Long-Distance Radical Cation Migration through A/T Base Pairs in DNA: An Experimental Test of Theory

It is widely accepted that radical cations (holes) can migrate long distances in duplex DNA by a series of relatively short-range steps (hops). The mechanism for the short-range migration is not clearly understood. At one extreme, the radical cation is localized on guanines (G) and undergoes a unistep migration to a distant G by superexchange through a bridge of intervening A/T base pairs. Alternatively, the radical cation can reside on the bases of the A/T bridge, even though this appears to be prohibited by differences in oxidation potentials measured for the isolated DNA bases. We report experiments on DNA oligonucleotides in which GG steps are separated by (A/T)n bridges (n = 2−5) and a radical cation is introduced by irradiation of a covalently linked anthraquinone derivative. Quantitative assessment of the distance dependence of radical cation migration efficiency shows that it is incompatible with a mechanism that requires hole hopping exclusively by superexchange.