Purine tautomeric preferences and bond-length alternation in relation with protonation-deprotonation and alkali metal cationization.

Quantum chemical calculations were carried out for deprotonated (P) and protonated purine (PH) and for adducts with one alkali metal cation (PM and PM, where M is Li or Na) in the gas phase {B3LYP/6-311+G(d,p)}, a model of perfectly apolar environment, and for selected structures in aqueous solution {PCM(water)//B3LYP/6-311+G(d,p)}, a reference polar medium for biological studies. All potential isomers of purine derivatives were considered, the favored structures indicated, and the preferred sites for protonation/deprotonation and cationization reactions determined. Proton and metal cation basicities of purine in the gas phase were discussed and compared with those of imidazole and pyrimidine.