Acquirement of water-splitting ability and alteration of the charge-separation mechanism in photosynthetic reaction centers.

In photosynthetic reaction centers from purple bacteria (PbRC) and the water-oxidizing enzyme, photosystem II (PSII), charge separation occurs along one of the two symmetrical electron-transfer branches. Here we report the microscopic origin of the unidirectional charge separation, fully considering electron-hole interaction, electronic coupling of the pigments, and electrostatic interaction with the polarizable entire protein environments. The electronic coupling between the pair of bacteriochlorophylls is large in PbRC, forming a delocalized excited state with the lowest excitation energy (i.e., the special pair). The charge-separated state in the active branch is stabilized by uncharged polar residues in the transmembrane region and charged residues on the cytochrome binding surface. In contrast, the accessory chlorophyll in the D1 protein (Chl) has the lowest excitation energy in PSII. The charge-separated state involves Chl and is stabilized predominantly by charged residues near the MnCaO cluster and the proceeding proton-transfer pathway. It seems likely that the acquirement of water-splitting ability makes Chl the initial electron donor in PSII.