EPR investigation of water oxidizing photosystem II: detection of new EPR signals at cryogenic temperatures.
Experiments are described which allow the detection and characterization of new EPR signals in photosystem II (PSII). PSII has been extensively studied with the water oxidising complex (WOC) poised in the S1 and S2 states. Other stages in the cycle of water oxidation lack characteristic EPR signals for use as probes. In this study, experiments use multiple turnovers of PSII from an initial S1 state to allow new states of PSII to be studied. The first EPR signal detected, centered at g = 4.85 and termed the g = 5 signal, is suggested to be a new form of S2 probably formed by decay of S3 at cryogenic temperatures, but a novel form of oxidized non-heme iron cannot be fully excluded at present. The second signal is split around g = 2 and shows characteristics of signals formed by spin-spin interaction between two paramagnetic species. The split g = 2 signal is reversibly formed by illumination at <30 K of a sample containing the g = 5 signal. The g = 2 signal may be a form of the "S3" EPR signal previously only found in a variety of PSII preparations where oxygen evolution has been inhibited. Those "S3" signals are thought to arise from the interaction of an oxidized amino acid radical and the S2 state, i.e., S2X+. Illumination at higher temperatures or illumination at <30 K, followed by dark-adaptation at 77 K, removes the g = 5 signal and prevents subsequent detection of the g = 2 signal on illumination at <30 K. The most likely explanation of our data is that illumination at <30 K of centers containing the g = 5 species allows accumulation of an oxidized intermediate and that at higher temperatures electron transfer proceeds to re-form an EPR-silent S state equivalent to that initially trapped during sample preparation. Study of these signals should provide an important new insight into the WOC and PSII.