The prototropic molecular-zwitterion tautomerism of purine (Pur) in vacuum was investigated by means of semiempirical quantum mechanical AMI method with full optimization of all structural parameters. It was established that molecular-zwitterion family of Pur occupies the energetical range about 102 kcal/mol and includes 33 dipole-stable isomers – 4 molecular and 29 zwitterion ones. Molecular tautomers of Pur are formed by iminoproton migration, in the case of zwitterions carboprotons are involved as well. The N7H molecular tautomer is the main and practically the only tautomeric form of Pur in gase phase (99.8 % at the room temperature). Among the tautomer-zwitterions ilid form with the protons localization at N7 and N9 and C8H proton absence is the most favourable from energetic view. The authors relate this ilid form to molecular-kinetic mechanism of hydrogen-tritium exchange of C8H group with water under corresponding pH. In the solvent with universal mechanism of solvatation (ε > 1) there is the tautomeric Pur N9H (98%) ↔ Pur N7H (2 %) equilibrium, as is evidenced by quantitative estimation in the frame of Onzager's classical model. The comparison of calculated and experimental data demonstrates an agreement between them.