Natural circular dichroism in achiral crystals evidenced

Chirality is a property of objects (or molecules) that makes them non-superimposable on their mirror image — much like the left and right hands. They may look very similar, but no matter how you rotate your left hand, it won’t fit into a right-hand glove.

In chemistry and biology, many molecules are chiral. For example, most of the amino acids and sugars in our bodies exist in one specific ‘handed’ form. This is crucial because biological systems are incredibly selective — one version of a molecule might act as a helpful drug, while its mirror image could be ineffective or even harmful.

Chirality also affects how molecules interact with light. Chiral substances can rotate the polarization of light - a phenomenon widely exploited in pharmaceuticals, food testing, and chemical research.

Theorists have long predicted that certain crystals, even those without chirality (that is, not "handed"), could still exhibit optical activity, or natural circular dichroism. This effect is rooted not necessarily in chirality itself but in the specific symmetries of the crystal structure — essentially, how the atoms are arranged in space. Until now, scientists have struggled to see this effect with visible light due to a birefringence, which alters light polarization, and hence masks the dichroic signal.

Now a team from the University of Bordeaux and Sorbonne University used the ESRF’s X-rays of ID12 to search for natural circular dichroism in achiral crystals. They studied copper and iron salts that crystallizes in two specific non-chiral symmetry groups: 4̅2m and 4̅. “The beamline ID12 is where X-ray natural circular dichroism was pioneered to probe chirality in element selective manner. This technique remains the only tool we have to unambiguously observe this effect in achiral crystals”, explains Andrei Rogalev, scientist in charge of the beamline and co-author of the publication.

The team tuned X-rays to specific energies corresponding to absorption edges of copper and iron and measured a difference in absorption of right- and left-circularly polarized X-rays. The experimental findings are exactly what the theory had predicted: the dichroism changed with the rotation of the crystal, in a way dictated by the symmetry of the crystal structure. In crystals with 4̅ symmetry, they observed an additional angular ‘phase shift’ induced by the translation operations of the crystal’s space group.

This study confirms that natural circular dichroism is not exclusive to chiral systems, challenging long-standing general beliefs and demonstrating that optical activity can emerge purely from symmetry in non-centrosymmetric achiral crystals. By using X-rays rather than visible light, the researchers avoided problems caused by birefringence and showed that X-ray natural circular dichroism is a powerful way to explore symmetry-driven effects in solid-state physics and chemistry – with potential applications ranging from fundamental science to the design of novel functional materials.

Reference:

Duverger-Nédellec E., et al., Journal of the American Chemical Society 2025 147 (26), 22296-22301. DOI: 10.1021/jacs.5c03574

Text by Montserrat Capellas Espuny