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