Eur. Phys. J. D 35, 391–398 (2005) DOI: 10.1140/epjd/e2005-00205-7 T HE EUROPEAN P HYSICAL JOURNAL D Positive and negative ion formation via slow electron collisions with 5-bromouridine S. Denifl, P. Candori a , S. Ptasi´ nska, P. Lim˜ ao-Vieira b , V. Grill, T.D. M¨ ark c , and P. Scheier d Institut f¨ ur Ionenphysik, Center for Molecular Biosciences, Leopold Franzens Universit¨at, Technikerstr. 25, 6020 Innsbruck, Austria Received 5 April 2005 / Received in final form 27 May 2005 Published online 26 July 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. Cation and anion formation is studied experimentally upon inelastic low energy electron in- teractions with 5-bromouridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are deter- mined using a non-linear least square fitting procedure. The anion yields are investigated in the electron energy range from about 0 to 14 eV. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced. PACS. 34.80.Ht Dissociation and dissociative attachment by electron impact – 87.14.Gg DNA, RNA – 87.50.-a Effects of radiation and external fields on biomolecules, cells and higher organisms 1 Introduction Halouracils can substitute thymine in the genetic sequence of cellular DNA without changing the normal gene ex- pression in non irradiated cells [1]. Thereby, the sensitiv- ity of living cells to X-rays is increased by a factor of four [2]. Radiation therapy following the incorporation of 5-halouridines (a halouracil attached to the sugar moi- ety, such as 5-bromouridine), acting as radio-sensitizers, has been suggested as a promising means in cancer treat- ment [3,4]. However, the molecular processes by which such radio sensitizers operate have not been unraveled so far. In general the damage to the living genome can be characterized as direct or indirect processes [5]. Di- rect damage occurs by immediate energy deposition to the DNA or RNA and its closely bound water molecules of the irradiated cells. Indirect damage is ascribed to the highly reactive hydroxyl radicals formed by the interac- tion of the primary radiation with the aqueous surround- ing. Two third of the damage to the genome is expected to be indirect damage and one third direct [5]. Since the discovery that low energy electrons [6,7] and photons [8] with energies as low as 7 eV can effi- a Permanent address: Department of Chemistry, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy. b Permanent address: Department of Physics, New Univer- sity of Lisbon, Quinta da Torre, 2829-516 Caparica, Portugal. c Adjunct Professor at: Department of Plasma Physics, Comenius University, 84248 Bratislava, Slovakia. d e-mail: paul.scheier@uibk.ac.at ciently induce single and double strand breaks in plas- mid DNA the contribution of especially low energy elec- trons for DNA damage has been considered to be par- ticularly significant. The number of low energy (<20 eV) secondary electrons per 1 MeV deposited primary quan- tum is 10 4 [9]. Within the last years theoretical and exper- imental studies were performed to investigate the specific action of low energy electrons with isolated and condensed biomolecules including DNA/RNA-bases [10–13], amino acids [14,15], sugar [16] and nucleosides [17]. Moreover, isolated gas phase halogenated bases were also investi- gated extensively [18–25] with respect to the interaction with low energy electrons. The most studied halouracils are 5-chlorouracil and 5-bromouracil where the hydro- gen at the C5 position is replaced by a halogen atom (Cl and Br, respectively). The electron interaction with 5-bromouracil (BrUra) was studied for the isolated gas phase molecule [21–24], in the condensed phase [26] and even for BrUra paired with adenine [27]. In contrast to isolated nucleobases which possess a negative vertical elec- tron affinity, for halogenated nucleobases a stable par- ent anion was observed in the gas phase. This result is consistent with the positive electron affinity reported for halouracils [25]. Compared with non-halogenated pyrimi- dine bases the cross-sections for dissociative electron at- tachment (DEA) of the halogenated molecules were more than two orders of magnitude higher [18,22,23]. However, for 5- and 6-chlorouracil the positive ion mass spectra re- vealed just the opposite tendency, i.e., higher yield of frag- ment ions in the case of the non halogenated species [19].