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    STUDIA PHYSICA - Issue no. SPECIAL ISSUE / 2001  

  Abstract:  DNA is a critical cellular target for oxidative processes induced by physical and chemical stresses. It is known that the direct effect of ionizing radiation on DNA results mainly in base ionization and may lead to mutation, carcinogenesis and cell death. The degradation of DNA induced by laser and ionizing radiation (photon beam), is analyzed in this paper. The laser degradation of DNA is a singlet oxygen process, while the ionizing radiation degradation is a radicalic one. A series of lesions among the major bases degradation product has been measured in isolated DNA (8-OxodGuo) exposed to gamma radiation in aerated aqueous solution. Degradation can be accounted for by the formation of hydroxyl radicals upon radiolysis of water (indirect effect). The relative contribution of the two mechanisms in damaging DNA depends on the type of radiation. Single strand breaks and base damage seem to be mainly produced by the attack of hydroxyl radicals on DNA, whereas double strand breaks result predominantly of direct energy deposition. The four bases are degraded in high yield. When DNA is irradiated in solution, single strand breaks are mainly due to the abstraction of an H atom from the 4ĺ position of 2ĺ-deoxyribose by the attack of OH radicals produced by water radiolysis. Quantification of the modified bases showed the guanine is the preferential target. Ionizing radiation produces very low quantities of degradation products of DNA if compared with photochemical pathway. When absorb the light, the photosensitizers reach an excited state able to abstract one electron from the DNA bases, producing the related radical cation, guanine being the preferential target. Ionizing radiation induce several types of DNA modifications, including chain breaks, DNA-protein cross-links, oxidized DNA bases. A high value for 8-OxodGuo degradation observed in control cells treated with either MB or AO by comparison with the others conditions. Ionizing radiation produces very low quantities of 8-OxodGuo, if compared with photosensitizers.  
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