Nanoscopic aspects of radiobiological damage: Fragmentation induced by secondary low‐energy electrons

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Abstract
I. Introduction 349 II. Experimental Methods 352 A. Thin‐Film Preparation 352 B. Electron‐Stimulated Desorption (ESD) of Ions and Neutral Species 353 C. Analysis by Electrophoresis 354 III. Interpretation of the Dependence of Ion and Neutral Yields on Incident Electron Energy 354 IV. Results and Discussion 356 A. Desorption of Ions and Neutral Species from Water‐Ice Films Induced by Low‐Energy Electrons (LEEs) 356 B. Anion ESD from Thin Films of Deoxyribose Analogs 358 C. Anion ESD from Thin Films of DNA Bases 359 D. Neutral‐Species Desorption from Short Single‐DNA Strands Induced by LEEs 360 E. Sequence‐Specific Damage Induced by LEE Impact on Oligonucleotides 362 F. Anion ESD from the Peptide and Disulfide Bridges of Proteins 364 G. LEE Damage to Plasmid DNA 365 V. Summary and Conclusions 366 Acknowledgments 367 References 367 Low‐energy electrons (LEEs) are produced in large quantities in any type of material irradiated by high‐energy particles. In biological media, these electrons can fragment molecules and lead to the formation of highly reactive radicals and ions. The results of recent experiments performed on biomolecular films bombarded with LEEs under ultra‐high vacuum conditions are reviewed in the present article. The major type of experiments, which measure fragments produced in such films as a function of incident electron energy (0.1–45 eV), are briefly described. Examples of the results obtained from DNA films are summarized along with those obtained from the fragmentation of elementary components of the DNA molecule (i.e., thin solid films of H2O, DNA bases, sugar analogs, and oligonucleotides) and proteins. By comparing the results of these different experiments, it is possible to determine fundamental mechanisms that are involved in the dissociation of biomolecules and the production of single‐ and double‐strand breaks in DNA, and to show that base damage is dependent on the nature of the bases and on their sequence context. Below 15 eV, electron resonances (i.e., the formation of transient anions) play a dominant role in the fragmentation of all biomolecules investigated. These transient anions fragment molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment. © 2003 Wiley Periodicals, Inc., Mass Spec Rev 21:349–369, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mas.10034