Mechanism of photoinduced electron transfer in photosystem II reaction centers

The shape of the EPR spectrum of the triplet state of photosystem II reaction centers with a singly reduced primary acceptor complex Q_AFe²⁺ was studied. It was shown that the spectroscopic properties do not significantly change when the relaxation of the primary acceptor is accelerated and when the magnetic interaction between the reduced quinone molecule Q_A and the nonheme iron ion Fe²⁺ is disrupted. This observation confirmed the earlier conclusion that the anisotropy of the quantum yield of the triplet state is the main cause of the anomalous shape of the EPR spectrum. A scheme of primary processes in photosystem II that is consistent with the observed properties of the EPR spectrum of the triplet state is discussed.