exists in acid solution. Our spectroscopic results indicate the tetragonal geometry for the [Cu(en)(His)] + , the geometry slightly deviated from square pyramidal for the [Cu(dien)(His)] + and strongly deviated from square pyramidal towards trigonal bipyramidal for the [Cu(Me 5 dien)(His)] + species. The coordination modes in these mixed-ligand complexes are discussed.