Abstract: Using the CAMB3LYP method of density functional theory, we optimized the geometric configuration of R-Cysteine (R-Cys) molecule in gas phase at 6-31G (d) basis set, theoretically studied the charge transfer characteristics between fragments of R-Cys system during the electron excitation, and calculated the charge transfer percentage between fragments based on the excited state density of relaxation and nonrelaxation. The results show that for SH fragment, the charge transfer percentage from S0 to S3 is 57.96%. For COOH fragment, the charge transfer percentages of the excited states from S0 to S1—S5 are negative, and the qualitative results of charge transfer are consistent. For NH₂ fragments, the charge transfer percentages from S0 to S1 and S4 are 698% and 3145%, respectively.

Key words: cysteine, electron density, charge transfer, theoretical calculation