Abstract: We studied the optical isomerism reaction of two kinds of the most stable configurations of methionine molecules based on amino group as proton transfer bridge by using the B3LYP method of density functional theory and the MP2 method of perturbation theory. The results show that there are two channels a and b in the optical isomerism reaction  of methionine molecules based on amino group as proton transfer bridge. The dominant reaction channel of
the configuration one is channel a, the ratedetermining step is the first elementary reaction, and the free energy barrier is 264.2 kJ/mol that is generated by the transition state of proton transfer from the chiral C to the amino N. The dominant reaction channel of the configuration two is the channel a, the ratedetermining step is the second elementary reaction, and the free energy barrier is 266.1 kJ/mol that is generated by the transition state of proton transfer from the chiral C to the amino N after carboxyl group isomerism. The reaction rate constants of the optical isomerism ratedetermining step of two configurations of methionine molecules are 3.04×10^-34,1.41×10^-34 s^-1, respectively.

Key words: optical isomerism, methionine(Met), perturbation theory, density functional theory, chirality, transition state