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An electron spin resonance investigation of the free-radical distribution in {gamma}-irradiated frozen samples of single stranded DNA (ssDNA) and double stranded DNA (dsDNA) is reported. Computer analysis of the ESR spectra of {gamma}-irradiated ssDNA in D{sub 2}O at 100 K suggests a more uniform distribution of the radical ions on the DNA bases than found for dsDNA, with the approximate ssDNA composition: thymine anion, T{sup {sm_bullet}{minus}} (30-35%), cytosine anion, C{prime}{sup {sm_bullet}{minus}} (20-28%), guanine cation, G{sup {sm_bullet}+} (26-28%), and adenine cation, A{sup {sm_bullet}+} (8-17%), with small amounts of purine anions or pyrimidine cations. Upon annealing {gamma}-irradiated dsDNA the authors find that the electron transfers from C{prime}{sup {sm_bullet}{minus}} to T and the T{sup {sm_bullet}{minus}} formed protonates at C-6 to produce the 5,6-dihydrothymin-5-yl radical (TH{sup {sm_bullet}}, TD{sup {sm_bullet}}). The rate of deuteration of T{sup {sm_bullet}{minus}} is shown to be 20-fold less than the rate of protonation. Electron transfer in ssDNA appears to be less facile than in dsDNA. Hole transfer from adenine to guanine in dsDNA is nearly complete at 100 K, but appears less complete in ssDNA at 100 K. The results suggest that DNA strandedness, which affects DNA conformation, base stacking and cross strand interactions, is important in determining the initialmore » ion radical sites as well as the subsequent ion radical transfer and reactions. A calculation of the ion radical abundances as a function of base transfer, starting from a random distribution in a model DNA strand, suggests base pairing greatly augments hole and electron transfer through the DNA strand and that on average ion transfer over only ca. 2 bases is sufficient to produce the ion distribution found in ssDNA and dsDNA. 40 refs., 8 figs., 1 tab.« less

An ESR study of the relative distribution of ion radicals formed in DNA equilibrated with D{sub 2}O and {gamma}-irradiated at 77 K is presented. The ESR spectra of irradiated DNA and polynucleotides (poly(dG){center dot}poly(dC) and poly(dAdT){center dot}poly(dAdT)) were obtained and employed in a computer-assisted analysis for the individual ion-radical distribution. Analysis of spectra as a function of power allowed the separation of the spectra of the pyrimidine anions (T{sup {sm bullet}{minus}}, C{sup {sm bullet}{minus}}) from the spectra of the purine cations (G{sup {sm bullet}+}, A{sup {sm bullet}+}). The spectra of the mononucleotide ion radicals, dCMP{sup {sm bullet}{minus}}, dTMP{sup {sm bullet}{minus}}, dGMP{sup {sm bullet}+}, and dAMP{sup {sm bullet}+}, were produced in 8 M LiCl glasses. In addition, the spectra of the ion radicals of all of the mononucleotide ion radicals except dAMP{sup +} were simulated by using hyperfine and g tensors from the literature. Basis spectra derived from (1) power saturation experiments, (2) polynucleotide and mononucleotide spectra, (3) spectra of mononucleotides alone, and (4) anisotropic simulations were used to fit the spectra of DNA by use of a linear least-squares analysis. Each of the four separate analyses confirms that the cytosine anion dominates the spectra of DNA at 100 K. Threemore » analyses included the cationic composition, and they strongly favor the guanine cation over the adenine cation. An average of the authors results gives the DNA ion radicals' relative to abundances as ca. 77% C{sup {sm bullet}{minus}}, 23% T{sup {sm bullet}{minus}} for the anions and >90% G{sup {sm bullet}+} for the cations about equal amounts of anions and cations are present. No difference in results is found for DNA irradiated in frozen D{sub 2}O solutions or simply exchanged at 100% D{sub 2}O humidity.« less