Electron spin resonance of irradiated DNA

An electron spin resonance study of deoxyribonucleic acid (DNA) after irradiation with U. V. light or ionizing radiation has identified the main free radical as a hydrogen addition product of the base thymine. The unpaired spin has been located at the C₅ position from its hyperfine interaction of 20.80.25em0ex±0.3 gauss with the -CH₃ group attached to C₅ and confirmed by deuteration of the methyl group. Hydrogen atom addition at the C₆ was proven by exposing the DNA to D₂O vapors before irradiation in which case the 37.760.25em0exhfs of the CH₂ was reduced to the value characteristic of CHD. The amount of water vapor increased the amount of thymine radical as did increasing the intensity of irradiation and the sample temperature during irradiation from 77^∘ to 195^∘K. The quantum yield was ~0.25em0ex5×10⁻⁴ radicals/photon adsorbed which, within experimental error, is as large as for any stable photoproduct of DNA. Paramagnetic metal ions bound to the DNA in the ratio of one per ten phosphates reduced the amount of free radical formed by a factor of about six. Denaturing the DNA does not change the resonances observed. DNA from different animals shows different amounts of the thymine radical and these differences are not understood. It is not known what stable photoproduct is formed by the thymine radical when it disappears at room temperature in moist samples. Four other radicals are characterized, but not identified, by subtracting the calculated thymine spectrum from the observed spectra.