Estimation of Life Times and Diffusion Distances of Radicals Involved in X-Ray-Induced DNA Strand Breaks or Killing of Mammalian Cells

We have used a mammalian tissue culture system to calculate the life times and diffusion distances in DNA scissions as well as cell killing for the three main products of water radiolysis: OH, H, and$e{}_{{\rm aq}}{}^{-}$. Using various alcohols as radical scavengers, the average life time for OH in DNA single-strand breaks was calculated to be about 4× 10^-9 sec. Using the same data and published rate constants, the apparent life time of H atoms was calculated to vary from about 2× 10^-7 to 4× 10^-6 sec and, similarly, the calculated life time of the hydrated electron was found to vary more than was the case for OH. From these life times, the radical diffusion distances were estimated to be approximately 60 Å for OH, which is reasonable, but the values for both H and$e{}_{{\rm aq}}{}^{-}$ were unrealistically large, i.e., 880-4040 Å for H and 9590-19810 Å for$e{}_{{\rm aq}}{}^{-}$. In cell killing, the OH radical life time was estimated to be about$8.7\times 10^{-9}$ sec which gives an average diffusion distance for this radical of about 93 Å. Our data support the idea that OH is the radical species primarily responsible for the indirect effect in radiation injury measured as DNA single-strand breaks or cell killing, and that H and$e{}_{{\rm aq}}{}^{-}$ are not significantly involved.