title: Combination is the dominant free radical process initiated in DNA by ionizing radiation: an overview based on solid-state EPR studies.
abstract: An overview of the early processes initiated in DNA by ionizing radiation is given from the perspective of studies done by solid-state EPR with the focus on radical combination. Comparisons with free radical formation and trapping in crystalline pyrimidines (1-methylcytosine, thymine, 1-methylthymine, 1-methyluracil, and cytosine monohydrate) provide insight into the processes occurring in DNA. Between 25 and 50% of low LET ionizations in fully hydrated DNA at 4 K lead to trapped free radicals, the remaining unobserved radicals are assumed to have combined. The majority of the radicals trapped in DNA at 4 K (G approximately 0.3 mumol/J) are believed to be in clusters. Based on the value of G, it is argued that the range of holes and bound electrons in DNA at 4 K are, in the main, limited to within the cluster diameter, approximately 4 nm. Proton transfer across hydrogen bonds promotes radical trapping and inhibits combination but is thermally reversible. Warming to room temperature mobilizes the reversibly trapped radicals and gives additional combination (50-80% of those trapped at 4 K). The yield of free radicals, after anneal, is sufficient to account for the yield of single-strand breaks produced by direct effects.
authors.affiliation: Department of Biophysics, University of Rochester, NY 14642.
authors.forename: W A
authors.initials: WA
authors.lastname: Bernhard
authors.forename: N
authors.initials: N
authors.lastname: Mroczka
authors.forename: J
authors.initials: J
authors.lastname: Barnes
mesh_terms: D004247:DNA / Q000528:radiation effects*; D004249:DNA Damage; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals
chemical_list: D005609:Free Radicals; D004247:DNA