Making it easier to distinguish mirror image molecules

Some molecules exist in two forms that are identical in structure but are mirror images of each other, such as our right and left hands. These are called chiral molecules. Their two mirror image forms are called enantiomers. Chirality is especially relevant in biological molecules, as it can cause a variety of effects in the body. It is therefore essential in biochemistry and toxicology, but also in drug development, to separate enantiomers from each other, so that, for example, only the desired variant ends up in a drug. Now researchers from PSI, EPFL and the University of Geneva have jointly developed a new method that can better distinguish enantiomers and thus separate them from each other: helical dichroism in the X-ray domain.

The currently established method of distinguishing between enantiomers is called circular dichroism (CD). In this approach, light with a certain property – called circular polarization – is sent through the sample. This light is absorbed by the enantiomers to varying degrees. CD is widely used in analytical chemistry, in biochemical research, and in the pharmaceutical and food industries. With CD, however, the signals are very weak: the light absorption of two enantiomers differs by slightly less than 0.1 percent. There are several strategies to amplify the signals, but they are only suitable if the sample is available in the gas phase. However, most studies in chemistry and biochemistry are conducted in liquid solutions, mainly in water.

In contrast, the new method uses the so-called spiral dichroism, or HD for short. The effect underlying this phenomenon is found in the shape of the light rather than its polarization: the wavefront is curved in a spiral shape.

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

For this experiment, it was crucial to create X-ray light with exactly the right properties. The researchers succeeded with so-called spiral zone plates, a special type of diffractive X-ray lenses through which they sent the light before it hit the sample.

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

Jérémy Rouxel, an EPFL researcher and the lead author of the new study, further explains: “Spiral dichroism creates a completely new kind of light-matter interaction. We can use it perfectly to distinguish between enantiomers.”

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

Text: Paul Scherrer Institute/Laura Hennemann

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