Prototropic changes in cationic base-pair adducts. I. Guanine protonation

AM 1 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. © 1992 John Wiley & Sons, Inc.