Atmospheric oxygen is generated by water oxidation at the MnCa cluster of the protein-pigment complex, photosystem II located in the thylakoid membrane. At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophyll
a, P680 in photosystem II and P700 in photosystem I of cyanobacteria, algae, or plants, and a pair of bacteriochlorophyll
a, P870 in purple bacterial RCs. These pairs differ greatly in their redox potentials for one-electron oxidation,
Em. For P680,
Em is 1100-1200 mV, but for P700 and P870,
Em is only 500 mV. Calculations with the linearized Poisson-Boltzmann equation reproduce these measured
Em differences successfully.
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