Cyclic peptides. XVIII. 13C spin-lattice relaxation times of (X-pro-Y)2 cyclic hexapeptides

¹³C spin-lattice relaxation times (T₁'s) of four cyclic hexapeptides of sequence, (X-L -Pro-Y)₂, are reported. The T₁'s of the protonated carbons, which undergo dipolar relaxation, are interpreted qualitatively in terms of the overall tumbling motion of the molecule and in terms of internal motion. It is found that three of the cyclic hexapeptides, those which adopt all-trans β-conformers, tumble isotropically and appear to lack internal motion in the peptide backbone. The method of Torchia and Lyerla was applied to these compounds in order to compare the mobility of the proline rings. The results show that the sequence and particular type of β-turn present affect the internal motion of the Pro ring. Data on a fourth cyclic hexapeptide, which occurs in a conformation with two-cis X-Pro bonds, suggests that internal motion of the backbone contributes an additional frequency component to the motion of the Y residue α-carbons. A consideration of the mobility of the proline rings in the conformer with two-cis peptide bonds revealed that they are significantly more rigid in the two-cis structure than in the all-trans.