Theoretical study of the OH reaction with cytosine

The adduction reactions and hydrogen abstraction reactions of OH radical with cytosine have been studied by using density functional theory (DFT). The results indicate that the OH radical additions to the C5 and C6 sites are thermodynamically and kinetically more favorable than to the N3 and C4 sites. The H42 atom abstraction reaction has an energy barrier ∼2.51 kcal/mol (without zero-point correction), which is the smallest among the three hydrogen atom abstraction reactions. And the H42-abstracted radical is energetically most favorable in the hydrogen abstraction reactions. However the energy of the H42 abstraction products, the H42-abstracted radical and the water molecule, is about 8.79 kcal/mol higher than that of the C6-hydroxylated radical. So in comparison with the OH radical addition to the sites C5 and C6 of cytosine, the H42 atom abstraction reaction has less reaction probability.