Mechanisms of DNA damage initiated by 3 - 100 eV electrons

Full text: Most of the energy deposited by ionizing radiation in living tissue is immediately channeled into production of positive and negative ions, radicals, and non-thermal secondary electrons with initial kinetic energies well below 100 eV; the latter are ballistic precursors of solvated/hydrated electrons that have been a traditional focus in the biological radiation sciences. Although low energy electrons are known to decompose small molecules via resonant and non-resonant mechanisms, their ability to induce substantial yields of strand lesions in large supercoiled DNA has been observed only recently. Here, I will present the experimental methods and results of our previous as well as current measurements of single (SSB), double (DSB), and even multiple (MSB) strand breaks initiated in DNA by 3-100 eV electron impact. We find that the electron-energy dependent yields of DNA SSBs and DSBs possess a threshold near 4 - 6 eV, and a strong resonance-like peak near 10 eV. This peaked signature is caused by the decay of transient molecular anion (TMA) states, i.e. resonances, similar to those formed in small molecules regardless of their state of aggregation. Here, the TMA states are thought to localize on the DNA's basic structural components (bases, deoxyribose, phosphate, hydration water), and dissociate within several femtoseconds (∼ 10-14 s) into various reactive anion and radical fragments. The subsequent reactions of these fragments within their local environment, on similar time-scales, are believed to compound the final DNA damage. For electron energies above 15 eV the SSB and DSB yields rise to a level that remains roughly constant up to 100 eV electron energy, and are attributed to non-resonant dissociative electronic excitations, such as dipolar dissociation, or the numerous ionization and dissociation pathways that are known to occur at such higher energies. For electron energies above ca. 18 eV we also observe production of short linear DNA fragments, i.e. MSBs, whose yields have a plateau near 25 eV, after which they rapidly increase with increasing electron energy above ca. 30 eV. These MSBs are attributed to multiple DSBs along the path (nano-track) of a single electron across several plectonemic windings within the supercoiled DNA plasmid. We propose that secondary-electron initiated ultra-fast events play a crucial role in the nascent stages of DNA radiolysis, and already induce lethal damage long before their thermalization (10-12 s), or the diffusion limited reactions (10-12 to 10-9 s) of other thermal secondary species produced along ionizing radiation tracks in living cells