HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn(4)CaO(5) cluster, with participation of the redox-active tyrosine residue (Y(Z)) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between Y(Z) and D1-His190 likely renders Y(Z) kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at Y(Z) remains elusive owing to the transient nature of its intermediate states involving Y(Z)⋅. Herein, we employ a combination of high-resolution two-dimensional (14)N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the Y(Z) residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.