Transition metal complexes of bis(imidazol-2-yl) derivatives of dipeptides

Copper(II), nickel(II) and zinc(II) complexes of the dipeptides containing chelating bis(imidazolyl) agents at the C-termini (GlyLeu-BIMA, LeuGly-BIMA, PheGly-BIMA and AlaPro-BIMA; BIMA=bis(imidazol-2-yl) methylamine) were studied by potentiometric, UV-VIS, EPR and MALDI-MS techniques. The imidazole nitrogen donor atoms were described as the primary metal binding sites forming stable mono- and bis-(ligand) complexes in the acidic pH range. The formation of dinuclear complexes was detected in equimolar solutions of copper(II) and dipeptides and various isomeric forms of these species ([Cu2L2](4+) and [Cu2H-2L2](2+)) were indentified by EPR measurements. Deprotonation and co-ordination of the amide groups of dipeptides took place in slightly alkaline solution resulting in the formation of [MH-2L] complexes of copper(II) and nickel(II). Metal ion co-ordination of one of the imidazole nitrogen donor atoms promoted the ionization of the pyrrole type N(1) H group of imidazole in copper(II) complexes and resulted in the formation of a trinuclear complex ([Cu3H-6L2]). The latter species contains negatively charged bridging imidazolato residues.