The particular sequence of nucleobases in a DNA oligomer determines both the efciency of hopping and the specic location and nature of the damaging chemical reaction.In aqueous solution, DNA is a polyanion because of the negative charge carried by its phosphate groups.Counterions to the phosphate groups play an important role in facilitating both hopping and the eventual reaction of the radical cation with HO.Irreversible reaction of a radical cation with HO in duplex DNA occurs preferentially at the most reactive site.In normal DNA, comprising the four common DNA nucleobases G, C, A, and T, reaction occurs most commonly at a guanine, resulting in its conversion primarily to oxo,dihydroguanine. Both electronic and steric effects control the outcome of this process.If the DNA oligomer does not contain a suitable guanine, then reaction of the radical cation occurs at the thymine of a TT step, primarily by a tandem process.The oxidative damage of DNA is a complex process, inuenced by charge transport and reactions that are controlled by a combination of enthalpic, entropic, steric, and compositional factors.These processes occur over a broad distribution of energies, times, and spatial scales.Chemical reactions whose outcome alters the DNA of living systems play a major role in processes such as mutagenesis, carcinogenesis, and aging.Aerobic organisms are subject to oxidizing conditions resulting from normal metabolism or from exposure to light or ionizing radiation.Under these conditions, DNA is susceptible to loss of an electron from one of its aromatic nucleobases.Oxidative damage of DNA is a complex process inuenced by charge transport and by reactions that are controlled by a combination of enthalpic, entropic, steric, and compositional factors.These reactions occur over broad distributions of energies, times, and spatial scales.Emergence of a complete picture will require additional exploration of the structural, kinetic, and dynamic properties of DNA.The <a href="http://www.molbioglobal.com/archives/298"></a>
possibility that DNA could serve as an efcient charge conduit sparked experimental and theoretical studies focused on exploring that possibility and its implications to biology and to nanoelectronic technologies where a DNA wire might be exploited as a selforganizing conductor.The B may hop reversibly through the duplex DNA until it is trapped in a chemical reaction with HO resulting in a damaged nucleobase that is symbolized as O generally as X.Experiments in our laboratory rely on photooxidation by a covalently linked anthraquinone derivative photosensitizer.The electronically excited AQ is capable of converting any of the four common DNA bases to its radical cation. The AQ is rapidly consumed by reaction with O to form superoxide and regenerate the AQ, which leaves a B with sufcient time to hop and, eventually, to become trapped by an irreversible reaction.The damaged base is revealed by subsequent chemical or enzymatic treatment that causes cleavage of the DNA at the damaged site. In carefully controlled experiments, the distance dependence of charge transfer is related to the amount of strand cleavage measured at sites remote from the AQ.The GG steps are highly reactive sites where reaction of a guanine radical cation results in its conversion primarily to oxo,dihydroguanine and other oxidation products.