DNA Damage DOI: 10.1002/anie.200600303 Selective Excision of C5 from d-Ribose in the Gas Phase by Low-Energy Electrons (0–1 eV): Impli- cations for the Mechanism of DNA Damage** IlkoBald,JaninaKopyra,andEugenIllenberger* Sugar is the central unit within a nucleotide connecting the DNA base with the phosphate group, which itself couples to the neighboring nucleotides within single-stranded DNA. The study of the excitation, ionization, and fragmentation of biomolecular systems is essential for the understanding of many problems in the area of life sciences such as the mechanism of radiation damage in cellular systems or the action of radiosensitisers used in tumor therapy. The passage of high-energy radiation through dense media such as water or a living cell leaves a trace of free electrons. These secondary electrons are created in numbers (510 4 per MeV of deposited energy [1] ) that makes them the most abundant radiolytic species. In the course of thermal- ization they can induce further ionization or excitation processes, but they can also efficiently attach at specific energies (resonances) and sites to DNA, forming transient negative ions that subsequently dissociate (dissociative elec- tron attachment, DEA). [2] Ample evidence exists that DEA with its unique features plays an important role in the nascent states of cellular DNA radiolysis. [2] To date, these phenomena have been investigated at two extremes of DNA complexity, namely, plasmid DNA and isolated nucleobases in the gas phase. Experiments on plasmid DNA have demonstrated that low-energy electrons can efficiently induce single-strand breaks (SSBs), as well as double-strand breaks (DSBs). [3] In the very low-energy domain (0–3 eV), below the threshold of electronic excita- tion, only SSBs are observed. [4] In these experiments it became apparent that the efficiency of both DSBs and SSBs as a function of the primary electron energy exhibits a resonant behavior, indicating that the formation of negative- ion resonances is the initial step. Studies on isolate nucleobases (NBs) in the gas phase [5–11] have demonstrated that they undergo DEA in the range of roughly 6–9 eV and also at much lower energies (< 3 eV) where SSBs are observed. [5] While the high-energy feature leads to loss of H  and further fragment ions associated with the rupture of the NB ring structure, [5–7] the low-energy resonance exclusively leads to the loss of neutral hydrogen with the excess charge remaining on the nucleobase. In a recent theoretical study [12] modeling a section of DNA composed of cytosine, sugar, and the phosphate group, an interesting mechanism for electron-initiated strand breaks was proposed. The calculations predict a low-lying anionic potential energy surface that connects the initial p* anion state of the base to a s* state in the backbone. An electron captured by a DNA base may thereby be transferred to the backbone, leading to rupture of the C O bond between the phosphate and the sugar. On the other hand, very recent experiments on thymidine (thymine coupled to sugar) [13] indicate that such an electron transfer is not operative; instead it appears that sugar moiety itself has a pronounced ability to capture low-energy electrons with subsequent fragmentation. For the detailed investigation of the response of sugar following electron attachment we use d-ribose (C 5 H 10 O 5 ) and some isotopically labeled analogues (1- 13 C, 5- 13 C, C,1-D). For simplicity we will use the term ribose for d- ribose throughout this manuscript. A previous study by the Innsbruck Laboratory on deoxyribose (C 5 H 10 O 4 ) revealed that electron capture at energies already close to 0 eV induces a variety of fragmen- tation reactions. [14] As we shall demonstrate, isotopic labeling enables us to identify the underlying decomposition process and to specify the site of the target molecule involved. This provides essential information for the molecular process of DNA damage by low-energy electrons. The experiments were carried out in a crossed electron molecular beam arrangement consisting of an electron source, an oven, and a quadrupole mass analyzer (QMA). [15] The components were housed in a ultrahigh-vacuum chamber at a base pressure of 10 8 mbar. A well-defined electron beam generated from a trochoidal electron monochromator [16] (resolution 90–120 meV fwhm) intersected orthogonally with an effusive molecular beam consisting of ribose mole- cules. They emanated from a resistively heated oven directly connected to the reaction chamber by a capillary. At a temperature of about 370 K (measured by a platinum resistance) the density of intact ribose molecules was high enough to yield a reasonable negative-ion signal. The generated anions were extracted by a small electric field towards the entrance of the QMA where they were analyzed and detected by a single-pulse counting technique. The energy scale was calibrated using the well-known resonance in SF 6 near 0 eV generating metastable SF 6  . To prevent ion– molecule reactions involving SF 6  ions, the flow of the calibration gas was switched off prior to each measurement. Ribose and the 5- 13 C analogue were obtained from Sigma Aldrich (stated purity 98 and 99%, respectively), [1- 13 C]ribose and [C,1-D]ribose were obtained from Cam- bridge Isotope Laboratories, Inc. (stated purity 99 and 98%, respectively). All samples were used as delivered. [*] Dipl.-Chem. I. Bald, Dr. J. Kopyra, [+] Prof. Dr. E. Illenberger Institut für Chemie und Biochemie Physikalische und Theoretische Chemie Freie Universität Berlin Takustrasse 3, 14195 Berlin (Germany) Fax: (+ 49)30-838-55378 E-mail: iln@chemie.fu-berlin.de [ + ] Permanent address: Chemistry Department, University of Podlasie 08-110 Siedlce (Poland) [**] This research was supported by the Deutsche Forschungsgemein- schaft, the EU via the Network EPIC, and the Freie Universität Berlin. I.B. is a fellow of the Studienstiftung des Deutschen Volkes, and J.K. acknowledges support from the EIPAM program of the European Science Foundation. Angewandte Chemie 1 Angew. Chem. Int. Ed. 2006, 45,1–6 # 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü