Unraveling the thermal decomposition and chemical ionization of methionine using ion mobility spectrometry and computational chemistry

Methionine, an essential amino acid, plays a crucial role in various biological processes and exhibits antioxidant properties, significantly impacting the health and well-being of humans, livestock, poultry, and fisheries. This study focuses on the thermal decomposition products of methionine utilizing ion mobility spectrometry (IMS) in conjunction with computational chemistry. This research focuses on analyzing the thermal decomposition products of methionine using ion mobility spectrometry (IMS) combined with computational chemistry. The IMS spectra of methionine were acquired under normal conditions and after subjecting the samples to elevated temperatures (280 °C) for different durations (5, 10, and 15 s) prior to analysis. By comparing the IMS spectra with those of pure compounds, analyzing changes in peak intensities over elapsed time, and employing a two-reference method to predict the masses of ionic species, the study aimed to identify and characterize the thermal degradation products of methionine at this temperature. Density functional theory (DFT) was employed to further interpret the IMS spectra to predict the fragments generated during decomposition. To achieve this, the decomposition pathways of methionine and protonated methionine from the N, O, and S centers are considered comprehensively. Consequently, four potential energy surfaces (PESs) are constructed with suitable details. The obtained PESs revealed that the saddle points and produced fragments in protonated molecules are more stable than neutral molecules compared to the respective reactant. This led us to conclude that the initial high impact between the hydronium ion and methionine plays a crucial role in the fragmentation process. The theoretical findings align well with the IMS spectra. Moreover, the methodology employed in this study can be applied to exactly interpret the IMS spectra of any similar molecular system.