Quantum chemical calculations cytosine

As mentioned in the Introduction, tautomerism plays a major role in the chemistry of DNA and RNA bases. Colominas et al have made an extensive study of the tautomeric equilibria of guanine and cytosine (see Figure 5). The importance of protonation on the equilibria was also studied. To begin with all possible tautomers were considered. The least stable species were then screened out using increasingly accurate quantum chemical calculations with a SCRF solvent term. FEP calculations were then used for the most important equilibria. In the neutral cytosine molecule, four important tautomers are based on the amine form shown in Figure 5 as cytosine I. Another 2 tautomers are based on the imino form shown in the Figure 5 as cytosine II. In the gas phase, an admixture of forms is predicted, with the depicted cytosine I structure the most stable. In solution, all tautomers are destabilised with respect to this tautomer, which then dominates by about 20 kJ/mol. 

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