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

The results of an ab initio post Hartree-Fock study of the relative stabili ties and mechanism of intramolecular proton transfer in isolated and monohy drated thiocytosine are reported. The geometries of local minima and transi tion states were optimized without symmetry restrictions by the gradient pr ocedure at the HF and MP2 levels of theory and were verified by energy seco nd derivative calculations. The standard 6-31G(d) basis set was used. The s ingle 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 tauto mers for cytosine and thiocytosine both in the gas phase and in polar media (PCM solvation model). Interaction of thiocytosine tautomers with one wate r molecule decreases the difference in the energies of the two most stable tautomers significantly but does not change the order of the stability of t automers. However, five water molecules forming a first solvation shell cha nge the stability pattern. The heights of the barriers of sulfur-involved p roton transfers in thiocytosine are much smaller than analogous barriers in cytosine. In contrast, corresponding barriers for the complexes with one w ater molecule are higher in thiocytosine. (C) 2001 Elsevier Science B.V. Al l rights reserved.