Abstract
The primary donor Chl of PSII, P680, has an extremely high oxidation potential (Eox) of ~1.2 V, which enables water oxidation. How such a high potential was achieved in the evolution of PSII remains unanswered. We have investigated the cause of the high P680 potential using a quantum chemical method. The dielectric-constant (ε) dependence of Chl Eox was estimated by DFT calculations. A sharp increase in Eox with decreasing ε was observed at ε < 5, whereas Eox was rather constant at higher ε. From this relationship, the Eox of Chl in ideal hydrophobic proteins (ε = 2) was estimated to be ~1.5 V. This indicates that the Eox of Chl is originally very high in nonpolar environments. Indeed, the protein environment around P680 is significantly hydrophobic in comparison with that around bacterial P, suggesting that reducing the polarity around P680 was a major factor to achieve its high potential.
