Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A1. At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P700+/A1−, it decays with t1/e = 23 µs, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t1/e = 200 ns in untreated photosystem 1, due to forward electron transfer from A1− to one of the iron-sulfur centers. This rate is unchanged in Fe-SA/B-depleted PS1 but is lost when the iron-sulfur center Fe-SX is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t1/e = 1.3 µs, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P700 → A1 → Fe-SX → Fe-SA/B.
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