RELATIVE ABUNDANCES OF PRIMARY ION RADICALS IN GAMMA-IRRADIATED DNA - CYTOSINE VS THYMINE ANIONS AND GUANINE VS ADENINE CATIONS

An ESR study of the relative distribution of ion radicals formed in DNA equilibrated with D2O and gamma-irradiated at 77 K is presented. The ESR spectra of irradiated DNA and polynucleotides (poly[dG].poly[dC] and poly[dAdT].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.-, C.-) from the spectra of the purine cations (G.+, A.+). The spectra of the mononucleotide ion radicals, dCMP.-, dTMP.-, dGMP.+, and dAMP.+, were produced in 8 M LiCl glasses. In addition, the spectra of the ion radicals of all of the mononucleotide ion radicals except dAMP.+ 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. Three analysis include the cationic composition, and they strongly favor the guanine cation over the adenic cation. An average of our results gives the DNA ion radicals' relative abundances as ca. 77% C.-, 23% T.- for the anions and > 90% G.+ for the cations; about equal amounts of anions and cations are present. No difference in results is found for DNA irradiated in frozen D2O solutions or simply exchanged at 100% D2O humidity.