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X-rays of water to close gaps in our understanding

Friday 12 Jun 20

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Kristoffer Haldrup
Senior Researcher
DTU Physics
+45 22 98 37 20
By bringing newly developed X-ray methods into play, researchers at DTU will close gaps in our knowledge of how water participates in and controls chemical reactions. Thanks to DKK 12.5 million in funding from Novo Foundation's new "NERD" program, researchers can now attack this scientific challenge at an unprecedented level of detail.

In the field of water science, much detailed knowledge is lacking in the description of the structural role of water in chemical reactions at the fundamental molecular level.

No one can doubt that water is important. Without water, no life as we know it. Water is the "universal solvent" in which all the biochemical reactions that make up our cells and body take place. And water is also a highly important solvent for a wide range of chemical reactions.

‘Water surrounds us on all sides and water molecules are intriguingly simple, the well-known H2O with just two hydrogen atoms and a single oxygen atom. Therefore, it is also very easy to come to the conclusion that water is well understood as a liquid and as a solvent for chemical and biochemical reactions. But the reality is that we still have a lot of unanswered questions about the physical and chemical properties of water at the molecular level”, says Kristoffer Haldrup, Senior Researcher at DTU Physics.

It is these gaps in our knowledge of water that Senior Scientist Kristoffer Haldrup from DTU Physics wants to address with advanced X-ray methods in the project, which, over the next 6 years, has been supported with a total of DKK 12.5 million from the Novo Nordisk Foundation's new "NERD"-program.

The researchers have long known that one of the most important properties of water is the hydrogen bonds that water molecules can form with other molecules and with each other. Bonds that are directional and relatively long-range, and at the same time are quite complex and much more fragile than normal chemical bonds.

"One of the main reasons we know so little about the basic structures of water is that the hydrogen bonds form and disappear sometimes billions of times per second"
Kristoffer Haldrup

For some molecules in solution, the bonds cause water to pack tightly around the molecule and protect it from chemical reactions. However, for other molecules, which may be nearly similar, the effect is just the opposite and the solvated molecules act as if they were "water-repellent" and the water molecules keep their distance and pack only loosely, thus making the molecules more accessible to chemical reactions. However, it is often extremely difficult to predict this packing and in particular how it changes during a chemical reaction.

"There is a huge gap in our knowledge of what is actually going on at the molecular level during a chemical reaction, and even just how the water molecules pack with other water molecules has been under intense scientific discussion for decades," Kristoffer explains.

“One of the main reasons we know so little about the basic structures of water is that the hydrogen bonds form and disappear sometimes billions of times per second, and common experimental methods therefore only see a muddy average of the molecular details offered".

Kristoffer and his colleagues will meet this challenge by using advanced X-ray methods at so-called synchrotrons and X-ray lasers kilometer-long research facilities that Denmark contributes to in Hamburg and in Grenoble, France, among other places.

“Using these facilities, we can generate extremely brief and insanely intense flashes of X-rays. Flashes so short that the scattered X-rays from the sample contains a snapshot of how the molecules sit relative to each other within this extremely short time span”, Kristoffer Haldrup says.

Hundreds of thousands of such individual scattering patterns are then pieced together into a kind of "molecular movie" where the researchers can follow the details of the “molecular dance” with water molecules interacting with other molecules, and by comparing with advanced computer simulations one can map out what forces drive the water molecules, and thus what may determine the course of a given chemical reaction.

These ultra-fast, X-ray-based methods have been developed by Kristoffer Haldrup and a world-wide network of colleagues and collaborators over the last decade and are now so robust that they can measure even tiny changes in the molecular structure of water around other molecules.

The project will also examine exactly how the interaction with water can modify the properties of photo-active molecules, e.g. changing their properties from being lousy materials for harvesting solar energy to being quite excellent for just that purpose.

“With the knowledge we gain about the detailed structural dynamics of water and the insight into how these processes influence solvation, we can help pave the way for new designs of dye-based solar cells and in the longer run obtain a better understanding of how water influences also protein interactions,” Kristoffer explains.

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