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 ( RO)( R^ O) PO₂ , with the unpaired electron localized on one or both of the nonester oxygens. A similar radical, ( HO)( RO) PO₂ , 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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Bernhard et al. (1973) studied this question.