Rapid Evolution of the Photosystem II Electronic Structure during Water Splitting

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A Mn4Ca cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) is used to observe the process of oxygen formation in real time. These experiments reveal that the oxygen evolution step, initiated by three sequential laser flashes, is accompanied by rapid (within 50 μs) changes to the Mn Kβ XES spectrum. However, no oxidation of the Mn4Ca core above the all-Mn^IV state is detected to precede O-O bond formation, and the observed changes are therefore assigned to O-O bond-formation dynamics. We propose that O-O bond formation occurs prior to the transfer of the final (fourth) electron from the Mn₄Ca cluster to the oxidized tyrosine Tyr_Z residue. This model resolves the kinetic limitations associated with O-O bond formation and suggests an evolutionary adaptation to avoid releasing harmful peroxide species.