Proton-coupled electron-transfer processes in photosystem II probed by highly resolved g -anisotropy of redox-active tyrosine Y_Z

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• to provide insight into the mechanism of Y_Z• 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 g_X-component along the phenolic C-O bond of Y_Z• 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• upon reduction.