Oxidation of DNA by long-range charge transport

Since the structure of DNA was elucidated, it has been proposed that charge might move through the stacked base pairs of the double helix. Here it is demonstrated that photoactivated rhodium and ruthenium intercalators can promote oxidation of guanines in 5′-GG-3′ sites from up to 200 Å away. Longrange oxidative damage to guanine doublets in DNA is shown to compete for oxidation with the repair of thymine dimers, demonstrating that electronic “holes” generated on genomic DNA might not of necessity cause DNA damage, but could also be funneled onto proteins or other oxidizible sites. The upper distance limits and sequence effects on long-range charge transfer were examined further by targeting intercalating photooxidants to specific sites on restriction fragments using an appended triplex-forming oligonucleotide. Charge migration occurs in both directions from the intercalator and on both DNA strands of the target over 25–38 base pairs. Targeting oxidative damage by triplex formation extends DNA charge transport studies to significantly longer DNA sequences with a strategy that does not require covalent attachment of the photooxidant to the DNA being probed. Photoexcited rhodium complexes were also used to explore charge transport through DNA within nucleosome core particles, the fundamental packing unit of DNA within eukaryotic cells. Although histone proteins inhibit intercalation of a noncovalent rhodium complex, they do not protect DNA from charge transfer damage through the base pair stack. Using the rhodium photochemistry, the oxidation of guanine by photoexcited rhodium complexes inside of nuclei from cultured human cells was examined and compared with the oxidative damage on bare genomic DNA. Oxidation occurs preferentially at 5′-GG-3′ sites, indicative of charge transport chemistry, and at protein-bound sites that are inaccessible to rhodium. Thus, charge transport acts to induce base damage from a distance on DNA within the nucleus, and direct interaction of an oxidant is not necessary to generate a base lesion at a specific site. These observations indicate that charges can migrate along DNA within in the cell, requiring a reconsideration of cellular mechanisms for DNA damage and repair, and presenting new avenues for exploration in the design of DNA-based drugs and therapies.