Synthesis and utilization of 13C and 15N backbone-labeled proline: NMR study of synthesized oxytocin with backbone-labeled C-terminal tripeptide amide

Summary. The ¹³C and ¹⁵N backbone-labeled proline was prepared using Oppolzer’s method based on application of a sultam as chiral auxiliary. This isotopomer was used in the synthesis of the ¹³C, ¹⁵N backbone-labeled C-terminal tripeptide amide fragment of neurohypophyseal hormone oxytocin. Finally, this tripeptide amide was coupled by segment condensation with N-Boc- or N-Fmoc-tocinoic acid, followed by N-deprotection with TFA or piperidine. The labeled oxytocin exhibited biological activity identical with that of natural oxytocin. A detailed ¹H, ¹³C and ¹⁵N NMR study confirmed the assigned oxytocin conformation containing a β-turn in the cyclic part of the molecule, stabilized by H-bond(s) that can be perturbed by the C-terminal tripeptide amide moiety as indicated by comparison of NMR data for both the tocine ring in oxytocin and tocinoic acid.