In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P700+ and phylloquinone acceptors A1A- and A1B- in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe-4S clusters, FX, FA, and FB, and the PsaC protein subunit (the so-called P700-A1 core), in which phylloquinone molecules A1A and A1B serve as the terminal electron acceptors. It was shown that in the P700-A1 core preparations, the average distance between the centers of the P700+A1- ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is ~25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe-4S clusters FX, FA, and FB are discussed.
Keywords: electron spin echo envelope modulation (ESEEM); electron transfer; iron–sulfur clusters; photosystem I; trehalose matrix.