Abstract: We studied reaction mechanism and water solvation effect of chiral transition of tryptophan molecules of two most stable configurations by
 using the B3LYP method of density functional theory, the MP2 method of perturbation theory, and SMD model method of selfconsistent reaction field (SCRF) theory. The results show that there are three chiral transition channels a,b and c of tryptophan molecules of two configurations. The dominant reaction channel of configuration one is channel a, and the stepdetermining free energy barrier is 2567 kJ/mol.  The dominant reaction channels of configuration two are channel b and c,  and the stepdetermining free energy barriers are 258.8, 256.7 kJ/mol, respectively. The stepdetermining free energy barriers come from the transition state of proton transfer from the chiral C to the amino N. The water solvation effect reduces the stepdetermining energy barrier to 113.4 kJ/mol for the dominant reaction channel of the configuration one. Monomer tryptophan molecule is stable. The chiral transition of tryptophan can molecules can proceed slowly in water solvent environment.

Key words: density functional theory； transition state； perturbation theory； selfconsistent reaction field, tryptophan, chirality