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)center dot] in dAdo and its 2'-deoxyribonucleotides leads to formation of deoxyribose sugar radicals in remarkably high yields. Illumination of A(-H)center dot 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 similar to 5, photo-excitation of A(-H). in dAdo at 143 K produces mainly C5'. whereas only C3'. is observed at high pH similar to 12. C-13 substitution at C5' in dAdo yields C-13 anisotropic couplings of (28, 28, 84) G whose isotropic component 46.7 G identifies formation of the near planar C5'.. A beta-C-13 16 G isotropic coupling from C3'. is also found. These results are found to be in accord with theoretically calculated C-13 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.