Protonation vs. ionization on intermolecular proton transfer processes in Guanine-Cytosine Watson-Crick base pair

Single and double proton transfer processes in ionized and protonated Guanine-Cytosine base pair are studied with the B3LYP theoretical method. The main analogy between both ionization and protonation arises from the positive charge on Guanine which leads to a remarkable stability of the N1-N3 proton transferred structure. The major difference comes from the asymptote behavior of the product, which is significantly less stable for the protonated system than for the ionized one. The possible implications on mutagenesis are discussed.