Site of Azido Substitution in the Sugar Moiety of Azidopyrimidine Nucleosides Influences the Reactivity of Aminyl Radicals Formed by Dissociative Electron Attachment

In this work, electron-induced site-specific formation of neutral pi-type aminyl radicals (RNH.) and their reactions with pyrimidine nucleoside analogs azidolabeled at various positions in the sugar moiety, e.g., at 2'-, 3'-, 4'-, and 5'- sites along with a model compound 3-azido-1-propanol (3AZPrOH), were investigated. Electron paramagnetic resonance (EPR) studies confirmed the site and mechanism of RNH center dot formation via dissociative electron attachment-mediated loss of N-2 and subsequent facile protonation from the solvent employing the N-15-labeled azido group, deuterations at specific sites in the sugar and base, and changing the solvent from H2O to D2O. Reactions of RNH center dot were investigated employing EPR by warming these samples from 77 K to ca. 170 K. RNH center dot at a primary carbon site (5'-azido-2',5'-dideoxyuridine, 3AZPrOH) facilely converted to a sigma-type iminyl radical (R=N center dot) via a bimolecular H-atom abstraction forming an alpha-azidoalkyl radical. RNH center dot when at a secondary carbon site (e.g., 2'-azido-2'-deoxyuridine) underwent bimolecular electrophilic addition to the C5=C6 double bond of a proximate pyrimidine base. Finally, RNH center dot at tertiary alkyl carbon (4'-azidocytidine) underwent little reaction. These results show the influence of the stereochemical and electronic environment on RNH center dot reactivity and allow the selection of those azidonucleosides that would be most effective in augmenting cellular radiation damage.