Progress in Characterization of the Photosystem II Oxygen Evolving Complex Using Advanced EPR Methods

We have utilized both continuous wave (CW) EPR and pulsed EPR spectroscopies to characterize the structure and function of the oxygen-evolving complex (OEC) of Photosystem II (PSII). Parallel polarization CW EPR spectroscopy of the dark-stable S₁-state of the OEC in PSII particles isolated from the cyanobacterium Synechocystis sp. PCC 6803 reveals a ΔMs = 0 signal, centered at a g-value of approximately 12, with 18 or more well-resolved ⁵⁵Mn hyperfine lines. This signal provides direct evidence for the presence of a tri- or tetranuclear coupled Mn cluster in the S₁-state. Electron spin echo (ESE) envelope modulation (ESEEM) and ESE-ENDOR experiments indicate that substrate water and small alcohol derived ligands are bound to the Mn cluster in the S₂-state of the cycle. Progress in modeling the magnetic interaction between the photo-oxidized Y^•_Z tyrosine and the Mn cluster is described, along with new pulsed EPR evidence supporting recent models that suggest Y^•_Z is directly involved in the water oxidation chemistry.