Influence of D and L amino‐acid residues on the conformation of peptides in solution: A carbon‐13 nuclear magnetic resonance study of cyclo (prolyl‐leucyl)

The 13 C chemical shifts and spin‐lattice relaxation times are reported for cyclo ( L ‐Pro‐ L ‐Leu) and cyclo ( L ‐Pro‐ D ‐Leu). The chemical shifts of the D and L leucyl residues in the cyclic peptides differ from each other by 1.8 and 3.6 parts per million for the α and β carbons, respectively. The α‐carbons of the prolyl residues differ by 1.0 ppm as a consequence of proximity to a D or an L leucyl residue. The 13 C spin‐lattic relaxation time( T 1 ) of the prolyl residues, but not the leucyl residues, in both compounds are indicative of difference in conformational equilibria within the pyrrolidine ring in the L ‐ L isomer as compared to the L ‐ D isomer. Anisotropic overall molecular reorientation is not responsible for the differences observed in the T 1 values. The differences in T 1 values and chemical shifts between cyclo ( L ‐Pro‐ L ‐Leu) and cyclo ( L ‐Pro‐ D ‐Leu) appear to result from a difference in conformations of the two diketopiperazine rings.