Nanosecond electron transfer kinetics in photosystem I as obtained from transient EPR at room temperature

Transient EPR spectra of photosystem I (PS I) in spinach chloroplasts and PS I particles prepared fromSynechococcusare presented. Two consecutive spectra are observed after the laser pulse. The decay time of the first spectrum is equal to the rise time of the second spectrum. The two spectra represent sequential charge‐separated states in the electron transfer chain and the time constant of the electron transfer step between them is found to bet1/e= 260 ± 20 ns for theSynechococcusPS I particles as well as for the spinach chloroplasts. The first spectrum is assigned to the P+700A−1pair, where A−1is the second electron acceptor, probably a quinone‐like molecule. The second spectrum covers the region of the P+700signal and may be part of the spectrum due to the coupled radical pair P+700(FeS), where (FeS)−is the first iron‐sulfur center along the electron transfer chain.