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.

Effects of Plasma Treatment on the Surface and Photocatalytic Properties of Nanostructured SnO2-SiO2 Films.

In this work, we study the effects of treating nanostructured SnO2-SiO2 films derived by a sol-gel method with nitrogen and oxygen plasma. The structural and chemical properties of the films are closely investigated. To quantify surface site activity in the films following treatment, we employed a photocatalytic UV degradation test with brilliant green. Using X-ray photoelectron spectroscopy, it was found that treatment with oxygen plasma led to a high deviation in the stoichiometry of the SnO2 surface and even the appearance of a tin monoxide phase. These samples also exhibited a maximum photocatalytic activity. In contrast, treatment with nitrogen plasma did not lead to any noticeable changes in the material. However, increasing the power of the plasma source from 250 W to 500 W led to the appearance of an SnO fraction on the surface and a reduction in the photocatalytic activity. In general, all the types of plasma treatment tested led to amorphization in the SnO2-SiO2 samples.

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.

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.

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.

Can the Electron-Accepting Properties of Odorants Be Involved in Their Recognition by the Olfactory System?

The present study examines the possible importance of the electron-accepting properties of odorant molecules and, in particular, the formation and decay of temporary negative ions via low-energy electron attachment as a possible contribution toward understanding odorant recognition by olfactory receptors (ORs). Fragments formed by dissociative electron attachment (DEA) of mustard oil odorants represented by a series of isothiocyanates are studied experimentally using DEA spectroscopy and DFT calculations. Relative intensities for the most abundant fragment species, S- and SCN-, are found to be characteristic of structurally similar odorants under investigation. This novel approach for the investigation of odorants may contribute to understanding the initial stages of the olfactory process and may provide a means to distinguish between odorants and their interactions with the olfactory receptor system.

Low-Energy Electron Interaction with Melatonin and Related Compounds.

The electron attaching properties and fragmentation of temporary negative ions of melatonin and its biosynthetic precursor tryptophan are studied in vacuo using dissociative electron attachment (DEA) spectroscopy. The experimental findings are interpreted in silico with the support of Hartree-Fock and density functional theory calculations of empty orbital energies and symmetries, and evaluation of the electron affinities of the indolic molecules under investigation. The only fragment anions formed by DEA to melatonin at incident electron energies below 2 eV are associated with the elimination of a hydrogen atom (energetically favored from the NH site of the pyrrole ring, leaving the ring intact) or a CH3· radical from the temporary molecular negative ion. Opening of the pyrrole ring of melatonin is not detected over the whole electron energy range of 0-14 eV. The DEA spectra of l- and d-tryptophan are almost identical under the present experimental conditions. The adiabatic electron affinity of melatonin is predicted to be -0.49 eV at the B3LYP/6-31+G(d) level, indicating that the DEA mechanism in melatonin is likely to be present in most life forms given the availability of low energy electrons in living systems in both plant and animal kingdoms. In particular, H atom donation usually associated with free-radical scavenging activity can be stimulated by electron attachment and N-H bond cleavage at electron energies around 1 eV.

Why Can Unnatural Electron Acceptors Protect Photosynthesizing Organisms but Kill the Others?

The polychlorinated compounds captafol (CPL) and 2,6-dichloroisonicotinic acid (INA) are able to protect plants acting as a fungicide or an inductor of plant resistance, respectively. At the same time, CPL and INA are dangerous for the respiratory organisms, i.e. mammalians, bacteria, and fungi. The high electron-withdrawing ability of these compounds enables them to serve as unnatural electron acceptors in the cellular ambient near to electron transport pathways located in the thylakoid membrane of chloroplasts or in the mitochondrial respiratory chain. Low-energy electron attachment to CPL and INA in vacuo leads to formation of many fragment species mainly at thermal electron energy as it is shown using dissociative electron attachment spectroscopy. On the basis of the experimental findings, assigned with the support of density functional theory calculations it is suggested that the different bioactivity of CPL and INA in respiratory and photosynthetic organisms is due to the interplay between the dissociative electron attachment process and the energies of electrons leaked from the electron transport pathways.

Role of Resonance Electron Attachment in Phytoremediation of Halogenated Herbicides.

This study is aimed to point out the important role played by resonance electron attachment in reductive dehalogenation, in particular in phytoremediation of organic pollutants under conditions of excess negative charge. To model enzymatic reactions occurring in reductive conditions, low-energy electron capture by the halogenated herbicides atrazine and bromoxynil was studied in vacuo using electron transmission spectroscopy. A variety of decay channels of the temporary molecular negative ions was discovered by means of dissociative electron attachment spectroscopy. The experimental results were interpreted with the support of quantum-chemical calculations. Dehalogenation of atrazine and bromoxynil was found to be the dominant decay of the molecular negative ions formed at thermal energies of the incident electrons. It is concluded that formation of negative ions by electron donation in enzymatic active centers followed by their dissociation along the σ bond can be considered as the main mechanism of reductive dehalogenation.

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.

Dissociative Electron Attachment to Resveratrol as a Likely Pathway for Generation of the H2 Antioxidant Species Inside Mitochondria.

The electron-attaching properties of polyphenolic compound resveratrol were studied in vacuo by means of dissociative electron attachment (DEA) spectroscopy and in silico using density functional theory calculations. The most intense fragments generated by DEA to isolated resveratrol at thermal electron energy are semiquinone anions and neutral hydrogen molecules. On the basis of the present experimental and theoretical data, a new molecular mechanism for the antioxidant activity of resveratrol is presented. It is suggested that the activity of resveratrol in living cells is driven by dissociative attachment of electrons "leaked" from the respiratory chain to this polyphenolic molecule, followed by the formation of the H2 antioxidant species inside mitochondria and participation in mitochondrial energy biogenesis.

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.

Dissociative Electron Attachment to Anthralin to Model Its Biochemical Reactions.

The antipsoriatic drug anthralin (dithranol) is known to be extensively accumulated inside mitochondria of keratinocytes and to interact with the electron flow of the respiratory chain. Primary products of the one-electron reduction of polyphenolic anthralin observed in vivo are its dehydrogenated anions, which are formed by H-atom abstraction. The same species are mainly generated at low electron energies by dissociative electron attachment (DEA) to anthralin molecules in vacuo. A likely mechanism for the biochemical transformations of anthralin under reductive conditions in vivo is hypothesized on the basis of its DEA properties. The involvement of excited electronic states generated by ultraviolet irradiation of skin is discussed.

Relation between electron scattering resonances of isolated NTCDA molecules and maxima in the density of unoccupied states of condensed NTCDA layers.

The empty-level structure of the 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) molecule is characterized by means of dissociative electron attachment (DEA) experiments in the gas phase coupled with DFT calculations. Distinct maxima in the anion currents generated by electron attachment to NTCDA, as a function of incident electron energy, are ascribed to capture of incident electrons into empty orbitals, i.e., the process referred to as shape resonance. The empty orbital energies of gas-phase NTCDA shifted to 1.2 eV lower energy reproduce satisfactorily the maxima of the unoccupied electronic states of a multilayer NTCDA film measured by means of the very low energy electron diffraction method and the total current spectroscopy measurement scheme. The present results indicate that the empty levels of individual NTCDA molecules are stabilized in the solid state, but their relative energies remain nearly unaltered. The stabilization energy in multilayer film of NTCDA molecules is likely due to attractive polarization forces. Fragmentation of the gas-phase NTCDA temporary parent anions via the DEA mechanism, the other issue of the present investigation, leads to the rupture of the bonds between the end carbonyl groups and the naphthalene core, and occurs at incident electron energies above 2 eV. Possible chemical changes in condensed NTCDA molecules initiated by the DEA mechanism under conditions of electron transport through the film are discussed.