Capture of an electron by ions in methionine and norleucine molecules

The relative cross sections of processes taking place when H$^+$ and He$^{2+}$ ions with an energy of 6$z$ keV ($z$ is the ionic charge) capture an electron from molecules of C$_5$H$_{11}$NO$_2$S methionine (proteogenic amino acid) and C$_6$H$_{13}$NO$_2$ norleucine (nonproteogenic amino acid) are measured by time-of-flight mass spectrometry (a methionine molecule transforms into a norleucine molecule by substituting the CH$_2$ group for the S heteroatom). The fragmentation pattern of resulting molecular ions is established from correlation analysis of the detection times of all fragment ions. The results are compared with experimental data for fragmentation of the same molecules ionized by electrons and photons. In these amino acids, the pattern of molecular ion fragmentation is found to depend on the type of molecule ionization. However, the detachment cross section of the COOH neutral group or residue (neutral or charged) $R$ of a side chain of the amino acid is invariably among the largest. The relative cross sections of capture with single and double ionization of molecules are measured.