The effect of carboxylate groups on the complexation of metal ion with oligopeptides – Potentiometric investigation.

Iron(II), iron(III), cobalt(II), zinc(II) cadmium(II) and lead(II) complexes of Asp2 and Asp3 and other Asp/Glu containing ligands were studied by potentiometry. Our goal was to study the influence of β- and γ-carboxylate groups as well as the increased negative charge of the ligands on the complex formation processes. Mainly 1:1 species are present, the formation of bis(ligand) complexes is not typical for these systems. It can be concluded that the effect of side chain carboxylate groups on the stability of complexes is especially significant in the case of lead(II) and cadmium(II). The greater the number of carboxylic groups there are in the system, the more stable complexes are formed with the studied metal ions. The influence of β-carboxylate groups of aspartic acid is unambiguous, they are directly bounded to the metal ion, which results in increased stability of the complexes. On the other hand, the repulsion between the negative charged residues becomes conspicuous resulting less stable species in the case of peptides containing glutamic acid.