ELECTRON-TRANSFER FROM THE ACCEPTOR A(1) TO THE IRON-SULFUR CENTERS IN PHOTOSYSTEM-I AS STUDIED BY TRANSIENT EPR SPECTROSCOPY

The electron transfer in photosystem I (PS I) from the secondary accep tor A(1) to the iron-sulfur centers is studied by X-band transient EPR with a time resolution of similar to 50 ns. Results are presented for a series of different PS I preparations from the cyanobacterium Synec hococcus 6301 ranging from whole cells to core particles in which the iron-sulfur centers have been successively removed. In addition, resul ts from PS I preparations from spinach and the cyanobacterium Synechoc ystis 6803 are presented. In all samples containing iran-sulfur center s, two consecutive spin-polarized EPR spectra are observed. The two si gnals have previously been assigned to the charge-separated states P(7 00)(+.)A(1)(-.) and P-700(+.)(FeS)(-), where (FeS) is one of the three iron-sulfur centers, F-X, F-A, or F-B [Bock, C., van der Est, A., Bre ttel, K., & Stehlik, D. (1989) FEBS Lett. 247, 91-96]. In agreement wi th this, the second spectrum is not observed in the sample in which th e iron-sulfur centers have been removed. For (P-700-F-X), core particl es which do not contain F-A and F-B, the second spectrum can unambiguo usly be assigned to the pair (P700FX-)-F-+.. In all samples containing F-X, the transition from the first to the second spectrum occurs with t(1/e) approximate to 280 ns (t(1/2) approximate to 190 ns) both in t he presence and absence of F-A and F-B, which strongly suggests that t his phase reflects electron transfer from A(1)(-.) to F-X in intact PS I. For the spinach particles and for the cyanobacterial (P-700-F-X) c ore particles, the EPR data show evidence for the presence of a fracti on of the reaction centers in which electron transfer from A(1)(-.) to F-X is faster than the response time of the spectrometer, in agreemen t with optical data for the same samples. It is shown that when this f raction is taken into account the early spectrum extracted from the EP R data sets shows no net spin polarization and is identical for all sa mples except the (P-700-A(1)) core particles. Possible reasons for the differences in this sample are discussed.