A MULTIPLE-RADICAL MODEL FOR RADIATION ACTION ON DNA AND THE DEPENDENCE OF OER ON LET

We have a developed a multiple-radical model of the chemical modificat ion reactions involving oxygen and thiols relevant to the interactions of ionizing radiations with DNA. The treatment is based on the Alper and Howard-Flanders equation but considers the case where more than on e radical may be involved in the production of lesions in DNA. This mo del makes several predictions regarding the induction of double strand breaks in DNA by ionizing radiation and the role of sensitizers such as oxygen and protectors such as thiols which act at the chemical phas e of radiation action via the involvement of free radicals. The model predicts a decreasing OER with increasing LET on the basis that as rad ical multiplicity increases so will the probability that, even under h ypoxia, damage will be fixed and lead to lesion production. The model can be considered to provide an alternative hypothesis to those of 'in teracting radicals' or of 'oxygen-in-the-track'.