Pyrolysis of valine and leucine at 500°C: identification of less-volatile products using gas chromatography-Fourier transform infrared spectroscopy-mass spectrometry

Abstract

The products of valine and leucine pyrolysis at 500°C under nitrogen atmosphere have been analyzed using the methods of high-performance liquid chromatography and gas-chromatography-Fourier transform infrared spectroscopy-mass spectrometry with auxiliary computer simulation of IR spectra. The amino acids do not decompose completely under the above conditions, and are recovered by 10% (Val) and 2% (Leu). Among their pyrolysis products several carboxylic acids, primary and secondary amides have been identified, as well as a number of less-volatile compounds resulting from amino acid intermolecular condensation. Major condensation pathway is the formation of cyclic dipeptides piperazine-2,5-diones with the yields of 5% for Val and 3.5% for Leu, which can further undergo thermal dehydrogenation, dealkylation, rearrangement into hydantoins, loss of HNCO, etc. The formation of small amounts of bicyclic amidines hexahydroimidazo[1,2-a]pyrazine-3,6-diones via amino acid condensation has been also observed. Pathways of their thermal decomposition also include dehydrogenation, dealkylation and loss of HNCO.