Although electrons having enough energy to ionize or electronically excite DNA have long been known to cause strand breaks (i.e., bond cleavages), only recently has it been suggested that even lower-energy electrons (most recently 1 eV and below) can also damage DNA. The findings of the present work suggest that, while DNA bases can attach electrons having kinetic energies in the 1 eV range and subsequently undergo phosphate-sugar O-C sigma bond cleavage, it is highly unlikely (in contrast to recent suggestions) that electrons having kinetic energies near 0 eV can attach to the phosphate unit's P=O bonds. Electron kinetic energies in the 2-3 eV range are required to attach directly to DNA's phosphate group's P=O pi orbital and induce phosphate-sugar O-C sigma bond cleavages if the phosphate groups are rendered neutral (e.g., by nearby counterions). Moreover, significant activation barriers to C-O bond breakage render the rates of both such damage mechanisms (i.e., P=O-attached and base-attached) slow as compared to electron autodetachment and to other damage processes.

We extended our earlier study on single strand break (SSB) formation in DNA induced by low-energy electrons that attach to DNA bases' π*-orbitals. In particular, we examined a range of electron energies (E) representative of the Heisenberg width of the lowest π*-resonance state of cytosine, and we considered how the SSB rates depend on E and on the solvation environment. Moreover, we evaluated the adiabatic through-bond electron transfer rate with which the attached electron moves from the base, through the deoxyribose, and onto the phosphate unit. Our findings show that the SSB rate depends significantly on the electron energy E and upon the solvation environment near the DNA base. For example, in solvation characterized by a dielectric constant of 4.9, the rates range from 10 0 to 10 7 s -1 as the electrons' kinetic energy varies from 0.2 to 1.5 eV. We also find that the rate of through-bond electron transfer is not the factor that limits SSB formation; rather, it is the rate at which a barrier is surmounted on the anion's energy surface and it is this barrier that depends on E and on solvation.

In earlier studies on damage to model DNA systems caused by low-energy electrons, we considered electrons that attach either to cytosine's lowest π*-orbital or to a PO π*-orbital of a phosphate unit. We examined a range of electron kinetic energies (E) (e.g., representative of the Heisenberg width of the lowest π*-resonance state of cytosine), and we determined how the rates of cleavage of the sugar−phosphate C−O σ-bond depend on E and on the solvation environment. In the PO attachment study, we showed that electrons of ca. 1.0 eV could attach to form a π*-anion, which then could break either a 3‘ or 5‘ O−C σ-bond connecting the phosphate to either of two attached sugar groups. In the present study, we extend the base-attachment aspect of our work and consider electrons having kinetic energies below 1 eV attaching to thymine's lowest π*-orbital, again examining the energy and solvation dependence of the resulting rates of C−O σ-bond cleavage.