For decades considerable efforts have been devoted to study the interaction of low-energy electrons (LEE) with biomolecules, particularly DNA. LEE were found to temporarily localize on specific bonds, thus forming transient negative ions (TNI), which decay into destructive channels and may thus inflict considerable damage to the genome. However, the exact nature of the mechanism, the TNI involved and their decay channels, remain unclear. In the present experiments, the energy dependence of the yields of single and double strand breaks (SSB and DSB) and crosslinks induced by electron impact on plasmid DNA films is measured in the 2-20 eV range. The yield functions exhibit two strong maxima, which are interpreted as due to the formation of core-excited resonances of the bases, and their decay into the autoionization channel, resulting in p ? p* electronic transitions of the bases, followed by electron transfer to the C-O s* bond in the phosphate group. Occupancy of the s* orbital ruptures the C-O bond of the backbone via dissociative electron attachment, producing a SSB. From a comparison of our results with those of other works, including theoretical calculations and electron-energy-loss spectra of the bases, the 4.6 eV peak in the SSB yield function is attributed to the resonance decay into the lowest electronically excited states of the bases; in particular, those resulting from the  transitions 13A' (p2 ? p) and 13A'' (n2 ? p3*)  of thymine and 13A' (p ? p*) of cytosine. The strongest peak at 9.6 eV in the SSB yield function is also associated with electron capture by excited states of the bases, resulting mostly from a multitude of higher-energy p?p* transitions. The DSB yield function exhibits strong maxima at 6.1 and 9.6 eV. The peak at 9.6 eV is probably related to the same resonance manifold as that leading to SSB, but that at 6.1 eV may be particuarly associated with decay into the electronic state 13A' (p ? p*) of cytosine via autoionization. The yield function of crosslinks is dominated by a broad peak extending over the 3.6-11.6 eV range with another, sharper one at 17.6 eV. The different line shape of the latter yield function, relative to those for SSB and DSB, appears to be due to the formation of reactive radical sites in the initial supercoiled configuration of the plasmid, which react with the circular form (i.e., DNA with a SSB) to produce a crosslink.

DNA strand breaks and cross links induced by transient anions formed by incident electrons with energies in the range 2-20 eV

248th National Meeting (2014)

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