Novel soft X-ray spectrometer enables individual steps of photosynthetic water oxidation to be observed

Sketch of the Photosystem II. Credit: SLAC

HZB scientists have developed a novel spectrometer at BESSY II that enables researchers to obtain detailed insights about catalytic processes in metalloenzymes. Their international collaboration was successful in delineating individual steps in the catalytic oxidation of water to dioxygen in photosystem II. They published their study in the journal Structural Dynamics. Photosystem II is a part of the of photosynthetic electron transport chain, a process that is responsible for the conversion of solar energy to chemical energy in plants, algae and cyanobacteria.

Photosystem II is a large protein complex that has four manganese and one calcium atoms at its active catalytic centre. Sunlight triggers the process that splits water into oxygen, protons and electrons, which are subsequently utilized to generate carbohydrates that all life depends on.

One of the biggest challenges in understanding this process is observing the individual intermediate steps involved in the four photon-four electron enzymatic cycle. Up until now, it was impossible to investigate experimentally the electronic structure and the chemical reactions occurring at the individual manganese atoms with soft x-ray spectroscopy, because of the difficulties in detecting the small signals and issues with X-ray damage to the delicate biological materials. “And even though spectroscopy using soft X-rays represents one of the most direct methods of mapping  the bonds around the manganese atoms, it was previously impossible to carry out these kinds of measurements”, explains Dr. Philippe Wernet from HZB.

Now an international collaboration with HZB and leading experts in photosystem II research around Junko Yano at the LBNL in Berkeley, and others in the USA, in Sweden, and in France has made important progress using an X-ray free-electron laser at the Linac Coherent Light Source (LCLS) at Stanford (USA), and a novel spectrometer that was developed and tested at the HZB.

Spectrometer developed at HZB

The spectrometer contains a reflection zone plate that acts as a lens for the X-rays, likewise developed at HZB. The spectrometer and reflection zone plate made it possible for the first time to investigate metalloenzymes at extremely low concentrations while in solution, thus in their natural environment.

Intermediate steps observed

The research teams were particularly interested in how the electronic structure of the manganese atoms situated at the centre of the enzyme change. This is because the intermediate steps of the reactions leading up to the actual splitting of water can be firmly identified using that information. The team was able to access some intermediate steps which were generated in situ by visible laser excitation of photosystem II. “With our method we can investigate how nature is able to convert solar energy into chemical energy in plants and algae so successfully", says Markus Kubin, HZB, first author of this study that has now been published in Structural Dynamics.

Other catalytic metal complexes in biological and inorganic systems can also be investigated with the newly developed spectrometer.

Publication in Structural Dynamics 4, 054307 (2017); Soft X-ray Absorption Spectroscopy of Metalloproteins and High-Valent Metal-Complexes at Room Temperature Using Free-Electron Lasers; Markus Kubin, Jan Kern, Sheraz Gul, Thomas Kroll, Ruchira Chatterjee, Heike Löchel, Franklin D. Fuller, Raymond G. Sierra, Wilson Quevedo, Christian Weniger, Jens Rehanek, Anatoly Firsov, Hartawan Laksmono, Clemens Weninger, Roberto Alonso-Mori, Dennis L. Nordlund, Benedikt Lassalle-Kaiser, James M. Glownia, Jacek Krzywinski, Stefan Moellerc, Joshua J. Turnerc, Michael P. Minittic, Georgi L. Dakovskic, Sergey Koroidovf,h, Anurag Kawdeh, Jacob S. Kanady, Emily Y. Tsui, Sandy Suseno, Zhiji Han, Ethan Hill, Taketo Taguchi, Andrew S. Borovik, Theodor Agapie, Johannes Messinger, Alexei Erko, Alexander Föhlisch, Uwe Bergmann, Rolf Mitzner, Vittal K. Yachandra, Junko Yano, Philippe Wernet

doi: 10.1063/1.4986627