Conformational Mobility of the Pyrrolidine Ring of Proline in Peptides and Peptide Hormones as Manifest in Carbon 13 Spin-Lattice Relaxation Times

Abstract The spin-lattice relaxation times (T1) of carbon 13 in natural abundance were determined for proline, N-acetyl-prolineamide, glycylproline, cyclo(triprolyl), and a series of proline-containing peptide hormones. The data are interpreted in terms of rapid interconversion of proline between various ring-puckered forms. The nature of the puckering depends upon the type of group attached to proline. In proline, the β, γ, and δ carbon atoms are appreciably more mobile than the α carbon atom, suggesting rapid interconversion between a number of ring-puckered forms. In melanocyte-stimulating hormone release-inhibiting hormone (Pro-Leu-Gly-NH2) and its dimethylamido analog, the γ carbon atom has the greatest mobility, suggesting a rapid endo-exo interconversion at this position. In Gly-Pro and in acetyl-Pro-NH2, the cis and trans conformers of proline have very similar T1 values, indicating very little dependence of the dynamic proline ring conformation on the conformation at the peptide bond; in both isomers, the proline ring interconverts rapidly between half-chair conformers puckered at Cβ and Cγ. In thyrotropin-releasing hormone (lGlu-His-Pro-NH2), the proline ring conformation is similar to that in Gly-Pro, and independent of the state of ionization of the histidine residue. Oxytocin [see PDF for equation] , lysine-vasopressin [see PDF for equation] , [Ile5]angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), and luteinizing hormone-releasing hormone ( lGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) all have proline in the trans conformation about the peptide bond, and the δ carbon atom has a mobility comparable to that of the α carbon atom. The latter reflects severe steric constraints at Cδ due to the neighboring residues. In oxytocin and angiotensin II, the β carbon atom is considerably more mobile than Cα or Cδ. In luteinizing hormonereleasing hormone, the proline ring apparently undergoes rapid interconversion between half-chair forms puckered at Cβ and Cγ, whereas in lysine-vasopressin, the mobility of Cγ is greatest