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

Abstract

Copper(), nickel() and zinc() 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() and dipeptides and various isomeric forms of these species ([Cu 2 L 2 ] 4ϩ and [Cu 2 H Ϫ2 L 2 ] 2ϩ ) 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 Ϫ2 L] complexes of copper() and nickel(). 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() complexes and resulted in the formation of a trinuclear complex ([Cu 3 H Ϫ6 L 2 ]). The latter species contains negatively charged bridging imidazolato residues.

Metal ion selectivity of oligopeptides

“…The results obtained for the amino acid derivatives have already been reviewed, 166 while the most recent data on the metal complexes of dipeptide-BIMA ligands have been published recently. 167,168 It is clear from these studies that the presence and metal ion coordination of bis(imidazolyl) residues significantly enhances the metal binding affinity of these peptides, but the amide functions are the major metal binding sites in basic solution.…”

New peptide-type tripodal ligands and their metal complexes

“…The measured higher molecular weights are due to the reduction of Cu 2+ to Cu + and the corresponding uptake of one proton per metal ion during the ionization process. Identical observations were reported in the literature related to MALDI-TOF MS studies of other copper(II) complexes, formed with cyclen/cyclam derivatives [121], some pseudo-peptides [122] and peptides [123]. Since trinuclear complexes may have several protonation states, the copper(II)-L 1 3:2 system presents a complicated, but rather obvious speciation behavior (Figure 4.3).…”