| Abstract: | The 1H-nmr spectra (270 MHz) of cyclic di- and tri-L -azatidine-2-carboxylic acid cy-clo(L -Aze)2 and cyclo(L -Aze)3] were determined in CDCl3 and D2O and computer simulated. The spectral results were compared with those obtained with cyclo (L -Pro)2 and cyclo (L, -Pro)3. In CDCl3 and D2O solution, the four membered ring of cyclo (L -Aze)2 is puckered with the α-proton in a pseudo-axial position, and the ? angle is smaller in absolute value than ?60°, as found for cyclo (L -Pro)2,. The puckering of the four-membered ring of cyclo(L, -Aze)3 in CDCl3 has the α-proton in a pseudo–equatorial position and ? angle larger in absolute value than ?60°, in agreement with cyclo(L -Pro)3. In D2O, cyclo(L -Aze)3 was found to interconvert rapidly between different conformers. In the azetidine cyclic peptides studied, the range of values found for the ? angles was smaller than in the related proline cyclic peptides, indicating greater rigidity in the four-membered ring. |
