In this report we present an electron spin resonance (ESR) investigation of the yields of the individual free radicals formed in γ-irradiated frozen DNA as a function of hydration and dose at 77 K. Analysis of the ESR spectra taken at low hydration shows that the ion radical composition remains nearly constant with dose and that few secondary radicals are formed even at high doses (above about 50 kGy). For fully hydrated samples, the radical composition changes dramatically with dose. Thymine anion radical (<tex-math>${\rm T}^{-\bullet}$</tex-math>) is found in abundance at low doses but nearly disappears at higher doses, with a corresponding increase in the N-3 deuterated cytosine anion radical (<tex-math>${\rm C}_{{\rm D}}{}^{\bullet}$</tex-math>). Guanine cation radical (<tex-math>${\rm G}^{+\bullet}$</tex-math>) decreases at high doses, with a concomitant increase in secondary radical species (<tex-math>${\rm S}^{\bullet}$</tex-math>). Analysis of the dose-response data for G values (the yields in μM/J), k values (the destruction constants) and k′ values (a new constant that characterizes the change in G with dose) was performed for each of the DNA base free radicals present at 77 K. The G value for each of the base radicals increases with the hydration level. The k values for <tex-math>${\rm C}_{{\rm D}}{}^{\bullet}$</tex-math> and <tex-math>${\rm G}^{+\bullet}$</tex-math> increase slightly with hydration; however, that of <tex-math>${\rm T}^{-\bullet}$</tex-math> increases substantially. Destruction constants for neutral radicals such as <tex-math>${\rm TH}^{\bullet}$</tex-math> and <tex-math>${\rm C}_{{\rm D}}{}^{\bullet}$</tex-math> are found to be substantially smaller than those for ion radicals and provide an indication of the radical charge state. A negative k′ value for <tex-math>${\rm T}^{-\bullet}$</tex-math> and a positive k′ value for <tex-math>${\rm C}_{{\rm D}}{}^{\bullet}$</tex-math> are explained in terms of radiation effects that result in the formation of a deuterated cytosine base, i.e., <tex-math>${\rm C}({\rm N}3){\rm D}^{+}$</tex-math>, which greatly increases cytosine's electron affinity. The ratio of anion radical to cation radical concentrations is found to be about 1.6 and is invariant with hydration. A speculation on the imbalance based on hole-hole combinations in spurs is presented.

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