The alkoxy radical, RCH2Ȯ, as a free radical product in x-irradiated single crystals of nucleosides and nucleotides

Abstract

The RCH₂Ȯ radical is formed in x‐irradiated 3′‐cytidylic acid (3′CMP), 5‐chlorodeoxyuridine (ClUdR), 5‐bromodeoxyuridine (BUdR), adenosine⋅HCl (AR⋅HCl), and deoxyadenosine monohydrate (AdR⋅H₂O) owing to the net loss of hydrogen from O_5′ of the primary hydroxylic group. ESR measurements were made between 78 and 100 K on single crystals of 3′CMP, ClUdR, BUdR, and AR⋅HCl and on polycrystalline AdR⋅H₂O. The range of principal g values is g_max=2.054–2.093, g_int=2.005–2.009, g_min =1.995–2.000. The range of sum of the isotropic β‐hydrogen hyperfine couplings is A^β1_iso +A^β2_iso=141–156 G. The g tensors and A^β_iso values are used to determine which oxygen nonbonding orbital contains the bulk of the unpaired electron density. The unpaired electron 2p orbital symmetry axis is perpendicular to a plane determined by X(H) ⋅⋅⋅Ȯ–C. Of the two relevant hydrogen bonds in the undamaged molecule, the X–H⋅⋅⋅O bond remains intact whereas the hydrogen in the O–H⋅⋅⋅Y bond is displaced. The equation A^β_iso=B₀+B₂ cos²ϑ, with B₀ =0 and B₂=101±13 G describes the dependency of the β‐hydrogen hyperfine coupling on the torsion angle ϑ, between the unpaired electron symmetry axis and the C–H_β bond of the RCH₂Ȯ radical. Mechanisms of formation, factors in stabilization, and reactions of decay are discussed.