C5′- and C3′-sugar radicals produced via photo-excitation of one-electron oxidized adenine in 2′-deoxyadenosine and its derivatives

We report that photo-excitation of one-electron-oxidized adenine [A(-H)*] in dAdo and its 2'-deoxyribonucleotides leads to formation of deoxyribose sugar radicals in remarkably high yields. Illumination of A(-H)* in dAdo, 3'-dAMP and 5'-dAMP in aqueous glasses at 143 K leads to 80-100% conversion to sugar radicals at C5' and C3'. The position of the phosphate in 5'- and 3'-dAMP is observed to deactivate radical formation at the site of substitution. In addition, the pH has a crucial influence on the site of sugar radical formation; e.g. at pH approximately 5, photo-excitation of A(-H)* in dAdo at 143 K produces mainly C5'* whereas only C3'* is observed at high pH approximately 12. 13C substitution at C5' in dAdo yields 13C anisotropic couplings of (28, 28, 84) G whose isotropic component 46.7 G identifies formation of the near planar C5'*. A beta-13C 16 G isotropic coupling from C3'* is also found. These results are found to be in accord with theoretically calculated 13C couplings at C5' [DFT, B3LYP, 6-31(G) level] for C5'* and C3'*. Calculations using time-dependent density functional theory [TD-DFT B3LYP, 6-31G(d)] confirm that transitions in the near UV and visible induce hole transfer from the base radical to the sugar group leading to sugar radical formation.