Potassium-Uracil/Thymine Ring Cleavage Enhancement As Studied in Electron Transfer Experiments and Theoretical Calculations D. Almeida, † M.-C. Bacchus-Montabonel,* ,‡ F. Ferreira da Silva, † G. García, §,∥ and P. Lima ̃ o-Vieira* ,†,⊥ † Laborató rio de Colisõ es Ató micas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciê ncias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ‡ Institut Lumiè re Matie ̀ re, UMR5306 Universite ́ Lyon 1-CNRS, Universite ́ de Lyon, 69622 Villeurbanne Cedex, France § Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 113-bis, 28006 Madrid, Spain ∥ Centre of Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia ⊥ Centre of Earth, Planetary, Space and Astronomy Research, Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. ABSTRACT: We report experimental and theoretical studies on ring cleavage enhancement in collisions of potassium atoms with uracil/thymine to further increase the understanding of the complex mechanisms yielding such fragmentation pathways. In these electron transfer processes time-of-flight (TOF) negative ion mass spectra were obtained in the collision energy range 13.5−23.0 eV. We note that CNO − is the major ring breaking anion formed and its threshold formation is discussed within the collision energy range studied. Such a decomposition process is supported by the first theoretical calculations to clarify how DNA/RNA pyrimidine base fragmentation is enhanced in electron transfer processes yielding ion-pair formation. 1. INTRODUCTION Radiation-induced damage to biologically relevant molecular systems has recently come under great scrutiny by the international scientific community due to recent findings highlighting the lethality of low-energy electrons as a DNA/ RNA damaging agent. 1 However, much still remains to be unveiled regarding the exact molecular mechanisms that occur in the nascent stages of DNA/RNA damage by the incident radiation, in particular the role of third bodies in electron capture processes by the double-helix molecular constituents. Indeed, in such radiation-induced processes, secondary electrons are produced from the ionization events as well as from their release in the physiological medium from solvated and/or presolvated states. 2,3 Studying the damaging role of “bound” electrons to DNA/RNA may provide another route to better understand the nature of such processes that seem to be more attuned to the physiological environment than simple free electrons. In particular, studying electron transfer processes from donating atoms with biologically relevant molecules can provide valuable information on the role of such “bound” electrons. 4−8 A quite extensive set of theoretical 2,7,9−14 and experimen- tal 7,9−11,14−21 dissociative electron attachment (DEA) studies to pyrimidine nucleobases have gradually been published over the past few years. In the particular case of uracil/thymine, these DEA studies report the dehydrogenated parent anion formation as the dominant fragmentation pathway 2,9,22−24 which, together with theoretical calculations, has been extensively studied and reported to derive exclusively from hydrogen abstraction from the N1 and N3 sites. 9,15,18,20 However, for potassium collisions with such molecular targets, the results are significantly different. Several studies on the negative ion formation in potassium−molecule collisions have been performed 5,7,11,25−27 and, although the dehydrogenated parent anion is also one of the main fragments, the most intense fragment anion has been assigned to CNO − , which necessarily requires ring breaking and, hence, access to anionic states very different from those that result in dehydrogenation from the N-sites. 15,25 In particular, a recent study on the metastable decay of methylated pyrimidine derivative (tempo- rary negative) ions resulting in CNO − formation upon electron capture/transfer, have shown the relevance of hydrogen abstraction (from either N1 or N3) 11 in the site and bond selectivity decomposition mechanism. However, from a literature survey we note that theoretical descriptions of intramolecular decomposition processes are scarce or even absent within the context of electron transfer. As such, there has been an increased need to perform more comprehensive and detailed studies on how some of the most dominant fragments, apart from the dehydrogenated parent anion, are formed. Furthermore, the present electron transfer studies show that other fragments, as is the case of CNO − , are more relevant within the pyrimidine’s decomposition (than the Special Issue: Franco Gianturco Festschrift Received: March 31, 2014 Revised: May 12, 2014 Published: May 12, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 6547 dx.doi.org/10.1021/jp503164a | J. Phys. Chem. A 2014, 118, 6547−6552