中文摘要：

运用Gaussian98 MP2／6-311G（d，p）级的从头计算法计算了DNA分子中胞嘧啶碱基中H13质子的势能曲面。发现有两个势阱：其中一个对应正常的胞嘧啶，而另一个则对应它的顺式亚胺式互变异构体。质子在这两个势阱的束缚对热扰动是稳定的。质子在势阱中像单摆一样绕着离它最近的氮原子摆动，还可以远离氮原子与其它碱基形成氢键。估算出的H13质子从一个势阱到另一个势阱的穿透几率表明质子呆在其中一个势阱中的寿命大约是6×10^2a，这使胞嘧啶和它的顺式亚胺式互变异构体在室温气相实验中远不能达到热平衡。对这些结果的生物学意义作了讨论。

英文摘要：

The potential energy surface of the H13 proton in base cytosine of the DNA molecules is calculated 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 pendulum, 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 6×10^2 a. 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 results is discussed.