Abstract: We studied mechanism of chiral enantiomer transition of phenylalanine （Phe） molecules and catalysis of water molecules (clusters) at the MP2/6-311++G(3-df,2pd-)/WB97X-D/6-311++-G(-d,p-) level, and the bonding characteristics of the stationary points were analyzed by the atomic theory in molecules (AIM). The results show that the chiral enantiomer transition of Phe molecule is realized through a series of transition states of carboxyl hydroxyl rotation, proton transfer, carboncarbon bond rotation and amino turnover. The protons are transferred with amino nitrogen as the bridge in channela, and in channel b with carbonyl oxygen and amino nitrogen as the bridge. When two water molecules clusters are used as proton transfer medium, the reaction path of photon tranfer only with carbonyl oxygen as the bridge is added in channel b. The channel a has the advantage, where the intrinsic energy barrier of the ratedetermining step is 25971 kJ/mol and the apparent energy barrier is 27026 kJ/mol. The intrinsic energy barrier of the ratedetermining step is reduced to 126.47 kJ/mol by the catalysis of two water clusters, and the apparent energy barrier is reduced to 80.80 kJ/mol. When zeropoint vibration energy is considered, the enery barrier of proton transfer from amino nitrogen to carbonyl oxygen disappears. The chiral enantiomeric transition of Phe molecules can be realized by the catalysis of water molecules (clusters).

Key words: phenylalanine, chiral enantiomer, density functional theory, ab initio, activation energy