Ab Initio Treatment of Ion-Induced Charge Transfer ... - ACS Publications

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Ab Initio Treatment of Ion-Induced Charge Transfer Dynamics of Isolated 2‑Deoxy‑D‑ribose Marie-Christine Bacchus-Montabonel* Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne Cedex, France ABSTRACT: Modeling-induced radiation damage in biological systems, in particular, in DNA building blocks, is of major concern in cancer therapy studies. Ion-induced charge-transfer dynamics may indeed be involved in proton and hadrontherapy treatments. We have thus performed a theoretical approach of the charge-transfer dynamics in collision of C4+ ions and protons with isolated 2-deoxy-D-ribose in a wide collision energy range by means of ab initio quantum chemistry molecular methods. The comparison of both projectile ions has been performed with regard to previous theoretical and experimental results. The charge transfer appears markedly less efficient with the 2-deoxy-D-ribose target than that with pyrimidine nucleobases, which would induce an enhancement of the fragmentation process in agreement with experimental measurements. The mechanism has been analyzed with regard to inner orbital excitations, and qualitative tendencies have been pointed out for studies on DNA buiding block damage.

1. INTRODUCTION Ionizing radiation may induce lesions to DNA, including singleand double-strand breaks.1 For high-energy photons or electrons, most of the damage is not due to the primary radiation itself but is induced by the secondary particles generated along the track after interaction of the ionizing radiation with the biological medium.2 In particular, it has been shown that low-energy secondary electrons can cause severe damage to DNA via dissociative electron attachment, even at very low kinetic energies, well before they are thermalized or solvated.3−5 In addition to the secondary electrons, ionizing radiations also produce abundant ballistic low-energy ions and neutral radical fragments,6 and their interaction with the surrounding medium can drive important physicochemical reactions. Numerous studies have been focused on the DNA degradation induced by secondary electrons and free radicals, but important interest has developed recently on collisions of ions on biomolecular targets. Such secondary ions may be different species, with different charge state or energy with regard to their formation dynamics.7 Most of the recent experimental and theoretical investigations have been performed for keV kinetic energies relevant for the heavy-ion biological radiation damage in the region of the Bragg peak corresponding to maximum induced damage.8−16 Effectively, proton or heavy-ion-beam cancer therapy differs from conventional high-energy electron and photon therapy by delivering radiation doses with a maximum dose density deposited at the Bragg peak, shortly before the track end. This selectivity makes heavy-ion therapy a promising technique in cancer treatments with high efficiency for deep-seated tumors.17−20 However, secondary ions with hundreds of eV or even eV kinetic energies may be produced in proton or heavy-ion treatments. Specific physicochemical interactions with the biological medium could thus occur with regard to the collision energy range. In that © XXXX American Chemical Society

sense, a few experimental and theoretical studies have been performed recently at low energies.21−25 In order to explore the mechanism underlying ion-induced DNA damage at the molecular level, numerous studies focused on the behavior of the DNA building blocks in collisions with ions have been developed. Nucleobase targets have been particularly investigated experimentally8−10,22 and theoretically,16,23−25 but experiments on base−sugar complex nucleosides have been performed recently. They have shown that ioninduced nucleoside damage pathways, including base or sugar loss and disintegration of either part, are dominated by sugar damage.21 The crucial role of the deoxyribose moiety is even confirmed by experiments on protonated oligonucleotides.26 This conclusion is supported by experiments on isolated molecules showing an almost complete fragmentation of the sugar whatever the projectile ion.13,14 A theoretical study of ioninduced collisions with 2-deoxy-D-ribose (dR) molecular targets would be important to understand the mechanism of such reactions. Different processes are involved in reactions of ions with biomolecular targets: excitation and fragmentation of the target, direct ionization of the biomolecule, and also possible charge transfer from the multiply charged ion toward the molecular target. From the experimental point of view, excitation and fragmentation processes have been mainly investigated, and fragmentation cross sections are determined from mass spectra.8 However, these experimental measurements may not give any information on charge transfer between the incident Special Issue: Franco Gianturco Festschrift Received: August 27, 2013 Revised: October 4, 2013

A

dx.doi.org/10.1021/jp408570b | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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ion and the biomolecular target.11 On another hand, theoretical treatments on fragmentation dynamics of biological targets proceed on ionized species as collision processes are very fast and electron removal is almost instantaneous with regard to the fragmentation time.16 A better understanding of the ionization step would be determinant. We have thus developed an ab initio treatment in order to investigate the charge-transfer mechanism in collisions of ions with biomolecules. The consideration of collisions of different charged Cq+ carbon ions with pyrimidine nucleobases, uracil, thymine, and 5halouracil molecules presenting similar structures with different substituents provides a detailed analysis of the steric or electronic effects in the charge-transfer process.11,15,25,27 In order to compare these results to the behavior of the dR, we develop in the present paper a dynamical treatment of the charge transfer with the sugar target, considering first of all the same carbon projectile ion for a comprehensive analysis of the charge-transfer process in the whole series of biomolecular targets. The C4+ ion corresponding to efficient electron exchange has been chosen,8 and extensive comparisons are available as charge transfer with C4+ has been widely investigated with pyrimidine nucleobase targets.24,27 However, with regard to important experimental studies undertaken with protons involved in proton therapy treatments,13,14 we have considered also the charge transfer between protons and isolated dR. Those results could be compared with the previous collision with C4+, and a more detailed analysis of the inner orbital excitation could be developed for such a projectile ion as simpler molecular calculations are expected. The theoretical treatment is driven in the framework of the molecular representation of the collisions with ab initio molecular calculations of the potential energies and NACMEs.28 The collision dynamics has been performed in a wide energy domain, from keV to 100 eV energies corresponding to the kinetic energy range of secondary electrons, in order to investigate the region of the Bragg peak as well as possible specific behavior at low energies.

Figure 1. Geometry of the dR.

Figure 2. Geometry of the ion−dR system in the perpendicular approach.

and the anisotropy of the charge-transfer process may be investigated considering the orientation of the projectile ion with regard to this mean plane. In that sense, the chargetransfer process has been undertaken in three specific geometries, in a perpendicular approach along the z coordinate and in a planar approach along the x and y directions. The different molecular states involved in the process are calculated along the reaction coordinate R for the planar and perpendicular approaches. The potentials have been determined for a large number of R distances, every 0.1 Å in the interacting region, from 0.5 to 9 Å. The potentials have then been extrapolated to reach the asymptotic region. The geometry of the ground state of the dR has been optimized by density functional theory (DFT) calculations with the gradient-corrected BLYP functional33,34 and 6-311G** basis set. It has been kept frozen during the collision process. The molecular calculations have been carried out by means of the MOLPRO suite of ab initio programs.35 For all geometries, an all-electron calculation has been performed with no symmetries and using Cartesian coordinates with the 6311G** basis set for all atoms. The potential energies and nonadiabatic coupling matrix elements (NACMEs) have been determined by state-averaged CASSCF (complete active space self consistent field) calculations. Dynamical correlation effects are not taken into account at this level of theory, but we can expect a correct description of the relative energies of the different excited states, and some calculations at a higher MRCI (multireference configuration interaction) level of theory have been performed for a few critical points. Different active spaces have been considered and will be described in detail for each collision system.

2. THEORETICAL TREATMENT 2.1. Molecular Calculations. In the molecular description of the collisions, the charge-transfer process is described as the evolution of a quasi-molecular system formed by the projectile ion and the molecular target. For complex polyatomic targets, a simple model may be proposed by means of the one-dimension reaction coordinate approximation used in a number of cases.29−31 The ion−biomolecule system may thus be considered as a pseudodiatomic molecule. Its evolution is driven by the reaction coordinate corresponding to the distance between the center-of-mass of the biomolecule and the colliding ion. Such an approach is very simple and does not consider the internal motions of the biomolecule, but it has been shown to be reasonable for very fast collision processes where nuclear vibration and rotation periods are assumed to be much longer than the collision time. The geometry of the pyrimidine nucleobases is almost planar, constructed around a six-membered, ring32 and the anisotropy of the charge-transfer process may be investigated for different orientations of the projectile toward this ring plane, even for nonplanar targets such as thymine.24 The point is a bit different for the dR (C5H10O4), which is clearly a nonplanar biomolecule constructed around a five-membered ring with CH2OH and OH substituents (see Figure 1). However, a mean xy plane may be defined over the five-membered ring, as shown in Figure 2, B



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eigenfunctions φm of Hel for each spin and space symmetry with eigenvalues εm, where m is the number of electronic states

The charge-transfer process being driven mainly by nonadiabatic interactions in the vicinity of avoided crossings,36 the NACMEs between all pairs of states of the same symmetry have been calculated numerically by means of the finite difference technique37

Ψ(r , b , v , t ) =

m

∂ gmn(R ) = ⟨φm| |φn⟩ ∂R = ⟨φm(R )| lim

Δ→ 0

∑ am(b , v , t )φm[r , R(t )] × exp( −i

i

which, taking into account the orthogonality of the eigenfunctions |φm(R)⟩ and |φn(R)⟩ for m ≠ n, reduces to

εm[R(t ′)] dt ′)

⎛

∑ am(t )⎜⎝⟨φn|H el|φm⟩ − i vZ ⟨φn| R

m

(2-1)

(2-4)

⎞ vb ⟨φ |iLy|φm⟩⎟ exp( −i ⎠ R2 n

∫0

∂ |φ ⟩ ∂R m

t

(εm − εn) dt ′) (2-5)

involving the radial NACMEs ⟨φm|(∂/∂R)|φn⟩ between states of the same symmetry, as well as the rotational couplings ⟨φm| iLy|φn⟩ between molecular states of different space symmetry. In our case, only radial coupling matrix elements have to be considered as the molecular calculations are performed in the C1 symmetry group. The first term is reduced to zero in the adiabatic representation. The amplitudes of probabilities am are determined by integration of eq 2-5. The probabilities are given by P(b,v) = ∑m|am(b,v,∞)|2 with summation over all charge exchange channels. The cross section is then given by the expression σ(v) = 2π

∫0

∞

bP(b) db

(2-6)

The collision dynamics has been performed using the EIKONXS program based on an efficient propagation method.45 The coupled equations have been solved with a step size such that an accuracy of 10−4 for the symmetry of the S matrix is achieved. For both collision systems, the calculations have been carried out for laboratory energies from about 100 eV to 100 keV for planar and perpendicular orientations, taking into account all of the transitions driven by radial coupling matrix elements.

3. COLLISION OF CARBON IONS WITH DR The present calculation aims to exhibit a comprehensive comparison between the dR and the pyrimidine nucleobases studied previously in order to analyze the behavior of the different biological targets in collision with ions. In that sense, we have chosen to look at the collision with the C4+ carbon ion, which has been shown to drive important charge transfer8,11 and has been widely investigated in collisions with pyrimidine nucleobases.15,23,24 The calculation has been carried out as similarly as possible with regard to previous biological targets, using the same 6-311G** basis set and similar active spaces. For that purpose, the active space includes the six highest valence orbitals constructed mainly on the 2pz orbitals, perpendicular to the xy mean plan of the five-membered ring, centered on oxygen O1 of the ring and oxygen O3 of the CH2OH group and the 2px, 2py, 2pz orbitals of the colliding carbon ion with two active electrons. The process is driven by an excitation of the doubly occupied {(2pzO)2} level centered mainly on the 2pz orbital on the oxygen of the CH2OH group with delocalization on the oxygen O1 of the ring to the 2px, 2py, and 2pz orbitals of the colliding carbon atom, showing a strong nonadiabatic interaction between this {(2pzO)2} molecular state and the {2pzO2px},

(2-2)

The Hamiltonian of the system is given by H[r , R(t )] = T + H el[r , R(t )]

dan(t ) = dt

−i

The stability with regard to the differentiation step Δ has been tested, and a value of Δ = 0.0012 au has been chosen38 using the three-point numerical differentiation method for reasons of numerical accuracy. The center of mass of the dR molecule has been chosen as the origin of electronic coordinates. 2.2. Collision Dynamics. The collision dynamics has been developed using a semiclassical method in the framework of the sudden approximation assuming that the electronic transitions occur so fast that vibration and rotation motions remain unchanged. The total and partial cross sections, corresponding to purely electronic transitions, may then be determined by solving the collision equations considering the geometry of the molecular target fixed. Such a treatment is, of course, relatively crude, but it has proved its efficiency in a number of ion− diatomics39 and even ion−polyatomics collisions27 for energies higher than ∼10 eV/amu. Effectively, the collision time at such energies becomes much shorter than typical vibration time and a fortiori typical rotation time.40 To give an order of magnitude, the collision time Tcoll to travel a given distance has been evaluated to be about 102 times shorter than the corresponding vibration time Tvib in collisions of Cq+ ions on uracil at 1 keV12. This approach has been extended to lower collision energies23,24 using a recent analysis of time-dependent quantal wave packet and semiclassical approaches in ion−atom chargetransfer processes. Effectively, semiclassical methods have been shown to give quite reasonable cross sections down to 10−20 eV for ion−atom collisions.41,42 The discrepancy with quantal calculations appears only for eV and lower energies. We can thus expect a semiclassical approach to give a correct order of magnitude of the charge-transfer cross sections in a wide collision energy domain. Considering the geometry of the target fixed, the collision equations may be derived in the eikonal-sudden approximation.43,44 In this approach, the projectile follows straight-line trajectories, R(t) = b + vt with impact parameter b and velocity v, while the electronic motion is described by the timedependent Schrödinger equation ⎛ ∂ ⎞⎟ ⎜H[r , R(t )] − i × Ψ(r , b , v , t ) = 0 ⎝ ∂t ⎠

t

This leads to a set of coupled differential equations

1 |φ (R + Δ) − φn(R )⟩ Δ n

∂ 1 gmn(R ) = ⟨φm| |φn⟩ = lim ⟨φm(R )|φn(R + Δ)⟩ Δ→ 0 Δ ∂R

∫0

(2-3)

where T is the kinetic energy operator for the nuclear motion and Hel the Born−Oppenheimer electronic Hamiltonian. Equation 2-2 may be solved for each velocity v and impact parameter b by expanding the total wave function Ψ on the C



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{2pzO2py}, {2pzO2pz} charge-transfer levels. Such a feature, already observed in previous collisions with pyrimidine nucleobases, would lead to a relatively simple mechanism driven mainly by a direct excitation process; the double excitation process involving the π orbital of the six-membered ring of pyrimidine nucleobases is no longer observed. Further analysis of inner excitations would need to include deeper molecular orbitals in the active space leading to quite impracticable calculations for the C4+ + dR collision system. This point is analyzed in the next paragraph for collisions of dR with protons. The charge-transfer cross sections have been calculated in the [120 eV−108 keV] laboratory energy range. Calculations have been performed in the two limit orientations, in the xy plane, corresponding to the mean plane of the five-membered ring of the dR molecule, and in the perpendicular orientation along the z axis. The corresponding charge-transfer cross sections are displayed in Table 1. The perpendicular orientation is clearly

Figure 3. Charge-transfer cross sections averaged over the different orientations for C4+ + biomolecule collision systems: green, thymine; red, uracil; blue, 5-fluorouracil; magenta, 5-chlorouracil; yellow, 5bromouracil; black, dR. Charge-transfer cross sections over the perpendicular (light blue, dashed line) and the planar (light blue, full line) orientations for the C4+ + dR collisions system.

Table 1. Charge-Transfer Cross Sections in Perpendicular and Planar Orientation for the C4+ + dR Collision System (in 10−16cm2) σz perpendicular

velocity v (au)

Elab (eV)

0.02 0.03 0.04 0.05 0.07 0.1 0.15 0.2 0.3 0.4 0.5 0.6

120 270 480 750 1.47 × 103 3 × 103 6.75 × 103 12 × 103 27 × 103 48 × 103 75 × 103 108 × 103

1.85 1.81 2.61 2.25 3.29 3.32 3.83 3.86 2.83 2.11 1.63 1.28

× × × × × × × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

σxy plan 2.88 4.74 8.65 9.98 1.20 1.05 1.17 1.46 1.92 2.14 1.97 1.70

× × × × × × × × × × × ×

10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

σ mean value 1.07 1.14 1.74 1.62 2.24 2.19 2.50 2.66 2.38 2.12 1.80 1.49

× × × × × × × × × × × ×

to the order of magnitude of the charge-transfer cross sections: thymine and uracil, which correspond to charge-transfer cross sections up to 10 × 10−16 cm2, the 5-halouracil targets with cross sections lower by about a factor of 102−103, and the 2deoxyribose, whose charge-transfer cross sections are shown to be ∼10−4 × 10−16 cm2, which is lower than the thymine ones by about a factor 105! This result can be analyzed by taking into account that the relative fragmentation yield for a molecular target after impact with a projectile ion can be assumed to be inversely proportional to the electron capture cross section.13 This point has been widely exhibited in our previous theoretical studies.15,24,27 Effectively, if one considers experimental fragmentation yield with regard to charge-transfer cross sections, an interesting qualitative correlation may be pointed out, as shown for example on Cq+ + uracil collisions.27 Experimentally, almost total fragmentation may be observed for the C2+ + uracil collision system at low energy, and fragmentation cross sections appear to decrease with increasing collision velocity to stabilize at around vcoll = 0.4 au;8 in correspondence, theoretical charge-transfer cross sections are very low for C2+ + uracil and increase with collision velocity to stabilize between vcoll = 0.3 and 0.4 au.27 The behavior appears quite different, for example, for the C4+ + uracil system. The

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

preferred for the charge-transfer process, however with always very weak cross section values, reaching at the maximum 3.86 × 10−4 × 10−16 cm2. Such values are significantly lower than the charge-transfer cross sections calculated for the series of pyrimidine nucleobases presented in Table 2. The mean value between perpendicular and planar orientations is compared in Figure 3 to the charge-transfer cross sections averaged over the different orientations for pyrimidine and halouracil targets. We observe clearly three groups with regard

Table 2. Charge-Transfer Cross Sections Averaged over the Different Orientations for a Series of C4+−Biomlecule Collision Systems (in 10−16 cm2) velocity v (au)

Elab (eV)

C4+ + thymine

C4+ + uracil

C4+ + fluorouracil

C4+ + chlorouracil

C4+ + bromouracil

0.02 0.03 0.04 0.05 0.07 0.1 0.2 0.3 0.4 0.5 0.6

120 270 480 750 1.47 × 103 3 × 103 12 × 103 27 × 103 48 × 103 75 × 103 108 × 103

8.68 9.01 10.66 11.62 13.53 14.93 13.24 12.12 10.16 11.89 11.33

6.45 7.74 7.63 6.67 6.28 5.79 5.04 3.97 3.56 3.28 2.85

0.020 0.029 0.025 0.028 0.032 0.031 0.024 0.018 0.013 0.010 0.007

0.072 0.056 0.045 0.051 0.052 0.047 0.038 0.025 0.017 0.012 0.009

0.009 0.008 0.009 0.011 0.012 0.013 0.007 0.004 0.003 0.002 0.002

D



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fragmentation process is shown experimentally to be significantly less efficient, with a relative yield almost constant with collision velocity; in correspondence, calculations provide quite higher charge-transfer cross sections, almost constant also with collision velocity. Of course, such correlations are absolutely qualitative, but they can provide a general tendency for related processes, which may be useful in order to extract general features relevant for radiation damage. In the present case, so low values for the charge-transfer cross sections in C4+ + dR collisions would suggest a complete disintegration of the dR molecule in ion-induced collisions. This is effectively what has been observed experimentally,13,21 whatever the projectile ion or the collision energy. Our present calculation confirms this point and gives the order of magnitude of the effect, showing how important it is in dR−nucleobase clusters. To extend this discussion to more general collisions of carbon ions with biomolecules, we can remark that the charge-transfer cross sections in C4+ + dR collisions are about of the same order of magnitude as those in collisions of C6+ used in hadrontherapy treatments with 5-bromouracil.25 Such a halouracil molecule has been shown for a long time to present high radio sensitization properties46,47 widely used in radiation therapy, and dR might thus be expected to present, more or less, the same sensitivity to radiation.

Figure 4. Adiabatic potential energy curves of the 1A states of the H+ + dR collision system in the perpendicular geometry for different active spaces: dashed lines, (4,4); full lines, (6,5); dotted lines, (8,6); red, {2pzO3 1sH}; green, {2pzO1 1sH}; blue,{2pxyO3 1sH}; black, {(2pzO3)2}, entry channel.

4. COLLISION OF PROTONS WITH DR The charge transfer induced by collisions of protons with isolated dR has also been investigated with regard to recent experiments at keV energies.13,14 Calculations have been performed as in the previous case at the CASSCF level of theory with the 6-311G** basis set, but different active spaces have been considered in order to take account of inner excitations and analyze their influence on the charge-transfer process. For that purpose, several active spaces have been considered, including the HOMO constructed mainly on the 2pz orbitals, perpendicular to the xy mean plan of the sugar ring, centered on oxygen O3 of the CH2OH group and the corresponding 2pxyO3 in-plan orbital, together with successively the 2pz and 2pxy orbitals on the oxygen O1 of the ring and on O4 and O2 oxygen atoms of the hydroxyl groups corresponding to deeper shells with, of course, the 1s orbital of the incident proton. Taking account of the closed-shell inner molecular orbitals, calculations have thus been performed successively with 4−8 active orbitals and 4−12 active electrons corresponding to (4,4), (6,5), (8,6), (10,7), and (12,8) active spaces by including inner excitations. The corresponding potential energy curves with 4−6 active orbitals in the perpendicular orientation are presented in Figure 4. A full calculation may be performed, and important avoided crossings are clearly exhibited between the entry channel {(2pzO3)2} and the successive molecular levels. The mechanism shows successive charge-transfer steps involving inner molecular levels, but as previously pointed out for the C4+ + dR system, no double excitation may be observed. The corresponding charge-transfer cross sections have been determined in the [123 eV−100 keV] laboratory energy range for each active space. The cross sections in the perpendicular orientation for the successive active spaces are presented in Figure 5. From the present results, it appears clearly that the consideration of the 2pzO1 orbital centered on the oxygen of the sugar ring is absolutely necessary in order to have a correct determination of the charge-transfer cross sections. Such a feature is in agreement with previous results on

Figure 5. Charge-transfer cross sections in the perpendicular orientation for C4+ + dR and H+ + dR collision systems: green, H+ + dR active space (4,4); red, H+ + dR active space (6,5); blue, H+ + dR active space (8,6); light blue, dashed lines, H+ + dR active space (12,8); black, C4+ + dR.

the C4+ + dR system, where a delocalization between 2pzO3 and 2pzO1 was observed. On the contrary, further inner orbitals do not significantly modify the charge-transfer cross sections, as shown in particular with five and six active orbitals. The calculation with eight active orbitals appears to give a somewhat higher charge-transfer cross section at higher energies, showing some influence of inner excitations; however, a limit appears to be reached by assuming the (6,5) active space to be adequate for the present calculation. The charge-transfer cross sections are significantly higher for the collision with protons than those previously determined with the C4+ projectile. This may also be observed when looking at the anisotropy of the process. The cross sections in perpendicular and planar orientations are given in Figure 6 and Tables 1 and 3, for both projectile ions. For both collision systems, the charge transfer is preferred in the perpendicular geometry; the effect is very important for the H+ + dR system for which clearly the cross sections are an E



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of the DNA damage. First of all, the charge-transfer cross sections are very low in collisions with dR, whatever the projectile ion considered. Almost complete disintegration of the dR is thus expected in nucleobases−dR clusters, and the sugar block appears particularly sensitive for radiation damage. Collision with carbon ions would lead even to a more efficient dissociation of dR than collisions with protons. With regard to the mechanism, both collision processes are favored in an orientation perpendicular to the sugar ring. Successive single charge transfer is observed, with excitation of an electron from the HOMO and delocalization toward the oygen of the fivemembered sugar ring.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 6. Comparison of the charge-transfer cross sections for C4+ + dR and H+ + dR collision systems: green, H+ + dR planar orientation; red, H+ + dR perpendicular orientation; blue, H+ + dR mean value; light blue, C4+ + dR planar orientation; magenta, C4+ + dR perpendicular orientation; black, C4+ + dR mean value.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M.F. Politis (LAMBE, Université d’Evry val d’Essonne, France) for fruitful discussions. We acknowledge support from the HPC resources of CCRT/CINES/IDRIS under the Allocation 2013-[i2013081566] made by GENCI [Grand Equipement National de Calcul Intensif] as well as from the COST actions MP1002 Nano-IBCT, CM0805 Chemical Cosmos and CM1204 XLIC.

Table 3. Charge-Transfer Cross Sections in Perpendicular and Planar Orientation for the H+ + dR Collision System (in 10−16cm2) velocity v (au)

Elab (eV)

0.07 0.08 0.09 0.1 0.15 0.2 0.3 0.4 0.5 0.6 0.7 1.0 1.5 2.0

123.4 161.1 203.9 251.8 566.5 1 × 103 2.26 × 103 4 × 103 6.3 × 103 9.1 × 103 12.34 × 103 25.18 × 103 56.65 × 103 100.7 × 103

σz perpendicular 0.116 0.103 0.092 0.084 0.057 0.041 0.023 0.015 0.010 7.47 × 5.64 × 2.88 × 1.31 × 7.43 ×

−3

10 10−3 10−3 10−3 10−4

σxy plan 3.14 3.15 3.01 3.05 2.58 2.50 5.56 5.51 4.66 3.90 3.28 2.02 1.04 6.17

× × × × × × × × × × × × × ×

■

σ mean value 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4

0.060 0.053 0.048 0.043 0.030 0.022 0.014 0.010 7.48 × 5.37 × 4.46 × 2.45 × 1.17 × 6.80 ×

REFERENCES

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10−3 10−3 10−3 10−3 10−3 10−4

order of magnitude lower in the planar orientation. If we consider the mean values, the charge-transfer cross sections appear globally 2 orders of magnitude lower for a collision with C4+ than those with a proton. Taking always into account the inverse ratio between fragmentation yield and electron capture cross sections, this would suggest that the fragmentation with C4+ ions is markedly efficient and a bit less with proton projectiles. In collisions with protons, the charge-transfer cross sections remain however very low, 3 orders of magnitude lower than those in the C4+ + thymine collision system, and a preferred disintegration of the dR is expected in nucleobase− dR clusters.

5. CONCLUDING REMARKS The charge transfer in the collision of C4+ carbon ions and protons with dR has been investigated in a wide collision energy range using ab initio molecular calculations. Some qualitative tendency may be exhibited from a compared analysis of these charge-transfer calculations with previous theoretical and experimental results, which could be useful in the analysis F



Article

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