Spectroscopic and potentiometric studies on coordination abilities of tetrapeptides containing proline and tyrosine or phenylalanine

Abstract

The insertion of proline residues, a ‘break-point’ [1–3] in the second position of a tetrapeptide divides the ligand molecule into two fragments which are potentially able to interact with metal ions independently, i.e. the N-terminal aminoacid ( e.g. via NH 2, CO) and two C-terminal aminoacid residues ( via e.g. 2N −, COO −). The introduction of tyrosine to such a peptide sequence complicates solution equilibria distinctly [1, 3]. Phenylalanine, which is not able to use its side-chain for the direct metal ion binding, is a good analogue for tyrosine and both residues were used for the synthesis and the study of the coordination abilities of tetrapeptides. The spectroscopic and thermodynamic studies of cupric complexes with tetrapeptides: GlyProGlyTyr, GlyProTyrGly, TyrProGlyGly, GlyProPheGly, GlyProDPheGly and PheProGlyGly have shown that the metal peptide interaction always starts at the N-terminal (NH 2, CO donors). The tyrosine residue may involve its side-chain OH in the metal ion binding with formation of monomeric or dimeric species. The complex equilibria strongly depend on the position of tyrosine residue in the peptide sequence. The lack of the direct involvement of phenylalanine side-chain in the direct metal ion coordination leads to much simpler equilibria. The presence of proline residue in the ligand molecule causes very unusual coordination modes in the formal complex species with formation of unusually large chelate rings. The combination of spectroscopic and potentiometric approaches led to the satisfactory description of very complicated systems which may be used as reasonable spectroscopic and structural models for biosystems.