Proton NMR spectra of glutathione have been analyzed over the whole pH region. The spectra of the glutamic acid residue are of the ABCDX type at pHs below 2.5, of the AA′BB′X types in the pH region 2.5–9, and of the ABCC′X type at pHs higher than 9. For the cysteine part, spectra of the ABC type have been observed at pHs higher than 8, which change to spectra of the A2B type at pHs lower than 7. On the basis of these results the conformation of the glutathione molecule in solution has been discussed in detail.
The L-proline residue in oligopeptides has been shown to act as a 'break-point' in their co-ordination to copper(ii) ions leading to the formation of large chelate rings with the peptide locked in the (3-conformation.
Results are reported of a potentiometric and spectrophotometric study of the H + and Cu2+ complexes of the tetra peptides X-Gly-Gly-G ly, G ly-X-G ly-Gly, G ly-Gly-X-G ly, and Gly-Gly-G ly-X where X is the proline (Pro) and sarcosine (Sar) residue (Gly = glycine). All the tetrapeptides (HL) form the series of complexesand [CuH-,L] (charges omitted). The ligands Gly-X-Gly-Gly also form the bis-complex, [CuL,]. When inserted in a peptide chain the Pro and Sar residues cannot co-ordinate t o Cu2+ through their peptide nitrogens since they d o not possess ionizable protons. In addition the Pro residue tends t o force the peptide chain to form a 'pturn' 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 t o 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.
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