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-redu ced adenine are shown to depend upon the environment adenine is reduce d in. Reduction of 2'-deoxyadenosine in a PEG glass (polyethylene glyc ol, 12 M monomer) at 77 K probably gives rise to the neutral, N1-proto nated reduced adenine radical. Upon annealing to similar to 150 K, an irreversible protonation at C2 or C8 is observed, in agreement with pu lse 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 grou p, which assumes an sp(3) conformation with one N-H bond perpendicular to the ring plane. A second, irreversible protonation occurs at C5 up on annealing to 40-80 K. This adenine radical is identified by a disti nctive set of EPR and optical absorption spectra. The reaction pathway leading to protonation at C5 could be a significant trap for mobile e lectrons in X-irradiated, single-stranded DNA.