Canonical, enol and imino tautomers of cytosine were studied theoretically in the gas phase, in a microhydrated environment (1 and 2 waters) and in bulk water. The structures of isolated, mono- and dihydrated tautomers were determined at the RI-MP2 level with the TZVPP basis set. The relative energies of isolated tautomers were calculated up to the CCSD(T) level using the cc-pVTZ basis set and at the MP2 level using the aug-cc-pVQZ basis set. For the MP2 and CCSD(T) predictions, complete basis set estimates were obtained using various extrapolation techniques. One of the enol forms is the global minimum at all theoretical levels in the gas phase while the canonical form represents the first local minimum. Already two water molecules reverse the relative stability of these two tautomers making the canonical form the global minimum. The effect of bulk solvent on the relative stability of cytosine tautomers was examined from self-consistent reaction
field, Monte Carlo and molecular dynamics free energy calculations. Bulk solvent calculations unambiguously favored the canonical tautomer over the enol forms, in agreement with the trends found for the mono- and dihydrated cluster model. However, the bulk solvent results for relative energy changes differ from those of the cluster model. While the enol structure is predicted to be the least stable species in the bulk solvent, the microhydration model predicts it to be the first local minimum with a rather small energy difference (∼1 kcal mol−1) with respect to the global minimum.
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