CD and FTIR spectroscopic studies of Amadori compounds related to the opioid peptides

Abstract

Circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy have been used to investigate conformational effects of glycation on the secondary structure of opioid peptide Leu-enkephalin and on structurally related peptides in 2,2,2-trifluoroethanol (TFE) solution. CD spectral analysis of Leu-enkephalin-related Amadori compounds revealed that attachment of the protected or free sugar may influence not only the distribution of the backbone but also the side-chain conformation of the Tyr moiety. The amide I region of the FTIR spectra analysed by self-deconvolution and curve-fitting methods revealed that Leu-enkephalin is present as a mixture of β-sheet and γ-turn conformers in TFE solution, while its methyl ester likely adopts a β-turn conformation. FTIR spectroscopy has shown that no major spectral changes occur in the peptide part of glycated (Amadori) compounds as compared to parent peptides. The structurally related Tyr-Gly-Gly tripeptide derivatives contain amide I components at ca. 1630 and ca. 1645 cm-1 consistent with the presence of γ-turns with strong and weak 1 â†� 3 H-bondings, respectively. The attachment of the protected or free sugar moiety to pentapeptides appears to destabilize β-turns but not to affect H-bonded γ-turns. In the spectra of Amadori compounds containing a free sugar moiety, the component band at ca. 1730 cm-1 suggests the presence of the open-chain sugar form. Based on the studies presented herein, FTIR spectroscopy is shown to be a powerful tool for the structural analysis of glycated peptides, in particular for the detection of the keto form of the sugar and turn conformations of the peptide part of the molecule.