A theoretical study of the cis-syn cytosine-containing pyrimidine dimers in the gase phase and a water cluster and a tautomer-bypass mechanism for the origin of UV-incduced mutations

A quantum mechanical study of cis-syn cyclobutane cytosine-containing photo dimers including the normal and rare tautomeric forms of bases has been per formed using the ab initio method at HF/6-31G(d,p), MP2(fc)//HF/6-31 G(d,p) and MP2(fc)/6-31G(d,p) levels. It is predicted that in the gas phase all p hotodimers containing the rare imino form of cytosine are more stable than those containing its normal form. The Monte Carlo study of the hydration fo r cytosine-consisting dimers showed that the dimer containing the imino for m of cytosine is stabilized by water cluster more than that containing its amino forms. As a result, the imino form of cytosine in the cytosine-contai ning dimer directs the incorporation of adenine in the complementary strand during replicative bypass. Data obtained point to the cytosine tautomerism as a possible mechanism for the origin of W-induced mutation.