Abstract: Using density functional theory B3LYP, we investigat ed the chiral transition mechanism of the leucine (Leu) molecule in the non\|confined m onomer and water at the level of 6-311+G(d,p) basis sets. We drawed a Leu mo lecule chiral transition path reaction potential energy surface by finding struc tures of the extreme value points including the transition st ates and intermediates, and analyzed the geometric and electronic structure prop erties of extreme value points. The results show that the hydrogen atom o n the chiral carbon atom of SLeu molecule can transfer to the other si de of the carbon atom via the oxygen atoms of carboxyl atoms as a bridge, and ach ieve the chiral transition of Leu molecule from Stype to Rtype. On t his path there are four intermediates and five transition states. The maximum re action energy barrier is 57235 9 kJ/mol, which comes from the first transition state TS₁-S-Leu&1H₂O-1. The maximum reaction energy barrier of the first transition state TS₁-S-Leu of monomer Leu molecule is 145452 7 kJ/mol. As a result, a single water molecule as the H transfer bridge can reduce the reaction barrier.

Key words: leucine, transition state, chirality, water molecule