Abstract: Using B3LYP method of density functional theory (DFT), the geometry of methionine (Met) was optimized under the gas phase condition at the level of 6-311+G(2df) basis set, and under the PBE0/def2-TZVPP, using the method of time-dependent density functional theory (TDDFT), we obtained the molecular orbital (MO) characteristics, and hole-electron distribution isosurface maps of the chiral conversion reactants of Met and H₂O molecules with 1∶1,1∶2 mixed system in the implicit methanol solvent,  compared and analyszed electronic excitation characteristics with diagrams. The results show that the three qualitative analysis results are basically consistent, and the qualitative results of excitaed state S9 of S-Met-CH₃OH are inconsistent with those of S-Met-CH₃OH+1H₂O and S-Met-CH₃OH+2H₂O, the qualitative results of excited state S7 of S-Met-CH₃OH+2H₂O and S-Met-CH₃OH+1H₂O are inconsistent, and the water chain has a certain effect on the electronic excitation characteristics of S-Met-CH₃OH molecular system.

Key words: methionine, chiral molecule, mixed system, hole-electron, electronic excitation