Pathways of the Dissociative Electron Attachment Observed in 5- and 6-Azidomethyluracil Nucleosides: Nitrogen (N 2 ) Elimination vs Azide Anion (N 3 - ) Elimination.

5-Azidomethyl-2'-deoxyuridine (5-AmdU, ) 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 (π-aminyl (U-5-CH-NH), , and σ-iminyl (U-5-CH═N), ) formation. This work shows that these nitrogen-centered radicals are not formed via the reduction of the azido group in 6-azidomethyluridine (6-AmU, ). Radical assignments were performed using electron spin resonance (ESR) in supercooled solutions, pulse radiolysis in aqueous solutions, and theoretical (DFT) calculations. Radiation-produced electron addition to leads to the facile N loss, forming a stable neutral C-centered allylic radical (U-6-CH, ) through dissociative electron attachment (DEA) via the transient negative ion, TNI (U-6-CH-N), in agreement with DFT calculations. In contrast, TNI (U-5-CH-N) of , via facile N loss (DEA) and protonation from the surrounding water, forms radical . Subsequently, undergoes rapid H-atom abstraction from and produces the metastable intermediate α-azidoalkyl radical (U-5-CH-N). U-5-CH-N converts facilely to radical . N loss from U-6-CH-N is thermodynamically controlled, whereas N loss from U-5-CH-N is dictated by protonation from the surrounding waters and resonance conjugation of the azidomethyl side chain at C5 with the pyrimidine ring.