INTERMOLECULAR INTERACTIONS AND TAUTOMERISM OF NUCLEIC-ACID BASES AND THEIR ANALOGS

The structures and stabilization energies of guanine-cytosine, guanine-thymine and adenine-thymine base pairs formed by "normal" amino-oxo and "rare" amino-hydroxy tautomers of guanine and the "normal" tautomers of adenine and thymine are studied using a combined quantum-mechanical approach, an ab initio calculation for relative stabilities of the isolated tautomers together with a calculation of intermolecular interaction energies using a multipolar expansion method developed by Claverie and co-workers. It is shown that the interaction between the components of the base pairs decreases the relative stability of the pairs involving "rare" tautomers of guanine; for the isolated molecules the stability of both tautomers of guanine is found to be approximately the same. Our calculations include the base pairs formed by the "rare" hydroxy tautomer of guanine, whose existence has been reported in recent experimental studies of guanines isolated in an inert low-temperature matrix. Calculations for this amino-hydroxy form of guanine serve as a model for interactions of O6-methylguanine with pyrimidine bases. © 1990.