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).