Analysis of the spin-polarized electron spin echo of the [P700+A1-] radical pair of photosystem I indicates that both reaction center subunits are competent in electron transfer in cyanobacteria, green algae, and higher plants

The decay of the light-induced spin-correlated radical pair [P700+A1-] and the associated electron spin echo envelope modulation (ESEEM) have been studied in either thylakoid membranes, cellular membranes, or purified photosystem I prepared from the wild-type strains of Synechocystis sp. PCC 6803, Chlamydomonas reinhardtii, and Spinaceae oleracea. The decay of the spin-correlated radical pair is described in the wild-type membrane by two exponential components with lifetimes of 2−4 and 16−25 μs. The proportions of the two components can be altered by preillumination of the membranes in the presence of reductant at temperatures lower than 220 K, which leads to the complete reduction of the iron−sulfur electron acceptors FA, FB, and FX and partial photoaccumulation of the reduced quinone electron acceptor A1A-. The “out-of-phase” (OOP) ESEEM attributed to the [P700+A1-] radical pair has been investigated in the three species as a function of the preillumination treatment. Values of the dipolar (D) and the e...