A theoretical investigation of tautomeric equilibria and proton transfer in isolated and hydrated thiocytosine

Abstract

The results of an ab initio post Hartree–Fock study of the relative stabilities and mechanism of intramolecular proton transfer in isolated and monohydrated thiocytosine 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)/6-31+G(d,p)//MP2/6-31G(d) and MP2/6-311++G(d,p)//MP2/6-31G(d) approximations. The post Hartree–Fock ab initio theory predicts different distributions of the tautomers for cytosine and thiocytosine both in the gas phase and in polar media (PCM solvation model). Interaction of thiocytosine tautomers with one water molecule decreases the difference in the energies of the two most stable tautomers significantly but does not change the order of the stability of tautomers. However, five water molecules forming a first solvation shell change the stability pattern. The heights of the barriers of sulfur-involved proton transfers in thiocytosine are much smaller than analogous barriers in cytosine. In contrast, corresponding barriers for the complexes with one water molecule are higher in thiocytosine.