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 distanc es in duplex DNA by a series of relatively short-range steps (hops). The me chanism for the short-range migration is not clearly understood. At one ext reme, the radical cation is localized on guanines (G) and undergoes a unist ep migration to a distant G by superexchange through a bridge of intervenin g 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 difference s in oxidation potentials measured for the isolated DNA bases. We report ex periments on DNA oligonucleotides in which GG steps are separated by (A/T), bridges (n = 2-5) and a radical cation is introduced by irradiation of a c ovalently linked anthraquinone derivative. Quantitative assessment of the d istance dependence of radical cation migration efficiency shows that it is incompatible with a mechanism that requires hole hopping exclusively by sup erexchange.