In the present paper, we consider the formation of rare tautomeric forms of the neutral base pairs adeninethymine (A-T) and cytosine-guanine (C-G) in low-energy excited singlet electronic states. Ab initio calculations (6-31G basis set) have been carried out at the Hartree - Fock level of theory for the ground electronic states and using a configuration interaction among all single excitations (CIS) technique for the excited electronic states. The obtained results indicate that the double proton transfer is not a feasible process in the ground electronic states. For the excited singlet electronic states, which can be directly accessed upon photoexcitation, the excitation energy is localized in the π system of one of the monomers of the pair. In these states, especially in the A-T base pair, the double proton transfer becomes energetically more accessible. However, it is unlikely that the rare tautomer may live long enough to perturb the duplication of the genetic code. Our theoretical results also show the existence of charge-transfer excited electronic states in both A - T and C - G base pairs. These states are found at a considerable high energy in the region corresponding to the ground-state minimum-energy configuration. These structures, which can be accessed only upon internal conversion from another excited electronic state, have a remarkable minimum of energy in the region corresponding to a single proton transfer that eventually neutralizes the charge separation induced by the electronic transition. We discuss the possibility that such metastable structures may play a key role in altering the DNA unwinding and strand separation (that is, in mutagenesis).