IRREVERSIBLE PROTONATION SITES OF ONE-ELECTRON-REDUCED ADENINE - COMPARISONS BETWEEN THE C5 AND THE C2 OR C8 PROTONATION SITES

The reversible and irreversible protonation sites of one-electron-reduced adenine are shown to depend upon the environment adenine is reduced in. Reduction of 2'-deoxyadenosine in a PEG glass (polyethylene glycol, 12 M monomer) at 77 K probably gives rise to the neutral, N1-protonated reduced adenine radical. Upon annealing to similar to 150 K, an irreversible protonation at C2 or C8 is observed, in agreement with pulse radiolysis studies in aqueous solution (Candeias, L. P.; Steenken, S. J. Phys. Chem. 1992, 96, 937-944). Upon reduction at 4 K in a high-salt glass containing Li+ or Mg2+, 5'AMP protonates at the amino group, which assumes an sp(3) conformation with one N-H bond perpendicular to the ring plane. A second, irreversible protonation occurs at C5 upon annealing to 40-80 K. This adenine radical is identified by a distinctive set of EPR and optical absorption spectra. The reaction pathway leading to protonation at C5 could be a significant trap for mobile electrons in X-irradiated, single-stranded DNA.