One-electron oxidation of DNA: thymine versus guanine reactivity.

One-electron oxidation of anthraquinone (AQ)-linked DNA oligonucleotides containing A/T base pairs with repeating TT steps results in the distance-dependent reaction of the resulting radical cation and base damage at the TT steps that is revealed by subsequent reaction as strand cleavage. However, the inclusion of a remote guanine or GG step inhibits the reaction at thymine and results in predominant reaction at the guanine bases. For the oligomers examined in this work, the results reveal that the specific sequence of nucleobases determines the distance dependence, location of reaction and the efficiency of radical cation migration. In particular, a sequence of A/T base pairs can behave either as a trap, shuttle or barrier, depending on the context of the entire oligomer. The A/T sequences act as a shuttle when reaction occurs at a remote G or GG step and the same sequence of A/T bases acts as a barrier when there is more than one GG step in the sequence. In contrast, the A/T steps act as a trap in sequences that lack guanines.