Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons

Abstract

We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10(-15)- 10(-14) cm2, giving an effective cross section of the order of 10(-18) cm2 per nucleotide. Their strong variation with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of pi* shape resonances in the bond-breaking process. Furthermore, the fact that the energy threshold for SSB production is practically zero implies that the sensitivity of DNA to electron impact is universal and is not limited to any particular energy range.

Absolute cross section for loss of supercoiled topology induced by 10 eV electrons in highly uniform /DNA/1,3-diaminopropane films deposited on highly ordered pyrolitic graphite

“…The separation of the recovered DNA into the undamaged super-coiled (SC), nicked circular (C), and linear (L) structural forms was performed by agarose gel electrophoresis. 24,26 The DNA samples and the agarose gels were stained with SYBR Green I in concentrations of ×100 and ×10 000, respectively. The samples were passed on 1% agarose gel Tris-acetic acid-EDTA (TAE) buffer at 100 V for 7 min, then at 75 V for 68 min (5 V cm −1 ).…”

“…22 While measurement of LEE induced damage under such conditions is possible (e.g., Refs. [23][24][25][26] and has even allowed measurement of more transferable cross sections, 22 by varying film thicknesses, there exists a need for absolute cross sections for DNA damage induced by LEEs. Such values can be derived from experiments on uniform plasmid DNA films, which can be recovered from their substrate for post-irradiation analysis.…”

Stable Valence Anions of Nucleic Acid Bases and DNA Strand Breaks Induced by Low Energy Electrons

“…below 5 eV), shape resonances localized at nucleobases are suspected to be responsible for the observed strand damage [18,22]. Due to the development of more sensitive techniques to assay SSB and DSB in DNA, the 0-4 eV range of incident electrons energies were studied by Martin and coworkers [18,22] (Figure 21-4). This experimental picture could be reproduced by a model that simulates the electron capture cross-section as it might appear in DNA owing to the * anion states of the bases.…”

Interaction of low-energy electrons with the purine bases, nucleosides, and nucleotides of DNA

“…The observation that slow electrons cause damage to DNA, including single-and double-strand breaks, [1][2][3][4][5] has stimulated considerable interest in gas-and condensed-phase studies of low-energy electron interactions with the constituents of DNA and RNA. 6 Peaks in the damage rate as a function of electron energy indicate that resonant processes are involved.…”