The Ligand Environment of the S2 State of Photosystem Ii: A Study of the Hyperfine Interactions of the Tetranuclear Manganese Cluster by 2D Hyscore Spectroscopy.‡

Abstract

The solar water-splitting protein complex, photosystem II (PSII), catalyzes the light-driven oxidation of water to dioxygen in Nature. The four-electron water oxidation reaction occurs at the tetranuclear manganese-calcium-oxo (Mn4Ca-oxo) cluster that is present in the oxygen-evolving complex (OEC) of PSII. The mechanism of light-driven water oxidation has been a subject of intense interest and the OEC of PSII has been studied extensively by structural methods. While the recent X-ray crystal structures, single crystal EXAFS and EPR spectroscopy investigations provide a model for the geometry of the catalytic Mn4Ca-oxo cluster, there is limited knowledge of the protein environment that surrounds the catalytic site. It is suggested that the binding and activation of the substrate water molecules at the Mn4Ca-oxo cluster in the OEC of PSII are facilitated by key amino acid residues that could be ligated to the catalytic cluster. In this study, we demonstrate the application of two-dimensional (2D) hyperfine sublevel correlation spectroscopy to determine the magnetic couplings of the S2 state of PSII. We utilize 2D difference spectroscopy to facilitate unambiguous assignments of the spectral features arising from the substrate molecules and surrounding amino acid residues in the S2 state of PSII. The results presented here, for the first time, identify previously unknown ligands to the catalytic cluster and provide avenues for the assignment of residues by site-directed mutagenesis and the refinement of computational and mechanistic models of PSII.®€ This study is supported by the Office of Basic Energy Sciences, United States Department of Energy (DE-FG02-0ER06-15).