Free Radical Distribution in Gamma-irradiated DNA

Summary Previous E.S.R. investigations performed on purine pyrimidine nucleotide complexes, γ-irradiated at 25°c, showed the occurrence of complete spin transfers from the purine to the pyrimidine nucleotides. The dTMP · dCMP complex has now been investigated under the same conditions and with the following results: among the various dTMP radicals, only the H-addition radicals can be transferred to the dCMP. This result is in agreement with the mechanism suggested previously, according to which, for a transfer to occur the donor molecule must go through an anionic phase during its radical evolution, for, while the ṪH radicals arise from the evolution of T− anions, the other dTMP radicals do not. As a result, they cannot be involved in the transfer and remained localized on dTMP. The validity of this mechanism was again confirmed by the fact that, in the dGMP · dAMP · dTMP · dCMP complex, the two types of transfer take place simultaneously: complete transfers from purine to pyrimidine nucleotides and incomplete transfer from dTMP to dCMP. These results were then applied to the interpretation of the spin migration within the DNA, γ-irradiated at 25°c. In this molecule, as in the four nucleotide complex, all the radiation damage was localized on the pyrimidine nucleotides. No purine signal was detectable. The modifications of the DNA spectra as a function of several experimental factors were finally studied. They were ascribed to changes in the relative intensities of the pyrimidine patterns in the spectrum.