Femtosecond responses of hydrated DNA irradiated by ionizing rays focus on the sugar-phosphate part

Abstract

In this article, we investigate the mechanisms of DNA ionization upon irradiation by 0.5 meV alpha particles. We focus on the sugar-phosphate group and its hydration shell. In radiation chemistry, the term quasi-direct effect refers the physical and chemical responses taking place after irradiation of solvent molecules pertaining to the solvation shells of solutes. The molecular mechanisms accounting for the quasi-direct effect are actually largely elusive, especially for those prevailing in the early timescales (< 10–12 s). We report Real-Time Time-Dependent Auxiliary Density Functional Theory simulations carried out within the framework of hybrid QM/MM scheme (Quantum Mechanics/Molecular Mechanics) with polarizable and non-polarizable embedding. Ten water molecules from the solvation shell of DNA backbone are independently irradiated. We find that during the first femtoseconds after irradiation, the holes formed on the irradiated water remain at their sites of formation. Electrostatic induction within the environment does not significantly impact charge migrations. We address the hypothesis that charge migration driven by electron correlation is responsible for an ultrafast H2O+ to DNA charge transfers, which would account for a quasi-direct effect. We find that pure charge migration at fixed nuclear positions is not responsible for the quasi-direct effect when considering sugar-phosphate solvation shells.