The isomerization of cytosine: Intramolecular hydrogen atom transfer mediated through formic acid

The catalytic ability of H2O and HCOOH to facilitate the tautomerism of KA to KIt isomer has been studied. It is shown that the direct tautomerism (path A) is unlikely because of the high activation free energy, whereas the presence of H2O and HCOOH (paths B and C) significantly contributes to decreasing the activation free energy. Meanwhile, the conventional transition state theory followed by Wigner tunneling correction is applied to estimate the rate constants. The rate constant with Wigner tunneling correction for direct tautomerization is obviously smaller than that of HCOOH-mediated tautomerization, which is the most plausible mechanism. Finally, another important finding is that the ratio of reaction rates between direct tautomerism reaction and catalyst-induced tautomerism increases with the increase of the catalyst concentration at a given temperature. The results of the present study demonstrate the feasibility of acid catalysis for DNA bases isomerization reaction that would otherwise be forbidden.