The electron transfer in photosystem I (PS I) from the secondary acceptor A1 to the iron-sulfur centers is studied by X-band transient EPR with a time resolution of ~50 ns. Results are presented for a series of different PS I preparations from the cyanobacterium Synechococcus 6301 ranging from whole cells to core particles in which the iron-sulfur centers have been successively removed. In addition, results from PS I preparations from spinach and the cyanobacterium Synechocystis 6803 are presented. In all samples containing iron-sulfur centers, two consecutive spin-polarized EPR spectra are observed. The two signals have previously been assigned to the charge-separated states P700 +•A1 -• and P700 +•(FeS)−, where (FeS) is one of the three iron-sulfur centers, Fx, Fa, or Fb [Bock, C., van der Est, A., Brettel, K., & Stehlik, D. (1989) FEBS Lett. 247,91-96]. In agreement with this, the second spectrum is not observed in the sample in which the iron-sulfur centers have been removed. For (P700-Fx), core particles which do not contain Fa and Fb, the second spectrum can unambiguously be assigned to the pair P700 +•Fx -. In all samples containing Fx, the transition from the first to the second spectrum occurs with t1/e ≈280 ns (t1/2 ≈ 190 ns) both in the presence and absence of FA and Fb, which strongly suggests that this phase reflects electron transfer from A1 -• to Fx in intact PS I. For the spinach particles and for the cyanobacterial (P700-Fx) core particles, the EPR data show evidence for the presence of a fraction of the reaction centers in which electron transfer from A1 -• to Fx is faster than the response time of the spectrometer, in agreement with optical data for the same samples. It is shown that when this fraction is taken into account the early spectrum extracted from the EPR data sets shows no net spin polarization and is identical for all samples except the (P700-A1) core particles. Possible reasons for the differences in this sample are discussed.
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