Comparative study of the thermal decomposition behaviour of different amino acids and peptides

Abstract

Thirteen nitrogen-containing compounds, including 2 peptides and 11 amino acids, were analysed for their thermal decomposition behaviour using TGA/DSC coupled with MS and FTIR.

Various decomposition courses have been found. Glycine, cysteine, cystine, proline, D- and L-threonine decomposed in two stages, whereas arginine, asparagine, glutamine, histidine and tryptophan decomposed in three stages. The peptides of glycine both decomposed in four decomposition steps, except for the glycylglycine under oxidative conditions, which decomposed in three stages. Proline proved to be exceptional, as it needed only one decomposition step under nitrogen to decompose (99.5% mass loss).

The decomposition products of amino acids and peptides described in literature could be confirmed in this work. These were mainly carbon dioxide, water, ammonia, hydrocyanic acid and nitrogen oxides. In traces, also isocyanates and nitriles could be detected, but no clear identification of these substances could be made due to superposition of the signals. For example, in the absorption spectrum at 3000 cm⁻¹, a superposition of the hydrocarbons and amines occurred and in the two sulphur-containing amino acids cysteine and cystine at 2100 cm⁻¹, a superposition of the resulting sulfur compounds could be observed.

It has also been shown that with increasing chain length of the peptides, in this case of glycine, the molecule becomes thermally more stable. Concerning cystine, something similar was observed for cystine, this is due to the strong sulphur bonding of the monomers. In addition, glycine and its peptides have been shown to have doubled or tripled the volume of the resulting carbon foam formed as length increased, a feature interesting for future research involving intumescent materials.