Proton Transfer in Guanine-Cytosine Base Pairs in B-DNA.

A double proton transfer reaction in a guanine-cytosine (GC) base pair has been proposed as a possible mechanism for rare tautomer (G*C*) formation and thus a source of spontaneous mutations. We analyze this system with free energy calculations based on extensive Quantum Mechanics/Molecular Mechanics simulations to properly consider the influence of the DNA biomolecular environment. We find that, although the G*C* rare tautomer is metastable in the gas phase, it is completely unstable in the conditions found in cells. Thus, our calculations show that a double proton reaction cannot be the source of spontaneous point mutations. We have also analyzed the intrabase H transfer reactions in guanine. Our results show that the DNA environment gives rise to a large free energy difference between the rare and canonical tautomers. These results show the key role of the DNA biological environment for the stability of the genetic code.