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 π-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· formation via dissociative electron attachment-mediated loss of N2 and subsequent facile protonation from the solvent employing the 15N-labeled azido group, deuterations at specific sites in the sugar and base, and changing the solvent from H2O to D2O. Reactions of RNH· were investigated employing EPR by warming these samples from 77 K to ca. 170 K. RNH· at a primary carbon site (5'-azido-2',5'-dideoxyuridine, 3AZPrOH) facilely converted to a σ-type iminyl radical (R═N·) via a bimolecular H-atom abstraction forming an α-azidoalkyl radical. RNH· 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· at tertiary alkyl carbon (4'-azidocytidine) underwent little reaction. These results show the influence of the stereochemical and electronic environment on RNH· reactivity and allow the selection of those azidonucleosides that would be most effective in augmenting cellular radiation damage.

Prehydrated One-Electron Attachment to Azido-Modified Pentofuranoses: Aminyl Radical Formation, Rapid H-Atom Transfer, and Subsequent Ring Opening.

Methyl 2-azido-2-deoxy-α-d-lyxofuranoside (1a) and methyl 2-azido-2-deoxy-ß-d-ribofuranoside (2) were prepared from d-xylose or d-arabinose, respectively. Employing ESR and DFT/B3LYP/6-31G* calculations, we investigated (i) aminyl radical (RNH·) formation and (ii) reaction pathways of RNH·. Prehydrated electron attachment to 1a and 2 at 77 K produced transient azide anion radical (RN3·-) which reacts via rapid N2 loss at 77 K, forming nitrene anion radical (RN·-). Rapid protonation of RN·- at 77 K formed RNH· and -OH. 15N-labeled-1a confirmed this mechanism. Investigations employing in-house synthesized site-specifically deuterated derivatives of 1a (e.g., CH3 (1b), C4 (1c), and C5 (1d)) established that (a) a facile intramolecular H atom transfer from C5 to RNH· generated C5· and RNH2. C5· formation had a small deuterium kinetic isotope effect suggesting that this reaction does not occur via direct H atom abstraction. (b) Subsequently, C5· underwent a facile unimolecular conversion to ring-opened C4·. Identification of ring-opened C4· intermediate confirms the mechanism of C5'· mediated unaltered base release associated with DNA-strand break. However, for 2, ESR studies established thermally activated intermolecular H atom abstraction by RNH· from the methyl group at C1. Thus, sugar ring configuration strongly influences the site and pathway of RNH· mediated reactions in pentofuranoses.