On the role of peptides in the pyrolysis of amino acids

Abstract

The pyrolysis of aspartic acid, asparagine, glutamic acid, glutamine, and pyroglutamic acid were studied. Each amino acid was first pyrolyzed at 300 °C to obtain a low temperature tar (LTT) and a low temperature char (LTC). The LTC was then pyrolyzed at 625 °C to obtain a high temperature tar (HTT) and a high temperature char (HTC). The LTT and HTT were analyzed by gas chromatography/mass spectrometry. Maleimide, succinimide and related compounds were observed from asparagine and aspartic acid. Glutarimide was the major pyrolysis product from glutamine and glutamic acid. A number of new products were identified. In order to explain the formation of reduction products such as succinimide and glutarimide, a disproportionation mechanism involving polypeptides, formed in the pyrolysis melt, is suggested. The product distributions appear to depend on the relative volatility and stability of the amino acids and their pyrolysis products. In addition, the thermal properties of asparagine and its pyrolysis products were studied by thermogravimetry/differential scanning calorimetry (TGA/DSC). On the basis of the appearance temperatures of various products including ammonia and water and the temperatures at which the lower molecular weight products evaporate, it was concluded that asparagine (and glutamine and possibly other amino acids) thermally first polymerize to polypeptides which, at higher temperatures, subsequently degrade to lower MW products.