Research - Project Aims, Hypotheses being Tested #1 - The aim is to improve our understanding of hole and excess-electron trapping and detrapping by DNA. The following hypotheses are being tested. 1) Radical combination reactions, persisting hours after energy deposition at 4K, are propagated by tunneling. 2) Thermally activated detrapping, of radicals stably trapped at 4K, is “gated” by reversible proton transfer. 3) The activation energy for hole detrapping is substantially less than that for electron detrapping. 4) The range of the electrons and holes is governed by a three dimensional walk, not a one-dimension walk as would be the case if electron/hole transfer is only between stacked bases. #2 - The aim is to determine the reactions that lead to the formation of radicals that are stably trapped in DNA at room temperature. Radical trapping and reactions are being measured in a series of DNA crystals, testing our working model. Imbedded in the model are a number of specific hypotheses, for example: 1) that holes are initially trapped on sugar and guanine in a ratio of ~1:3 and 2) that while most (~70%) of the electrons are initially trapped by reversible proton transfer to one-electron reduced cytosine, there is an important fraction (~20%) that are trapped by irreversible proton transfer to the C5-C6 bond of reduced cytosine and thymine. #3 - The aim is to correlate stable end products with their free radical precursors. The hypothesis that all (>99%) of the stable end products produced in DNA are derived from free radical precursors is being tested and the reaction pathways elucidated. For crystalline DNA irradiated at 4K, the prediction that the ratio of strand breaks to base damage is unusually large (about 1:1) will be tested. #4 - The aim is to detect multiply damaged sites and determine their yields. These experiments are designed to optimize the chances of measuring the yields of specific types of multiply damaged sites. They test, among other things, the hypothesis that some stable sites of reductive damage are spatially correlated with a nearby site of sugar oxidation.