Pavel V. Shchukin

Specific formation of negative ions from leucine and isoleucine molecules

Dissociative electron attachment (DEA) to gas phase leucine (Leu) and isoleucine (Ile) molecules was studied using experimental and quantum-chemical methods. The relative partial cross sections for DEA have been measured using crossed electron/molecular beams technique. Supporting ab initio calculations of the structure, energies of neutral molecules, fragments, and negative ions have been carried out at G3MP2 and B3LYP levels in order to interpret the experimental data. Leu and Ile exhibit several common features. The negative ionic fragments from both molecules are formed in the electron energy range from 0 to approximately 14 eV via three resonances (1.2, 5.5, and 8 eV). The relative partial cross sections for DEA Leu and Ile are very similar. The dominant negative ions formed were closed shell negative ions (M-H)(-) (m/z=130) formed preferentially via low electron energy resonance of 1.23 eV. Additional negative ions with m/z=115, 114, 113, 112, 84, 82, 74, 45, 26, and 17 have been detected.

High resolution mass analysis of N- and C-terminal negative ions resulting from resonance electron capture by aliphatic amino acids

High mass resolving power was applied to study resonance electron capture by glycine, alanine, and valine, and accurate mass measurements helped to distinguish between some negative ions having the same nominal masses. It was established that the C- and N-terminal negative ions of the same nominal masses were formed at different electron energies from different resonance states. The typical fragmentation pathways in deprotonated amino acids via loss of water initiated by collisional activation were not observed upon resonant electron capture by aliphatic amino acids. Instead, [M-18](-) negative ions in the vicinity of 5 eV were found to be associated with simultaneous loss of either ammonia and a hydrogen atom or an amino group and a hydrogen molecule.

Slow decay of negative molecular fluorofullerene ions in the electron autodetachment process

The mean lifetimes of negative molecular ions of C60 fullerene and its fluoroderivatives C60F18 and C60F36 with respect to the autodetachment of electrons have been measured as functions of the ionizing electron energy by the method of mass spectrometry of electron resonance capture. The lifetimes of negative ions in the compounds under investigation are within the one-second range, and an increase in the number of addends leads to an increase in the lifetimes of negative ions by 1.5–2.5 orders of magnitude.

Electron ionization and dissociation of aliphatic amino acids

We present experimental and theoretical study of electron ionization and dissociative ionization to the gas phase amino acids valine, leucine, and isoleucine. A crossed electron/molecular beams technique equipped with quadrupole mass analyzer has been applied to measure mass spectra and ion efficiency curves for formation of particular ions. From experimental data the ionization energies of the molecules and the appearance energies of the fragment ions were determined. Ab initio calculations (Density Functional Theory and G3MP2 methods) were performed in order to calculate the fragmentation paths and interpret the experimental data. The experimental ionization energies of parent molecules [P](+) 8.91 ± 0.05, 8.85 ± 0.05, and 8.79 ± 0.05 eV and G3MP2 ionization energies (adiabatic) of 8.89, 8.88, and 8.81 eV were determined for valine, leucine, and isoleucine, respectively, as well as the experimental and theoretical threshold energies for dissociative ionization channels. The comparison of experimental data with calculations resulted in identification of the ions as well as the neutral fragments formed in the dissociative reactions. Around 15 mass/charge ratio fragments were identified from the mass spectra by comparison of experimental appearance energies with calculated reaction enthalpies for particular dissociative reactions.

Study of fragmentation pathways of metastable negative ions in aliphatic dipeptides using the statistical theory.

RATIONALE Dipeptide molecules are good model systems for investigation of resonant reactions of low-energy electrons with proteins. The present work is devoted to the study of the processes of formation and fragmentation of negative ions in aliphatic dipeptides glycyl-glycine and glycyl-alanine. The metastable decays of negative ions were detected in these objects, and have been investigated with the aim of clarification of the mechanisms of fragmentation. METHODS The effective yield curves for negative ions as functions of electron energy were obtained using a magnetic sector mass spectrometer rebuilt for generation and detection of negative ions. For analysis of the observed metastable decays statistical and thermochemical approaches have been used. RESULTS The ions structures, the enthalpies of formation of neutral and charged particles, and the rate constants of dissociative reactions have been found. CONCLUSIONS Comparison of the experimental results with theoretical data leads to the conclusion that metastable ion decay proceeds with minimal structural changes avoiding complicated rearrangements and isomerization processes.

Formation of negative ions via resonant low-energy electron capture by cysteine and cystine methyl esters

The processes of resonance low-energy free electron attachment to methyl esters of some sulfur-containing amino acids were studied. The long-lived molecular negative ions of cystine dimethyl ester formed in the valence state via the Feshbach nuclear excited resonance mechanism were detected by mass spectrometry. The reactions of disulfide bond dissociation were identified in an electron energy range of 0—1 eV. They can be considered as model reactions regarding processes of peptide decomposition due to the resonance interaction with low-energy electrons. Predissociation of short-lived molecular ions of cysteine methyl ester formed by capture of electrons with energies of ~1.6 eV is accompanied by the intra-ionic transfer of negative charge from the carbonyl group to the sulfur atom leading to the elimination from the latter of hydrogen atom.

Resonant electron capture by aspartame and aspartic acid molecules

RATIONALE The processes for dissociative electron capture are the key mechanisms for decomposition of biomolecules, proteins in particular, under interaction with low-energy electrons. Molecules of aspartic acid and aspartame, i.e. modified dipeptides, were studied herein to define the impact of the side functional groups on peptide chain decomposition in resonant electron-molecular reactions. METHODS The processes of formation and decomposition of negative ions of both aspartame and aspartic acid were studied by mass spectrometry of negative ions under resonant electron capture. The obtained mass spectra were interpreted under thermochemical analysis by quantum chemical calculations. RESULTS Main channels of negative molecular ions fragmentation were found and characteristic fragment ions were identified. CONCLUSIONS The СООН fragment of the side chain in aspartic acid is shown to play a key role like the carboxyl group in amino acids and aliphatic oligopeptides. Copyright

Resonant electron capture by orotic acid molecules

Resonant electron attachment by orotic acid molecules (6-COOH-uracil) are studied in the energy range of 0–14 eV via negative ion mass spectrometry. Molecular ions, whose lifetimes relative to electron autodetachment are found to be ~300 μs are recorded in the region of thermal electron energies; they form in the valence state through a vibration-excited resonance mechanism. Unlike unsubstituted uracil, most dissociative processes occur in the low-energy region of <4 eV and are due to carboxylic anions. An absolute cross section of 2.4 × 10−17 cm2 is found for the most intense fragment ions [M–H]– at an output energy of 1.33 eV. The kinetics of decarboxylation is considered for these ions. This could be a model reaction for the last stage of uridine monophosphate biosynthesis.