EPR characterisation of the triplet state in photosystem II reaction centers with singly reduced primary acceptor Q A

Abstract

The triplet states of photosystem II core particles from spinach were studied using time-resolved cw EPR technique at different reduction states of the iron–quinone complex of the reaction center primary electron acceptor. With doubly reduced primary acceptor, the well-known photosystem II triplet state characterised by zero-field splitting parameters |D| = 0.0286 cm −1, |E| = 0.0044 cm −1 was detected. When the primary acceptor was singly reduced either chemically or photochemically, a triplet state of a different spectral shape was observed, bearing the same D and E values and characteristic spin polarization pattern arising from RC radical pair recombination. The latter triplet state was strongly temperature dependent disappearing at T = 100 K, and had a much faster decay than the former one. Based on its properties, this triplet state was also ascribed to the photosystem II reaction center. A sequence of electron-transfer events in the reaction centers is proposed that explains the dependence of the triplet state properties on the reduction state of the iron–quinone primary acceptor complex.