Two paths of proton transfer of the molecular structures of guanine tautomers were calculated by the B3LYP method at the 6-311++G** level

both in the gaseous and in the aqueous phases

with full geometry optimization（Path A is intramolecular proton transfer

Path B is water-assisted intermolecular proton transfer）.The geometry and electronic structure of the tautomers of guanine and their transition states were obtained.The PCM（polarized continuum model） solvate theory model was employed for the aqueous solution calculations

and the influences of the solvent effects on the geometry

energy

charge and activation energy were discussed.The calculation results indicate that the activation energies of Path B is about half of Path A.The molecular structure of transition states of Path B is a six-member ring

and that of Path A is a four-member ring.