Track chirality in real time

Chiral molecules exist in two forms called enantiomers, which are mirror images of each other and cannot be superimposed 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 bind to only one enantiomeric form of a target molecule. Consequently, identification and control of chirality is often key to designing (bio)chemical compounds, for example in the food, fragrance and pharmaceutical industries.

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

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

In a collaboration with the group of Jérôme Lacour at the University of Geneva and Francesco Zinna at the University of Pisa, the researchers used the new method to investigate the magnetic switching dynamics of so-called “iron-based spin-crossover complexes”. an important class of metallurgicalorganic molecules with promising applications in magnetic data storage and processing equipment. After decades of research, the deactivation mechanism of their magnetic state has remained unsolved, despite its importance for magnetic data storage.

By conducting a time-resolved circular dichroism experiment, the researchers found that the loss of magnetization is caused by a twisting of the molecule’s structure that disrupts its chiral symmetry. Remarkably, the team was also able to slow the decay of the magnetic state by suppressing the spinning motion in modified complexes.

“These groundbreaking experiments show that time-resolved circular dichroism is ideally suited to capture the molecular motion that many (bio)chemical processes“, says Malte Oppermann. “This provides a new way to investigate challenging dynamic phenomena, for example the ultrafast rotations of synthetic molecular motors and the conformational changes of proteins and enzymes in their natural liquid environment.”

The study is published in Natural Chemistry†