PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

EPR characterisation of donor triplet state in Photosystem II with singly reduced primary acceptor Q_A. W. Onno Feikema¹, Peter Gast^*,1, Irina Klenina², Ivan Proskuryakov², ¹ Department of Biophysics, Leiden, The Netherlands² Institute of Basic Biological Problems RAS, Pushchino, Russia

ABSTRACT- The structure of the Photosystem II reaction center (PS II RC) is similar to that of the bacterial RC, but many functional properties are different. These differences are most probably related to the generation by PS II of high redox potentials that are sufficient to oxidize water molecules. One such dissimilarity is in the characteristics of the RC triplet state, populated when physiological electron transfer is disrupted. It is generally believed that to observe the triplet state of the PS II RCs by EPR, double reduction or removal of the primary quinone acceptor is demanded. In contrast, the triplet state in bacterial RCs is populated also with singly reduced primary acceptor. Here we report the first observation of the PS II triplet state in RCs with singly reduced quinone acceptor, detected with time-resolved cw EPR at low temperatures. In accordance with earlier optical studies, this triplet demonstrates a short lifetime and strong temperature dependence, disappearing at T>100 K. Its EPR spectral shape is best rationalized by an assumption of anisotropic triplet yield, caused by hyperfine, magnetic dipolar and exchange interaction anisotropy and short lifetime of the precursor radical pair (RP) state. A novel sequence of electron transfer events in PS II RCs is proposed, which explains the triplet state properties. The first stage of this sequence is electron transfer from the P_A half of the chlorophyll dimer to the monomeric chlorophyll B_A and then to the pheophytin molecule. Spin evolution in this first RP is suppressed by strong exchange interaction, and is limited to the second RP. This second RP, P_A⁺Pheo^-, in singly reduced RCs is short-lived due to electrostatic effect of the reduced quinone. The role of such mechanism in protecting PS II RCs from the long-lived triplet state formation at room temperature, and consequent singlet oxygen formation, will be discussed.

KEY WORDS: triplet state, photosystem II, epr, quinone redox state