Investigation of pyrolysis residues of poly(amino acids) using matrix assisted laser desorption ionization-time of flight-mass spectrometry

Matrix assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) has been used to analyze the pyrolysis products of poly(proline), poly(phenylalanine), poly(leucine) and methionine enkephalin-Arg-Phe oligopeptide. Each was individually pyrolyzed in a quartz tube in the temperature range of 245-285 °C using a nitrogen atmosphere. After pyrolysis, the residue in the pyrolysis tube was removed and analyzed by MALDI-TOF-MS. Based on a difference of 18 mass units between the major peaks from the original peptides and the pyrolysis residues, cyclic oligomers were postulated. Digestion of the pyrolysis residues with cyanogen bromide, which selectively cleaves methionine-containing peptides, was used to substantiate the presence of cyclic product in the pyrolyzates.