The Site of Azido Substitution in a Pyrimidine Nucleobase Dictates the Type of Nitrogen-Centered Radical Formed after Dissociative Electron Attachment

Azido(N₃)-nucleosides are known for their antiviral and anticancer properties. Recently, N₃-nucleosides have attracted attention for augmenting radiation damage in tumor cells via dissociative electron attachment (DEA) reactions mediated by the simplest and the most potent reducing species (the electron). To investigate the effect of N₃ group substitution at specific sites of the pyrimidine base-ring (C4, C5, and C6) on the DEA reaction, we employed both commercially available and in-house-synthesized N₃-nucleosides. We conducted a comprehensive study combining electron paramagnetic resonance (EPR) spectroscopy at a low temperature, picosecond pulse radiolysis in an aqueous solution under ambient conditions, and DFT calculations. For N₃ substitution at C4, EPR studies and DFT calculations established a stable azide anion radical (R-N₃^•-) formation, after the addition of radiation-produced electrons. For N₃ substitution at C5, the DEA leads to a π-type aminyl radical (R-NH^•) formation. For N₃ substitution at C6, a conjugated iminyl σ-radical (R═N^•) is formed via DEA. R═N^• is in equilibrium with R-NH^•. Thus, this work reports a significant finding: the stabilization and reactivity of each type of nitrogen-centered radical (RN₃^•-, RNH^•, R═N^•) are determined by the position of N₃ substitution on the pyrimidine base-ring.