Configurations of morphiceptins by proton and carbon-13 NMR spectroscopy

Abstract

As part of our program to study the structure-activity relationship of peptide opioids, we have undertaken the spectroscopic examination of two proline-containing peptides: morphiceptin, Tyr-Pro-Phe-Pro-NHj, and a highly selective morphiceptin analogue, Tyr-Pro-(NMe)Phe-D-Pro-NH2[(NMe)Phe], Using high-resolution *H and 13C NMR experiments, we have assigned two of the four discernible configurational isomers observed for both morphiceptin and the (NMe)Phe analogue. The largest isomer amounting to 55% for morphiceptin and 65% for the (NMe)Phe analogue has been assigned as the all-trans isomer. The second configurational isomer accounting for 25% in both molecules studied adopts a cis conformation about the Tyr-Pro amide bond. The other isomers could not be unambiguously assigned since they are present in small amounts with overlapping and poorly resolved resonances. The exchange between the two major configurational isomers, cis/trans isomerization about the Tyr-Pro amide bond, proved to be extremely slow as established by chemical exchange measurements.Despite the two proline residues, the linear tetrapeptides do not exhibit clear conformational preferences. The measured NOE's, involving intraresidue or interresidue backbone atoms, are indicative of conformational averaging. The experimental examination of these morphiceptins is the first step in the development of structure-activity relationship for these molecules.

Morphiceptin Analogues Containing a Dipeptide Mimetic Structure: An Investigation on the Bioactive Topology at the μ-Receptor

“…These peptides contain a Pro residue in position 2, and consequently cis/trans isomerization occurs at the Tyr 1 -Pro 2 peptide bond. [8][9][10] In fact, previous results of 1 H NMR spectroscopic studies performed on morphiceptin and endomorphins in aqueous and DMSO solutions indicated the existence of a cis/trans equilibrium with a predominance of the trans isomer in all cases. [8][9][10][11] The cis/trans configuration around the Tyr 1 -Pro 2 amide bond is of particular significance for morphiceptin and endomorphins bioactivity because this bond is located within the biologically important Nterminal tripeptide message sequence.…”

“…[8][9][10] In fact, previous results of 1 H NMR spectroscopic studies performed on morphiceptin and endomorphins in aqueous and DMSO solutions indicated the existence of a cis/trans equilibrium with a predominance of the trans isomer in all cases. [8][9][10][11] The cis/trans configuration around the Tyr 1 -Pro 2 amide bond is of particular significance for morphiceptin and endomorphins bioactivity because this bond is located within the biologically important Nterminal tripeptide message sequence. 12 To elucidate the active configuration around this peptide bond, extensive conformational studies have been performed on the endogenous peptides morphiceptin, endomorphins, and their analogues.…”

“…In this context, morphiceptin (H-Tyr-Pro-Phe-Pro-NH 2 ) and the endomorphins [endomorphin-1 (H-Tyr-Pro-Trp-Phe-NH 2 ) and endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH 2 )] are opioid peptide agonists with high selectivity for μ opioid receptors. These peptides contain a Pro residue in position 2, and consequently cis/trans isomerization occurs at the Tyr 1 -Pro 2 peptide bond. − In fact, previous results of 1 H NMR spectroscopic studies performed on morphiceptin and endomorphins in aqueous and DMSO solutions indicated the existence of a cis/trans equilibrium with a predominance of the trans isomer in all cases. − The cis/trans configuration around the Tyr 1 -Pro 2 amide bond is of particular significance for morphiceptin and endomorphins bioactivity because this bond is located within the biologically important N-terminal tripeptide message sequence . To elucidate the active configuration around this peptide bond, extensive conformational studies have been performed on the endogenous peptides morphiceptin, endomorphins, and their analogues. − ,− Briefly, while for morphiceptin there is general consensus about a Pro 2 cis configuration in the bioactive structure, Pro 2 configuration in endomorphin-1 bioactive structure has been reported both as trans 9,15 and cis .…”

“…12 To elucidate the active configuration around this peptide bond, extensive conformational studies have been performed on the endogenous peptides morphiceptin, endomorphins, and their analogues. [8][9][10][11][13][14][15][16][17][18] Briefly, while for morphiceptin there is general consensus about a Pro 2 cis configuration in the bioactive structure, 14 Pro 2 configuration in endomorphin-1 bioactive structure has been reported both as trans 9,15 and cis. 11 Similarly, endomorphin-2 active derivatives were shown to possess both trans 17 and cis 16,18 configuration.…”

“…Furthermore, morphiceptin has a second proline residue at position 4; therefore, the cis/trans isomerization occurs at both Tyr 1 -Pro 2 and Phe 3 -Pro 4 peptide bonds. 8 In an attempt to obtain additional information on the importance of cis/trans isomerization at proline peptide bonds of morphiceptin and on the overall structural requirements of µ-opioid selective ligands, we synthesized and biologically evaluated new analogues in which a previously reported dipeptide mimetic structure 19 has been introduced in the peptide sequence (Figure 1). Since in this dipeptide mimetic the amide bond is replaced by a single rotable bond between the C-1′ and the C-2 atom of the pseudo-dipeptide, no cis-trans isomerization can occur.…”