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Conformational studies of cyclic pentapeptides by proton magnetic resonance spectroscopy.

The proton magnetic resonance spectra of c-(-Gly-L-Ala-Gly-Gly-L-Pro-) (I) and four analogous cyclopentapeptides are presented. At ambient temperature the spectra contain two sets of resonances which correspond to two different molecular conformations of the peptides. The relative concentrations of the two forms depend on the peptide, the solvent, and the temperature. For the two molecular species of peptide I in DMSO solution, the NMR. data imply that the peptide linkage involving the nitrogen of proline is respectively in the cis- and the trans-form, and both conformations contain intramolecular hydrogen bridges. Replacement of L-alanine in I by L-cysteine leaves the molecular conformations essentially unalteed. On the other hand substitution of L-proline by L-proline, or replacement of the two glycines in positions 3 and 4 by two sarcosyl residues gives rise to markedly different types of peptide backbone conformation.