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Comment Processing & Handling

A more-sensitive way to differentiate chiral compounds

By Gerald Ondrey |

Circular dichroism (CD) is the established method for distinguishing between enantiomers — the optical isomers of molecules that are mirror images of each other. In this approach, circularly polarized light is passed through the sample and is absorbed differently by the enantiomers. Although CD is widely used in analytical chemistry, in biochemical research, and in the pharmaceutical and food industries, the signals are very weak: the light absorption of two enantiomers differs by just under 0.1%. There are various strategies for amplifying the signals, but these are only suitable if the sample is available in the gas phase. Most studies in chemistry and biochemistry, however, are carried out in liquid solutions.

Now, Swiss researchers from the Paul Scherrer Institute (PSI; Villigen; www.psi.ch), EPF Lausanne (www.epfl.ch) and the University of Geneva (www.unige.ch) have demonstrated a new method, which was reported in a recent issue of Nature Photonics. The new method exploits so-called helical dichroism (HD), which relies on the shape (helical) of the radiation’s wavefront, rather than its polarization.

At the Swiss Light Source (SLS) at PSI, the researchers were able, for the first time, to show that enantiomers can also be distinguished from each other using helical X-ray light. At the cSAXS beamline of SLS, they demonstrated this on a sample of the chiral metal complex iron-tris-bipyridine in powdered form (diagram). 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 thus fulfills the prerequisite for applications in chemical analysis. The ability to distinguish enantiomers is also an important tool when separating them.

The researchers were able to create light with the desired properties with spiral-zone plates, diffractive X-ray lenses through which the X-rays pass before hitting 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 thus an orbital angular momentum. The beams we create in this way are also referred to as optical vortices,” says PSI researcher Benedikt Rösner, who designed and fabricated the spiral zone plates for this experiment.

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