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Abstract
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) was 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 MnIV state was detected to precede O-O bond formation, and the observed changes were therefore assigned to O-O bond formation dynamics. We propose that O-O bond formation occurs prior to the transfer of the final (4th) electron from the Mn4Ca cluster to the oxidized tyrosine YZ residue. This model resolves the kinetic limitations associated with O-O bond formation, and suggests an evolutionary adaptation to avoid releasing of harmful peroxide species.
Highlights
Photosynthetic water oxidation is a fundamental process that sustains the biosphere
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 changes to the Mn Kβ XES spectrum
The reaction catalyzed by the Mn4Ca cluster of photosystem II (PS II) during photosynthesis holds a special place, as the ability to biosynthesize O2 from H2O occurred only once during evolution
Summary
Rapid Evolution of the Photosystem II Electronic Structure during Water Splitting. Katherine M. The reaction catalyzed by the Mn4Ca cluster of photosystem II (PS II) during photosynthesis holds a special place, as the ability to biosynthesize O2 from H2O occurred only once during evolution The development of this process dramatically altered our planet by generating the oxygen-rich atmosphere we live in today. The tyrosine residue (TyrZ) located between P680 and the OEC is oxidized by the special pair to form TyrZ, which is subsequently reduced by the OEC on a microsecond timescale This process drives the water-splitting reaction [2]. O─O bond formation likely occurs on a microsecond timescale during the S3-to-S0 transient step of the catalytic cycle, culminating in O2 evolution Direct monitoring of this transient process, has proven challenging, and details remain elusive. A preeminent report by Babcock, Blankenship, and Sauer, as well as recent studies by Nilsson et al, support a rate of TyrZ reduction with t1=2 ∼ 1 ms following three flashes and associates this rate
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