A theoretical study on tautomerization processes of dehydrated and monohydrated cytosine

Some dehydrated and monohydrated tautomeric forms of cytosine enclosed in an energy range of about 20 kcal/mol were investigated at B3LYP/6-311+G(2df,2p) density functional level of theory with the aim to evaluate the interconversion barriers between them and to establish if it possible explain the coexistence, experimentally proven, of some keto-amino and enol-amino forms of free bases. Results indicated that, the clearing of the activation barriers involved in gas-phase is unlikely but, the presence of a coordination water molecule acts as a catalyst making quite easier the transformation processes. Limited to the two most stable keto-amino and enol-amino tautomers, interconversion was studied in the presence of two explicit water molecules. From the obtained data it appeared clear that the influence of the second water molecule as well as the introduction of solvent effects in reducing the height of the tautomerization barriers for these bihydrated systems is quite small.