Summary
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.
<p>Light-driven water oxidation in algae, cyanobacteria, and higher plants generates dioxygen that supports life on Earth. The water-oxidation reaction is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII) that is comprised of the tetranuclear manganese calcium-oxo (Mn<sub>4</sub>CaO<sub>5</sub>) cluster, with participation of the redox-active tyrosine residue (Y<sub>Z</sub>) and a hydrogen-bonded network of amino acids and water molecules. Y<sub>Z</sub> mediates successive proton-coupled electron transfer (PCET) reactions that are essential for the oxidation of water to dioxygen at the Mn<sub>4</sub>CaO<sub>5</sub> cluster. It has been proposed that the strong hydrogen bond between Y<sub>Z</sub> and and its conjugate base, D1-His190, likely renders Y<sub>Z</sub> kinetically and thermodynamically competent leading to highly efficient water oxidation.<sup>1</sup> However, a detailed understanding of PCET at Y<sub>Z</sub> remains elusive due to the transient nature of its intermediate states. In this study, we utilize a combination of high-resolution two-dimensional (2D) <sup>14</sup>N hyperfine sublevel correlation (HYSCORE) spectroscopy and density functional theory (DFT) methods to investigate the electronic structure of a bioinspired artificial photosynthetic reaction center, benzimidazole-phenol porphyrin (BiP–PF<sub>10</sub>), that mimics the PCET process at the Y<sub>Z</sub> residue of PSII. The results of these studies underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.</p>
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