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Abstract
Terminally protected pentapeptides with 2 histidines (Ac-HHVGD- and Ac-HVGDH-) and the terminally free peptides containing both internal aspartyl and C-terminal histidyl residues (FDAH and VIDAH) have been synthesized, and copper(II) complexes studied by potentiometric, UV-Vis, CD, and EPR spectroscopic techniques in solution. Both thermodynamic and spectroscopic data reveal that side chain donor atoms of aspartyl and histidyl residues have a significant contribution to the metal binding affinity of peptide molecules. In the case of terminally protected peptides, the role of the imidazole-N donor functions is reflected in the enhanced stability of the 3N and 4N coordinated copper(II) complexes. The amino and -carboxylate groups of FDAH and VIDAH create a very effective metal binding site with the (, , ) and (, , , ) coordination modes including the N-termini, while the histidine sites are available for the formation of the (, , ) binding mode resulting in the preference of dinuclear complex formation.
Highlights
Metal complexes of peptides are promising and frequently used models of the active centres of various metalloenzymes
It was found that the presence of β-carboxylate functions of aspartyl residues always results in the enhanced thermodynamic stability of peptide complexes and in the case of the Xaa-Xaa-Asp-Xaa sequences the coordination of β-carboxylate-O donor is able to prevent the deprotonation of subsequent amide functions
In this paper we report the synthesis and the results obtained for the copper(II) complexes of four new peptide ligands: (a) the terminally protected pentapeptides including one aspartyl and two histidyl moieties, Ac-HisHisValGlyAsp-NH2 (Ac-HHVGDNH2), Ac-HisValGlyAspHis-NH2 (Ac-HVGDH-NH2), and (b) the terminally free peptides containing internal aspartyl and C-terminal histidyl residues, PheAspAlaHis (FDAH) and ValIleAspAlaHis (VIDAH)
Summary
Metal complexes of peptides are promising and frequently used models of the active centres of various metalloenzymes. The terminal and Bioinorganic Chemistry and Applications amide donor functions are separated in these peptides and it generally results in very complicated solution equilibria including the formation of polynuclear species [16, 17]. The complexity of these systems is best exemplified by the metal ion interactions of glutathione [18]. The tetrapeptide HisValGlyAsp corresponds to the 78–81 amino acid fragment of the zinc binding site of the enzyme CuZn-SOD [19] and its copper(II) and zinc(II) complexes were studied by potentiometric and spectroscopic methods both for the free and terminally protected peptides. In this paper we report the synthesis and the results obtained for the copper(II) complexes of four new peptide ligands: (a) the terminally protected pentapeptides including one aspartyl and two histidyl moieties, Ac-HisHisValGlyAsp-NH2 (Ac-HHVGDNH2), Ac-HisValGlyAspHis-NH2 (Ac-HVGDH-NH2), and (b) the terminally free peptides containing internal aspartyl and C-terminal histidyl residues, PheAspAlaHis (FDAH) and ValIleAspAlaHis (VIDAH)
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