Mechanism of charge transport in DNA: Internally-linked anthraquinone conjugates support phonon-assisted polaron hopping

A series of anthraquinone-containing (AQ) DNA conjugates was prepared. In each case, the AQ is linked to the 2'-oxygen of a uridine. Physical and spectroscopic data suggest that the AQ is intercalated in the duplex DNA on the 3'-side of the uracil. Irradiation of the AQ-DNA conjugates with UV light results in piperidine-requiring strand cleavage at GG steps of both the AQ-containing strand and its complement. The AQ-conjugates were designed to have GG steps disposed symmetrically about the AQ intercalation site. The distance dependence of reaction efficiency at GG steps following AQ irradiation was measured by means of an AQ-containing 71 mer having 7 GG steps. The efficiency of reaction in this sequence falls off exponentially with a distance dependence of 0.071 Angstrom(-1). AQ-containing conjugates were prepared that incorporate 7,8-dihydro-8-oxoguanines (8-OxoG) at various locations. 8-OxoC has a lower oxidation potential than any of the normal DNA bases and serves as a trap for the migrating radical cation. The 8-OxoG is a very effective trap when the radical cation must migrate through it to reach the GG step, it is a less effective trap when it is on the strand complementary to the GG step. These findings support the mechanism. for long-range radical cation migration described as phonon-assisted polaron hopping.