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, 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 (π-aminyl (U-5-CH 2 -NH • ), 2 , and σ-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 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 4 leads to the facile N 3 - loss, forming a stable neutral C-centered allylic radical (U-6-CH 2 • , 5 ) through dissociative electron attachment (DEA) via the transient negative ion, TNI (U-6-CH 2 -N 3 •- ), in agreement with DFT calculations. In contrast, TNI (U-5-CH 2 -N 3 •- ) of 1 , via facile N 2 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 α-azidoalkyl radical (U-5-CH • -N 3 ). U-5-CH • -N 3 converts facilely to radical 3 . N 3 - loss from U-6-CH 2 -N 3 •- is thermodynamically controlled, whereas N 2 loss from U-5-CH 2 -N 3 •- is dictated by protonation from the surrounding waters and resonance conjugation of the azidomethyl side chain at C5 with the pyrimidine ring.