Pyrolysis of alanine and α-aminoisobutyric acid: identification of less-volatile products using gas chromatography/Fourier transform infrared spectroscopy/mass spectrometry

Abstract

In addition to volatile low-molecular-weight decomposition compounds, α-aminoisobutyric acid (Aib) and alanine (Ala) pyrolysis at 500°C under nitrogen atmosphere leads to less-volatile products resulting from amino acid intermolecular condensation. The major pathway is the formation of cyclic dipeptides piperazine-2,5-diones with the yields of 1% for Aib and 68% for Ala. To identify other pyrolysis products, they have been extracted by chloroform and analyzed by means of the coupled technique of gas chromatography/Fourier transform infrared spectroscopy/mass spectrometry with auxiliary computer simulation of IR spectra and 1H and 13C nuclear magnetic resonance spectroscopy. In the case of Aib, the condensation has been also found to produce a linear dipeptide Aib-Aib, which undergoes further decarboxylation and the loss of H 2NCH(CH 3) 2 and hydroxyl moiety. Formation of small amounts of the bicyclic amidines hexahydroimidazo[1,2- a]pyrazine-3,6-diones via amino acid condensation has been detected as well. The presence of α-hydrogen atom in Ala residue facilitates dehydrogenation reactions for the related PD and bicyclic amidine, whereas in the case of Aib the dehydrogenated compounds have not been found. Further pathways of PD pyrolysis can lead to 4- and 5-membered lactams and hydantoins.