PROTOTROPIC CHANGES IN CATIONIC BASE-PAIR ADDUCTS .1. GUANINE PROTONATION

AM1 calculations have been used to study the effects of protonation on the structures, energies, and, in some cases, proton transfer reactions of guanine cytosine base pairs. Protonation at the guanine O6-position, or at various ring sites, leads to a relatively facile conversion to a surprisingly stable complementary base pairs following proton transfer to the cytosine 3-position. In the case of O6-protonation, this constitutes a direct route to guanine enolization. It is suggested that the spontaneous formation of apyrimidinic sites in nucleic acids take place via prior protonation of guanine moieties in the opposite strand.