Ground-Breaking Experiments: Tracking Chirality in Real Time

Enantiomers are mirror copies of each other that are non-superposable, similar to a pair of hands. Enantiomers can have negative impacts in (bio)chemical phenomena, although sharing the majority of chemical and physical features. A protein or enzyme, for example, may bind just one enantiomeric version of a target molecule. As a result, chirality is frequently used in the creation of (bio)chemical molecules, such as in the food, fragrance, and pharmaceutical sectors.

The most common method for determining chirality is circular dichroism, which uses the difference in how chiral materials absorb left- and right-circularly polarized light to directly identify pairs of enantiomers. Circular dichroism's chiral reaction may also assist clarify a molecule's conformation, a fact that has made it a prominent analytical tool in (bio)chemical sciences.

Circular dichroism, on the other hand, has a restricted temporal resolution and spectral range. Researchers lead by Malte Oppermann at EPFL have created a novel time-resolved device that monitors circular dichroism changes in fractions of a picosecond (one trillionth of a second), allowing them to "capture" ultrafast pictures of a molecule's chirality during its (bio)chemical activity. This allows researchers to determine the conformational motion that underlies the conversion of absorbed light energy and capture the chirality of photoexcited molecules.

The researchers used the new method to investigate the magnetic-switching dynamics of so-called "iron-based spin-crossover complexes" – an important class of metallo-organic molecules with promising applications in magnetic data storage and processing devices – in collaboration with Jérôme Lacour's group at the University of Geneva and Francesco Zinna's group at the University of Pisa. Despite its usefulness for magnetic data storage, the deactivation process of their magnetic state has remained unsolved after decades of research.

The researchers revealed that the loss of magnetization is caused by a twisting of the molecule's structure, which alters its chiral symmetry, using a time-resolved circular dichroism experiment. Surprisingly, by limiting the twisting motion in modified complexes, the team was able to slow down the deterioration of the magnetic state.

“These ground-breaking experiments show that time-resolved circular dichroism is uniquely suited to capture the molecular motion that drives many (bio)chemical processes,” explains Malte Oppermann. “This offers a new way for investigating challenging dynamic phenomena – for example, the ultrafast rotations of synthetic molecular motors, and the conformational changes of proteins and enzymes in their native liquid environment.”