Primary photochemistry of Photosystem I in chloroplasts. Dynamics of reversible charge separation in open reaction centers at 25 K |
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Authors: | Mark S. Crowder Alan Bearden |
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Affiliation: | Department of Biophysics and Medical Physics, and Division of Biology and Medicine, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 U.S.A. |
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Abstract: | The reduction rate of oxidized reaction center chlorophyll of Photosystem I after laser-flash excitation at 25 K has been determined for D-144 subchloroplast fragments and chloroplasts. A maximum of 40% of Photosystem I reaction centers undergo irreversible charge separation (P-700, Cluster A: P-700+, Cluster A?) at 25 K, a percentage which is independent of laser-flash intensity. The remaining reaction centers in chloroplasts and D-144 fragments undergo reversible charge separation with biphasic recombination. Similar amplitudes and time constants (chloroplasts, 49 μs (61%); D-144 fragments, 90 μs (67%)) were obtained for the fast component, while the slower component differed considerably in time (chloroplasts, 2.9 ms; D-144 fragments, 170 ms). It is known that Fe-S Cluster A is photoreduced in less than 1 ms at 25 K. Data obtained support a model for Photosystem I involving a single intermediate in the decay path between the reduced primary electron acceptor (A?1) and P-700+ and a second intermediate in the decay path between a reduced secondary electron acceptor and P-700+. Dual laser-flash experiments to determine rate constants for these processes are included. |
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Keywords: | Photosystem I Reaction center Charge separation P-700 ESR PS photosystem D-144, particles LDAO particles TSF-1 PS I particles obtained by treatment with Triton X-100 CIDEP chemically induced dynamic electron polarization |
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