The primary electron donor of photosystem(PS) I (P700), consisted of a dimer of chlorophyll a-type pigments, generates strong reduction power upon illumination which is utilized for CO₂ fixation in the dark. Though numerous data have been accumulated for the P700 redox potential, which is one of the keys for understanding the molecular basis of highly efficient photosynthetic light reaction, over decades, the reported values are heavily scattered. A part of these scattering might be arisen from inaccuracy in the measurements. It is also possible that the P700 redox potential itself varies significantly among oxygenic photosynthetic organisms, though little attention is generally paid for such possibility due to highly conserved amino acid sequences of core polypeptides surrounding P700 throughout species.
Here, the P700 redox potentials were determined systematically for oxygenic photosynthetic organisms belong to different phyla by highly accurate spectroelectrochemical means recently developed by us. We revealed that the P700 redox potential is varied by about 70 mV among the organisms depending roughly on its phylum. The P700 redox potential was lowest for primitive cyanobacteria and the potential shifted gradually to positive direction in an order of red algae, cyanobacteria, green algae and higher plants. We will discuss possible causes for the shifts of the P700 redox potential in the light of kinds of in vivo electron donor proteins for P700⁺ reduction they employed and its redox potential.