Theoretical study of the excited states of the photosynthetic reaction center in photosystem II: Electronic structure, interactions, and their origin

The excited states of the chlorophyll 6-mer in the photosystem II (PSII) reaction center (RC) were investigated theoretically using ab initio quantum chemical calculations, and the results are compared with those of the bacterial reaction center (bRC). A significant difference in the peak at the lowest energy in the absorption spectra arises from the structural asymmetry of the special pair (SP). The origin can be traced back to the structural difference in the CD helix. The low-lying excited states are characterized as a linear combination of the excited states of the chlorophyll monomers, which verifies the applicability of exciton theory. Analysis of the molecular interactions clearly explains the cause of the constructive/destructive interferences in the state transition moment. The protein electrostatic potential (ESP) decreases the energy of the charge-transfer (ChlD1 → PheoD1) state. The ESP also localizes the HOMO distribution to the PD1 moiety and increases the ionization potential.

Highlights► Ab initio calculations of the excited-states of the photosynthetic reaction centers. ► Asymmetry of the special pair causes the spectral change in PSII. ► Structural differences in the CD helix are the origin of the structural asymmetry. ► Protein electrostatic potential stabilizes charge-transfer (ChlD1 → PheoD1) state. ► Protein electrostatic potential increases ionization potential of the special pair.