Abstract: We studied optical isomerism mechanism and water solvation effect from gaseous phase S-isoleucine to R-allo-isoleucine 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 channels a,b and c in the reaction. The optical isomerism reaction requires three elementary reactions in the channel a and c, four elementary reactions in the channel b. Channel a is the main reaction channel and the stepdetermining Gibbs free energy barrier is 255.0 kJ/mol, which is generated by the transition state of proton transfer from the chiral carbon to the amino N. The stepdetermining reaction rate constant is 1.25×10^-32 s^-1. The water solvation effect enables the stepdetermining energy barrier to reduce to 114.1 kJ/mol and the reaction rate constant to increase to 2.73×10^-7 s^-1. It shows that water environment has better catalytic effect on Sisoleucine optical isomerization.

Key words: allo\, transition state, optical isomerism, isoleucine, density functional theory, chirality, isoleucine, perturbation theory, selfconsistent reaction field (SCRF)