Abstract :
The tautomeric equilibrium between the keto–amino and enol forms of cytosine was investigated in aqueous solution. Calculations in solution were performed using both combined discrete/self-consistent reaction field (SCRF) and SCRF methods. Complexes containing a cluster of seven water molecules filling up the first solvation shell of cytosine were used for combined discrete/SCRF calculations. Results indicate that SCRF methods overestimate the stability of the enol tautomers by about 10 kcal/mol. Thus, the free energies of tautomerization of the enol forms determined from combined discrete/SCRF methods are about 16–19 kcal/mol, whereas the estimates derived from the SCRF methods are about 6–8 kcal/mol.
