Electrogenic reduction of the primary electron donor P700⁺ in photosystem I by redox dyes

The kinetics of reduction of the photo-oxidized primary electron donor P700⁺ by redox dyes N,N,N',N'tetremethyl-p-phenylendiamine, 2,6-dichlorophenol-indophenol and phenazine methosulfate was studied in proteoliposomes containing Photosystem I complexes from cyanobacteria Synechocystis sp. PCC 6803 using direct electrometrical technique, In the presence of high concentrations of redox dyes, the fast generation of a membrane potential related to electron transfer between P700 and the terminal iron-sulfur clusters F(A)/F(B) was followed by a new electrogenic phase in the millisecond time domain, which contributes approximately 20% to the overall photoelectric response. Tills phase is ascribed to the vectorial transfer of an electron from the redox dye to the protein-embedded chlorophyll of P700⁺. Since the contribution of this electrogenic phase in the presence of artificial redox dyes is approximately equal to that of the phase observed earlier in the presence of cytochrome c₆, it is likely that electrogenic reduction of P700⁺ in vivo occurs due to vectorial electron transfer within RC molecule rather than within the cytochrome c₆-P700 complex.