Article pubs.acs.org/JPCA
Temperature Dependence of the Dissociative Electron Attachment to 2‑Thiothymine Janina Kopyra Siedlce University, Faculty of Sciences, 3 Maja 54, 08-110 Siedlce, Poland
Hassan Abdoul-Carime* Univ. Lyon, Université Claude Bernard Lyon 1, CNRS/IN2P3, Institut de Physique Nucléaire de Lyon UMR5822, F-69003, LYON, France
Piotr Skurski Department of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdańsk, Poland ABSTRACT: We report the temperature dependence for the dissociation of 2-thiothymine induced by low energy electrons. Although hot molecules favor dissociative electron attachment (DEA) initiated by shape/coreexcited resonances, here we demonstrate that, in contrast, the dipole bound mediated DEA is inhibited, by decreasing the accessibility for the excess electron to the dipole bound anion formation channel. In addition, from this research the estimation of the change in the cross sections for the fragments production via the shape/core-excited resonances can be extended to temperatures at biological relevance.
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INTRODUCTION Improving methods for cancer treatments remains a tremendous challenge for nuclear medical research. In particular, chemo-radiotherapy that combines the use of ionizing radiation and the incorporation of cytotoxic agents within tumorous cells, has been developed as an alternative to the traditional surgery or chemotherapy. Halogenated analogues of thymine (e.g., bromouracil) employed as sensitizing drugs have been observed to increase the sensitivity of living cells to X-ray radiation1 without modifying the normal gene expression in nonirradiated cells.2 The mechanism by which these radiosensitizers3,4 operate has been attributed to the interaction of prehydrated secondary electrons produced along the ionization tracks of the primary energetic particles.5 These electrons induce the reduction of the halogenated nucleobase, producing the reactive uracil-yl radical,6 which is believed to be the precursor responsible for strong enhancements of the genotoxic damage.7 Recently, it has been shown that cisplatin bonded to guanine in DNA increases the lethality of the nucleic acid by a factor of as high as 48 and the mechanism involves resonant processes induced by low energy electrons.8−10 Thiolated nucleobases are also found to be candidates as sensitizers for radio/phototherapy treatments.11−13 Thiothymine (TT) exhibits almost the same structure as the canonical thymine with the exception that the oxygen atom at the C2 © 2016 American Chemical Society
position is substituted by a sulfur atom as shown in the inset of Figure 1. In a previous paper, we have shown that the capture of low energy electrons causes dissociation of thiothymine, resulting in the formation of various species.14 In addition to
Figure 1. Measured pressure at the collision chamber plotted in the natural logarithm scale as a function of 1/T, exhibiting a linear dependence according to the Clausius−Clapeyron equation. The solid line corresponds to the linear fit providing a slope of 14727.5 K. The structure of thiothymine is shown in the inset. Received: June 28, 2016 Revised: August 23, 2016 Published: September 1, 2016 7130
DOI: 10.1021/acs.jpca.6b06512 J. Phys. Chem. A 2016, 120, 7130−7136
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The Journal of Physical Chemistry A
Figure 2. Ion yield of (TT-H)− (M: 141 amu), SCN− (M: 58 amu), and S− (M: 32 amu) as a function of the incident electron energy and at different evaporation temperatures.
the dehydrogenated parent anion (TT-H)− observed at incident electron energies below 2 eV, the thiocyanide anion, SCN−, is a very abundant species produced via more complex fragmentation of the molecule. A priori, thiothymine could be a potential good candidate as a radiosensitizing drug. However, in these experiments, the measurements have been undertaken at relatively high temperature (ca., 400 K) and thus not directly biologically relevant. Therefore, it is desirable to investigate the efficiency of electrons to fragment thiothymine at lower temperatures. Here, we report and discuss the interaction of low energy electrons with 2-thiothymine (TT) evaporated at different temperatures. The fragmentation of thiothymine by electrons leads to the formation of three dominant anionic species: SCN−, S−, and (TT-H)−, as already shown, and we will below discuss the unexpected temperature effect on the production of these fragments.
thio- and methyluracils, do not indicate any decomposition products.16 Thus, here, the molecules are likely to evaporate intact. Tautomeric structures, with CS and CSH, of thiothymine may a priori coexist in the gas phase. The second tautomer has been found to be higher in energy by 0.4 eV.14 Although, there are no experimental data for thiothymine, infrared spectra of thermally evaporated thiouracil and their methylated derivatives do not show SH vibration stretching bands.16 Therefore, it is very likely that only the lowest conformation, i.e., CS, is produced in the present gas phase experiment. This is confirmed by the calculated energetics of the different tautomers of thiothymine, indicating that the lowest energy isomer is separated by the kinetic barrier of 1.5 eV from the second low energy structure. Hence, this latter conformation is not likely to contribute, even at T = 444 K.14 When the chamber is baked free of material up to 460 K, the pressure increases only up to 4 × 10−8 mbar. Thus, the increase of the pressure in the chamber with respect to the background pressure, ΔPmeas, with the temperature is directly proportional to the number of evaporated molecules. As shown in Figure 1, ΔPmeas plotted as a function of 1/T follows the Clausius− Clapeyron equation ln(ΔPmeas) = C − Bmeas/T,17 indicating that the investigated material directly sublimates from the solid. From this present experimental data, Bmeas is measured to be 14727.5 K, which is found to be lower by about 8% than that obtained by thermochemistry measurements (Btherm = 15754.5 K18). This difference is likely to be attributed to the accuracy of our pressure gauge in the chamber, and therefore, we will use the thermochemistry data to recalibrate the measured pressure: ΔP = A·ΔPmeas·exp(−(Bthem − Bmeas)/T), “A” being a scaling factor. Negative ions that are produced after collision with electrons are extracted from the interaction area by a small draw-out-field (395 K) it is the high energy feature. This is also observed when the yield functions of (TT-H)− are compared at ∼0 and 0.7 eV. In experiments that require thermal evaporation of molecules, and particularly in this work, the increase of the ion yield can a priori reflect the increase of the number of evaporated neutral molecular targets. Therefore, to understand the temperature effect to the electron−molecule interaction, the number of produced ions, Nmeas ion , must be normalized by the number of neutral targets. In a recent work it has been shown that the absolute DEA cross sections can be evaluated knowing the experimentally measured parameters such as Pmeas, T, and 33 Nmeas ion :
presence of the calibration gas avoiding unwanted reactions such as dissociative electron transfer with the investigated molecules that would produce an additional contribution to the anion signal near 0 eV.19
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RESULTS AND DISCUSSION Low energy electron impact on thiothymine produces three dominant negatively charged fragments detected at m/z 141, 58, and 32. They are attributed to the dehydrogenated thiothymine (TT-H)−, the thiocyanate SCN−, and the sulfur S− anions, respectively. The mechanism for the formation of these fragments has been discussed elsewhere14 and in the following, we provide the essential findings. Regardless of the operative temperature, a quick inspection of the yield functions show resonance features typical of dissociative electron attachment (Figure 2). For the SCN− and the S− anions the resonances are clearly localized around 0.3 and 3.0−6.0 eV, whereas for the (TT-H)− anion, the features are less well resolved and appear at around 0 eV, broad peak at 0.7 eV, and a shoulder at 1.3 eV (additional small structure near 3.5 eV will not be discussed here). The features in the yield function of the (TT-H)− anion have been assessed to shape resonances and vibrational Feshbach resonances. In the shape resonances, the attractive forces (due to the interaction of the excess electron with the neutral molecule) combine with the repulsive centrifugal force caused by the electron’s nonzero angular momentum to produce an effective potential for the electron. This potential has an attractive well and a centrifugal barrier and thus a shape resonance arises from electron being trapped behind the barrier. In our case, the shape resonance state formation involves the localization of an excess electron in the empty π* orbital of thiothymine. If this state mixes with a dissociative σ* state, unimolecular dissociation takes place. The structure observed near 0 eV in the (TT-H)− yield function may be associated with such a shape resonance by comparison with the results reported from 6-aza-2-thiothymine experiments.20 The structures observed in the energy range of 0.5−1.5 eV (peaks located at 0.7 eV, 1.0 and 1.3 eV) resemble those reported from DEA to thymine,21,22 uracil,23 and thiouracil.24 They have been attributed to the VFRs or dipole bound mediated dissociative electron attachment involving the cleavage of bond at the nitrogen site. For the dipole bound mediated DEA the excess electron must be first captured into diffuse orbital of the dipole bound anion25,26 either by direct means (500 meV vibrational Feshbach resonances) before being trapped by the long-range field.28 Then dipole bound (DB) state can couple to some valence states transferring the electron29,30 before ongoing into molecular dissociation. A priori the high dipole moment of thiothymine (5.41 D) ensures the formation of dipole bound states.14 The production of the thiocyanide anion, SCN−, associated with the formation of the C4NOH6 neutral counterpart is found to be exothermic. The fragmentation of TT driven by the high electron affinity of the thiocyanide radical (3.57 eV31,32) can already be observable from 0 eV. Although the structures observed around 3−6 eV can be directly compared to those reported from 6-aza-2-thiothymine20 and attributed to shape and core excited resonances, those visible at 0.3 eV in the yield function can be associated with the DB mediated DEA, as we shall discuss below.
σT(E ,T ) = Cnst
meas Nion (E ,T ) T · meas Ielec (E) ΔPmeas(T ) ·e−ΔB / T
(1)
where Cnst is a proportional constant coefficient and Imeas elec (E) is the measured electron current. This constant factor has been shown to be proportional to various parameters such as the geometry of the beams, detection efficiencies of the transmitted electrons and the detection efficiencies of the ions by the detector, the overlap of the electron and the effusive gas beams, the scaling factor for the gas pressure measurements (see the Experimental Procedure section), and other physical constants.33 Although accessible, we do not consider the absolute cross section in this work but rather σT(E,T)/Cnst, which also reflects the characteristics of the electron-thiothymine interaction. Furthermore, from (1) σT(E,T) can be integrated over a given energy range specific to a given dissociation mechanism: σ(T).34 Thus, for the SCN− and the S− anion the chosen energy ranges are 0−0.5 eV (ER1) and 2.5−6.5 eV (ER2), the latter being clearly attributed to shape/core-excited resonance.14 For the (TT-H)− anion the energy ranges are less well-defined but nevertheless it is possible to estimate 0−0.5 eV (ER1) and 0.5−1.6 eV (ER3), the latter energy range being associated with VFRs.14 Prior to the evaluation of the integrated cross section, the ion yields are traditionally fitted with Gaussian functions.35 Thus, σT/Cnst has a temperature dependence exclusively through the measured parameters. Panels (a)−(c) of Figure 3 present σ(TT‑H)−/Cnst (ER1 black squares, ER3 red circles), σSCN−/Cnst (ER1 black squares, ER2 red circles), and σS−/Cnst (ER1 black squares, ER2 red circles), respectively. It is clear that σSCN−/Cnst (ER2), σS−/Cnst (ER2), and σ(TT‑H)−/Cnst (ER1) show a slight increase with an increase of the temperature. Surprisingly, σ(TT‑H)−/Cnst (ER3), σSCN−/ Cnst (ER1), and σS−/Cnst (ER1) exhibit a negative temperature dependence. The temperature effect on shape/core-excited resonances initiated DEA has been investigated for a large number of molecules.36,37 Typically, according to the semiclassical model that considers the extension of the Franck−Condon overlap region induced by the increase of the vibrationally excited states and the autodetachment, the increase of the DEA cross section and a shift of the resonance peaks toward lower energies can be 7132
DOI: 10.1021/acs.jpca.6b06512 J. Phys. Chem. A 2016, 120, 7130−7136
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completely understood yet. On the contrary, it has been shown that including rotational degrees of freedom in the treatment of DBA causes a significant changes in the electron binding and the scattering features of the problem.41,42 Therefore, in the following we wish to foresee some possible consequences of the rotational excitation of thiothymine upon electron attachment. At a given temperature the population of rotational states of a neutral molecule follows the Boltzmann distribution as ϖj,kPj· Pk/s·Q,17 with s, Q, Pj, and Pk being respectively the symmetry number,43 rotational partition function, and the Boltzmann distributions (Pj = exp(−Bj(j + 1)/kBT) and Pk = exp(−(A − B)k2/kBT), kB the Boltzmann constant, j and k the quantum numbers associated with j2 and jz. Here the symmetry-top approximation is found to be relevant for the description of thiothymine in regard to the calculated rotational constants A, 3145.29 MHz, B, 915.62 MHz, and C, 712.34 MHz (MP2 augcc-pVDZ+4s3p14). Figure 4a presents the population of a given Figure 3. σ/Cnst plotted as a function of temperature for (a) the (TTH)− anion at 0 eV (red circles) and 0.7 eV (black squares), (b) the SCN− anion at 0.3 eV (black squares) and 4.0 eV (red circles), and (c) the S− anion 0.3 eV (black squares) and 4.0 eV (red circles). The black squares correspond to VFRs, and the red circles, to DEA initiated by the shape/core-excited resonances. The error bars have been overestimated to 50%. Panel d shows the number of weighted accessible states, Σϖj (see text). All the curves are plotted in the logarithm scale.
essentially attributed to the effect of vibrational excitation of the target molecule.36,38,39 On the contrary, theories predict little effect of the rotational states of the target molecules.36 From our measurements, the increase of σ/Cnst with the temperature is observed for the S− and the SCN− anions in the energy range 2.5−6.0 eV (ER2). The σ/Cnst parameter measured for the (TT-H)− anion near 0 eV (ER1) exhibits also a positive temperature corroborating the attribution of this feature to a shape/core-excited resonance.14 In contrast, for the 0.7 eV resonance in the production of the dehydrogenated thiothymine anion associated with VFRs, the σ/Cnst(ER3) decreases with temperature (Figure 3A). A similar observation has been recently reported for thiouracil,40 for which the negative temperature dependence of the ion fragments has been suggested to be a characteristic of the VFRs. However, this present observation conflicts with theory that predicts no significant effect on cross section within the 300− 450 K range but instead an increase at much higher temperature (700 K).28 This scattering theory describing the initial step for VFRs concerns the formation of the ground state dipole bound anion (E0db). The potential energy curve (PEC) of the covalent anion used for the scattering problem is usually obtained by a shift of the PEC of the neutral molecule by E0db along a particular coordinate, i.e., N−H coordinate when the dehydrogenated parent anion is explored.27,28 In the present experiments, and in general, in experiments requiring thermal heating, the molecules are not initially prepared solely in the ground state but also in different vibrational and rotational states prior to the electron interaction. Therefore, in principle, the interacting electron having transferred some energy quanta into vibrational states (and rotational states) of the neutral hot molecule can be trapped by the long-range electric field not only into the ground state of the dipole bound anion as postulated traditionally by the VFRs theory24,28 but also a priori into any of the accessible rotationally excited DB states. The influence of the rotational excited states to the VFRs is not
Figure 4. (a) Boltzmann probability to populate a rotational j states as the function of j at different temperatures: 173 K, purple; 273 K, orange; 298 K, black; 373 K, red; 423 K, blue; 433 K, green. The total number of accessible “k” states for a given j state is provided in the inset. (b) Calculated binding energy, Edb, of the electron in the dipole bound anion as a function of the rotational j state with l = 4 and k = 0. For j = 140, Edb is plotted as the function of the quantum number “k” (inset).
rotational j state ϖj after integrating overall accessible k substates for different temperatures: T = 173 K (purple), T = 273 K (orange), T = 298 K (black), T = 373 K (red), T = 423 K (blue), and T = 433 K (green). From these curves it is possible to find the most probable populated state at a given temperature, e.g., at T = 173 K the most populated state is jmp = 34 whereas at T = 423 K this is jmp = 54. Although the hot molecule can be found in various rotational states, all of them will not support dipole bound states according to the balance between the binding energy of the electron and the centrifugal potential induced by the rotation of the molecule. The different accessible dipole bound states, Edb(j,k), can be evaluated as a function of j states using the Clary’s rotationally adiabatic theory.44 This model, successfully developed to discuss the rotationally resolved experimental photodetachment spectra of the CH2CHO− and the CH2CN− anions, has been shown to predict relatively accurately Edb for various molecular systems, among them DNA nucleobases, e.g., thymine.26 For instance the binding energy of the electron in the dipole bound state of 7133
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binding energy for the dipole bound state decreases to 74 meV. As the temperature increases, jmp shifts toward higher values and the probability for occupying this state decreases following the Boltzmann distribution. Finally, at T = 0 K (not shown) the most probable populated rotational state corresponds to jmp = 0 with a binding energy for the dipole bound state of 79 meV. Bearing in mind that the theoretically highest j value for allowing a dipole bound to be formed is jmax = 143 (i.e., Edb = 0), it is obvious that the number of accessible (j, k) states decreases with the increase of T. Thus, integrating ϖj for j values up to jmax provides the probability for a dipole bound state to be accessible: for (T = 173 K, ∑ϖj = 99.89%), (T = 298 K, ∑ϖj = 98.94%), and (T = 423 K, ∑ϖj = 96.79%). In fact, from the experimental point of view the highest j value may be slightly lower than 143. Indeed, for very weakly bound electrons, the external electric field of 1 V/cm may suffice to prevent from the formation of the dipole bound anion.26 The total cross section for the production of a given fragment via VFRs must be weighted by the statistical population of molecular states:49
thymine has been calculated to be 69−72 and 88 meV, respectively,26 and higher level of theory,45 whereas the experimental measurements provide a value of 68 meV.26 In brief, the electron−molecule interaction is described by the Hamiltonian: H = Bj2 + (A − B)jz2 + l2/(2r2) + V(r,θ), j and jz are the rotational angular momentum and the projection along the symmetry axis of the molecule respectively (j and k are the associated quantum number), A and B are the rotational constants, and l is the relative orbital momentum of the electron relative to the molecule. From the theory, the lowest rotational adiabatic potential energy curve, εad(r), is obtained by diagonalizing the Hamiltonian H in the spherical harmonic basis set {Ylm}. Finally, the DB state, Edb, is obtained by resolving a one-dimension Schrödinger equation, in this work, using a standard numerical method. The electrostatic longrange interaction potentials V(r,θ) defined in refs 26, 44, and 46 uses the experimental or calculated electrical dipole moment (5.41 D), polarizability (15.58 Å3), quadrupole moment (14.9 au),47,48 and rotational constants given above. Figure 5 shows
jmax k max
σT(T ) =
∑ ∑ ϖj ,k(T ) σj ,k(T ) j=0 k=0
(2)
where jmax and kmax are the highest values for which DBAs are still forming and σj,k(T) are the cross sections for the associated VFRs calculated by the scattering theory. The exact estimate of the total cross section taking into account all possible σj,k values is quite a tedious work. In the following we will foresee and discuss a possible scenario. It has to be noted, however, that the maximum of the dipole binding energy is only 79.2−79.9 meV, which is small in comparison to the difference between two consecutive vibrational states. For instance, for thiouracil, the difference between two consecutive vibrational states has been calculated to be 450 meV.24 Because the first step for the VFR consists of the transition from a dipole bound state to a valence state in a given vibrational mode, the fact that the dipole binding energy is smaller than Δν ensures that no ro-vibrational overlap between two consecutive vibrational modes can take place. Thus, after transferring energies to some vibrational states, the excess electron is trapped in the most probable rotational excited state jmp (Figure 4a) and only a small number of j states around jmp (for which the change in the energy level is very small; i.e., Edb varies by 2 meV) contribute to σT(T). With this assumption it is likely that σj,k(T) remains almost constant. Figure 3d (green stars) shows a decrease of ∑ϖ with an increase of the temperature. From our assumption, the VFRs cross section must also decreases in qualitative agreement with the experimental observation. We also report probability Pjmp, associated with the most probable state jmp (Figure 3D, orange stars), which also exhibits a small negative temperature dependence, e.g., Pjmp=39(T=298 K) = 1.26%, Pjmp=47(T=433 K) = 1.08%. We can also evaluate ∑ϖ(373 K)/∑ϖ(433 K) to be 1.64. This ratio is, however, smaller than the σT(373 K)/σT(433 K) found for SCN− and S−, ca. 4.7 and 12.4, respectively (Figure 3b,c black squares). Although this model is qualitatively in reasonable agreement with the experimental measurements, it is not sufficient to fully describe the negative temperature dependence. Indeed, in VFRs, it is admitted that the incoming electron transfers energy to the molecular vibrational states but no information is available on the amount of energy that is also transferred to the rotational states of the hot molecule. This latter would certainly lead to a shift of the presently calculated
Figure 5. Rotational adiabatic potential obtained by diagonalizing the interaction Hamiltonian (for j = k = 0, l = 4; see text) plotted as a function of the electron-molecule distance, r. The lowest potential energy curve presents a well deep enough for binding an excess electron in a dipole bound state (Edb = 79.19 meV) whereas the second PEC is purely repulsive. In the inset the normalized radial wave function associated with the lowest PEC is plotted.
the two first calculated adiabatic PEC. The ground state dipole bound corresponding to the lowest PEC (black) leads to the formation of the dipole bound state, and the associated diffuse radial wave function is presented in the inset of the figure. We found E0db(j=0,k=0) = 79.19 meV, which agrees well with the value of 79.9 meV calculated with more sophisticated methods (MP2 aug-cc-pVDZ+4s3p).14 Figure 4b presents the calculated Edb as a function of j (and for k = 0), and in the inset is the kdependency of Edb for j = 140. We also mention that from l = 4 to l = 8, the binding energy varies by less than 1%, but the higher the l value, the longer is the calculation time. Thus, we use an l value of 4 to calculate Edb in Figure 4b. The quadratic behaviors of the Edb dependencies permit us to express the binding energy as Edb(j,k) = Bj(j + 1) + (A − B)k2 + E0db, as shown by Clary.44 As can be seen, for j > jmax = 143 or for j = 140 and k > 18 the electron is no longer bound as a dipole bound anion, i.e., Edb(j,k) = 0. From this analysis, it is easy to calculate the number of accessible kmax for a given j state: e.g., for j < 80, kmax increases as 2j + 1. Finally, it is each of these (j, k) accessible states that can a priori form a dipole bound anion and thus contribute to the VFRs. Therefore, it is necessary to take all the accessible (j, k) states into account. The results presented in Figure 4 tell us that at a temperature of T = 173 K (purple squares, Figure 4a) the most probable jmp is found to be 37 with a probability of 1.6% and the associated 7134
DOI: 10.1021/acs.jpca.6b06512 J. Phys. Chem. A 2016, 120, 7130−7136
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The Journal of Physical Chemistry A “j” states to higher values, in consequence reducing the number of accessible states for the dipole bound anion formation. The difference between model and experiment found above suggests that the contribution of σj,k(T) for different (j, k) sets of values must be taken into account in eq 2. It is not clear at this stage whether all valence states can be equally accessible and this is likely not the case. Indeed, the cross sections must necessarily increase as the temperature decreases because the absolute value of the ∑ϖ(373 K)/∑ϖ(433 K) ratio (Figure 3d) is found substantially lower than the experimentally obtained σT(373 K)/σT(433 K) ratio (Figure 3a−c black squares). Finally, as the deposited energy leads to the excitation of vibrational state, it is not clear even from the actual theory28 whether the amount of energy may lead to the tautomerism form of thiothymine found in structure optimization14 prior to electron attachment to form a dipole bound state. From this study, dipole bound mediated DEA appears to be a less favorable mechanism as the temperature increases. Therefore, the fact that the yield of SCN− and S− increases as the temperature decreases can be a signature of DB mediated DEA process. With anticipation, it is observed that (in log scale) the production of the SCN− and the S− anions via shape resonance processes increase by a factor of 4 (Figure 3b,c, red circles) in average in the temperature range 298−433 K whereas it decreases by a factor of 40 in the case of VFR (Figure 3b,c, black squares).
environments), although it seems likely that the analogous short-lived surface anionic states might be formed (e.g., as in small water clusters). In addition, this general feature may have some potential application in other field of physics such as cold plasma.
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AUTHOR INFORMATION
Corresponding Author
*H. Abdoul-Carime. E-mail:
[email protected]. Phone: (33)47243359. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Polish Ministry of Science and Higher Education, The University of Lyon 1, and the Centre National de Recherche Scientifique (CNRS/IN2P3). H.A.C. acknowledges support for a visit to Siedlce University, Siedlce (Poland) from the European Union via the COST Action MP1002 (Nano-IBCT).
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REFERENCES
(1) Szybalski, W. X-ray sensitization by halopyrimidines. Cancer Chemother. Reports 1974, 58, 539−557. (2) Zamenhof, S.; DeGiovanni, R.; Greer, S. Induced gene unstabilization. Nature 1958, 181, 827−829. (3) Sevilla, M. D.; Failor, R.; Zorman, G. Radicals formed after electron attachment to 5-halouracils in aqueous glasses. J. Phys. Chem. 1974, 78, 696−699. (4) Abdoul-Carime, H.; Huels, M. A.; Illenberger, E.; Sanche, L. Sensitizing DNA to secondary electron damage: resonant oxidative radicals from 5-halouracils. J. Am. Chem. Soc. 2001, 123, 5354−5355. (5) Mücke, M.; Braune, M.; Barth, S.; Förstel, M.; Lischke, T.; Ulrich, V.; Arion, T.; Becker, U.; Bradshwa, A.; Hergenhahn, A. A hitherto unrecognized source of low-energy electron in water. Nat. Phys. 2010, 6, 143−146. (6) Abdoul-Carime, H.; Huels, M. A.; Illenberger, E.; Sanche, L. Formation of negative ion from gas phase halo-uracils by low energy (0−18eV) electron impact. Int. J. Mass Spectrom. 2003, 228, 703−716. (7) Rivera, E.; Schuler, R. H. Intermediates in the reduction of 5halouracils by eaq-1. J. Phys. Chem. 1983, 87, 3966−3971. (8) Rezaee, M.; Alizadeh, E.; Cloutier, P.; Hunting, D. J.; Sanche, L. A single subexcitation-energy electron can induce a double-strand break in DNA modified bt platinum chemotherapeutic drugs Chem. ChemMedChem 2014, 9, 1145−1149. (9) Alizadeh, E.; Orlando, T. M.; Sanche, L. Biomolecular damage induced by ionizing radiation: the direct and indirect effects of lowenergy electron on DNA. Annu. Rev. Phys. Chem. 2015, 66, 379−398. (10) Kumar, A.; Sevilla, M. D. Low Energy Electron (LEE) Induced DNA Damage: Theoretical Approches to Modelling Experiments. Handbook of Computational Chemistry Vol. IV: Applications − Biomolecule;Shukla, M., Leszczynski, J., Eds.; Springer: Berlin, 2015. (11) Cui, G.; Fang, W.-H. State-specific heavy-atom effect on intersystem crossing in 2-thiothymine: a potential photodynamic therapy photosensitizer. J. Chem. Phys. 2013, 138, 044315. (12) Herak, J. N.; Sankovic, K. Hütterman, Thiocytosine as a radiation energy trap in a single crystal of cytosine hydrochloride. Int. J. Radiat. Biol. 1994, 66, 3−9. (13) Attard, N. R.; Karran, P. UVA photosensitization of thiopurine and skin cancer in organ transplant recipients. Photochem. Photobiol. Sci. 2012, 11, 62−68. (14) Kopyra, J.; Freza, S.; Abdoul-Carime, H.; Marchaj, M.; Skurski, P. Dissociative attachment to gas phase thiothymine: experimental and theoretical approaches. Phys. Chem. Chem. Phys. 2014, 16, 5342−5348.
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CONCLUSION In general, an increase of the temperature of the target molecule is observed to promote the dissociation process in the case of DEA initiated by shape/core-excited resonances. In contrast, it is reported here that for DB supported DEA, or the VFR mechanism, heating the molecule reduces the dissociation process. When the molecule is heated, the Boltzmann statistic indicates that different rotational states can be accessible; however, all these states cannot support the formation of dipole bound anion. Therefore, the probability to find such states decreases with temperature. The present model does not aim to provide a complete description of the VFRs. However, it is qualitatively in reasonable agreement with our observation. A complete understanding of the negative temperature dependence requires the calculation of the cross sections σj,k associated with all accessible states and this must be a tremendous challenge for further theories to describe the process. As already observed in electron−thiouracil interaction experiments, the negative temperature dependence is confirmed in the present work for fragments arising from dipole bound mediated DEA. This approach of temperature dependence of the anionic yields can therefore be a good means for the characterization of VFRs processes. As a consequence, the structure observed near 0 eV for SCN− anion production must arise from dissociative electron attachment mediated by the formation of a dipole bound state. From the point of view of potential application, the cross sections for the fragments production at biologically relevant temperatures are accessible. However, it is clear that dipole bound mediated DEA seems to provide a much higher fragmentation cross sections at biological temperature than at usual gas phase experiments (i.e., 420 K). It is noteworthy that the results obtained from the experiments for which the molecules are in isolation may not be directly applicable to the condensed phase situation. Particularly, dipole bound systems cannot exist in the condensed phase (and thus in biological 7135
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Article
The Journal of Physical Chemistry A
the approximation of a perfect gas, T/Pmeas represents the density of the target molecule. Furthermore, for this experimental set-up, Cnst/ Imeas is found to be 2.1185 × 10−26 (for a transmitted electron beam of 14 nA); the cross section, σ, can then be directly estimated in cm2 unit, with an accuracy of about 50%. (34) Note that the temperature dependences of σT(E,T)/Cnst and σT(T)/Cnst are similar. (35) The authors in ref 23 use multiple (up to 12) Gaussian lines for the deconvolution of their well-defined anion yields, although they mentioned that a Lorentzian profile fits better a resonance peak than a Gaussian profile. (36) O’Malley, T. F. Theory of dissociative electron attachment. Phys. Rev. 1966, 150, 14−29. (37) Christophorou, L. G.; Olthoff, J. K. Advances in Atomic, Molecular and Optical Physics; vol 44, Electron Collisions with molecules in gases: Applications to plasma diagnostics and modelling; Kimura, M., Itikawa, Y., Eds.; Academic Press: New York, 2001 and references herein. (38) Lehr, L.; Miller, W. H. A classical approach to dissociative electron attachment DA: application to temperature effects in the DA cross section of CF3Cl. Chem. Phys. Lett. 1996, 250, 515−522. (39) Rosa, A.; Brüning, F.; Kumar, S. V. K.; Illenberger, E. Dissociative attachment to SF6−: selective IR excitation versus thermal activation. Chem. Phys. Lett. 2004, 391, 361−365. (40) Kopyra, J.; Abdoul-Carime, H. Unusual temperature dependence oft he dissociative electron attachment cross section of 2thiouracil. J. Chem. Phys. 2016, 144, 034306−1−7. (41) Garrett, W. R. Critical binding of an electron to a rotationally excited dipolar system. Phys. Rev. A: At., Mol., Opt. Phys. 1971, 3, 961− 972. (42) Garrett, W. R. Critical binding and electron scattering by symmetric-top polar molecules. J. Chem. Phys. 2014, 141, 164318−1− 5. (43) Zhou, Y.-J.; Lv, J.; Yu, K.; Ma, J. J.; Guo, D. S. Supramolecular structures in three thiouracil derivatives: 5,6-trimethylene-2-sulfanylidene-1,2-dihydropyrimidin-4(3H)-one, 2-(4-fluorobenzylsulfanyl)-5,6trimethylenepyrimidin-4(3H)-one and methyl 2-{[2-(4-fluorobenzylsulfanyl)-5,6-trimethylenepyrimidin-4-yl]oxy}acetate. Acta Crystallogr., Sect. C: Struct. Chem. 2014, C70, 416−420. Thiouracil crystallizes in the space group C2/m consisting of one symmetry-independent molecule corresponding to the mirror plane. Thus the symmetry number is 1. Although, there is no crystallographic information on thiothymine, according to the structure of the molecule, the assumption of symmetry number of one is plausible. (44) Clary, D. C. Photodetachment of electron from dipolar anions. J. Phys. Chem. 1988, 92, 3173−3181. (45) Oyler, N.; Adamowicz, L. Theoretical ab initio calculations of the electron affinity of thymine. Chem. Phys. Lett. 1994, 219, 223−227. (46) Edwards, S. T.; Johnson, M. A.; Tully, J. C. Vibrational Fano resonances in dipole-bound anions. J. Chem. Phys. 2012, 136, 154305− 1−10. (47) Schneider, W. C.; Halverstadt, I. F. The dipole moments of thiouracil and some derivatives. J. Am. Chem. Soc. 1948, 70, 2626− 2631. (48) Kryachko, E.; Nguyen, M. T.; Zeegers-Huyskens, T. Thiouracils: acidity, basicity and interaction with water. J. Phys. Chem. A 2001, 105, 3379−3387. (49) Kharchenko, V.; Dalgarno, A.; Zygelman, B.; Yee, J.-H. Energy transfer in collision of oxygen atoms in the terrestrial atmosphere. J. Geophys. Res. 2000, 105 (A11), 24899−24906.
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