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Novel carbohydrate‐induced modification of peptides: crystal structure and NMR analysis of ester‐bridged bicyclic galactose‐enkephalin adduct containing imidazolidinone moiety

Comparative studies based on x‐ray crystallography and NMR spectroscopy were used for structural characterization of the novel minor, imidazolidinone moiety containing, product 2b of the Maillard reaction obtained in vitro by using the galactose‐modified endogenous opioid pentapeptide leucine‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Leu) 1. The x‐ray analysis uniquely defined the molecular structure as cyclo‐{N‐{[2‐[‐4)‐D‐galacto‐pentitol‐1‐yl]‐4‐(4‐hydroxybenzyl)‐5‐oxoimidazolidin‐1‐yl‐(1 → O]acetyl}glycyl‐L‐phenylalanyl‐L‐leucyl‐} (3), having an 18‐membered ring with an ester bond between the secondary (C4′) hydroxyl group of a D‐galacto‐pentitolyl residue and the C‐terminal carboxy group of leucine‐enkephalin. The absolute configuration of the new chiral centre at the imidazolidinone moiety was established as C2(S), indicating a cis arrangement of C2 and C4 substituents at the 5‐membered heterocyclic ring. The NMR analysis of compound 2b carried out in CH3CN‐d3 and DMSO‐d6, indicated the existence of two isomers in solution, differing only in the position of the ester group in the molecule. NMR data for the minor isomer (13%–16%) are in agreement with structure 3. The migratory tendency of the peptidyl group from the primary (2b) to the secondary hydroxyl group (3) of a D‐galacto‐pentitolyl residue in methanol/water solution was confirmed by RP HPLC analysis. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd.