The Distance That a Radical Formed by Ionizing Radiation Can Diffuse in a Yeast Cell

Abstract

An equation ob-tained previously by Zirkle and Tobias, which relates the dose of ionizing radiation and the magnitude of the indirect effect produced on molecules, is derived on somewhat different grounds. A parameter, [rho], having the significance of an average distance which the free radicals produced by the radiation can diffuse, is evaluated from data for the inactivation of enzymes in wet and in dry cells. The parameter is found to be about 30 [image] if it be assumed that the free radicals react with large biological molecules on every encounter; if the probability of reaction on encounter is as low as 10-3, then [rho] will be of the order of a few hundred [image]. Even if the probability of reaction is not known, the equation given can be used to estimate the indirect effect expected on a given molecule. The value of [rho] being in any case a small fraction of the dimensions of a cell, no support is found for any concept of indirect action which visualizes the effects of an ionization diffusing throughout a cell. Instead, a picture rather like that of target theory is indicated, but with a target volume having hazy boundaries of thickness between tens and hundreds of [image]ngstroms.