Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session C1: Interaction of Slow Electrons with Biomolecules |
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Chair: Ilya Fabrikant, University of Nebraska Room: Burnham Yates Conference Center Ballroom I |
Wednesday, May 18, 2005 1:30PM - 2:06PM |
C1.00001: Temporary Negative Ions and Bond-breaking in Bio-molecules Invited Speaker: Strand-breaks in DNA are known to be induced by electron impact through the dissociative electron attachment (DEA) process. These discoveries have stimulated gas phase studies of electron collisions with the components of DNA as well as other bio-molecules such as the amino acids. We describe the various types of resonances possessed by the DNA/RNA bases, focusing on the role played by vibrational Feshbach resonances (VFRs) produced by the supercritical electric dipole moments of these molecules. We have proposed that the sharp structure in the DEA cross sections arises from mixing of VFRs with temporary valence anion states of $\Sigma $ symmetry, and we offer theoretical support for this picture. Finally, we discuss the current status of measurements of absolute DEA cross sections in these compounds. [Preview Abstract] |
Wednesday, May 18, 2005 2:06PM - 2:42PM |
C1.00002: Destruction of uracil and thymine at subexcitation energies Invited Speaker: Free electron attachment to gas phase uracil (U) and thymine (T) leads exclusively to the formation of fragment anions. In the electron energy range between 1 and 3 eV the attachment cross section of the most dominant products (U-H)$^{-}$ and (T-H)$^{-}$ reveals several narrow resonances [1]. By using partially deuterated T with deuterium connected to all carbon atoms it is possible to show that all these resonances originate from the abstraction of hydrogen from the two nitrogen sites [2]. However, in DNA the hydrogen atom where T is connected to the sugar is missing and the other H atom is part of a hydrogen bridge to adenine. Attachment cross sections for the H abstraction from thymidine and 1-methyl-thymine show a single asymmetric resonance at about 2 eV and enable us to distinguish between the two nitrogen positions. For the formation of H$^{-}$ from U and T the attachment cross section shows several resonances in the energy range between 5 and 12 eV. Experiments with partly deuterated T show that the different peaks in the H$^{-}$ ion yield can unambiguously be associated to abstraction from the different molecular sites [3]. The energy dependence for H$^{-}$ abstraction from the carbon sites shows a remarkable resemblance to the energy dependence of strand breaks observed in plasmid DNA [4] suggesting that this reaction may be an important initial step towards strand breaks. In collaboration with Sylwia Ptasinska, Stephan Denifl, Stefan Feil, Manuel Winkler, Barbara Mroz, Michael Probst, Stefan Matejcik, Department of Plasma Physics, Bratislava; Eugen Illenberger, Institute of Chemistry, FU-Berlin; Tilmann M\"ark, and the Center of Molecular Biosciences Innsbruck Team. \newline \newline [1] S. Denifl et al., J. Phys. Chem. A 108 (2004) 6562 \newline [2] H. Abdoul-Carime et al., Phys. Rev. Lett. 92 (2004) 168103 \newline [3] S. Ptasi\~nska et al., Angew. Chem. Int. Ed. (2005) in print \newline [4] B. Boudaiffa et al., Science 278 (2000) 1658 [Preview Abstract] |
Wednesday, May 18, 2005 2:42PM - 3:18PM |
C1.00003: Low energy electron induced fragmentation and reactions of DNA and its molecular components Invited Speaker: Much research has been stimulated by the recognition that ionizing radiation can, in condensed matter, generate large numbers of secondary electrons with energies less than 20 eV [1] and by the experimental demonstration that such electrons may induce both single and double strand breaks in plasmid DNA [2]. Identifying the underlying mechanisms involves several research methodologies, from further experiments with DNA to studies of the electron interaction with the component `sub-units' of DNA in both the gas and condensed phases [3]. In particular, understanding electron-induced \underline {strand break} damage, the type of damage most difficult for organisms to repair, necessitates study of the sub-units of DNA back-bone, and here Tetrahyrofuran (THF) and its derivatives, provide a useful model for the furyl ring at the centre of the deoxyribose sugar. In this contribution, we review with particular reference to DNA and related molecules, the use of electron spectroscopy and mass spectrometry to study electron-induced fragmentation and reactions in thin molecular solids. We describe a newly completed instrument that combines laser post-ionization with a~time-of-flight mass analyzer~for~highly sensitive ion and neutral detection. Use of the instrument is illustrated with results for THF and derivatives. Anion desorption measurements reveal the role of transient negative ions (TNI) and Dissociative Electron Attachment in significant molecular fragmentation and permit effective cross sections for this electron-induced damage to be obtained. The neutral yield functions also illustrate the importance of TNI, mirroring features seen in recently measured cross sections for electron induced aldehyde production in THF [4]. \newline \newline 1. J. A. Laverne and S. M. Pimblott, Radiat. Res. \textbf{141}, 208 (1995) \newline 2.~B. Boudaiffa, et al, Science \textbf{287}, 1658 (2000) \newline 3.~L. Sanche. Physica Scripta. \textbf{68}, C108, (2003) \newline 4.~S.-P. Breton, et al.,J. Chem. Phys.\textbf{ 121}, 11240 (2004) [Preview Abstract] |
Wednesday, May 18, 2005 3:18PM - 3:54PM |
C1.00004: Theory of Damage to DNA by Low-energy Electrons Invited Speaker: Stimulated by the observations of Boudaiffa et al that low-energy electrons can induce strand breaks in DNA (probably via core-excited resonance states) and knowing of the work of Burrow et al on resonances in DNA bases, we explored the possibility that even lower-energy shape resonances could cause such damage. In a series of publications, we employed stabilization-type electronic structure methods and potential energy landscape exploration techniques to address this question. The systems we examined included small fragments of DNA each of which included at least one base, one sugar, and two phosphate units. We found that electron attachment to form a low-lying $\pi $* shape resonance of a base unit could cause a sugar-phosphate C-O bond to be rendered susceptible to fragmentation over barriers ranging from 5 to 25 kcal mol$^{-1}$ but only when the phosphate group has a counter cation nearby. The rates of C-O bond rupture can be as high as 10$^{10 }$s$^{-1}$ which, considering the ca. 10$^{14 }$s$^{-1}$ detachment rates of the $\pi $* resonance, suggest a yield of one strand break in 10$^{4}$ attached electrons. [Preview Abstract] |
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