A quantum mechanical study of the ground state of the normal and rare tautomeric forms of pyrimidine bases and their photohydrates has been performed using the semiempirical Hamiltonians AM1 and PM3 with full geometry optimization. Unlike cytosine, it is predicted that both conformations of the rare imino form (trans and cis) of cytosine photohydrate are more stable than their amino form (1,2). The HF, MP2 (fc) and DFT nonempirical methods predict an analogous result (3). Hydration simulation of these compounds by the Monte Carlo method showed that the amino tautomer of cytosine is more stabilized by a water cluster than the imino one, while cytosine photohydrate is predicted to be more stable in its imino form (2). The interaction between the water and Cyt photohydrate does not change the order of the relative stabilities of the considered tautomers from that predicted for the gas phase. The solvent-induced stabilization for the trans conformation increases the relative population of the imino tautomers with respect to the situation in the gas phase. The possible biological significance of this prediction may be important, since the rare tautomer of cytosine photohydrate can form a base pair with adenine that leads to the G-C -> A-T (U) or C -> T (U) transitions because of the replication error. The data obtained serve as evidence that the tautomerism is a possible mechanism of UV-light mutagenic effect.

References and Footnotes

1. V. I. Danilov, J. L. Alderfer, Biopolymers and Cell (in Russian) 15, 487 (1999).
2. V. I. Danilov, J. J. P. Stewart, A. Les and J. L. Alderfer, Chem. Phys. Lett., submitted for publication.
3. V.I. Danilov and A. Les, Polish J. Chem., in press.