17 research outputs found
Resonance electron interaction with five-membered heterocyclic compounds: Vibrational Feshbach resonances and hydrogen-atom stripping
Low-energy (0â15 eV) resonance electron attachment to a series of five-membered heterocyclic rings (isoxazole, imidazole, pyrazole, pyrrole, 1-methyl-, and 2-methylimidazole) is studied under gas-phase conditions by means of electron transmission spectroscopy and dissociative electron attachment spectroscopy (DEAS). Experimental spectral features are assigned on the basis of Hartree-Fock and density functional theory calculations. Sharp features, with a width of less than 0.1 eV, observed in the electron transmission spectra of imidazole, pyrazole, and pyrrole close to 0.45 eV, i.e., well below the energy of their lowest-lying Ïâ shape resonances detected at 1.90, 1.87, and 2.33 eV, respectively, are associated with formation of negative ion states bound by long-range electron-molecule interactions. Effective range theory calculations which include both dipolar and polarization interactions support this interpretation. In addition to the general observation of cleavage of the NâH bond at incident electron energies close to 2 eV, elimination of as many as three hydrogen atoms from the molecular negative ions is detected at higher energies by DEAS with the only exception of methylated imidazoles. This complex process is associated with ring opening and formation of diatomic hydrogen as one of the neutral fragments, as indicated by the calculations to satisfy the energetic requirements. The present results are of importance for understanding the basic mechanisms of damages caused in living tissues by high-energy radiations
Low-Energy Electron Capture by 6-Aza-2-thiothymine: Investigations by Electron Attachment and Electron Transmission Spectroscopies
The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out
Dissociative Electron Attachment to Resveratrol as a Likely Pathway for Generation of the H<sub>2</sub> 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 âleakedâ
from the respiratory chain to this polyphenolic molecule, followed
by the formation of the H<sub>2</sub> antioxidant species inside mitochondria
and participation in mitochondrial energy biogenesis
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
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
Interruption of the inner rotation initiated in isolated electron-driven molecular rotors
Fragmentation of chlorpyrifos by thermal electron attachment: a likely relation to its metabolism and toxicity
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
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 Ï bond
can be considered as the main mechanism of reductive dehalogenation
Fragmentation of chlorpyrifos by thermal electron attachment: 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 the ETS at 2.4, 3.1 and 4.30 eV, ascribed to empty \uf073* MOs, an higher-lying \u3c0* MO and a core-excited state, respectively. Two stable \uf070* 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 dissociation of stable (vibrationally excited) negative ion states formed by electron attachment into the \u3c0* LUMO and LUMO+1. The CPF\u2013 molecular anions (not observed in the present study) are expected to decay by fast dissociation to give the most abundant ([CPF \u2013 HCl]\u2013) species, which in turn dissociates on the microseconds time scale 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 direct decomposition of CPF\u2013. 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 into biotransformation of CPF by cytochrome P450 enzymes in a reductive environment producing toxic species and precursors of the main metabolites is briefly discussed