TheL-proline residue as a ‘break-point’ in metal–peptide systems

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Abstract
Results are reported of a potentiometric and spectrophotometric study of the H+ and Cu2+ complexes of the tetrapeptides X-Gly-Gly-Gly, Gly-X-Gly-Gly, Gly-Gly-X-Gly, and Gly-Gly-Gly-X where X is the proline (Pro) and sarcosine (Sar) residue (Gly = glycine). All the tetrapeptides (HL) form the series of complexes [CuL], [CuH–1L], [CuH–2L], and [CuH–3L](charges omitted). The ligands Gly-X-Gly-Gly also form the bis-complex, [CuL2]. When inserted in a peptide chain the Pro and Sar residues cannot co-ordinate to Cu2+ through their peptide nitrogens since they do not possess ionizable protons. In addition the Pro residue tends to force the peptide chain to form a ‘β-turn’ and so adopt a ‘bent’ conformation. These studies demonstrate the formation of a large chelate ring when tetrapeptides containing Pro (and, to a smaller extent, Sar) in the second or third positions co-ordinate to Cu2+. This ring spans the terminal residues of the peptide chain and locks the peptide into a ‘bent’ or ‘horse-shoe’ shaped conformation. Cu2+ could therefore play an important role in activating oligopeptides (e.g. neuropeptides) containing proline.