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-induced Mutations

A quantum mechanical study of cis-syn cyclobutane cytosine-containing photodimers including the normal and rare tautomeric forms of bases has been performed using the ab initio method at HF/6-31G(d,p), MP2(fc)//HF/6-31G(d,p) and MP2(fc)/6-31G(d,p) levels. It is predicted that in the gas phase all photodimers containing the rare imino form of cytosine are more stable than those containing its normal form. The Monte Carlo study of the hydration for cytosine-consisting dimers showed that the dimer containing the imino form 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-containing 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 UV-induced mutation.