COMBINATION IS THE DOMINANT FREE-RADICAL PROCESS INITIATED IN DNA BY IONIZING-RADIATION - AN OVERVIEW BASED ON SOLID-STATE EPR STUDIES

An overview of the early processes initiated in DNA by ionizing radiat ion is given from the perspective of studies done by solid-state EPR w ith the focus on radical combination. Comparisons with free radical fo rmation and trapping in crystalline pyrimidines (1-methylcytosine, thy mine, 1-methylthymine, 1-methyluracil, and cytosine monohydrate) provi de insight into the processes occurring in DNA. Between 25 and 50% of low LET ionizations in fully hydrated DNA at 4K lead to trapped free r adicals, the remaining unobserved radicals are assumed to have combine d. The majority of the radicals trapped in DNA at 4K (G similar to 0.3 mu mol/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 4K ar e, in the main, limited to within the cluster diameter, similar to 4 n m. Proton transfer across hydrogen bonds promotes radical trapping and inhibits combination but is thermally reversible. Warming to room tem perature mobilizes the reversibly trapped radicals and gives additiona l combination (50-80% of those trapped at 4K). The yield of free radic als, after anneal, is sufficient to account for the yield of single-st rand breaks produced by direct effects.