Synthesis and cis–trans isomerism in novel Leu-enkephalin-related peptidomimetics containing N-glycated glycine residues

Abstract

The influence of a new peptoid residue on amide-bond stereochemistry has been explored via the synthesis and NMR analysis of novel peptidomimetics related to the endogenous opioid pentapeptide Leu-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH). The compounds studied include protected and unprotected enkephalin analogues N-alkylated at either the second (Gly²) or both the second and the third (Gly²,Gly³) amino acid residues with a 6-deoxy-D-galactose moiety. The syntheses of the mono- (7) and the bis-glycated pentapeptide (11) were performed in a stepwise manner in solution by employing N-glycated glycine as the building block. The relative populations of the cis and trans isomers in the compounds studied were estimated by NMR spectroscopy. In the fully protected N-glycated dipeptide 3 the most abundant isomer (64%) was shown to contain a cis Tyr¹-(X)Gly² amide bond. NMR analysis of mono-N-glycated pentapeptide 6 provided evidence that elongation of the peptide chain disfavours the cis and augments the trans isomer population (cis∶trans 35∶65). For the unprotected monoglycated peptides 5 and 7 both the α- and β-pyranose forms of the galactose moieties were detected, the β-pyranose tautomer being the most abundant (≈70%). Removal of the protecting groups decreased the proportion of cis-rotamers relative to the corresponding protected peptides 3 and 6. The NMR spectra of enkephalin-related peptides 8–11, which contain two N-glycated glycine residues, were extremely complex; both proximal and distal isomerization effects were observed. For the bis-glycated tripeptide 8 significant amounts of both the cis and trans rotamers were observed for the Tyr¹-(X)Gly² and (X)Gly²-(X)Gly³ peptide bonds. In the fully protected pentapeptide 10 the configurational equilibrium was markedly shifted in favour of the trans isomers; only 15% of the cis isomer was observed for the Tyr¹-(X)Gly² bond, whereas the (X)Gly²-(X)Gly³ bond was completely in the trans configuration.