The complex PC,(pop)~-abstracts hydrogen atoms m~d electrons from DNA upon photolysis into the da* ~pa excited state (pop = P2OsH~-). In daplex DNA, the hydrogen atoms are abstracted from the 4'-and 5'-positions of the deoxyr~bose functionality, and electrons are abstracted by tunneling from the guanine nucleobase. At high Mg 2+ concentrations, the Pt2(pop)]-tetraanion can associate more intimate]y with the duplex, and both hydrogen atom and electron transfer are efficient; however, at low Mg z+ ,:oncentrations, the complex is situated far from the duplex so that the only efficient pathway is electron tunneling. Therefore, the guanine/sugar ratio decreases with increasing Mg ~*. The electron transfer pathway can also be examined in the absence of the hydrogen, transfer pathway in thermal reactions where the electron is abstracted from guanine by Ru(bpy).~ + (bpy = 2,2'-bipyridine). These reactions can be initiated dectrochen~ically by potentiation of the complex to the 3 + form, which produces catalytic enhancem~',:~ts in cyclic vokammograms. These enhancements show that guanine muttiplets are more reactive than dispersed guanines. Finally, binding of