Intermolecular condensation products formed during the pyrolysis of peptides

Mass Spectrometry (MS) analysis of pyrolysis products of simple peptides has revealed several non-volatile thermal degradation products at masses lower than the precursor peptide. In addition to these products, many other signals were also observed at higher masses than the precursor peptide, and their characterization is the focus of this study. Here we report on the observation of homo and hetero condensation peptide products formed during the pyrolysis of peptides. The observed peptide condensation products are formed between two, three or even four peptides. Tandem MS (MS/MS) analyses of these products showed that C-terminal to N-terminal intermolecular bonding is preferred during pyrolysis when combining two peptides, rather than involving crosslinking between basic and acidic side chain groups like arginine and aspartic acid. These observations are rationalized by steric hindrance effect and known pKa values of the peptide C- and N-termini and amino acid side groups like aspartic acid and arginine. Pyrolysis of a standard N-acetylated peptide showed no detectable condensation and/or crosslinked products, even in peptides with basic side groups, providing further evidence for the C-terminus to N-terminus intermolecular bonding between peptides under pyrolytic conditions.