Study of production mechanism of DNA clustered damage in aqueous solution

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Purpose of the study is to test our hypothesis that double strand breaks (DSB) of DNA, as a type of clustered damage, are mainly produced in a transiently-existing area (hot area) in which radicals are produced with high density by many energy deposition events in localized region. By changing the energy of monochromatic photons irradiated to aqueous samples, we can control the dimension of "hot area! Produced by the secondary photoelectron produced in water. We already observed that efficiency of DSB production become larger with lower energy photon, while single strand break (SSB) exhibit opposite dependence.During the term of the study, we have observed that the X-ray photon energy dependence of strand bread induction in the presence of high concentration of radical scavenger is the same with the case of the solution without scavenger. This seems to suggest that our hypothesis is effective in intracellular environment. In order to examine the production efficiency of double strand breaks in living cells, we have established a method to determine the number of double strand break in the cellular DNA without any treatment with restrictive digestion enzyme, by utilizing pulse field gel electrophoresis. We will soon have the answer of the test of our hypothesis.We have also investigated the production of DNA strand breaks by monochromatic vacuum UV photons (150 - 180 nm) in aqueous solution. We have found that VUV photon energy dependence of the production of DSB and SSB is different, indicating that production mechanism for the production of both type of strand breakage is different.