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In this report we present an electron spin resonance (ESR) investigation of the yields of the individual free radicals formed in gamma-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 (T-.) 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 (CD.). Guanine cation radical (G+.) decreases at high doses, with a concomitant increase in secondary radical species (S.). Analysis of the dose-response data for G values (the yields in microM/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 CD. and G+. increase slightly with hydration; however, that of T-. increases substantially. Destruction constants for neutral radicals such as TH. and CD. 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 T-. and a positive k' value for CD. are explained in terms of radiation effects that result in the formation of a deuterated cytosine base, i.e., C(N3)D+, 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.

Radicals from Relationship withA series of peroxyl radicals (ROO') with substituent groups of varying electron withdrawing power have been investigated using electron paramagnetic resonance spectroscopy. Sixteen carbon-based radicals have been produced, identified and reacted with ' '0-labelled molecular oxygen to produce the corresponding peroxyl radical. Carboncentred radical precursors include alkyl, aromatic, halocarbon, lipid and alcohol radicals. The oxygen-17 hyperfine couplings are shown to monitor closely the spin density distribution on the oxygen atoms in the peroxyl radicals. The experimental 170 couplings suggest a value of -5.7 mT for the anisotropic coupling parameter, B , in good agreement with the theoretical value calculated from improved wavefunctions. An increase in the magnitude of the terminal oxygen coupling is shown to correlate with an increase in the reactivity of peroxyl radicals. In addition, the Taft substituent parameter (o*) is also correlated with the oxygen-17 couplings. The results allow estimation of peroxyl radical reactivity from oxygen-17 couplings and suggest that complexed superoxide ion may be more reactive than free 0;-. The cysteine sulphonyl peroxyl and thiol peroxyl radicals are discussed in light of the results for carbon-based peroxyl radicals. The sulphonyl peroxyl radical is suggested to be highly reactive and the thiyl peroxyl radical has unique properties which suggest it is of a different nature.

This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).