Cai, Z. and Sevilla, M. D. Electron and Hole Transfer from DNA Base Radicals to Oxidized Products of Guanine in DNA. Radiat. Res. 159, 411–419 (2003).
An investigation of electron and hole transfer to oxidized guanine bases in DNA is reported. Guanine in DNA was preferentially oxidized by Br2·− at 298 K to 8-oxo-7,8-dihydro-guanine (8-oxo-G) and higher oxidation products. HPLC-EC analysis of irradiated DNA shows that the formation of 8-oxo-G could not be increased above the ratio of one 8-oxo-G to 127 ± 6 bp regardless of dose. 8-oxo-G can be produced only at low levels because it is further oxidized to other species. These oxidation products of guanine have been extensively investigated and identified by others. Our electron spin resonance studies suggest that at 77 K 8-oxo-G is a trap for radiation-produced holes, but certain further oxidation products of 8-oxo-G (Gox) are found to be efficient electron traps. Gamma irradiation of oxidized DNA samples in frozen (D2O) aqueous ices and glassy 7 M LiBr solutions resulted in radicals formed by electron attachment to the Gox sites that were monitored by electron spin resonance spectroscopy (ESR) at 77 K. These ESR spectra suggest that those oxidation products of 8-oxo-G containing α-diketo groups account for the electron traps (Gox) in oxidized DNA with oxaluric acid a likely major trap. Electron transfer from DNA anion radicals to Gox was followed by monitoring of their ESR signals with time at 77 K. Using typical values for the tunneling constant β estimates of the relative amount of Gox to base pairs were obtained. Radicals formed by UV photolysis of oxidized DNA in 8 M NaClO4 glassy aqueous solutions were also investigated. The 8-oxo-G cation accounts for less than 10% of all the radicals observed after either γ irradiation of oxidized DNA in frozen (D2O) aqueous solution or UV photolysis of oxidized DNA in 8 M NaClO4 glassy aqueous solutions. We estimate hole transfer distances of about 7 ± 1 bp at 1 min from G· to 8-oxo-G.
