The back-reaction kinetics in Photosystem I (PS I) were studied on the μs-to-s time scale in cyanobacterial preparations, which differed in the number of iron-sulfur clusters to assess the contributions of particular components to the reduction of P700/+. In membrane fragments and in trimeric P700-F(A)/F(B) complexes, the major contribution to the absorbance change at 820 nm (ΔA820) was the back-reaction of F(A)/- and/or F(B)/- with lifetimes of ~10 and 80 ms (~10% and 40% relative amplitude). The decay of photoinduced electric potential (Δψ) across a membrane with directionally incorporated P700-F(A)/F(B) complexes had similar kinetics. HgCl2-treated PSI complexes, which contain F(A) but no F(B), retain both of these kinetic components, indicating that neither can be assigned uniquely to a specific acceptor. These results suggest that F(A)/- reduces P700/+ directly and argue for a rapid electron equilibration between F(A) and F(B), which would eliminate their kinetic distinction in a back-reaction. In PsaC-depleted P700-F(X) cores, as well as in P700- F(A)/F(B) complexes with chemically reduced F(A) and F(B), the major contribution to the ΔA820 and the δψ decay is a biphasic back-reaction of F(X)/- (~400 μs and 1.5 ms) with some contribution from A1 (~10 μs and 100 μs), the latter of which is variable depending on experimental conditions. The δA820 decay in a P700-A1 core devoid of all iron- sulfur clusters comprises two phases with lifetimes of 10 μs and 130 μs (2.7:1 ratio). The biexponential back-reaction kinetics found for each of the electron acceptors may be related to existence of different conformational states of the PSI complex. In all preparations studied, excitation at 532 nm with flash energies exceeding 10 mJ gives rise to formation of antenna 3Chl, which also contributes to ΔA820 decay on the tens-of-μs time scale. A distinction between ΔA820 components related to back-reactions and to 3Chl decay can be made by analysis of flash saturation dependencies and by measurements of kinetics with preoxidized P700.
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