Gas-phase chemistry and lasers characterize role of secondary electrons in DNA fragmentation

Produced when ionizing radiation and high-energy particles pass through biological materials, secondary electrons are known to cause DNA fragmentation. Numerous techniques for understanding how these low-energy electrons interact with DNA have emerged, including the use of gas-phase iodide ion-molecule clusters, in which electrons are photodetached from the iodide ions producing low-energy free electrons with well-defined energies that then attach to the nucleobases.

Researchers leveraged the power of gas-phase chemistry to characterize how one form of nucleobases, pyrimidines, break apart in the presence of secondary electrons. Described in The Journal of Chemical Physics, the team investigated the excited states of thymine and cytosine when coupled with iodide ions using laser photodissociation spectroscopy above their electron detachment thresholds, for the first time. In this manner they learned about the character of the initial anion state, formed when a low-energy electron collides with the nucleobase, and observed two different types of states depending on the electron energy.

Both iodide-thymine and iodide-cytosine clusters exhibited prominent dipole-bound excited states near 4.0 electronvolts, the nucleobases’ vertical detachment energy. Like the previously studied iodide-uracil cluster, iodide-thymine clusters exhibited a second, higher-energy excited state centered at 4.8 eV that corresponded to a transition into an antibonding orbital and deprotonated nucleobase anion production.

Iodide-cytosine clusters, however, yielded a trio of weaker excited states. The iodide photofragment in these clusters displayed a distinctively flat profile above the cluster’s photodetachment energy.

The team to concluded that there are several ways low-energy electrons can attach themselves to nucleobase molecules: Intracluster electron transfer dominates at energy levels near the vertical detachment energy, while nucleobase-centered excitations are predominant near antibonding transitions. Calculations using time-dependent density functional theory suggested that charge-transfer transitions from iodide to the nucleobases may contribute to the cluster absorption profile across the scanned spectral region.

Source: “Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations,” by Edward Matthews, Rosaria Cercola, Golda Mensa-Bonsu, Daniel M. Neumark, and Caroline E. H. Dessent, Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5018168 .