Detection of diastereomer peptides as the intermediates generating d -amino acids during acid hydrolysis of peptides

In this study, we investigated whether the amino acid residues within peptides were isomerized (and the peptides converted to diastereomers) during the early stages of acid hydrolysis. We demonstrate that the model dipeptides l -Ala- l -Phe and l -Phe- l -Ala are epimerized to produce the corresponding diastereomers at a very early stage, prior to their acid hydrolytic cleavage to amino acids. Furthermore, the sequence-inverted dipeptides were generated via formation of a diketopiperazine during hydrolytic incubation, and these dipeptides were also epimerized. The proportion of diastereomers increased rapidly during incubation for 0.5–2 h. During acid hydrolysis, C-terminal residues of the model dipeptides were isomerized faster than N-terminal residues, consistent with the observation that the d -amino acid values of the C-terminal residues determined by the 0 h-extrapolating method were larger than those of the N-terminal residues. Thus, the artificial d -amino acid contents determined by the 0 h-extrapolating method appear to be products of the isomerization of amino acid residues during acid hydrolysis.