Intramolecular proton transfer in monohydrated tautomers of cytosine: An ab initio post-Hartree-Fock study

Abstract

The results of an ab initio post-Hartree–Fock study of the molecular structures, relative stabilities, and mechanisms of intermolecular proton transfer in isolated and monohydrated cytosine complexes are reported. The geometries of the local minima and transition states were optimized without symmetry restrictions by the gradient procedure at the HF and the 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 were 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. All values of the total energies were corrected for scaled zero-point energy contributions. The post-Hartree–Fock ab initio theory predicts the height of the proton-transfer barrier for monohydrated cytosine complexes to be approximately three times lower for the tautomeric oxo-hydroxo reaction compared with non-water-assisted processes. The influence of polar media (Onsager's self-consistent reaction-field model) slightly changes these values according to the order of the stability of the tautomers in a polar solution. The interaction with one water molecule changes the order of the relative stability of cytosine tautomers from the gas phase to the one which corresponds to the experimentally measured relative stabilities in polar solutions. In contrast to guanine, we did not find significant water influence on the NH2-nonplanarity phenomena. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 855–862, 1998