Pathways of the Dissociative Electron Attachment Observed in 5-and 6-Azidomethyluracil Nucleosides: Nitrogen (N2) Elimination vs Azide Anion (N3-) Elimination

5-Azidomethyl-2???-deoxyuridine (5-AmdU, 1) has been successfully employed for the metabolic labeling of DNA and fluorescent imaging of live cells. 5-AmdU also demonstrated significant radiosensitization in breast cancer cells via site-specific nitrogen-centered radical (??-aminyl (U-5-CH2???NH???), 2, and ??- iminyl (U-5-CH???N???), 3) formation. This work shows that these nitrogen-centered radicals are not formed via the reduction of the azido group in 6-azidomethyluridine (6-AmU, 4). Radical assign-ments were performed using electron spin resonance (ESR) in supercooled solutions, pulse radiolysis in aqueous solutions, and theoretical (DFT) calculations. Radiation-produced electron addition to 4 leads to the facile N3??? loss, forming a stable neutral C-centered allylic radical (U-6-CH2???, 5) through dissociative electron attachment (DEA) via the transient negative ion, TNI (U-6-CH2-N3??????), in agreement with DFT calculations. In contrast, TNI (U-5-CH2???N3??????) of 1, via facile N2 loss (DEA) and protonation from the surrounding water, forms radical 2. Subsequently, 2 undergoes rapid H-atom abstraction from 1 and produces the metastable intermediate ??-azidoalkyl radical (U-5-CH???-N3). U-5-CH???- N3 converts facilely to radical 3. N3??? loss from U-6-CH2???N3?????? is thermodynamically controlled, whereas N2 loss from U-5-CH2??? N3?????? is dictated by protonation from the surrounding waters and resonance conjugation of the azidomethyl side chain at C5 with the pyrimidine ring.