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 Mn(4)Ca cluster (relative to PSI and PbRC), the position of P680 close to the luminal edge of transmembrane alpha-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).