The Role of ¹πσ* States in the Formation of Adenine Radical-Cations in DNA Duplexes

Photoinduced damage of DNA is a well-known but still far from fully understood phenomenon. Electronic structure methods are here employed to investigate potential roles of πσ^∗ states in initiating photodamage, and ways in which πσ^∗-state driven photochemistry might evolve with increasing molecular complexity. The study starts with the bare 9H-adenine molecule and progresses through to a model double-helix DNA duplex in aqueous solution. Relative to the gas phase, aqueous solvation is predicted to stabilize the ¹πσ^∗ states of these systems when exciting at the respective ground state equilibrium geometries, but to have relatively little effect on the asymptotic NH bond strengths. But the study also re-emphasises the potential importance of rival σ^∗ ← π excitations, wherein a solute π electron is promoted to a σ^∗ orbital localized on an OH bond of a complexing H₂O molecule, as a route to forming parent radical cations – as have recently been observed following near UV photoexcitation of double-helix adenine-thymine duplexes in water (Banyasz et al., 2018). The subsequent deprotonation of such radical cations offers a rival low energy route to NH bond fission and radical formation in such duplexes.