Peptide Conformation. 17. cyclo-(L-Pro-L-Pro-D-Pro). Conformational Analysis by 270- and 500-MHz One- and Two-Dimensional ¹H NMR Spectroscopy

The 270- and 500-MHz ¹H NMR spectra of cyclo-(L-Pro₂-D-Pro) in various solvents have been analyzed with the aid of one- and two-dimensional (2-D) NMR techniques; 500-MHz 2-D-J-resolved spectra were used to determine starting parameters for chemical shift values and to assign overlapping spin multiplets of the individual nuclei in the 21-spin system. 2-D spin-echo-correlated spectra yield connectivities between coupled spins. In particular, many small long-range couplings have been detected by this technique. Stereochemical assignment of geminal proline protons has been performed by 2-D nuclear Overhauser enhancement (2-D NOE) spectroscopy and by interpretation of aromatic solvent-induced shift (ASIS) effects, coupling constants, and carbonyl anisotropy. A twisted-boat backbone conformation similar to the structure determined by X-ray analysis was derived from the data.