Abstract: Using the B3LYP mehtod of density functional theory (DFT), the author optimized the geometry of gas phase methionine (Met) molecule at the level of the 6-311+G(2df) base set, and analyzed the characteristics of each elementary reaction in the process of chiral transformation of the Met+2H₂O molecular system. Under M06-2X/def2-TZVP, the time-dependent density functional theory (TDDFT) was used to obtain the molecular orbitals (MO), natural transition orbitals (NTO) and isosurface maps of hole-electron distribution of a transition state system in the process of the chiral transition of the Met+2H₂O molecular system in the implicit solvent methanol, and the electronic excitation characteristics were studied. The results show that with the help of double water molecules, H atom migrates to form transition states, which makes the Met molecular system structure and reactants change significantly, as well as the molecular function changes.

Key words: methionine, graphic analysis, elementary reaction, transition state, electronic excitation