Near-IR absorbance changes and electrogenic reactions in the microsecond-to-second time domain in photosystem I

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.