How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870

At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophyll a (Chla), P700 in photosystem I (PSI) and P680 in photosystem II (PSII) of cyanobacteria, algae, or plants, and a pair of bacteriochlorophyll a (BChla), P870 in purple bacterial RCs (PbRCs). These pairs differ greatly in their redox potentials for one-electron oxidation, E_m. For P680, E_m is 1,100-1,200 mV, but for P700 and P870, E_m is only 500 mV. Calculations with the linearized Poisson-Boltzmann equation reproduce these measured E_m differences successfully. Analyzing the origin for these differences, we found as major factors in PSII the unique Mn₄Ca cluster (relative to PSI and PbRC), the position of P680 close to the luminal edge of transmembrane α-helix d (relative to PSI), local variations in the cd loop (relative to PbRC), and the intrinsically higher E_m of Chla compared with BChla (relative to PbRC).