Motion of the hydrogen bond proton in cytosine and the transition   between its normal and imino states
  The potential energy surface of the H13 proton in base cytosine of the DNA molecules is calculated {\it ab initio} at the Gaussian98 MP2/6-311G(d,p) level. Two potential wells are found. One corresponds to the normal cytosine, while the other corresponds to its imino tautomer. The bindings of the proton in these wells are stable enough against the thermo-disturbance. The motions of the proton in these wells are oscillations around the nearest nitrogen atom like the pendula, and may move far away from the nitrogen atom to form the hydrogen bond with other bases. The estimated tunneling probability of the H13 proton from one well to another well shows that the life time of the proton staying in one of these wells is about 6$\times10^2$ yr. It is too long to let tautomers of cytosine be in thermodynamical equilibrium in a room temperature gas phase experiment. The biological significance of these result is discussed. 
Zhen-Min Zhao, Qi-Ren Zhang, Chun-Yuan Gao, Yi-Zhong Zhuo