Pulse EPR Spectroscopic Characterization of the S3 State of the Oxygen-Evolving Complex of Photosystem II Isolated from Synechocystis

The S-3 state is the last semi-stable state in the water splitting reaction that is catalyzed by the Mn4O5Ca cluster that makes up the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent high-field/frequency (95 GHz) electron paramagnetic resonance (EPR) studies of PSII isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus have found broadened signals induced by chemical modification of the S-3 state. These signals are ascribed to an S-3 form that contains a five-coordinate Mn-IV center bridged to a cuboidal (Mn3O4Ca)-O-IV unit. High-resolution X-ray free-electron laser studies of the S-3 state have observed the OEC with all-octahedrally coordinated Mn-IV in what is described as an open cuboid-like cluster. No five-coordinate Mn-IV centers have been reported in these S-3 state structures. Here, we report high-field/frequency (130 GHz) pulse EPR of the S-3 state in Synechocystis sp. PCC 6803 PSII as isolated in the presence of glycerol. The S-3 state of PSII from Synechocystis exhibits multiple broadened forms (approximate to 69% of the total signal) similar to those seen in the chemically modified S3 centers from T. elongatus. Field-dependent ELDOR-detected nuclear magnetic resonance resolves two classes of 'Mn nuclear spin transitions: one class with small hyperfine couplings (vertical bar A vertical bar approximate to 1-7 MHz) and another with larger hyperfine couplings (vertical bar A vertical bar approximate to 100 MHz). These results are consistent with an all-Mn-4(IV) open cubane structure of the S-3 state and suggest that the broadened S-3 signals arise from a perturbation of Mn4A and/or Mn3B, possibly induced by the presence of glycerol in the as-isolated Synechocystis PSII.