Comparisons between\nthe Direct and Indirect Effect\nof 1.5 keV X‑rays and 0–30 eV Electrons on DNA: Base\nLesions, Stand Breaks, Cross-Links, and Cluster Damages

The interaction of low energy electrons (LEEs; 1–30\neV)\nwith genomic material can induce multiple types of damage that may\ncause the loss of genetic information, mutations, genome instability,\nand cell death. For all damages measurable by electrophoresis, we\nprovide the first complete set of <i>G</i>-values (yield\nof a specific product per energy deposited) induced in plasmid DNA\nby the direct and indirect effects of LEEs (<i>G</i><sub>LEE</sub>) and 1.5 keV X-rays (<i>G</i><sub>X</sub>) under\nidentical conditions. Low energy photoelectrons are produced via X-rays\nincident on a tantalum (Ta) substrate covered with DNA and placed\nin a chamber filled with nitrogen at atmospheric pressure, under four\ndifferent humidity levels, ranging from dry conditions to full hydration\n(Γ = 2.5 to Γ = 33, where Γ is the number of water\nmolecules/nucleotide). Damage yields are measured as a function of\nX-ray fluence and humidity. <i>G</i><sub>LEE</sub> values\nare between 2 and 27 times larger than those for X-rays. At Γ\n= 2.5 and 33, <i>G</i><sub>LEE</sub> values for double strand\nbreaks are 27 and 16 times larger than <i>G</i><sub>X</sub>, respectively. The indirect effect contributes ∼50% to the\ntotal damage. These <i>G</i>-values allow quantification\nof potentially lethal lesions composed of strand breaks and/or base\ndamages in the presence of varying amounts of water, i.e., closer\nto cellular conditions.