Electron-Impact Ionization of the Glutamic Acid and Glutamine Molecules

The yield of positive ions formed as a result of the electron-impact dissociative ionization of glutamic acid (Glu-Acid) and glutamine (Glen) molecules in the gaseous phase has been studied both experimentally and theoretically. The experiment was performed using an MX-7304A monopole mass spectrometer in a mass number interval of 10–150 Da. The mass spectra of Glu-Acid and Gln molecules at various temperatures and the dynamics of the ionic fragment yield in an interval of initial substance evaporation temperatures of 310–430 K were studied, and the specific features of the relevant ionic fragment formation at the electron impact were analyzed in detail. Ab initio calculations of ionization potentials for glutamic acid and glutamine molecules were performed in the adiabatic approximation and on the basis of binding energies for the HOMO and LUMO orbitals of neutral molecules. The cross-sections of the single-electron ionization of both molecules by the electron impact were calculated in the framework of the binary encounter Bethe model and using the Gryzi ́nski formula. The calculated molecular constants were shown to agree well with the obtained experimental data. (author)