A theoretical investigation of tautomeric equilibria and proton transfer in isolated and monohydrated cytosine and isocytosine molecules

The results of an ab initio post Hartree-Fock (HF) comparable study of the relative stabilities and mechanisms of intramolecular proton transfer in isolated and monohydrated cytosine and isocytosine molecules are reported. The geometries of local minima and transition states were optimized without symmetry restrictions by the gradient procedure at the HF and MP2 levels of theory and were verified by energy second derivative calculations. The standard 6-31G(d) basis set was used. The single point calculations have been performed at the MP4(SDQ)/6-31G(d,p)/MP2/6-31G(d) and MP2/6-311 + + G(d,p)//MP2/631G(d) approximations. The post HF ab initio theory predicts different distributions of the tautomers for cytosine and isocytosine both in the gas phase and in polar media (Onsager's self-consistent reaction field model). The interactions with one water molecule change the order of relative stability of cytosine and isocytosine tautomers to those corresponding to the experimentally measured relative stabilities in polar solutions. Ab initio calculations predict the heights of the proton transfer barriers for monohydrated nucleic bases to be approximately three times lower compared with non-water-assisted processes. (C) 1999 Elsevier Science B.V. All rights reserved.