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

Abstract

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.

Secondary Electron Attachment-Induced Radiation Damage to Genetic Materials

“…Experimental techniques including ESR studies of irradiated DNA-model compounds and of irradiated hydrated DNA, picosecond pulse radiolysis of DNA-model compounds, and theoretical studies have unequivocally established that pyrimidine bases are more electron affinic than the purine bases. ,,,, Therefore, the C5-modified pyrimidine (especially thymine) nucleosides are being extensively explored for use as radiosensitizers. New base and nucleoside derivatives are synthesized that have similar or better DEA yields in the 0–3 eV region, and these modified nucleosides would not lose the C5-modification of the thymine base by thymidylate synthetase. − These electron-mediated base-radicals would augment formation of the radical species that are precursor to the DNA-lesions, and in addition, the incorporated modified nucleosides could affect the repair processes to augment the radiation-induced cell death. These results show how radiation-produced electrons could be used to improve the chemoradiotherapeutic efficacy. − Several platinum-based radiosensitizers are also being investigated …”

“…New base and nucleoside derivatives are synthesized that have similar or better DEA yields in the 0–3 eV region, and these modified nucleosides would not lose the C5-modification of the thymine base by thymidylate synthetase. − These electron-mediated base-radicals would augment formation of the radical species that are precursor to the DNA-lesions, and in addition, the incorporated modified nucleosides could affect the repair processes to augment the radiation-induced cell death. These results show how radiation-produced electrons could be used to improve the chemoradiotherapeutic efficacy. − Several platinum-based radiosensitizers are also being investigated …”

Quasi-Free Electron-Mediated Radiation Sensitization by C5-Halopyrimidines

“…Thus, some groups are synthesizing halopyridine-like molecules with similar or even better DEA efficiency, but without the drawbacks of dehalogenation by thymidylate synthetase. In other words, new base and nucleoside derivatives are synthesized that have similar or better DEA yields in the 0–3 eV region than the haloprimidines; in addition, these derivatives would not lose the C5-modification of the thymine base by thymidylate synthetase. − Based on our previous works and on the current work, our work predicts that the DEA process of these halopyridine-like molecules in solution should predominantly be governed by e qf – .…”

Pathways of the Dissociative Electron Attachment Observed in 5- and 6-Azidomethyluracil Nucleosides: Nitrogen (N2) Elimination vs Azide Anion (N3–) Elimination

“…We have compared the black spectrum in Figure A with our reported spectra ,− of the π-aminyl radical observed from azidonucleosides and azidosugars at 77 K in the dark. This comparison establishes that spectrum 2A does not show the line components due to axially symmetric anisotropic N-atom at the wings.…”

“…Neutral π-type aminyl radicals (RNH • ) are biologically important and their generation via DEA to azidonucleosides has been investigated by our group through the use of synthesis, electron spin/paramagnetic resonance (ESR/EPR) spectroscopy, and theoretical calculations. ,,− These studies have led to the following general findings: (a) base-modified azidopyrimidine nucleosides are found to augment radiation damage to cancerous cells via radiation-produced site-specific electron-mediated π-aminyl (RNH • (U-5-CH 2 –NH • ), 2 )/σ-iminyl (RN • (U-5-CHN • ), 3 ) radicals (Figure ) and (b) the site of azido substitution in the sugar moiety of azidosugars and in the azidopyrimidine nucleoside influences the reactivity (intramolecular vs intermolecular H-atom abstraction) of the aminyl radical. For example, the addition of radiation-produced electrons to 5-azidomethyl-2′-deoxyuridine (5-AmdU, 1 Figure ) leads to the formation of RNH • , 2 .…”