Comparison of Theoretical and Experimental Determinations of cis/trans Proline Isomer Ratios: Effect of Molecular Charge State

The resonances of the 1H and 13C n.m.r. spectra of the peptide Pro-Pro-Pro in aqueous solutions have been assigned to specific nuclei by employing one- and two-dimensional techniques and pH titrations. Minimum energy conformations of the tripeptide in different ionization states were estimated by conformational energy calculations with two effective dielectric constants. The cis/trans isomer ratios of the lowest energy conformers obtained for each set of conditions were compared to the isomer ratios measured from the 13C n.m.r. spectra of the peptide in the cationic, zwitterionic and anionic forms. The theoretical predictions did not agree with the experimental data. The conformations and geometry of triprolyl sequences in the uncharged pentapeptide Ac- Gly-Pro-Pro-Pro-Gly-NHMe were investigated for different isomer configurations of the proline residues. The structure of the molecule was characterized by its conformational energy and by two other parameters estimated from the atomic coordinates of each conformation: the distance between atoms located at the end groups, and the direction cosine between vectors defined at the N- and C-terminal ends of the molecule. About 90% of the statistical weight of the lowest energy conformers corresponded to extended geometries of the triprolyl sequence.