Nuclear Instruments and Methods in Physics Research B 235 (2005) 392–396 www.elsevier.com/locate/nimb
Fragmentation of adenine induced by collision with slow F2+ions R. Bre´dy
a,*
, J. Bernard a, L. Chen a, B.Wei a,b, A. Salmoun c, T. Bouchama d, M.C. Buchet-Poulizac a, S. Martin a
a
d
Laboratoire de Spectrome´trie Ionique et Mole´culaire (UMR CNRS 5579), Universite´ Claude Bernard Lyon1, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne CEDEX, France b Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China c Universite´ Cadi Ayyad, Faculte´ des Sciences, Semlalia BP 2390 Marrakech, Morocco Laboratoire de Spectroscopie Laser, Faculte´ de Physique USTHB, El-Alia BP 32, Bab-Ezzouar 16111 Alger, Algeria Available online 4 May 2005
Abstract We present a study of ionization and fragmentation of gaseous DNA base adenine induced by collision with 36 keV F2+ ions. Coincidence measurements between the outgoing projectiles F+, the recoil ions and the number of ejected electrons during the interaction have been performed. Charged adenine and its fragments mass distribution have been analyzed by time of flight spectrometry. The time of flight spectrum presents a peak of stable mono-charged adenine as well as the signature of an important fragmentation in mostly light charged fragments. Results have been interpreted as a function of the number of ejected electrons, which is related to the initial charge state of adenine before fragmentation. The coincidence spectrum between fragments provides information on some particular dissociation channels. Ó 2005 Elsevier B.V. All rights reserved. PACS: 87.14.g; 87.14.Gg; 34.70.+e; 34.50.Gb; 82.30.Fi Keywords: Biomolecules; DNA base; Fragmentation; Collision; Electron; Highly charged ion
1. Introduction Interaction of ionizing radiations and particles with biological systems can create important dam*
Corresponding author. Tel.: +33 4 72 44 81 81; fax: +33 4 72 43 15 63. E-mail address:
[email protected] (R. Bre´dy).
ages or modifications to the cellular DNA and lead, as an example, to a higher cancer risk. However, the same effects can be used in a positive way in radiotherapy. Thus, it is important to understand the ionization mechanisms as well as the energy deposition at the molecular level. Several studies involving low energy electrons or protons colliding with DNA or RNA bases [1–5] have
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.212
R. Bre´dy et al. / Nucl. Instr. and Meth. in Phys. Res. B 235 (2005) 392–396
shown that the dissociation of the molecule can occur well below the ionization energy threshold. There are also few studies on fragmentation of biomolecular target [6] or DNA bases [7] induced by slow highly charged ions. It is now well established that, due to the large amount of potential energy carrying by highly charged ions, collisions with such projectile are an efficient way to produce charged target with low excitation energy. In the following, we discuss results on the ionization and fragmentation of gaseous DNA base adenine (C5N5H5) induced by collision with F2+ ions at 36 keV. The adenine molecule is based on a hexagonal and pentagonal structure composed of C and N atoms with a NH2 molecule and H atoms attached to it. The existence of several tautomeric forms of adenine is not discussed in this paper.
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by time of flight spectroscopy and detected by two multi-channel plates followed by a multianode composed of 121 pixels. The signals delivered by the multianode detector are sent to four multi-hit time-to-digital converter (TDC 3377 Lecroy). For singly charged ions like Cþ m ðm ¼ 1–11Þ the detection efficiency has been estimated to be 75%. Multi-coincidence measurements between a charge selected projectile, the electrons and the recoil ions are performed in event-by-event mode data acquisition. For fragmentation processes the initial charge of the adenine parent (C5N5H5)r+ is determined using the electron number conservation law r = n + s where s refers to the number of electrons stabilized by the outgoing projectile (e.g. s = 1 for F2+ ! F+ collisions).
3. Results and discussion 2. Experimental setup The experimental setup has been described elsewhere [8] and only the main features are presented for the purpose of this paper. Briefly, the 19F2+ ion beam (36 keV) is delivered by a 10 GHz electron cyclotron resonance ion source (Nanogan III: an upgrade of Nanogan). After collimation the ion beam enters the interaction chamber through a 500 lm diameter hole and crosses an effusive jet of gaseous adenine created by evaporation of an adenine powder pure at 99% (from Aldrich-chemie) in an oven at 130 °C. This temperature ensures sufficient target density without thermal fragmentation of adenine. In these conditions, the background vacuum inside the chamber is below 108 mbar. After the interaction, the outgoing projectile is charge selected by an electrostatic analyzer and detected by a channeltron electron multiplier with an efficiency of about 100% in the energy range considered in this work. The ejected electrons and charged adenine or fragments are extracted from the interaction region by a 250 V/cm electric field. After extraction, the electrons are accelerated at 17 keV toward a semi-conductor detector (PIPS). The signal delivered by the PIPS is proportional to the energy deposition inside the detector and thus by the number of electrons that hit the detector. The recoil ions are analyzed
Only collisions in which one electron has been stabilized by the projectile are considered in this work: F2+ + C5N5H5 ! F+ + (C5N5H5)r+ + ne (n = r 1). The projectile-electron-recoil ions coincidences spectrum (EL-RI) (Fig. 1(a)) is obtained by plotting, for each detected projectile F+, the number of ejected electrons as a function of the recoil ions time of flight (TOF). A projection of the EL-RI spectrum along the Y- and X-axis gives, respectively, the TOF (Fig. 1(b)) and the ejected electron number (Fig. 1(c)) spectra. If several fragments were detected for a same event only the heaviest one, i.e. the fragment with the longest TOF, was taken into account. Moreover, this is important to note that the background signal of the PIPS is used to generate a signal even if there is no electron ejected during the interaction but when a coincidence is detected between a projectile and a recoil ion. This is the origin of a peak at zero electron that allows us to take into account events where no electrons are ejected. The TOF spectrum can easily be understood as several groups of peaks Gi representing charged fragments (CxNyHz) that are composed of i = x + y carbon and nitrogen atoms (1 6 i 6 10) with a various number z of H atoms. The TOF spectrum presents an important peak of stable mono-charged adenine (i = 10) as well as the signature of fragmentation
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Fig. 1. Projectile-electron-recoil ions coincidences spectrum (EL-RI) with its projections for F2+ (36 keV) – adenine collisions. Only events leading to outgoing F+ projectiles are selected. (a) EL-RI spectrum, the number of ejected electrons (n) during the interaction is plotted as a function of the TOF; (b) recoil ion time of flight (TOF); (c) number of ejected electrons spectrum.
in mostly light mono-charged fragments with less than 5 heavy atoms (i < 5). Due to the adenine structure a large number of fragments with the same ratio ‘‘mass over charge’’ exists, therefore some of the peaks can not be attributed to a single element. For fragmentation, the maximum intensity is obtained for CNHþ 2 fragment. For groups Gi with more than four heavy atoms (i > 4), the total intensity decreases with increasing number of heavy atoms except for the group G8. Doubly charged adenine (C5N5H5)2+ is also observed. This is confirmed by the electron-recoil ions coincidence where (C5N5H5)2+ is in coincidence with 1 ejected electron. The relatively small intensity of the doubly charged adenine peak and the large number of fragments imply a weak stability of the molecule. From the spectra of Fig. 1, the relative yield Rf for fragmentation of the adenine molecule has been measured (Rf 0.51). Rf is defined as Rf = Yf/Ytotal with Yf the sum of the peak integral over all the fragments and Ytotal the sum of all the peak integrals. The partial TOF spectra associated to n = 0, 1, 2 and 3 ejected electrons, i.e. r = 1, 2, 3 and 4, respectively, are shown in Fig. 2. For single electron capture (n = 0) the dominant line on the
TOF spectrum corresponds to singly ionized adenine. As n increases, the initial charge of adenine increases and the TOF spectrum is globally shifted towards lighter fragments. This is explained by the fact that collisions with a larger number of ejected electrons occur mainly at small impact parameters. Thus, more energy is deposited into the target leading to a fragmentation into smaller pieces. This also indicates that lighter fragments are more likely to carry out a charge. The relative yield Rrf for fragmentation of a charged adenine parent (C5N5H5)r+ has been measured. Rrf ¼ Y rf =Y f is defined as the sum of the peak integral Y rf of fragments that are detected with n = (r 1) ejected electron divided by the sum Yf. We obtain Rrf ¼ 0.52; 0.35; 0.09 and 0.03 for r = 1, 2, 3 and r P 4, respectively. This indicates that most of the fragments are related to a singly charged parent (C5N5H5)+. For a particular fragment, partial projection of the EL-RI spectrum along the X-axis provides the distribution of ejected electrons associated to this fragment. These electron distributions show evident structure effects in adenine fragmentation. Results will be presented in a forthcoming paper.
R. Bre´dy et al. / Nucl. Instr. and Meth. in Phys. Res. B 235 (2005) 392–396
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Fig. 3. TOF correlation spectrum for fragments (RI–RI). Horizontal axis: TOF of the heavy fragment; vertical axis: TOF of the correlated light fragments.
Fig. 2. TOF spectra associated to a given number n of ejected electrons.
In order to have a better understanding of the fragmentation processes, correlation between fragments has been obtained. When at least two charged fragments have been detected, the TOF of the light fragments is plotted as a function of the TOF of the heavy fragment. The correlation spectrum between fragments (RI–RI) is presented in Fig. 3. This spectrum implies that the adenine molecule has been left in a charge state r P 2 after the interaction. However, it can be calculated from Fig. 2 that most of the correlation (75%) comes from doubly charged adenine. It appears from Fig. 3 that many combinations between fragments are possible, each of these combinations appearing as a spot in the correlation spectrum. Most of the fragments are also correlated with H+ (not shown on Fig. 3). The more intense spots correspond to correlation between fragments of groups G2–Gi (i = 2–6). One noticeable spot is the correlation between the G6 group and the G2 group. Beside this
spot (G6–G2) no other correlation is observed with the G6 group. The spot is attributed to the fragmentation of adenine in the Diazine molecule (C4N2H4)+ with a (CNH2)+ fragment and the emission of a neutral N2. The extra proton in the total mass of the parent indicates that the adenine jet is not a pure (C5N5H5) jet but that extra protons are attached to it. The G3, G4 and G5 groups are mainly correlated to molecules of the G2 and G3 groups as well as singly charged atoms. Still, the dominant light fragment is composed of two heavy atoms (i = 2), the fragmentation is probably accompanied by emission of neutral fragments. In order to reduce the amount of energy deposited to the adenine, future experiments with highly charged ions of charge q+ at lower energy are planned. One can expect to observe heavier fragments as well as different fragmentation patterns by successively selecting the outgoing (q s)+ charged projectile (s = 1, 2, 3 . . . ). Indeed, it is well known that large impact parameter collisions with highly charged ions gently remove electrons from the target. For the study of these fragmentation processes, it is necessary to know the excitation energy of the adenine. In our future experiments, the excitation energy will be measured accurately by analyzing the energy loss of the projectile.
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4. Conclusion
References
Preliminary results have been presented for ionization and fragmentation of adenine induced by electron capture in collisions with slow F2+ ions. In this experiment, the recoil ions and the number of ejected electrons are detected in coincidence with F+ outgoing projectiles. The recoil ions time-offlight spectrum shows important fragmentation in mainly light fragments. The TOF spectra associated to a given number of ejected electrons indicate that the energy deposition on the adenine molecule can be important, leading to smaller fragments.
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Acknowledgement This work has been supported by the Re´gion Rhoˆne-Alpes under Grant No. 97027–223 of the Convention Recherche, Program Emergence.