| Abstract
| - Both monolayer and thick (20 μm) films of dry pGEM-3Zf(−) plasmid DNA deposited on tantalum foilwere exposed to Al Kα X-rays (1.5 keV) for various times in an ultrahigh vacuum chamber. For monolayerDNA, the damage was induced mainly by low energy secondary electrons (SEs) emitted from the tantalum.For the thick films, DNA damage was induced chiefly by X-ray photons. Different forms of plasmid DNAwere separated and quantified by agarose gel electrophoresis. The exposure curves for the formation of nickedcircular (single strand break, SSB), linear (double strand break, DSB), and interduplex cross-link forms 1 and2 were obtained for both monolayer and thick films of DNA, respectively. The lower limits of G values forSSB and DSB induced by SEs were derived to be 86 ± 2 and 8 ± 2 nmol J-1, respectively. These values are1.5 and 1.6 times larger than those obtained with 1.5 keV photons. The projected X-ray energy dependenceof the low energy electron (LEE) enhancement factor for the SSB and DSB in monolayer DNA is also discussed.This new method of investigation of the SE-induced damage to large biomolecules allows direct comparisonof the yield of products induced by high energy photons and LEEs under identical experimental conditions.
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