Unexpected Difference in the P700 Redox Potential Among Oxygenic Photosynthetic Organisms Revealed by Spectroelectrochemistry

The redox potential of the primary electron donor P700 of photosystem (PS) I has been determined for diverse organisms by spectroelectrochemistry with an optically transparent thin-layer electrochemical cell (OTTLE) to explore reasons for heavy scattering seen in the reported values of the P700 redox potential. The organisms submitted to examination were seven different organisms, four cyanobacteria including a primitive cyanobacteria Gloeobacter violaceus PCC7421, a red alga, a green algae and a higher plant. The P700 redox potential, determined within an error range of ±4 mV, showed a strong speciesdependence with a span over 70 mV. The causes for the strong species-dependence of the P700 redox potential are discussed in terms of the redox potentials of in vivo electron donors, cytochrome (cyt) c 6 and plastocyanin (PC).