Early chemical events and initial DNA damage.

Early chemical events (between 10(-15) and 10(-6) seconds) as they relate to the evolution of damage in radiation biology have been described in terms of a theoretical model. DNA is the target of concern in this model, and both indirect and direct effects have been explicitly accounted for in evaluating yields of strand breaks. In the indirect-effect considerations, a quantitative estimation of the time decay of water radical species--beginning with their production at 10(-14) seconds and leading to the interactions of hydroxyl radicals with DNA--has been a major focus. A method based on stopping-power theory and the Bragg rule has been described to account for direct effects. However, no attempt is made to follow all the chemical events that take place between the creation of initial (10(-6) seconds) damage and the observable strand break yields. The theoretical calculations refer to a simple aqueous system containing DNA molecules and scavenger (Tris). The theoretical results of strand break yields by different heavy charged particles are in good agreement with experimental cellular data under conditions of minimal enzymatic repair.