Ion mobility spectrometry and theoretical study for investigation of thermal decomposition, chemical ionization, and dimer formation of proline

Abstract

Chemical ionization and thermal decomposition of proline have been investigated by corona discharge ion mobility spectrometry (CD-IMS) and high-level ab initio calculations. Five main peaks were observed in the ion mobility spectrum of proline at low cell temperatures while only two remained at high temperatures. Experimental and theoretical evidences were collected to link the observed peaks to the related ionic species. Two peaks were assigned to the protonated proline and its symmetric proton bound dimer. Using a new mass-mobility correlation equation based on two standard mases, the other peaks were assigned to the pyrrolidine ring fragment ion with 70 amu, its asymmetric proton bound dimer with neutral proline, and a less stable protonated isomer of proline. The relative intensities of the peaks related to the fragment, parent, and the dimer ions were also explained based on the abundance of the ions in the IMS ionization region. In addition, the structure of the protonated proline was studied by considering ten stable proline conformers using ab initio calculations. Topical proton affinity values were then obtained for all the conformers. The results of the theoretical calculations showed that the proton transfer from ammonium reactant ion occurs preferably at nitrogen site of proline. The unimolecular fragmentation of protonated proline (116 amu) by eliminating H2O + CO (46 units) was also investigated by analyzing the ion mobility spectra backed by ab initio calculations. The results indicated that this reaction cannot happen in the ion mobility cell because of the high energy barrier in the reaction path. Hence, the fragment pyrrolidine ring ion is produced via thermal decomposition of proline in the injection port, followed by protonation in the ionization region.