Polycrystalline samples of 19 dinucleotides, barium diethyl phosphate, and 3′-cytidylic acid have been γ-irradiated and observed at 77°K by electron spin resonance. In 18 of the dinucleotides and barium diethyl phosphate a doublet having a hyperfine splitting of 117-124 G and g value (spectroscopic splitting factor) of 1.998-2.000 is observed. It is concluded that the doublet hyperfine interaction is due to31 P. The free radical postulated to account for the observed doublet is <tex-math>$({\rm RO})({\rm R}^{\prime}{\rm O}){\rm PO}_{2}$</tex-math>, with the unpaired electron localized on one or both of the nonester oxygens. A similar radical, <tex-math>$({\rm HO})({\rm RO}){\rm PO}_{2}$</tex-math>, is the postulated assignment for the observed 145-G hyperfine doublet observed in 3′-cytidylic acid. Depending on the individual dinucleotide, the free radical giving rise to the 120-G doublet after a dose of ∼10 MR accounts for from ∼0.1% to ∼5% of the total number of free radicals produced.

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