LONG-RANGE AND SHORT-RANGE OXIDATIVE DAMAGE TO DNA - PHOTOINDUCED DAMAGE TO GUANINES IN ETHIDIUM-DNA ASSEMBLIES

Short-range and long-range photoreactions between ethidium and DNA hav e been characterized. While no DNA reaction is observed upon excitatio n into the visible absorption band of ethidium, higher-energy irradiat ion (313-340 nm) leads both to direct strand cleavage at the 5'-G of 5 '-GG-3' doublets and to piperidine-sensitive lesions at guanine. This reactivity is not consistent with oxidation of guanine by either elect ron transfer or singlet oxygen as shown by comparison with reactions o f a rhodium intercalator and methylene blue, respectively. By covalent ly tethering ethidium to one end of a DNA duplex, we demonstrate the p resence of two distinct reactions, one short-range and the other long- range. The short-range reaction involves a covalent modification of gu anine by ethidium, based upon HPLC analysis of the nucleoside products and studies with ethidium derivatives. The long-range reaction is ent irely consistent with oxidation of guanine by DNA-mediated electron tr ansfer. The yield of this electron-transfer reaction is not attenuated with distance; equal yields of guanine damage are observed at a proxi mal (17 Angstrom Et-GG separation) and distal (44 Angstrom Et-GG separ ation) site. These results are quite similar to those previously obser ved with a covalently tethered rhodium photooxidant and underscore the unique ability of the DNA base stack to facilitate long-range electro n transfer so as to effect oxidative damage from a distance.