Theoretical investigation of hydrogen transfer mechanism in the guanine-cytosine base pair

We have studied the quantum-dynamics of the hydrogen bonds in the guanine-cytosine base pair. Due to the position of hydrogen atoms, different tautomers are possible: the stable Watson-Crick G-C, the imino-enol G*-C*, the imino-enol-imino-enol G#-C# and some zwitterionic structures. The common idea in the literature is that only the G-C and the G*-C* tautomers are stable with an estimate of G-C → G*-C* transition probability of 10−6-10−9 by the help of Boltzmann statistics. Here we show a detailed quantum theoretical study that suggests the following conclusion: G-C is the stablest tautomer, some partially charged systems (due to the movement of only one hydrogen atom) are important and a large amount of the imino-enol G*-C* (and less of the imino-enol-imino-enol G#-C# structure) tautomer is present at any time. The corresponding transition probabilities from different tautomers are not due to thermal passage, but they are a pure quantum phenomenon. These large probabilities definitively disprove the idea of these tautomers as mutation points. The mechanisms of passage from the G-C tautomer to the others have also been investigated.