Why are we interested in Photosystem II?
Why are we interested in Photosystem II?
Oxygenic photosynthesis is a biochemical process by which green plants, algae and cyanobacteria convert solar energy to carbohydrates. In this process, water is oxidized and dioxygen is formed via the photo-induced reaction:
2 H2O → O2 + 4e- + 4H+
The membrane protein photosystem II harbors the important water oxidation complex (also called Manganese cluster) which is mediating the splitting of water to molecular oxygen. Understanding the function of the metal cluster is crucial renewable energy production and of special interest for our group.
Overview of Photosystem II
Overview of Photosystem II
The photosynthetic splitting of water (i.e. oxygen evolution) is the source of nearly all of the O2 in the atmosphere, and takes place in the oxygen evolving complex (OEC), which is located in the multisubunit membrane protein complex Photosystem II (PSII). The OEC is a cluster of four Mn atoms and one Ca atom (Mn4CaO5), and has been shown via x-ray spectroscopy to be linked by mono- and di-μ-oxo or hydroxo bridges. The OEC cycles through five different oxidation states, known as Si states (i = 0 - 4), coupling the one-electron photochemistry of the reaction center with the four-electron chemistry of water oxidation. Although the structure and mechanism of Mn4CaO5 has been extensively studied by various methods, the precise molecular details OF mechanism remain elusive.
Time-resolved Crystallography combined with X-ray Spectroscopy
Time-resolved Crystallography combined with X-ray Spectroscopy
In our ongoing investigation of electronic and geometric structure and changes of the Mn4CaO5 cluster, we have made extensive use of X-ray and EPR spectroscopy and X-ray crystallography. We are using high-resolution absorption and emission X-ray spectroscopy, including Resonant Inelastic X-ray Scattering spectroscopy, to study crystals and solutions of PS II and inorganic models. We use X-ray absorption spectra at the K-and L-edges of the Mn cluster in its native and intermediates states at room temperature in a time-resolved manner, to capture short-lived intermediates and the step that includes the O-O bond formation. These techniques that we are developing, using both XFELs and synchrotron sources, will elucidate not only the structure of the Mn4CaO5catalytic complex, but also the changes in the structure as it cycles through the intermediate states, leading to an understanding of the mechanism. The insights gained from natural photosynthesis can then be applied in the design of inorganic-based energy conversion systems that split water, and will contribute to our search for cleaner, renewable carbon-neutral energy sources.
We are using XFELs and synchrotrons to systematically resolve the steps in the Kok cycle. Above is an image from our recent publication, 'Untangling the sequence of events during the S2 → S3 transition in photosytem II and implications for the water oxidation mechanism'. This paper focuses on the S2 → S3 transition. Utilizing the time dependence of our delivery method and the XFEL, we were able to resolve not only the stable intermediates (S2 and S3) but also time points between the S-states. Previously, we had seen the insertion of a new oxygen atom bridge between the Ca and Mn1 in the S3 state. By utilizing distinct time points, we are able to see when exactly this insertion happens. The goal is to collect data from many time points to make a molecular movie of the entire Kok cycle.
The Site of Photosynthetic Water Splitting: The Mn4O5/6Ca-µ-oxo cluster
The Site of Photosynthetic Water Splitting: The Mn4O5/6Ca-µ-oxo cluster
Changes in the electron density at the OEC during the Kok cycle. The omit map of the oxygen atom OX, which is inserted during the S2 → S3 transition as a bridging oxygen between Mn1 and Ca, is shown in orange. For reference, omit maps constructed by separately omitting the O5 atom are overlaid in blue at the same density levels. The density of OX increases gradually in the S2 → S3 transition and starts to decrease after 500 µs in the S3 → S0 transition with the density below noise level by 2000 µs. The red arrows show the first and last time points where we observe significant OX density. The populations (%) shown are modeled OX occupancies [except at 3F (2000 µs)] in the primary component (see text). Note that because we show the primary component here, the O5 and OEC populations are also changing. Mn atoms are shown as purple, Ca atoms as green and O atoms as red spheres.
The O – O Bond Formation Step
The O – O Bond Formation Step
The sequence of events (i–iv) leading to the first deprotonation event, the molecular oxygen release, the water insertion and the second deprotonation event. The OEC atoms are shown in purple (Mn), green (Ca) and red (O). The O1 channel is shown in red, the O4 channel is in blue and the Cl1 channel in green. The ligands of the OEC and the residues forming the water–proton channels are coloured based on the subunit they belong to (D1, blue; D2, green). Possible pathways for proton (cyan arrow), water (red dashed arrow), oxygen (red solid arrow) and electron (green arrow) transfer are depicted. Notable features are highlighted with black arrows. The right tan box shows the suggested models (model a and model b) for O–O bond formation. Oxygen highlighted with magenta indicates the candidate atoms for O–O formation.
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