Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-iron complex in photosystem II reaction centers

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Abstract
Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential E h = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo [unk] ( g = 2.0033, Δ H ≈ 13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0.13-0.16% methanol but is restored by reconstitution with plastoquinone A (alone or with β-carotene) although not with vitamin K 1 . TSF-II and TSF-IIa are found to contain ≈2 nonheme Fe atoms per reaction center. Incubation with 0.55 M LiClO 4 plus 2.5 mM o -phenanthroline (but not with 0.55 M LiClO 4 alone) decreases this value to ≈0.6 and completely eliminates the EPR doublet, but photoreduction of Pheo is not significantly affected. Partial restoration of the doublet (about 25%) was achieved by subsequent incubation with 0.2 mM Fe 2+ , but not with either Mn 2+ or Mg 2+ . The Fe removal results in the development of a photoinduced EPR signal ( g = 2.0044 ± 0.0003, Δ H = 9.2 ± 0.5 G) at E h = 50 mV, which is not observed after extraction with 0.16% methanol in hexane. It is ascribed to plastosemiquinone no longer coupled to Fe in photosystem II reaction centers. The results show that a complex of plastoquinone and Fe can act as the stable “primary” electron acceptor in photosystem II reaction centers and that the interaction of its singly reduced form with the reduced intermediary acceptor, Pheo [unk] , is responsible for the EPR doublet.

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