Regulated Electron Tunneling of Photoinduced Primary Charge-Separated State in the Photosystem II Reaction Center.

In initial events of the photosynthesis by higher plants, the photosystem II (PSII) generates photoinduced primary charge-separated (CS) state composed of reduced pheophytin (PheoD1-•) and oxidized special pair (P+•) in chlorophyll a (Chla) PD1/PD2 in the D1/D2 heterodimer, ultimately leading to the water oxidation at the oxygen-evolving Mn4CaO5 cluster by P+•. To understand the mechanism of the efficient generation of initially localized CS state (PD1+• PheoD1-•), we have characterized cofactor geometries and electronic coupling of the photoinduced primary CS state in quinone prereduced membrane of PSII from spinach using the time-resolved electron paramagnetic resonance method. It has been revealed that the electronic coupling between the charges is significantly weak in the CS state separated by 1.5 nm, showing the importance of regulated cofactor-cofactor electronic interaction between a vinyl substituent in PheoD1 and an accessory chlorophyll to inhibit the energy-wasting charge recombination after the primary electron-transfer processes.