Electron-driven processes in enantiomeric forms of glutamic acid initiated by low-energy resonance electron attachment.

Low-energy (0-14 eV) resonance electron interaction and fragment species produced by dissociative electron attachment (DEA) for enantiomeric forms of glutamic acid (Glu) are studied under gas-phase conditions by means of DEA spectroscopy and density functional theory calculations. Contrary to a series of amino acids studied earlier employing the DEA technique, the most abundant species are not associated with the elimination of a hydrogen atom from the parent molecular negative ion. Besides this less intense closed-shell [Glu - H]- fragment, only two mass-selected negative ions, [Glu - 19]- and [Glu - 76]-, are detected within the same electron energy region, with the yield maximum observed at around 0.9 eV. This value matches well the energy of vertical electron attachment into the lowest normally empty π* COOH molecular orbital of Glu located at 0.88 eV according to the present B3LYP/6-31G(d) calculations. Although the detection of asymmetric DEA properties a priori is not accessible under the present experimental conditions, "chirality non-conservation" can be associated with some decay channels. Evidently, the measured spectra for the L- and D-forms are found to be identical, the results, nevertheless, being of interest for the forthcoming experiments utilizing spin-polarized electron beam as a chiral factor in the framework of conventional DEA technique.

Glycyrrhetinic acid interaction with solvated and free electrons studied by the CIDNP and dissociative electron attachment techniques.

Electron transfer plays a crucial role in living systems, including the generation of reactive oxygen species (ROS). Oxygen acts as the terminal electron acceptor in the respiratory chains of aerobic organisms as well as in some photoinduced processes followed by the formation of ROS. This is why the participation of exogenous antioxidants in electron transfer processes in living systems is of particular interest. In the present study, using chemically induced dynamic nuclear polarization (CIDNP) and dissociative electron attachment (DEA) techniques, we have elucidated the affinity of solvated and free electrons to glycyrrhetinic acid (GA)-the aglicon of glycyrrhizin (the main active component of Licorice root). CIDNP is a powerful instrument to study the mechanisms of electron transfer reactions in solution, but the DEA technique shows its effectiveness in gas phase processes. For CIDNP experiments, the photoionization of the dianion of 5-sulfosalicylic acid (HSSA2-) was used as a model reaction of solvated electron generation. DEA experiments testify that GA molecules are even better electron acceptors than molecular oxygen, at least under gas-phase conditions. In addition, the effect of the solvent on the energetics of the reactants is discussed.

On delicate balance between formation and decay of tetracyanoethylene molecular anion triggered by resonance electron attachment.

Low-energy (0-15 eV) resonance electron interaction with isolated tetracyanoethylene (TCNE) molecules is studied in vacuo by means of dissociative electron attachment (DEA) spectroscopy. Despite this molecule being relatively small, the long-lived molecular anions TCNE- are formed not only at thermal electron energy via a vibrational Feshbach resonance mechanism but also via shape resonances with the occupation of the π4* and π5* molecular orbitals by an incident electron. Dissociative decays of TCNE- are mostly observed at incident electron energy above the π7* temporary anion state predicted to lie at 1.69 eV by means of B3LYP/6-31G(d) calculations combined with the empirical scaling procedure. Electron attachment to the π6* orbital (predicted at 0.85 eV) leads to the generation of long-lived TCNE- species, which can decay via two competing processes: extra electron detachment, which appears in hundreds of microseconds, or elimination of two cyano groups to form the [TCNE - 2(CN)]- negative fragment on a tens of microsecond timescale. The latter is accompanied by the generation of a highly toxic cyanogen molecule as a neutral counterpart. Since the electron transfer to the acceptor molecule TCNE plays a key role in the formation of single-molecule magnets, the present data are of importance to understand the long-term behavior and likely harmful effects produced by cyanide-based prospective materials.

Elementary processes triggered in curcumin molecule by gas-phase resonance electron attachment and by photoexcitation in solution.

Electron-driven processes in isolated curcumin (CUR) molecules are studied by means of dissociative electron attachment (DEA) spectroscopy under gas-phase conditions. Elementary photostimulated reactions initiated in CUR molecules under UV irradiation are studied using the chemically induced dynamic nuclear polarization method in an acetonitrile solvent. Density functional theory is applied to elucidate the energetics of fragmentation of CUR by low-energy (0-15 eV) resonance electron attachment and to characterize various CUR radical forms. The adiabatic electron affinity of CUR molecule is experimentally estimated to be about 1 eV. An extra electron attachment to the π1* LUMO and π2* molecular orbitals is responsible for the most intense DEA signals observed at thermal electron energy. The most abundant long-lived (hundreds of micro- to milliseconds) molecular negative ions CUR- are detected not only at the thermal energy of incident electrons but also at 0.6 eV, which is due to the formation of the π3* and π4* temporary negative ion states predicted to lie around 1 eV. Proton-assisted electron transfer between CUR molecules is registered under UV irradiation. The formation of both radical-anions and radical-cations of CUR is found to be more favorable in its enol form. The present findings shed some light on the elementary processes triggered in CUR by electrons and photons and, therefore, can be useful to understand the molecular mechanisms responsible for a variety of biological effects produced by CUR.

Dissociative Electron Attachment to Hexachlorobenzene.

Gas phase molecules of hexachlorobenzene (C6 Cl6 ) were investigated by means of dissociative electron attachment spectroscopy (DEAS). Three channels of molecular negative ions decay have been identified: abstraction of Cl- and Cl2- as well as electron detachment (τa ∼250 µs at 343 K). All three channels exhibit temperature dependence. The adiabatic electron affinity estimated using a simple but typically accurate Arrhenius model (EAa =1.6-1.9 eV) turns out to be much higher than the quantum-chemical predictions (EAa =0.9-1.0 eV). We discuss the possible reasons behind the observed discrepancy.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane.

Low-energy (0-15 eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ- are detected in a wide electron energy range (0-3 eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6 eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

Structural rearrangements as relaxation pathway for molecular negative ions formed via vibrational Feshbach resonance.

The low-energy (0-15 eV) resonance electron interaction with two organic acids, oxaloacetic and α-ketoglutaric, is studied under gas-phase conditions using dissociative electron attachment spectroscopy. The most unexpected observation is the long-lived (microseconds) molecular negative ions formed by thermal electron attachment via the vibrational Feshbach resonance mechanism in both compounds. Unlike oxaloacetic acid, for which only one slow (microseconds) dissociative decay is detected, as many as five metastable negative ions are observed for α-ketoglutaric acid. These results are analyzed using density functional theory calculations and estimations of electron affinity using the experimental electron detachment times. The results are of considerable interest for understanding the fundamental mechanisms responsible for the dynamics of highly excited negative ions and the transformation pathways of biologically relevant molecules stimulated by excess electron attachment.

Dissociative Electron Attachment to 2,3,6,7,10,11-Hexabromotriphenylene.

2,3,6,7,10,11-Hexabromotriphenylene (HBTP) and 2,3,6,7,10-pentabromotriphenylene (PBTP) were investigated by means of dissociative electron attachment spectroscopy (DEAS). The dominant decay channel of the transient molecular negative ions consists of elimination of Br- with resonances in the low electron energy region. Formation of long-lived parent anions with autodetachment lifetime τa = 310 µs is observed at thermal electron energies. The adiabatic electron affinities, EAa = 1.12 ± 0.1 eV in HBTP and 1.09 ± 0.1 eV in PBTP, evaluated using a simple Arrhenius approach are in good agreement with those predicted by DFT (XYG3/Def2-TZVPP//PBE0/Def2-TZVPP) calculations.

Ionizing radiation and natural constituents of living cells: Low-energy electron interaction with coenzyme Q analogs.

Resonance electron attachment to short-tail analogs of coenzyme Q10 is investigated in the electron energy range 0 eV-14 eV under gas-phase conditions by means of dissociative electron attachment spectroscopy. Formation of long-lived (milliseconds) molecular negative ions is detected at 1.2 eV, but not at thermal energy. A huge increase in the electron detachment time as compared with the reference para-benzoquinone (40 µs) is ascribed to the presence of the isoprene side chains. Elimination of a neutral CH3 radical is found to be the most intense decay detected on the microsecond time scale. The results give some insight into the timescale of electron-driven processes stimulated in living tissues by high-energy radiation and are of importance in prospective fields of radiobiology and medicine.

5-Nitro-2,4-Dichloropyrimidine as an Universal Model for Low-Energy Electron Processes Relevant for Radiosensitization.

We report experimental results of low-energy electron interactions with.

Electron stimulated ring opening in diphenylphthalide dicarboxylic acid: Its likely role in the unique properties of phthalide-based materials.

The electronic properties of diphenylphthalide dicarboxylic acid (DPDA) are studied under gas-phase conditions using dissociative electron attachment spectroscopy and in the condensed environment by means of total current spectroscopy. The experimental features are assigned with the support of density functional theory calculations of the energies of the lowest-lying anion states to describe both resonances responsible for low-energy (0-15 eV) electron attachment to the isolated molecule and the maxima in the density of unoccupied electronic states in the condensed ultrathin (up to 10 nm) films. Resonance electron attachment to DPDA is found to be followed by the opening of the γ-lactone ring in the molecular negative ions, an unusual mechanism leading to their stabilization. A similar mechanism is expected to be responsible for the unique properties of phthalide-based materials in the condensed state.

Fragmentation of chlorpyrifos by thermal electron attachment: a likely relation to its metabolism and toxicity.

The energies of formation and dissociative decays of temporary negative ions of the organophosphorus insecticide chlorpyrifos (CPF) are studied using electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DEAS) and quantum-chemical calculations. Three features are displayed by ETS at 2.4, 3.1 and 4.30 eV, which are ascribed to empty σ* MOs, a higher-lying π* MO and a core-excited state, respectively. Two stable π* anion states are predicted by the calculations. Most of the negative fragments are detected by DEAS at thermal energies of the incident electrons, being thus associated with the dissociation of stable (vibrationally excited) negative ion states formed by electron attachment into the π* LUMO and LUMO+1. The CPF- molecular anions (not observed in the present study) are expected to decay by fast dissociation to give the most abundant ([CPF - HCl]-) species, which in turn dissociates on the microsecond timescale, producing as much as six metastable peaks in the mass spectrum. The m/z = 196 and 169 negative fragments, structurally similar to the main metabolites of CPF, 3,5,6-trichloro-2-pyridinol and O,O-diethyl thiophosphate, respectively, are formed by the direct decomposition of CPF-. Active radicals able to abstract hydrogen atoms from lipid membranes are generated as neutral counterparts of the observed anion fragments. A likely involvement of DEA in the biotransformation of CPF by cytochrome P450 enzymes in a reductive environment producing toxic species and precursors of the main metabolites is briefly discussed.

Hypothesis for the Mechanism of Ascorbic Acid Activity in Living Cells Related to Its Electron-Accepting Properties.

Electron-accepting properties, and in particular resonance dissociative electron attachment (DEA) to ascorbic acid (AA), are investigated by means of DEA spectroscopy in vacuo. The experimental features are assigned in silico and discussed in relation to expected dissociative electron transfer processes in vivo with the support of density functional theory calculations and the polarizable continuum model. It is shown that formation of the two most abundant AA metabolites in living cells, namely monodehydroascorbic acid and dehydroascorbic acid, can be stimulated by cellular electron transfer to AA under reductive conditions. Prooxidant effects caused by AA are suggested to be mediated by hydroxyl radicals formation via the DEA mechanism. The involvement of excited electronic states under UV-irradiation in plants could open additional DEA channels leading to specific AA activity forbidden under dark state conditions.

Low-energy electron interaction with retusin extracted from Maackia amurensis: towards a molecular mechanism of the biological activity of flavonoids.

The antioxidant isoflavone retusin efficiently attaches low-energy electrons in vacuo, generating fragment species via dissociative electron attachment (DEA), as has been shown by DEA spectroscopy. According to in silico results obtained by means of density functional theory, retusin is able to attach solvated electrons and could be decomposed under reductive conditions in vivo, for instance, near the mitochondrial electron transport chain, analogous to gas-phase DEA. The most intense decay channels of retusin temporary negative ions were found to be associated with the elimination of H atoms and H2 molecules. Doubly dehydrogenated fragment anions were predicted to possess a quinone structure. It is thought that molecular hydrogen, known for its selective antioxidant properties, can be efficiently generated via electron attachment to retusin in mitochondria and may be responsible for its antioxidant activity. The second abundant species, i.e., quinone bearing an excess negative charge, can serve as an electron carrier and can return the captured electron back to the respiration cycle. The number of OH substituents and their relative positions are crucial for the present molecular mechanism, which can explain the radical scavenging activity of polyphenolic compounds.

Resonance electron attachment to tetracyanoquinodimethane.

Resonance interaction of low energy (0-14 eV) electrons with gas-phase 7,7,8,8-tetracyanoquinodimethane (TCNQ) was investigated using dissociative electron attachment (DEA) spectroscopy. Spectral features associated with formation of long-lived TCNQ molecular negative ions are detected at incident electron energies of 0.3, 1.3, and 3.0 eV. A variety of negative fragments is observed around 4 eV, and slow (microseconds) dissociative decay channels are detected at about 3 eV, in competition with simple re-emission of the captured electron. The average electron detachment time from the TCNQ(-) negative ions formed at 3 eV was evaluated to be 250 µs. The experimental findings are interpreted with the support of density functional theory (DFT) calculations of the empty orbital energies, scaled with an empirical equation, and by comparison with earlier electron transmission spectroscopy (ETS) data. A possible mechanism for the unusual formation of long-lived molecular anions above zero energy (up to 3 eV) is briefly discussed. The present results on the interactions between electrons and isolated TCNQ molecules could give more insight into processes observed in TCNQ adsorbates under conditions of excess negative charge. In particular, electron-stimulated surface reactions are hypothesized, likely occurring when condensed TCNQ molecules are exposed to electron beam irradiation.

Resonance electron attachment to plant hormones and its likely connection with biochemical processes.

Gas-phase formation of temporary negative ion states via resonance attachment of low-energy (0-6 eV) electrons into vacant molecular orbitals of salicylic acid (I) and its derivatives 3-hydroxy- (II) and 4-hydroxybenzoic acid (III), 5-cloro salicylic acid (IV) and methyl salicylate (V) was investigated for the first time by electron transmission spectroscopy. The description of their empty-level structures was supported by density functional theory and Hartree-Fock calculations, using empirically calibrated linear equations to scale the calculated virtual orbital energies. Dissociative electron attachment spectroscopy (DEAS) was used to measure the fragment anion yields generated through dissociative decay channels of the parent molecular anions of compounds I-V, detected with a mass filter as a function of the incident electron energy in the 0-14 eV energy range. The most intense negative fragment produced by DEA to isomers I-III is the dehydrogenated molecular anion [M-H](-), mainly formed at incident electron energies around 1 eV. The vertical and adiabatic electron affinities were evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. The same theoretical method was also used for evaluation of the thermodynamic energy thresholds for production of the negative fragments observed in the DEA spectra. The gas-phase DEAS data can provide support for biochemical reaction mechanisms in vivo.

Gas-phase dissociative electron attachment to flavonoids and possible similarities to their metabolic pathways.

The gas-phase empty-level structures and formation of anion states via resonance attachment of low-energy electrons to the flavonoids naringenin (III), quercetin (IV) and myricetin (V) and the smaller reference molecules chromone (I) and flavone (II) are investigated experimentally for the first time. Dissociative electron attachment spectroscopy (DEAS) is used to measure the fragment anion currents generated through dissociative decay channels of the molecular anions of compounds I­V, detected with a mass filter as a function of the incident electron energy in the 0­14 eV energy range. Due to the insufficient volatility of flavonoids III­V, the energies of vertical electron attachment associated with temporary occupation of the lower-lying virtual orbitals are measured with electron transmission spectroscopy (ETS) only in the smaller reference molecules I and II. The experimental findings are interpreted with the support of appropriate density functional theory calculations with the B3LYP functional. The experimental vertical electron attachment energies measured in the ET spectra of I and II are compared with the orbital energies of the neutral molecules scaled using an empirically calibrated linear equation. The vertical and adiabatic electron affinities are evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. The latter theoretical method is also used for evaluation of the most stable conformers of the neutral molecules, O­H bond dissociation energies and thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra. A possible role played by loss of an H(2) molecule from the parent molecular anion in vivo in the mitochondrial respiratory chain is briefly discussed.

Can mitochondrial dysfunction be initiated by dissociative electron attachment to xenobiotics?

Resonance attachment of low-energy electrons to xenobiotic molecules, 2,4-dichlorophenoxyacetic acid (2,4-D), dichlorodiphenyltrichloroethane (DDT) and dichlorodiphenyldichloroethylene (DDE), was investigated under gas-phase conditions by means of complementary experimental techniques. Electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS), in the 0-6 eV and 0-15 eV energy range, respectively, were applied with the aim of modeling the behavior of these pesticide molecules under reductive conditions in vivo. Formation of long-lived parent molecular anions and fragment negative ions was observed at incident electron energies very close to zero, in agreement with the results of density functional theory calculations. The gas-phase DEA process, analogous to dissociative electron transfer in solution, was considered as a model for the initial step which occurs in the intermembrane space of mitochondria when a xenobiotic molecule captures an electron "leaked" from the respiratory chain. A possible involvement of the fragments produced by DEA to the pesticides under investigation into cellular processes is discussed. It is concluded that the free radicals and potential DNA adducts formed by DEA are expected to be dangerous for mitochondrial functionalities, while several of the products observed could act as messenger molecules, thus interfering with the normal cellular signaling pathways.

Electron attachment to indole and related molecules.

Gas-phase formation of temporary negative ion states via resonance attachment of low-energy (0-6 eV) electrons into vacant molecular orbitals of indoline (I), indene (II), indole (III), 2-methylen-1,3,3-trimethylindoline (IV), and 2,3,3-trimethyl-indolenine (V) was investigated for the first time by electron transmission spectroscopy (ETS). The description of their empty-level structures was supported by density functional theory and Hartree-Fock calculations, using empirically calibrated linear equations to scale the calculated virtual orbital energies. Dissociative electron attachment spectroscopy (DEAS) was used to measure the fragment anion yields generated through dissociative decay channels of the parent molecular anions of compounds I-V, detected with a mass filter as a function of the incident electron energy in the 0-14 eV energy range. The vertical and adiabatic electron affinities were evaluated at the B3LYP∕6-31+G(d) level as the anion∕neutral total energy difference. The same theoretical method is also used for evaluation of the thermodynamic energy thresholds for production of the negative fragments observed in the DEA spectra. The loss of a hydrogen atom from the parent molecular anion ([M-H](-)) provides the most intense signal in compounds I-IV. The gas-phase DEAS data can provide support for biochemical reaction mechanisms in vivo involving initial hydrogen abstraction from the nitrogen atom of the indole moiety, present in a variety of biologically important molecules.

Empty-level structure and reactive species produced by dissociative electron attachment to tert-butyl peroxybenzoate.

The energy and nature of the gas-phase temporary anion states of tert-butylperoxybenzoate in the 0-6 eV energy range are determined for the first time by means of electron transmission spectroscopy (ETS) and appropriate theoretical calculations. The first anion state, associated with electron capture into a delocalized π* MO with mainly ring and carbonyl character, is found to lie close to zero energy, i.e., sizably more stable (about 2 eV) than the ground (σ*) anion state of saturated peroxides. Dissociative decay channels of the unstable parent molecular anions are detected with dissociative attachment spectroscopy (DEAS), as a function of the incident electron energy, in the 0-14 eV energy range. A large DEA cross-section, with maxima at zero energy, 0.7 and 1.3 eV, is found for production of the (m/e = 121) PhCOO(-) anion fragment, together with the corresponding tert-butoxy neutral radical, following cleavage of the O-O bond. Although with much smaller intensities, a variety of other negative currents are observed and assigned to the corresponding anion fragments with the support of density functional theory calculations.