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The principles of natures own light-driven water splitting catalyst, Photosystem II (PSII), can in the future inspire us to use water as electron and proton source to generate light-driven H₂ production. To mimic this challenging step, it is important to understand how the enzyme system can oxidize water. The mechanism of light-driven water oxidation in PSII is in this thesis addressed by EPR spectroscopy. P680⁺ is a strong oxidant formed by light-oxidation of the chlorophyll species P680 positioned in the center of PSII. The redox active tyrosine-Z (Y_Z) can reduce P680⁺ and the Y_Z^• radical is formed. This transient radical is further reduced by the CaMn₄-cluster, which is the binding site of the substrate water molecules. In a cyclic process called the S-cycle, this catalytic cluster accumulates four oxidizing equivalents to evolve one molecule of O₂ and to oxidize two molecules of water. We can induce the Y_Z^• radical at cryogenic temperatures in the different oxidation states of the catalytic S-cycle and observe this in metalloradical EPR signals. These metalloradical EPR signals are here characterized and used to deduce mechanistic information from the intact PSII. The "double nature" of these spin-spin interaction signals, so called split EPR signals, makes them excellent probes to both Y_Z oxidation and, when Y_Z^• is present, also to the S-states of the CaMn₄-cluster. The metalloradical EPR signals presented here, form a way to study the transient Y_Z^• radical in active PSII that has not been depleted of the catalytic metal cluster. This depleting method that has often been used in the past to study Y_Z is not representing studies of a mechanistically relevant material. The previously suggested disorder around Y_Z and accessibility to the bulk can be artifactual properties induced in the mechanistically defect PSII. On the contrary, our observation that proton coupled electron transfer from Y_Z to the light induced P680⁺ can occur in a high yield at cryogenic temperatures, suggests a well ordered catalytic site in the protein positioned for optimal performance. The optimized positioning of the redox components found in PSII might be a feature also important to build in an efficient water oxidizing catalyst.