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Published: 17 Sep, 2020

New findings on the water-splitting reaction in photosystem II

NEWS By combining isotope labeling, magnetic resonance, X-ray emission and crystallography, Casper de Lichtenberg discusses in his dissertation which water molecules, bound in the active site in photosystem II, are the reactive ones and how the water-splitting reaction proceeds. He defended his results on September 11 at Umeå University.

Text: Ingrid Söderbergh

Oxygenic photosynthesis is one of the most important chemical processes for life on Earth, as we know it. Through this process, the energy of sunlight is harvested, and is used to strip water of its hydrogen atoms and electrons which can be used in production of high energy molecules. Meanwhile the oxygen molecules are expelled into the air of the atmosphere that we breathe. This reaction is orchestrated by the protein photosystem II which can be found in the green parts of plants, algae and cyanobacteria. 

During the industrialization the use of fossil fuels was introduced to power our society. The accelerated use of these fuels leads to global warming that will soon result in an increase in average temperature of 2 degrees Celcius. Consequently, large efforts have been made to develop alternative fuel sources that are carbon neutral or carbon free.

Particularly photosystem II has received a great deal of attention, as the active site consists of metals (abundant in nature) and since the electrons and hydrogens produced in the catalytic process can be used to produce high-energy fuels. Thus, this system has become the prime example that light driven synthetic water oxidation catalysts should eventually be modelled after.

Unfortunately, the exact chemistry that occurs at the active site is still unclear since the identity of the water molecules that participate in the oxygen bond formation remain elusive. Thus, Casper de Lichtenberg had dedicated his PhD studies to the identification of these water molecules.

Eftersom fotosystem II utför sin reaktion i vatten, kan arbetet med att skilja vattnet som deltar i reaktionen från resten av vattnet liknas med att skilja en droppe vatten från resten av havet.

 “To move past this problem I have studied water splitting reaction in photosystem II with methods that exploit that the reactive waters are bound to the active site. By injecting isotope labelled water into the sample I can determine how quickly the label is incorporated into the site where the reactive waters sit, by measuring the magnetic resonance signal of the water ligands to the active site and by measuring the mass of the product oxygen. This provides direct chemical information about how the reactive waters are bound”, says Casper de Lichtenberg.

To properly rationalize this information it is generally helpful to know the physical and electronic structure of the active site where the reactive waters are bound. During his doctoral studies, Casper de Lichtenberg has also participated in experiments where the research team have identified the structure of all stable intermediates of the reaction cycle.”

“Furthermore, we have taken steps towards creating the first molecular movies of the water splitting reaction by picturing the structures at different time-points between these intermediates”.

Read the thesis

Press photos. Credit: David Naylor

 

About the dissertation

On Friday 11 September, Casper de Lichtenberg, Department och Chemistry at Umeå university, defended his thesis entitled: Time-resolved Structural and Mechanistic Studies of Water Oxidation in Photosystem II: water here, water there, water everywhere. 

The dissertation took place at 13:00, in room Glasburen, KBC building

The Faculty opponent was Professor Dr. Holger Dau, Department of Physics, Freie Universität Berlin, Berlin, Germany.

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