Microsolvation Redirects Electron-Induced Chemistry in Nucleobases

Low-energy electrons are a major source of radiation-induced damage to genetic material through attachment-induced bond rupture in DNA and RNA nucleobases. Here, we show that microsolvation fundamentally alters this process; even a single water molecule suppresses covalent bond rupture in electron-attached uracil. Using mass-selected hydrated uracil anions, we investigate fragmentation dynamics following photoexcitation over a wide energy range (0.5-5.5 eV). Rather than undergoing bond cleavage, electronically excited clusters relax via rapid internal conversion to a vibrationally hot ground state, followed by sequential water evaporation. Photofragment excitation spectroscopy reveals the electronic structures of individual hydrated uracil anions, while the observed solvent-loss patterns are quantitatively described by a stochastic evaporation model with an average water binding energy of ∼0.4 eV, independent of cluster size. These results demonstrate that immediate hydration qualitatively redirects the relaxation pathways of electron-attached nucleobases, with important implications for understanding electron-driven chemistry in aqueous biological environments.