Parallel Polarization EPR Studies of the Oxygen-Evolving Complex of Photosystem II
The oxygen-evolving complex of Photosystem II mediates the oxidation of water to molecular oxygen. According to a model devised by Kok and co-workers (1), the complex couples the four-electron water oxidation process to the single-electron primary photochemistry by cycling through a series of states, S0 – S4, as it transfers electrons to reduce the photo-oxidized primary donor. Four manganese ions are thought to be required for oxygen evolution activity, but the structural organization of the manganese ions and their oxidation states throughout the S-state cycle remain in question. Although the system has been thoroughly studied with conventional EPR, only two signals attributed to manganese have been reported, and both occur in the S2 state. The multiline signal, comprised of a number of hyperfine components centered near g=2, is consistent with an exchange-coupled mixed-valence manganese cluster (2). A second signal appears at an effective g value of 4.1 (3,4), and the correlation of the generation of this signal with an increase in the manganese X-ray absorption energy implies that the signal may originate from manganese (5). Recently proposed models for the structural organization of manganese and the origin of the S2 state EPR signals include the suggestion that the signals arise from different spin states of a single tetranuclear cluster (6), or that the signals arise from separate centers: for example, that the multiline signal is due to a mixed-valence binuclear cluster and the g=4.1 signal originates from a mononuclear Mn(IV) site (7).
KeywordsParallel Polarization Hyperfine Component Oxygen Evolution Activity Manganese Cluster Separate Center
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