The effect of visible light (410-550 nm) on radical composition and concentration in solid DNA and DNA constituents, previously γ-irradiated at -196°C in the dark in vacuo, was studied by electron spin resonance. For DNA, adenine, cytosine, and guanine, 70% or more of the radicals were removed by illumination at -196°C, for thymine and 2′-deoxy-D-ribose, about 20%. The decrease of the radical concentration for the corresponding nucleosides was smaller. For thymidine the radical concentration was unchanged by illumination. The combined effects of thermal annealing and illumination on γ-irradiated DNA were also investigated.

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