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Transient EPR spectra of photosystem I (PS I) in spinach chloroplasts and PS I particles prepared from Synechococcus are 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 be t 1/e = 260 ± 20 ns for the Synechococcus PS I particles as well as for the spinach chloroplasts. The first spectrum is assigned to the P+ 700 A− 1 pair, where A− 1 is the second electron acceptor, probably a quinone‐like molecule. The second spectrum covers the region of the P+ 700 signal 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.

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an "observer". The spin polarized transient EPR spectra are investigated using the coupled three-spin system P* I -Qp in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of PI -Qp and Qq following recombination of the radical pair (P = primary donor, I = primary acceptor, QA = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, Jp" in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of JPY .

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The radical pair P700.+Q.- (P700 = primary electron donor, Q = quinone acceptor) in native photosystem I and in preparations in which the native acceptor (vitamin K1) is replaced by different quinones is investigated by pulsed EPR spectroscopy. In a two-pulse experiment, the light-induced radical pair causes an out-of-phase electron spin echo, showing an envelope modulation. From the modulation frequency, the dipolar coupling, and therefore the distance between the two cofactors, can be derived. The observation of nearly identical distances of about 25.4 A between P700.+ and Q.- in all preparations investigated here leads to the conclusion that the reconstituted quinones are bound to the native A1 binding pocket. Since the orientation of the reconstituted naphthoquinone relative to the axis joining P700.+ and Q*- differs drastically from that of the native vitamin K1, it cannot be bonded to the protein in the same way as the native acceptor. This implies that the function of A1 as an electron acceptor does not depend on the orientation or hydrogen bonding of the quinone.