New Method Helps Differentiate Between Mirror Image Molecules

New Method Helps Differentiate Between Mirror Image Molecules

  • Technology
  • July 6, 2022
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  • 4 minutes read


Using a new method, scientists are better able to distinguish between mirror imaging substances. This is important, among other things, in drug development, because the two variants can cause completely different effects on the human body. Researchers from the PSI, EPFL and the University of Geneva describe the new method in the scientific journal Photonics of nature.

Some molecules exist in two forms that are structurally identical but are mirror images of each other, like our right and left hands. These are called chiral molecules. Its two forms of mirror image are called enantiomers. Chirality is especially relevant in biological molecules, as it can cause different effects on the body. Therefore, it is essential in biochemistry and toxicology, as well as in drug development, to separate the enantiomers from each other so that, for example, only the desired variant enters a drug. Now researchers at the PSI, EPFL and the University of Geneva have jointly developed a new method that makes it possible to better distinguish enantiomers and therefore better separate them from each other: helical dichroism in the X-ray domain.

The currently established method for distinguishing between enantiomers is called circular dichroism (CD). In this approach, light with a particular property, known as circular polarization, is sent through the sample. This light is absorbed to a different extent by the enantiomers. CD is widely used in analytical chemistry, biochemical research, and the pharmaceutical and food industries. In CD, however, the signals are very weak: the light absorption of two enantiomers differs by just under 0.1 percent. There are several strategies for amplifying signals, but they are only suitable if the sample is available in the gas phase. Most chemistry and biochemistry studies, however, are done in liquid solutions, mainly in water.

Instead, the new method exploits the so-called helical dichroism, or HD for short. The effect underlying this phenomenon is found in the shape of light rather than in its polarization: the wavefront curves into a helical shape.

At PSI’s Swiss Light Source SLS, researchers were able to successfully demonstrate for the first time that enantiomers could also be distinguished from each other by helical X-ray light. In SLS’s cSAXS light line, they demonstrated this in a sample of the iron-tris-bipyridine chiral metal complex powder, which researchers at the University of Geneva had made available. The signal they obtained was several orders of magnitude stronger than what can be achieved with CD. HD can also be used in liquid solutions and therefore meets an ideal prerequisite for applications in chemical analysis.

It was crucial for this experiment to create X-ray light with precisely the right properties. The researchers were able to achieve this with so-called spiral zone plates, a special type of diffractive X-ray lens through which they sent light before it reached the sample.

“With the spiral zone plates we were able, in a very elegant way, to give our X-ray light the desired shape and therefore an orbital angular momentum. The beams we create in this way are also called optical vortices, ”says PSI researcher Benedikt Rösner, who designed and fabricated the spiral zone plates for this experiment.

Jérémy Rouxel, an EPFL researcher and first author of the new study, also explains: “Helical dichroism provides a completely new type of light-matter interaction. We can exploit it perfectly to distinguish between enantiomers. ”

The study was made possible by funding provided through the European Research Council with the ERC Advanced Grant DYNAMOX, by the Swiss National Science Foundation with the National Center for Competence in Molecular Research Ultrafast Science and Technology (NCCR MUST) and by the German Scholar. Exchange Service (DAAD).

Reference: Rouxel JR, Rosner B, Karpov D, et al. Hard helical X-ray dichroism of disordered molecular media. Nat Photon. 2022. doi: 10.1038 / s41566-022-01022-x

This article has been republished from the following materials. Note: The material may have been edited by duration and content. For more information, contact the source cited.



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