Kinetics of aspartic acid isomerization and enantiomerization in model aspartyl tripeptides under forced conditions.

The aim of the present study was the determination of the isomerization and enantiomerization of aspartic acid (Asp) in tripeptides. Capillary electrophoresis (CE) assays were developed and validated allowing the simultaneous determination of the diastereomeric alpha-D/L-Asp and beta-D/L-Asp peptides. Rapid isomerization and enantiomerization were noted for peptides with the Phe-Asp-GlyOH sequence at pH 10 and 80 degrees C while Gly-Asp-PheOH proved to be more stable due to the steric influence of the phenyl side chain. A kinetic model assuming a central role of the succinimide intermediate was used to fit the concentration versus time data. In incubations of L-Phe-alpha-L-Asp-GlyOH the ratio of alpha-Asp/beta-Asp peptides was about 1:4 in agreement with literature data. With regard to L-Asp and D-Asp peptides an alpha-Asp/beta-Asp ratio of about 1:3 and 1:5, respectively, was observed. The stereochemistry of Phe at the X-1 position affected the ratio of L-Asp/D-Asp implying an effect of the stereochemistry of neighboring amino acids on Asp enantiomerization. Modeling only overall Asp enantiomerization rate constants in accordance to literature data were observed for Asp peptides. In case of the asparagine (Asn) peptide the data could only be fitted to the models considering a direct conversion of L-Asn to a D-configured succinimide via an alternative pathway.