DNA mutations are among the most exciting subject by which many scientists (chemists, biochemists, and biologists) have been attracted during the last 50 years. Although various hypotheses and models were proposed in the literature, the nature and the mechanism of DNA mutations are not quite clear yet. Any changes in the DNA sequence, the key to all genetic information, can generate mutations during the replication or repair processes. Therefore, understanding of DNA mutations is crucial not only to explain but also to predict and consequently to eliminate various diseases affecting human beings.

Prototropic tautomerism (amide-iminol and amine-imine tautomerism in DNA bases) is one of the phenomena responsible for spontaneous point mutations. During tautomeric interconversion, it is sufficient that a proton is transferred from one to the other conjugated site in DNA nitrogen bases that their ability to H-bond formation dramatically changes.

For pyrimidine bases, thymine (X = O) and cytosine (X = NH), their tautomeric preferences seem to be particularly sensitive to various internal and external effects. Since they contain two protons and four functions (two ring nitrogens and two exo-groups with heteroatoms), and rotations of the exo-groups take place, 13 tautomers-rotamers for thymine and 14 tautomers-rotamers for cytosine may be considered.

To understand how ionization influences tautomeric equilibria and pi-electron delocalization for these pyrimidine bases, in this work quantum-chemical calculations were performed for isolated neutral and ionized tautomeric forms of cytosine using the DFT(B3LYP) method and the 6-311+G(d,p) basis set. Change of tautomeric preferences upon ionization induces the point mutation of DNA.

Acknowledgments. DFT calculations were carried out at the Interdisciplinary Center for Molecular Modeling (ICM, Warsaw).

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