Molecular mechanisms of radiation induced DNA damage: H-abstraction and beta-cleavage.

Quantum mechanical simulations of hydrogen abstraction by hydroxyl radical from methanol and ethanol yield barriers that agree very well with those measured experimentally. Analysis of the multiconfigurational wavefunction indicates that the strength of the C-H bond is the electronic parameter that has a major contribution to the barrier for H-abstraction. Similar analysis applied to 2-deoxy-D-ribose shows that the strength of a C-H bond together with the steric accessibility of the hydrogen determine that H4 is the most susceptible hydrogen for abstraction by a hydroxyl radical. Quantum mechanical simulations of beta-cleavage show that a concerted mechanism in which a water molecule assists in the bond breaking process is more likely than a SN1 mechanism. However, the polar transition state suggests that the environment of the DNA and the surrounding water will have an important effect on the reaction.