Studies of excess electron and hole transfer in DNA at low temperatures

In this review investigations of hole and electron transfer processes in DNA after gamma-irradiation at low temperatures are described. These experiments suggest that at low temperatures DNA is a good ion radical trap, one which traps 30 to 60% of all electrons and holes produced by radiation. Electrons are trapped at the pyrimidine bases, cytosine and thymine, whereas holes are trapped mainly on guanine. Results show that all electrons in the hydration layer transfer to DNA and all holes in the first layer of waters transfer to DNA; holes in subsequent layers form hydroxyl radicals. After trapping, electron migration and hole transfer processes within DNA are limited to tunneling at low temperatures. Electron spin resonance studies have followed the time-dependent transport of electrons and holes from DNA base trap sites to acceptors such as intercalators or modified bases. Electron transfer through the DNA pi stack and between duplexes has been elucidated, as well as the effects of DNA hydration, complexing agents, base sequence, and H/D isotope exchange on electron-transfer distances and rates. Studies which vary the temperature have separated tunneling in DNA from activated mechanisms that dominate at temperatures near 200 K and above.