RELATIVE ABUNDANCE AND REACTIVITY OF PRIMARY ION RADICALS IN GAMMA-IRRADIATED DNA AT LOW-TEMPERATURES .2. SINGLE-STRANDED VS DOUBLE-STRANDED DNA

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 D2O 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.- (30-35%), cytosine anion, C'.- (20-28%), guanine cation, G.+ (26-28%), and adenine cation, A.+ (8-17%), with small amounts of purine anions or pyrimidine cations. Upon annealing gamma-irradiated dsDNA we find that the electron transfers from C'.- to T and the T.- formed protonates at C-6 to produce the 5,6-dihydrothymin-5-yl radical (TH., TD.). The rate of deuteration of T.- 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 standedness, with affects DNA conformation, base stacking and cross strand interactions, is important in determining the initial 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.