Thermal versus photochemical tautomerization of cytosine and guanine: a BLYP computational study along the IRC curves

We performed a theoretical study, at the BLYP/aug-cc-pVDZ theoretical level, of the mechanisms of H-transfer between biologically relevant tautomers of cytosine and guanine. The analysis of the ground- and excited state energy barriers of the conversions showed that in some cases they are lower as compared to the ground state reactions. The inclusion of one water molecule as a catalyst in the oxo-amino form of cytosine and guanine drastically changes the mechanism of the reactions in the ground as well as in the excited state. For example, the ground state reaction path of cytosine reduces the energy barrier from 165 to 60 kJ.mol(−1) and for guanine, this reduction is from 92 kJ.mol(−1) to 24 kJ.mol(−1). The results obtained are in support to the high photostability of the nucleic acid bases which guarantee the normal biological function of nucleic acids.