Rapid Evolution of the Photosystem II Electronic Structure during Water Splitting

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.

DIY XES—development of an inexpensive, versatile, and easy to fabricate XES analyzer and sample delivery system

“…This was done with a cumulative sample exposure of ~1 × 10 16 photons, or 5 × 10 15 photons for each state. This is in agreement with data achieved previously, only the detection system reduced the data collection requirement according to the 6‐fold increase in solid angle because of the new spectrometer design, and the sample delivery system conserved sample and significantly reduced the labor required.…”

“…From the spectra, we detected a shift in the Kβ 1,3 peak from S 1 to S 2 toward lower energy. This shift indicates an average higher oxidation state (reduced localized spin density) for the average Mn atom in the OEC which is in agreement with previous results . A first moment analysis was also conducted on the raw data and a one‐way Anova analysis was used to find our confidence interval of 99.9% that a shift toward oxidation occurs between these states.…”

“…The OEC, which features a unique Mn 4 CaO 5 core, was chosen as an example case because it has a very low Mn content (~500 μM) and is highly X‐ray damage sensitive . The preparation and characterization of these samples prior to beamtime followed previously reported protocols, with the exception of D 2 O buffer for final resuspension and centrifugation steps before freezing. At the beamline, a solution of 0.4 M sucrose, 5 mM CaCl 2 , 5 mM MgCl 2 , and 15 mM NaCl, 50 mM MES, pD 6.0 in D 2 O was used for resuspension of PS II membrane fragments.…”

“…XES measurements with benchtop sources are generally limited to concentrated samples, but new sources, such as metal liquid jets, are pushing those limits with even higher X‐ray fluxes. Time‐resolved XES (TRXES) measurements have also become more common with the development of X‐ray free‐electron lasers (XFELs), advanced synchrotron beamlines, and are becoming possible in a few laboratory based sources …”

Rationalizing the Geometries of the Water Oxidising Complex in the Atomic Resolution, Nominal S3 State Crystal Structures of Photosystem II

“…( Berkeley group) have studied this effect, for a wide range of OEC relevant Mn model compounds (ligands as above, including Mn oxides), as well as the OEC itself. Some more limited data, including Mn oxides, have been recently presented by Pushkar and colleagues . All groups see a closely clustering linear correlation for the model systems, between and formal Mn oxidation state (from +2 to +4).…”