Protein-backbone-modifications: Formation of imidazolines
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The formation of imidazolines within the backbone during heating of peptides and proteins was studied. In model experiments, the generation of 2-methyl-4-carboxy-imidazoline after incubation of N α -acetyl-β-aminoalanine was demonstrated by cation-exchange chromatography and NMR-studies. At 60 °C the highest concentrations of this imidazoline were measured at pH 9. With increasing temperature the pH-optimum of the imidazoline formation shifted to lower pH levels.
The reaction of N α -acetyl-dehydroalanine-methylester with ammonia generated β-aminoalaninoalanine, which was identified in comparison with synthesized references by HMQC-and1H-NMR-spectroscopy. β-Aminoalaninoalanine, a crosslink-product homologous to lysinoalanine, which reacts easily with the corresponding imidazoline. Thus, the formation of imidazolines in peptide backbones containing β-aminoalanine was shown unequivocally.
Further studies were conducted with peptides, synthesized according to the 10–28 sequence of bovine β-caseine with β-aminoalanine respectively β-alanine substituted for a phosphoserine residue. After incubation, the loss of one molecule of water from the β-aminoalanine containing peptide, but not from the native peptide or the peptide containing β-alanine, was detected by HPLC and MS. As a consequence, the loss of water was explained by cyclization and the formation of an imidazoline.
These investigations indicate that the formation of imidazolines—besides the formation of thiazolines—takes place during heating of peptides and proteins. Heterocyclic backbone-modifications are a form of posttranslational modifications, which may play a role during the processing of proteins and subsequent formation of unexpected compounds.
Key wordsImidazolines 2-methyl-4-carboxy-imidazoline β-aminoalaninoalanine β-aminoalanine protein-backbone modification
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