Abstract
In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry1–3. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition4–6, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.
| Original language | English |
|---|---|
| Pages (from-to) | 629-636 |
| Number of pages | 8 |
| Journal | Nature |
| Volume | 617 |
| Issue number | 7961 |
| DOIs | |
| State | Published - May 18 2023 |
| Externally published | Yes |
Funding
We thank K. Sauer (1931–2022) for his interest in this research and for many discussions about photosynthetic water oxidation. We thank R. Massad, M. Kretzschmar, P. Sinnott, J. Blaschke, A. Britz, S. Carbajo, C. de Lichtenberg, L.-C. Kao, L. Lassalle, D. Liebschner, D. Mendez, F. Moss, E. Pastor, C. Pham, B. Poon, K. D. Sutherlin and I. D. Young for support during sample preparation, data collection and processing. We thank the support staff at LCLS/SLAC, SACLA/Japan, SSRL and ALS. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES), Division of Chemical Sciences, Geosciences, and Biosciences of the Department of Energy (DOE) (J.Y., V.K.Y. and J.K.) for X-ray spectroscopy and crystallography data collection and analysis, and methods development for photosynthetic systems was supported by the National Institutes of Health (NIH; grants GM055302 (V.K.Y.) for photosystem II biochemistry, GM110501 (J.Y.) and GM126289 (J.K.) for instrumentation development for X-ray free electron laser (XFEL) experiments and GM117126 (N.K.S.) for development of computational protocols for XFEL data). N.K.S. acknowledges support from the Exascale Computing Project (grant 17-SC20-SC), a collaborative effort of the DOE Office of Science and the National Nuclear Security Administration. Germany’s Excellence Strategy (project EXC 2008/1-390540038 (A.Z., H.D. and S.H.)) coordinated by TU Berlin and the German Research Foundation via the Collaborative Research Center SFB1078 (Humboldt Universität zu Berlin), TP A5 (A.Z., H.D., M.I., R.H. and J.G.) and Vetenskapsrådet (grants 2016-05183 (J.M.) and 2020-03809 (J.M.)) as well as Energimyndigheten (grant 45421-1 (J.M.)) are acknowledged for support. R.H. acknowledges support from a Caroline von Humboldt Stipendium, Humboldt Universität zu Berlin. C.J.K. acknowledges support from the NIH (NRSA fellowship award F32GM142218). This research used resources of NERSC, a User Facility supported by the Office of Science, DOE (contract DE-AC02-05CH11231). XFEL data were collected at LCLS/SLAC, Stanford and SACLA, Japan. The XFEL experiments at SACLA were performed at BL2 with the approval of the Japan Synchrotron Radiation Research Institute (proposals 2018B8089, 2019A8081 and 2019B8067). Testing of crystals and various parts of the setup was carried out at synchrotron facilities that were provided by the ALS in Berkeley and SSRL in Stanford, funded by the DOE OBES. The SSRL Structural Molecular Biology Program is supported by the DOE OBER and the NIH (grant P41GM103393). Use of the LCLS and SSRL, SLAC National Accelerator Laboratory is supported by the DOE, Office of Science, OBES (contract DE-AC02-76SF00515), and structural biology work at the LCLS is supported by the NIH (grant P41GM139687; the Rayonix detector was funded by grant S10 OD023453). We thank K. Sauer (1931–2022) for his interest in this research and for many discussions about photosynthetic water oxidation. We thank R. Massad, M. Kretzschmar, P. Sinnott, J. Blaschke, A. Britz, S. Carbajo, C. de Lichtenberg, L.-C. Kao, L. Lassalle, D. Liebschner, D. Mendez, F. Moss, E. Pastor, C. Pham, B. Poon, K. D. Sutherlin and I. D. Young for support during sample preparation, data collection and processing. We thank the support staff at LCLS/SLAC, SACLA/Japan, SSRL and ALS. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES), Division of Chemical Sciences, Geosciences, and Biosciences of the Department of Energy (DOE) (J.Y., V.K.Y. and J.K.) for X-ray spectroscopy and crystallography data collection and analysis, and methods development for photosynthetic systems was supported by the National Institutes of Health (NIH; grants GM055302 (V.K.Y.) for photosystem II biochemistry, GM110501 (J.Y.) and GM126289 (J.K.) for instrumentation development for X-ray free electron laser (XFEL) experiments and GM117126 (N.K.S.) for development of computational protocols for XFEL data). N.K.S. acknowledges support from the Exascale Computing Project (grant 17-SC20-SC), a collaborative effort of the DOE Office of Science and the National Nuclear Security Administration. Germany’s Excellence Strategy (project EXC 2008/1-390540038 (A.Z., H.D. and S.H.)) coordinated by TU Berlin and the German Research Foundation via the Collaborative Research Center SFB1078 (Humboldt Universität zu Berlin), TP A5 (A.Z., H.D., M.I., R.H. and J.G.) and Vetenskapsrådet (grants 2016-05183 (J.M.) and 2020-03809 (J.M.)) as well as Energimyndigheten (grant 45421-1 (J.M.)) are acknowledged for support. R.H. acknowledges support from a Caroline von Humboldt Stipendium, Humboldt Universität zu Berlin. C.J.K. acknowledges support from the NIH (NRSA fellowship award F32GM142218). This research used resources of NERSC, a User Facility supported by the Office of Science, DOE (contract DE-AC02-05CH11231). XFEL data were collected at LCLS/SLAC, Stanford and SACLA, Japan. The XFEL experiments at SACLA were performed at BL2 with the approval of the Japan Synchrotron Radiation Research Institute (proposals 2018B8089, 2019A8081 and 2019B8067). Testing of crystals and various parts of the setup was carried out at synchrotron facilities that were provided by the ALS in Berkeley and SSRL in Stanford, funded by the DOE OBES. The SSRL Structural Molecular Biology Program is supported by the DOE OBER and the NIH (grant P41GM103393). Use of the LCLS and SSRL, SLAC National Accelerator Laboratory is supported by the DOE, Office of Science, OBES (contract DE-AC02-76SF00515), and structural biology work at the LCLS is supported by the NIH (grant P41GM139687; the Rayonix detector was funded by grant S10 OD023453).