Real-time chirality tracking

image: Artist’s impression of a time-resolved circular dichroism measurement of a photoexcited spin-crossing complex.
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Credit: Ella Maru Studio, Inc.

Chiral molecules exist in two forms, called enantiomers, which are mirror images of each other and not overlapping – much like a pair of hands. Although they share most chemical and physical properties, enantiomers can have adverse effects on (bio)chemical phenomena. For example, a protein or enzyme can only bind one enantiomeric form of a target molecule. Therefore, the identification and control of chirality is often essential for the design of (bio)chemical compounds, for example in the food, perfume and pharmaceutical industries.

A most common technique for detecting chirality is called circular dichroism, which measures how chiral samples absorb left and right circularly polarized light differently to directly identify pairs of enantiomers. Circular dichroism can also help resolve a molecule’s conformation through its chiral response – a feature that has made it a popular analytical tool in the (bio)chemical sciences.

However, circular dichroism has so far been limited in temporal resolution and spectral range. Researchers led by Malte Oppermann in Majed Chergui’s group at EPFL have developed a new time-resolved instrument that measures changes in circular dichroism in fractions of a picosecond (one trillionth of a second), meaning it can ” take” ultra-fast snapshots of a molecule’s chirality throughout its (bio)chemical activity. This captures the chirality of photoexcited molecules and resolves the conformational motion that drives the conversion of absorbed light energy.

In collaboration with the group of Jérôme Lacour from the University of Geneva and Francesco Zinna from the University of Pisa, the researchers used the new method to study the magnetic switching dynamics of the so-called “spin-crossing complexes at iron base” – an important class of metallo-organic molecules with promising applications in magnetic data storage and processing devices. After decades of research, the mechanism for turning off their magnetic state has remained unresolved, despite its importance for magnetic data storage.

By performing a time-resolved circular dichroism experiment, the researchers discovered that the loss of magnetization is driven by a twist in the molecule’s structure that distorts its chiral symmetry. Remarkably, the team was also able to slow the degradation of the magnetic state by suppressing the twisting motion in the modified complexes.

“These groundbreaking experiments show that time-resolved circular dichroism is particularly suited to capture the molecular motion underlying many (bio)chemical processes,” says Malte Oppermann. “It offers a new way to study difficult dynamic phenomena – for example the ultrafast rotations of synthetic molecular motors and the conformational changes of proteins and enzymes in their native liquid environment.”

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