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

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 (Mn4CaO5) cluster, with participation of the redox-active tyrosine residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. YZ mediates successive proton-coupled electron transfer (PCET) reactions that are essential for the oxidation of water to dioxygen at the Mn4CaO5 cluster. It has been proposed that the strong hydrogen bond between YZ and and its conjugate base, D1-His190, likely renders YZ kinetically and thermodynamically competent leading to highly efficient water oxidation.1 However, a detailed understanding of PCET at YZ remains elusive due to the transient nature of its intermediate states. In this study, we utilize a combination of high-resolution two-dimensional (2D) 14N 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–PF10), that mimics the PCET process at the YZ 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.