Ligand environment of the S2 state of photosystem II: a study of the hyperfine interactions of the tetranuclear manganese cluster by 2D 14N HYSCORE spectroscopy.

The solar water-splitting protein complex, photosystem II, catalyzes the light-driven oxidation of water to dioxygen in Nature. The four-electron oxidation reaction of water occurs at the tetranuclear manganese—calcium—oxo catalytic cl uster that is present in the oxygen-evolving complex of photosystem II. The mechanism of light-driven water oxidation has been a subject of intense interest, and the oxygen-evolving complex of photosystem II has been studied extensively by structural and biochemical methods. While the recent X-ray crystal structures and single-crystal EXAFS investigations provide a model for the geometry of the tetranuclear manganese-calcium-oxo catalytic cluster, there is limited knowledge of the protein environment that surrounds the catalytic cluster. In this study, we demonstrate the application of two-dimensional hyperfine sublevel correlation spectroscopy to determine the magnetic couplings of the catalytic cluster with the 14 N atoms of surrounding amino acid residues in the S 2 state of the oxygen-evolving complex of photosystem II. We utilize two-dimensional difference spectroscopy to facilitate unambiguous assignments of the spectral features and identify at least three separate 14 N atoms that are interacting with the catalytic cluster in the S 2 state. The results presented here, for the first time, identify previously unknown ligands to the catalytic cluster of photosystem II and provide avenues for the assignment of residues by site-directed mutagenesis and the refinement of computational and mechanistic models of photosystem II.