Chemical pathways of peptide degradation. VI. Effect of the primary sequence on the pathways of degradation of aspartyl residues in model hexapeptides.

The influence of the primary sequence on the degradation of Asp4 residues (e.g., formation of the cyclic imide and Asp-X and/or X-Asp amide bond hydrolysis) was investigated using Val-Tyr-Y-Asp-X-Ala hexapeptides. These reactions were proposed to involve cyclization, which would duly be sensitive to steric hindrance. The effects on the rates of individual degradation routes and product distribution under both acidic and alkaline conditions were assessed upon substitutions made on the C-terminal side (X) and on the N-terminal side (Y) of the Asp residue. As expected, the rate of intramolecular formation of cyclic imide and, thus, the product yield were most affected by the size of the amino acid on the C-terminal side of the Asp residue. However, such structural changes had little or no impact on the rate of Asp-X and Y-Asp amide bond hydrolysis. In the former case, the substituted site was one atom removed from the reaction site, accounting for the diminished steric effect observed. As for the latter, the site of substitution was not a participant in the reaction itself, and hence, the rate was unperturbed by this modification. Placing Ser and Val C terminally to the Asp residue prompted racemization and peptide bond hydrolysis to occur under alkaline conditions. N-Terminal substitution of Pro with Gly had no effect on the rate of isomerization via cyclic imide formation but greatly enhanced the rate of Y-Asp amide bond hydrolysis.