Formation of a cyclic imide in aspartyl or asparaginyl glycyl peptides induced by heating in the dry state*

Abstract
A common product was identified by HPLC from the heating of .alpha. and .beta. isomers of aspartyl glycine in the dry state. Fast atom-bombardment (FAB) mass spectrometry and Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) showed tthat the product is a cyclic imide. Cyclic imide formation also was shown to occur with the tripeptides Gly-Asp-Gly and Gly-Asn-Gly. The cyclization of aspartyl peptides depends upon protonation of the .beta.-carboxyl group and can be promoted by acidification or by heating the NH4+ salt in the dry state.