Why Can Unnatural Electron Acceptors Protect Photosynthesizing

Article pubs.acs.org/JPCB

Why Can Unnatural Electron Acceptors Protect Photosynthesizing Organisms but Kill the Others? Stanislav A. Pshenichnyuk*,†,‡ and Alexei S. Komolov‡ †

Institute of Molecule and Crystal Physics, Ufa Research Centre, Russian Academy of Sciences, Prospeκt Oktyabrya 151, 450075 Ufa, Russia ‡ St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russia S Supporting Information *

ABSTRACT: 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 electronwithdrawing 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. Lowenergy 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.

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INTRODUCTION The biological activity of compounds possessing the high electron-withdrawing ability as, for instance, halogenated or carboxyl-containing species, has been associated with the resonance electron attachment since pioneering work by Lovelock.1 Extra electron attachment to a xenobiotic molecule located in the biological media can be considered as its “activation” leading to generation of reactive species in vivo.2 This process occurs in those cellular compartments where quasi-free electron transfer takes place as, e.g., in mitochondria and chloroplasts, or in enzymatic active centers.3 The ability of the active molecules to capture cellular electrons can be described in terms of the competition with diatomic oxygen to accept these electrons, i.e., with a well-known process of oneelectron reduction of O2 that leads to formation of a superoxide anion. Electron affinity (EA) of O2 is estimated to be 0.45 eV,4 so that any xenobiotic species possessing EA exceeding this value can attach cellular electrons.5 The latter process is expected to initiate a cascade of biochemical reactions with involvement of the fragments formed by resonance dissociative electron attachment (DEA).6 DEA, i.e., an elementary process M + e− → M(*)−→ fragments, where M stands for a neutral molecule, M(*)− is the temporary negative ion unstable with respect to both electron detachment and dissociation, can be investigated in vacuo using mass-spectrometric technique to detect mass-selected negative ions as a function of incident electron energy. Quantumchemical calculations provide a suitable means to disclose the structures of negative and neutral fragments as well as to © 2017 American Chemical Society

associate the most likely energies of their formation with the energies of empty molecular orbitals (MOs) of the target molecule. The calculated virtual orbital energies (VOEs) of the neutral molecule should be corrected using the empirical linear law.7 The scaled VOEs obtained as a result are related to vertical attachment energies (VAEs; the negatives of vertical electron affinities), i.e., the energies of incident electrons suitable to produce temporary molecular anion states.8,9 Experimental studies of DEA to isolated bioactive molecules can be successfully applied to model dissociative electron transfer under cellular conditions,1,6,10 that is supposed to occur at the interface boundaries in the lipid membrane−aquatic cytosol−protein environment. The quasi-free cellular electrons are expected to be partly solvated at the interfaces with relatively low binding energies, i.e., are stabilized by 1.6 eV relatively to the vacuum level (VL), the reference zero energy under gas-phase experiments. Therefore, these electrons match well the “electron attachment window” for many polyatomic molecules possessing the stable negative ion states.11,12 When electron donation occurs in the hydrophobic media of enzymatic active centers, where substrate molecule is weakly bound by hydrogen bonds, the conditions are even closer to those of DEA experiment in vacuo13 as discussed for enzymatic reactions catalyzed by cytochrome P450.14 Transient anion (TA) states are expected to be stabilized in biological media by Received: November 29, 2016 Revised: January 7, 2017 Published: January 10, 2017 749

DOI: 10.1021/acs.jpcb.6b12007 J. Phys. Chem. B 2017, 121, 749−757

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The Journal of Physical Chemistry B

Figure 1. Fragmentation of captafol (CPL) via low-energy resonance electron attachment. Currents of mass-selected negative ions formed by dissociative electron attachment to CPL in vacuo as a function of incident electron energy (left panel). The likely structures of fragment negative ions and corresponding neutral counterparts (center). B3LYP/6-31+G(d) total energies (horizontal bars) of the fragments relative to that of the CPL neutral ground state (red bar), and the total energy of molecular negative ion CPL− (green bar) (right panel). 4H-PTI = 1,2,3,6tetrahydrophthalimide.

1.0−1.5 eV with respect to the gas-phase conditions.15,16 Therefore, in electron-molecule frame of reference both the electron and the TA are stabilized by approximately similar amounts. It makes possible to extrapolate gas-phase findings to model the electron attachment processes under cellular conditions. The electronic structure of vacant MOs remains practically undisturbed upon the transition from gas-phase to the condensed state,17 therefore, the energies of MOs for isolated molecule could be rigidly shifted as a whole to the low energy to predict the TA states in the condensed environment. Strictly speaking, the energies of both the transient anions and the electrons are different in biological media with respect to the gas phase, therefore distinctions should be made about the differences in their relative energies within the biological conditions. The very first step of the adverse bioactivity of harmful chemicals possessing high EAs is associated with generation of the active radicals by DEA causing a disruption of biomembranes and inactivation of enzymes.6,16,18 However, the observable physiological effects may be very different, especially in various organisms. In particular, among chlorinated compounds, the model toxicant carbon tetrachloride (CCl4) mainly causes liver damage. The first modern insecticide 1,1′-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene) (DDT) efficiently kills insects, and synthetic herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D) is a weedkiller. The present study reports on low-energy (0−14 eV) electron interaction and DEA properties for two unnatural electron acceptors captafol (CPL) and 2,6-dichloroisonicotinic acid (INA). CPL is known for its fungicidal activity19 but forbidden

in agriculture due to its harmful influence on human health.20 INA induces a systemic resistance in plants or provides protection against fungal and bacterial pathogens depending on the dose.21 The present experimental results are assigned on base of density functional theory (DFT) calculations with a goal to understand better the molecular mechanism of the bioactivity of compounds possessing high EAs in terms of their interaction with electrons “leaked” from the electron transport pathways. A difference between energies of the “leaked” electrons in mitochondria22 and chloroplasts23 is suggested to be the main reason for the different bioactivity of CPL and INA able to support the development of photosynthetic organisms but destructing mammalian, bacteria, and fungi whose intrinsic energetics is based on the respiration. The present work, therefore, contributes to the interconnection between molecular physics and the life sciences24 and expands application area of the DEA-related studies from the well-known effect of thermalized e− to induce DNA strand breaks25,26 to the new field of biological sciences related to the activity and metabolism of unnatural electron acceptors. The suggested mechanism attracts attention to the question why xenobiotic species used worldwide in agriculture to support plant growth and development can be dangerous for the respiratory organisms including humans.

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RESULTS AND DISCUSSION Fragmentation by Resonance Electron Attachment. The most intense signals observed in the DEA spectra of CPL and INA are presented in Figures 1 and 2, the complete data being reported in Supporting Information (Figure S1, Figure 750


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Figure 2. Fragmentation of 2,6-dichloroisonicotinic acid (INA) via low-energy resonance electron attachment. Currents of mass-selected negative ions formed by dissociative electron attachment to INA in vacuo as a function of incident electron energy (left panel). The likely structures of fragment negative ions and corresponding neutral counterparts (center). B3LYP/6-31+G(d) total energies (horizontal bars) of the fragments relative to that of the INA neutral ground state (red bar), and the total energy of molecular negative ion INA− (green bar) (right panel). A section of negative ion mass spectrum of INA recorded at thermal electron energy indicates slow (microseconds scale) elimination of HCl from INA−.

The observed DEA properties for CPL are in agreement with those for chlorine substituted compounds where the DEAmediated formation of Cl− is usually a dominant process.27,28 On the contrary, the most intense decay of INA− corresponds to elimination of a neutral HCl molecule generating the m/z = 155 anions at thermal energy. A hydrogen atom abstraction from the hydroxyl group is energetically favored (Table S2), although its migration toward the chlorine atom is necessary to be eliminated as HCl species. Besides zero-energy peak (2 orders of magnitude less intense than the m/z = 155 current), the signal of Cl− clearly shows maxima at incident electron energies 1.1, 3.6, and near 6 eV (Figure 2). The threshold of 0.15 eV is predicted for cleavage of the O−H bond to form the dehydrogenated m/z = 190 fragment observed at 0.5 eV. Formation of [INA−H]− at thermal energy likely originates from electron attachment to vibrationally excited molecules so that the energy deficit can be derived from the anion excess energy. The long-lived (mass spectrometrically detectable) molecular anions INA− are observed at thermal energy (referred to the VL energy). The electron detachment time from the INA− is estimated in the present work to be 70 μs (see Experimental and Computational Methods Section). B3LYP/6-31+G(d) adiabatic EA for INA is calculated to be 1.45 eV (Figure 2) that is sufficient to observe the long-lived INA−, since a precise correlation between EA and electron detachment times has been discovered.27−29 A broad signal recorded in the mass spectrum of INA at a seeming mass m1* = 125.8 (Figure 2) is ascribed to slow (microseconds) elimination of HCl from the INA−. Consecutive elimination of CO2 from the m/z = 155

S2, Table S1). CPL molecule consists of 1,2,3,6-tetrahydrophthalimide (4H-PTI) core substituted at the N-site by tetrachloroethane moiety via the sulfur atom bridge. CPL attaches exclusively thermal (zero energy) electrons in vacuo producing a variety of fragment species but is transparent for electrons with higher energies (up to 14 eV). B3LYP/631+G(d) thermodynamic energy thresholds (total energies of fragments relative to neutral ground state) reported in Figure 1 are all negative that makes possible fragmentation of CPL by attachment of thermal electrons. As many as four chlorine atoms in the tetrachloroethane moiety favor elimination of Cl2− (m/z = 70) species from the temporary negative ion CPL− generating closed-shell molecule as a neutral counterpart. Provided that an additional CC bond in the chlorinated group is formed upon elimination of the Cl2− species the process is exothermic producing energy release 2.10 eV (Table S2). Simple rupture of a single C−Cl bond is more probable by kinetic reasons and leads to formation of the most intense Cl− fragment (m/z = 35) and corresponding dechlorinated radical with smaller energy release (1.42 eV). Dehydrogenated 4H-PTI species formed by cleavage of the N−S bond are able to keep an electron during microseconds (that is a property of all fragment species detected through the mass spectrometer) producing a signal at the m/z = 150. Its neutral counterpart likely is an ·SCCl2CHCl2 radical. Less abundant fragment anions can be accounted for by elimination from the CPL− some closed-shell neutral molecules, namely trichloroethylene, 4H-PTI, 1,1,2,2-tetrachloroethane, and HCl to form, respectively, the m/z = 217, 198, 181, and 313 rearranged anions. 751


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The Journal of Physical Chemistry B anion to generate m/z = 111 negative fragment is supported by the observation of a metastable anion peak at m2*=79.2 at low electron energy (Table S1, Figure S2). Formation of the m/z = 111 anion is also clearly observed at 1.3 and 3.2 eV. Therefore, in contrast to CPL, INA is found to be not transparent for electrons with epithermal energies. Adiabatic EA of CPL is estimated to be even higher (2.23 eV as reported in Figure 1) than that of INA, nevertheless the long-lived molecular anions CPL− were not observed. This fact can be explained by the elongation to 2.96 Å of one of the C−Cl bonds in the B3LYP/ 6-31+G(d) optimized structure of CPL− (Figure S4), so that even thermal electron attachment to CPL is expected to produce Cl− effectively. Vacant Molecular Orbitals and Temporary Anion States. Maxima of anion currents observed in DEA spectra are associated with energies of the temporary electron-molecule bound states formed by extra electron addition to vacant MOs of the target molecule. Shape resonances8,9 with electron attachment to π* MOs usually have longer lifetimes with respect to detachment of an extra electron as compared with the σ* MOs.27,28 Therefore, their contribution to DEA spectra is usually expected to be more significant than that of the σ* states, which are often associated with purely repulsive potential energy surfaces but are additionally required to survive until dissociation is irreversible. CPL molecule possesses three π* MOs associated with two CO and one CC bonds of 4HPTI core, and four low-lying σ*C−Cl of the tetrachloroethane substituent. The predicted (see Experimental and Computational Methods Section) energy positions of these MOs belong to relatively narrow energy interval below 1 eV as shown in Figure 3. The lowest π* anion of the reference molecule phthalimide is found to be stable,29 but loss of the aromaticity in CPL pushes the π1*CO orbital to unbound region, the predicted VAE being 0.26 eV (Table S3). π2*CO state has a noticeable admixture of the σ*C−Cl MO and therefore its predicted position at 0.75 eV is questionable. Although the σ* scaling is expected to be less reliable than the π* scaling30 the lowest σ*C−Cl VAE of CPL is predicted to be positive (0.39 eV). In agreement, the lowest σ*C−Cl anions in a series of chloroalkanes including the reference compound 1,1,2,2tetrachloroethane are found to be unstable.31 Therefore, zeroenergy signals in the DEA spectra of CPL are expected to originate mainly from the s-wave attachment into the lowest anion state32 as discussed in a series of saturated chloro derivatives (see ref 33 and references therein). In other words, the zero peaks signals are attributed to occupation by the incoming electrons of vibrationally excited levels of the neutral molecules lying near the crossing point between potential curves of the neutral molecule and the anion.32 Additionally, a minor contribution from the π1* state cannot be ruled out, due to the uncertainty of the scaled VOE and to the shift to lower energy of the DEA maxima relative to the peaks in the total electron attachment cross section due to survival probability factor.8,27,28 INA molecule possesses four π* MOs, three being derived from the heterocyclic ring and one from CO double bond of the carboxyl group. Two low-lying σ* orbitals are associated with chlorine substituents. In contrast to CPL, these vacant MOs of INA occupy much broader energy interval, where the lowest two π* anion states are predicted to be stable as reported in Figure 3. The scaled VOE of the π1* MO is −1.07 eV (Table S3) in good agreement with the first vertical EA calculated by total energy differences (1.10 eV, Table S2). It

Figure 3. Schematic representation of the lowest B3LYP/6-31G(d) vacant molecular orbitals for neutral captafol (CPL) and 2,6dichloroisonicotinic acid (INA). Predicted positions of negative ion resonances, i.e., energies of incoming electrons generating temporary electron-molecule bound states by vertical electron attachment are shown by horizontal bars.

indicates adequacy of the semiempirical procedure used to scale the VOEs. Zero-energy signals in DEA spectrum of INA are associated with formation of vibrational Feshbach resonance8,27,28 with electron occupation of the π1* or π2* MOs. The observed maxima in the m/z = 111 (at 1.3 and 3.2 eV) and m/z = 35 (at 1.1 and 3.6 eV) currents can be ascribed by dissociation of anion states formed via π3* (predicted VAE = 1.38 eV) and π4* (predicted VAE = 3.69 eV) shape resonances, respectively. Observed shift of the DEA peaks to lower energies is understood in terms of survival probability factor.8,34 Breaking of a σ bond through the electron addition to a π* MO can be explained by the π*/σ* mixing caused by the outof-plane vibrations. However, in some cases fragmentation of molecular anions occurs directly through a σ* resonance, and does not require π*/σ* coupling.35 In the present case, σ1*C−Cl is predicted to lie at somewhat higher energy so that electron attachment to the π3* (scaled VOE = 1.38 eV) can be associated with the signals observed around 1.3 eV in INA DEA spectra. Suggested Mechanism of Biological Activity. The above findings are further used to foretell a behavior of CPL and INA in living tissues under reductive conditions. This approach is based on our assumption that temporary anions CPL− and INA− being formed by attachment of cellular electrons can dissociate mimicking DEA channels observed under gas-phase conditions as it has been discussed earlier.1,5,6,11,14,18,36 Indeed, the present calculations (Table S2) predict a strong (about 2 eV) stabilization of the CPL and INA anion states in aquatic media. In spite of the above-mentioned 752


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Figure 4. Suggested mechanism of bioactivity of captafol (CPL) and 2,6-dichloroisonicotinic acid (INA) under conditions of excess negative charge in the respiratory and photosynthesizing organisms. Penetration of unnatural electron acceptors (CPL or INA) into mitochondria and chloroplasts leads to withdrawal (red and green dotted arrows, respectively) of electrons from their usual pathways (black dashed arrows). Since energies of the “leaked” electrons in respiratory and photosynthetic electron transport chains are different, the diverse bioactivity of CPL and INA is attributed to formation of different fragment species by dissociative electron transfer (black solid arrows). Membrane complexes of the respiratory chain are labeled with roman numerals: (I) NADH dehydrogenase, (II) succinate dehydrogenase, (III) cytochrome bc1 complex, (IV) cytochrome c oxidase. P700 and P680 are the reaction centers of photosystems I and II, respectively. Pheo stands for pheophytin.

electrons “leaked” from the respiratory Complex III (cytochrome bc1 complex).5,22 These electrons are supposed to be weakly bound (binding energy of about 1.6 eV11) at the interface boundaries between protein complexes, lipid membrane and aquatic cytosol. The energies of these quasi-free electrons match well the “electron accepting window”, so that formation of negative ion resonances is energetically possible in biological media. It is worth mentioning that negative ion states are stabilized by 1.2−1.5 eV on going from gas-phase to the condensed state, i.e., the stabilization energy is close to the binding energy of electrons located at the interfaces.16,17 According to our data reductive dehalogenation of CPL by electrons “leaked” from mitochondrial respiratory chain can efficiently produce the active [CPL − Cl]· species able to disrupt biomembranes and to cause mitochondrial failure.18 Deprotonated radicals [4H-PTI−H]· formed by DEA to CPL as minor species (Table S1 and Table S2) can mediate mutations of mitochondrial DNA since teratogenic effect of thalidomide and related compounds is suspected.41 On the contrary, INA interaction with electrons “leaked” from the Complex III generates mainly neutral HCl species producing no evident toxic effect. Production of the harmful [INA−Cl]· radicals is 2 orders of magnitude less efficient, so that the dangerous effect of INA on respiratory organisms should be much weaker than that of CPL. The electron “leakage” in chloroplasts of photosynthesizing organisms occurs at (i) the first electron carrier intermediate (pheophytin), (ii) the primary quinone acceptor (QA), and (iii) cytochrome b559 at the electron acceptor side of photosystem II, where these electrons originate from the P680 excited state.23 The major site responsible for the electron attachment to cellular oxygen in plants is the 4Fe−4S complex on the stromal side of photosystem I.23 In other words, the electrons “leaked” from the photosynthetic pathways are excited by additional energy derived from the light-harvesting antenna complexes in comparison to electrons “leaked” from the Q0 or Q1 sites of mitochondrial Complex III.22 This additional energy can be estimated to lie in the 1−1.5 eV range since excitation energy of

complexity of the DEA energetics in cellular environment, the CPL and INA anion states are likely accessible for dissociative transfer of cellular electrons.5,11,16 CPL and INA are expected to behave in mitochondria and chloroplasts as unnatural electron acceptors stimulating electron leakage from the electron transport pathways since their predicted EAs exceed that of O2. Therefore, DEA is expected to play the important role as the molecular mechanism of bioactivity of CPL and INA which is not completely understood up to date. In fact, INA is supposed to inhibit ascorbate peroxidase and catalase, that leads to increasing of cellular H2O2 contents followed by activation of the defense-related genes. Since neither INA nor its metabolites possess antibiotic activity the mechanism of INA action is indirect. In other words, it is related to induction of the specific signal transduction pathways as it was reported for salicylic acid, a functional analog of INA.37,38 CPL is known to inhibit mycelial growth, to possess carcinogenic potential in animals and to cause DNA and membrane damage in human cells but exact mechanism of its bioactivity is unknown.39,40 A complete description of biological processes likely initiated by a variety of fragment species formed by dissociative electron transfer to CPL and INA is quite complicated. A simplified mechanism based on the involvement of the dominant CPL and INA fragments registered in our DEA experiments can be suggested. Isolated CPL molecules attach electrons with only thermal energy mainly generating the dechlorinated radicals as reported in Figure 1. Affinities of these radicals to neutral H atom (3.75 or 4.01 eV depending on the site of Cl− abstraction as shown in Table S4) are close to the threshold value 3.99 eV reported by Gregory to be sufficient to disrupt lipid membranes.6 Although H atom affinity of the [INA − Cl]· fragments is even higher (4.57 eV) their formation is 2 orders of magnitude less efficient than elimination of the neutral closed-shell HCl molecules (Figure 2). Attachment of epithermal (approximately 1.3 and 3.2 eV) electrons to INA leads to production of neutral HCl and CO2 species. Penetrating inside mitochondria xenobiotic molecules are expected to compete with cellular oxygen for interaction with 753


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Condon region. In the present case, when DEA to xenobiotics with high EA is expected to occur at the interface boundaries in the cell, polarization of the surrounding protein and lipid aggregations should be considered. It is however a complicated task and the corresponding approach is outside the scope of the present qualitative consideration. The reductive dehalogenation of halogen substituted compounds is approved to be the most probable mechanism of their metabolic transformations in plants55−57 and their toxicity in animals.58−61 Such behavior of halogenated xenobiotics in biological media is in line with both the processes observed in electrochemical studies in solution62−65 and the DEA properties studied in vacuo.6,18,66,67 Involvement of the excited electrons able to access higher-lying negative ion resonances has been discussed for anthralin.12 This compound is active in mitochondria of keratinocytes, and this activity can be reinforced by UV irradiation. This increased activity was associated with dehydrogenated anions formed by DEA to anthralin around 3 eV, whereas an H atom abstraction by thermal electrons accounts for the anthralin bioactivity in the absence of UV light.12

the chlorophyll dimer (P680) is 1.84 eV but only 1.38 eV is converted into the long-lived charge separated pair P680+·QA−. Charge separation in photosystem I with additional electron located at the iron−sulfur cluster stores only 0.99 eV from the total energy (1.77 eV) absorbed by P700.42 Therefore, in contrast to mitochondria the electrons “leaked” in photosystems are expected to occupy anion states lying 1− 1.5 eV above thermal energy, i.e., CPL is not able to interact with them. On the contrary, epithermal electron attachment to INA generates HCl and CO2 species able to induce the defense response in plants. In fact, 5-chlorosalicylic acid is known to control infection in Pinus radiata via indirect mechanism,43 but hydrogen chloride is the most intense species formed by DEA to this compound at electron energy 1.0 eV,44 very similar to that found for INA. Plants respond to a rise of carbon dioxide level by the closure of leaf stomata. It leads to elevation of the O2 content due to increased photosynthesis,45 that in turn increases production of superoxide generated by the interaction of “leaked” electrons with the O2.23 Therefore, a rise of CO2 content can initiate defense response in plants against the high level of superoxide. In this context increasing of cellular carbon dioxide level is associated with formation of neutral CO2 species by DEA to INA. Finally, the suggested molecular mechanism of the CPL and INA bioactivity in respiratory and photosynthesizing organisms is schematically summarized in Figure 4. Of course, the most crucial point of the present mechanism is linked with the fate of molecular negative ions formed under cellular environment by quasi-free electron attachment to xenobiotics. Namely, it is difficult to suggest a priori whether the so-formed molecular anions will dissipate their excess energy to surrounding media and thus escape dissociation.15 Or conversely, whether the neighboring molecules will increase DEA cross-section by one to 2 orders of magnitude as it was predicted for molecules embedded in the water clusters.46 Some recent experimental findings indicate prevention of dehydrogenation of microhydrated DNA bases by the DEA mechanism,47 however a cleavage of the N−H bond in the DNA base stimulated by electron attachment keeping the whole cluster anion undissociated cannot be completely ruled out. Actually, the problem is very complicated since the DEA efficiency is governed by many parameters. The phenomena are well understood from measurements of the absolute DEA cross-section for a series of chlorofluorocarbons isolated in or condensed on the rare gas matrices,48−52 as well as from observations of the electron-stimulated anion desorption.53,54 For instance, the measured DEA cross-section for CH3Cl has been found to increase by 4 to 6 orders of magnitude (with respect to the gas-phase value) when electron-attaching molecule is absorbed on the krypton substrate, or even more when it is embedded into the sandwich-like structure on the metal surface.48−50,52 The enhancement of the DEA crosssection can be well reproduced in the framework of R-matrix theory. This enhancement has been attributed to a significant increase of the survival probability factor caused by the lowering of the anion potential energy curve due to polarization of the environment by the intermediate anion state.52 From the other hand, a stabilization of the TA state can lead to decreasing of the DEA cross section due to reducing of the effective overlap between the anion and neutral ground states as reported for carbon tetrachloride,49 so that the TA state under the condensed media may even not be available in the Franck−

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CONCLUSIONS

Once introduced into living cells, exogenic compounds with electron affinities higher than that of molecular oxygen can act as unnatural electron acceptors, being able to withdraw cellular electrons from their usual pathways in mitochondria and chloroplasts. These compounds are able to compete with cellular O2 for attachment of quasi-free electrons. Dissociative electron transfer to xenobiotics is accompanied by fragmentation of the attaching molecules under reductive conditions in cells. Since energies of the electrons “leaked” in mitochondria and chloroplasts are expected to be different by 1−1.5 eV various fragment species can be formed in respiratory and photosynthesizing organisms. Some of these fragments are dangerous for cellular ambient but the others are expected to produce beneficial effects. CPL and INA are expected to be able to capture thermal electrons leaked from the mitochondrial respiratory chain. But only CPL effectively generates free radicals dangerous for cellular media. In contrast, CPL cannot attach epithermal electrons “leaked” in the photosystems, whereas attachment of the epithermal electrons to INA produces species likely responsible for induction of stress response in plants. The suggested mechanism provides a simplified description of bioactivity of synthetic herbicides and inductors of plant resistance usually possessing high electron affinities, as the model compounds CPL and INA.

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EXPERIMENTAL AND COMPUTATIONAL METHODS DEA Study in vacuo. An overview of DEA spectroscopy may be found elsewhere27,28 including the description of both the experimental details and means to estimate electron detachment time for the long-lived negative ions.10,68 Briefly, a magnetically collimated electron beam of a defined energy was passed through a collision cell containing a vapor of the substance under investigation, so that single-collision conditions were fulfilled. The electric current of the magnetically mass-selected negative ions was recorded as a function of the incident electron energy in the 0−14 eV energy range. The electron energy scale was calibrated with the SF6− signal at zero energy, generated by the attachment of the thermal electrons to 754


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SF6. The full width at half-maximum (fwhm) of the electron energy distribution was 0.4 eV, and the accuracy of the measured peak positions was estimated to be 0.1 eV. The substances under investigation are commercially available (Sigma-Aldrich) and were evaporated at 90 °C keeping the collision cell 10 °C hotter to prevent condensation. In silico Methods. The evaluation of the VOEs of the neutral molecule was conducted by means of the DFT calculations using the B3LYP hybrid functional and the standard 6-31G(d) basis set. The vertical electron affinity was calculated as the difference between the total energy of the neutral and the lowest anion state, both in the optimized geometry of the neutral state, using the standard 6-31+G(d) basis set. The adiabatic electron affinity was obtained as the energy difference between the neutral and the lowest anion state, each in its optimized geometry. Regardless of particular difficulties encountered for the description of anionic states, it has been demonstrated (see refs 7, 30, and references therein) that good linear correlations can be obtained between the energies of vertical electron attachment (VAEs) measured in electron transmission spectroscopy8,9 and the corresponding VOEs of the neutral molecules calculated with basis sets which do not include diffuse functions.69 In the present study, the linear equations VAE = 0.8065 × VOE + 0.9194 and VAE = 0.8111 × VOE + 1.60977 were employed to scale the B3LYP/631G(d) π* and σ*C−Cl VOEs, respectively. It should be noted that the σ* VAEs evaluated with this procedure are generally less reliable than the π* VAEs, due to the smaller number of experimental data available for calibration of the scaling equations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b12007. Complete set of the DEA spectra (including weak signals) and their digitized forms; B3LYP/6-31+G(d) energy thresholds and affinities for hydrogen atom; B3LYP/6-31G(d) energies of virtual molecular orbitals; and geometries and SOMOs for compounds under investigation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stanislav A. Pshenichnyuk: 0000-0001-5318-3638 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Saint-Petersburg State University (research grant 11.38.219.2014) and the Russian Foundation for Basic Research (grants #15-29-05786, #15-02-02809, and #17-0300196) for financial support. We are also grateful to the reviewers for useful comments and suggestions.

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REFERENCES

(1) Lovelock, J. E. Affinity of Organic Compounds for Free Electrons with Thermal Energy: Its Possible Significance in Biology. Nature 1961, 189, 729−732. 755


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