Theoretical study of the • H reaction with cytosine

We studied three possible reactions of H atom attacking the cytosine, using density functional theory (DFT) calculations. The results indicate that the H atom addition to the N-3 site of cytosine is energetically more favorable than to the C-5 or C-6 site. The reaction of addition to the C-6 site has an energy barrier of similar to 2.77 kcal/mol, which is similar to 2 kcal/mol higher than addition to C-5. The energy of C-5 H-adduct radical is also lower than that of C-6 H-adduct radical. From the point of view of both energetics and reaction kinetics, the addition of the H atom to the C-5 site is preferred to the addition to the C-6 site. (C) 2006 Wiley Periodicals, Inc.