Proton-Coupled Electron-Transfer Processes in Photosystem II Probed by Highly Resolved g-Anisotropy of Redox-Active Tyrosine YZ

The oxidation of a redox-active tyrosine residue Y-Z in photosystem II (PSII) is coupled with proton transfer to a hydrogen-bonded D1-His190 residue. Because of the apparent proximity of Y-Z to: the water-oxidizing complex and its redox activity, it is believed that Y-Z plays a significant role in water oxidation in PSII. We investigated the g-anisotropy of the tyrosine radical Y-Z(center dot) to provide insight into the mechanism of Y-Z(center dot) proton-coupled electron transfer in Nth-depleted PSII. The anisotropy was highly resolved by electron paramagnetic resonance spectroscopy at the W-band (94.9 GHz) using PSII single crystals. The g(x)-component along the phenolic C-O bond of Y-Z(center dot) was calculated by density functional theory (DFT). It was concluded from the highly resolved g-anisotropy that Y-Z loses a phenol proton to D1-His190 upon tyrosine oxidation, and D1-His190 redonates the same proton back to Y-Z(center dot) upon reduction.