PHOTON SCIENCE

A team of DESY researchers, led by DESY lead scientist and University of Kiel professor Melanie Schnell, has managed to control the generation of left- and right-handed organic molecules at the single quantum level using a sequence of microwave pulses, a long-standing challenge in chemistry. The phenomenon of handedness, or chirality, is critical for understanding the chemical foundations of life, as well as for the development of medicines. The results of the experiment are published in the journal Physical Review Letters.

Many types of molecular species in nature, for instance, biomolecules, such as sugars and amino acids, come in two forms. These forms – called enantiomers – are mirror images of one another, just like our left and right hands are. The phenomenon is called chirality, and the molecules that have this property are called chiral molecules. For the latter ones, this means that the left-handed and the right-handed molecules, also called left and right enantiomers, have the same atomic composition and even the same chemical bond connectivity, but they look as mirror images of one another. Due to that seemingly minor change, they behave differently upon interactions with other enantiomers. In many molecules, these enantiomers are highly stable for a long time, and bond breaking and new bond formation are required to transfer one enantiomer into the other one. However, some chiral molecules can rapidly interchange between the two enantiomers due to the quantum tunneling effect, and as a consequence, they exist in a quantum state where they are both left- and right-handed at once, like a Schrödinger cat, being alive and dead simultaneously.

All of these molecules are interesting to scientists because of a phenomenon also called the homochirality of life in living organisms. Many of the essential molecular species for life inside living organisms are, according to their type, predominantly left-handed – such as sugars – or right-handed – such as amino acids, the basic building blocks for proteins. Why some molecular species are set in one direction versus the other is a major mystery, as it is unknown where this asymmetry expressed in form of the homochirality of life arises from. Thus, some people believe that studying the way of making chirality using electromagnetic waves can shed light on the origin of such asymmetry. The homochirality of life has wide implications for biochemistry and medicine. For example, many compounds that are medicines in one handedness form are ineffective or even harmful in the other one.

For this study, the team used a compound called 3-fluorobenzyl alcohol that exists in a quantum mixture of two enantiomers. The team excited the molecules of this substance with a combination of tailored microwave pulses and managed to initiate a time-based control of the molecule where all of the molecules were suddenly left-handed, before switching to the other form, then back again, almost like a spring bouncing.

"If each molecule is acting like a spring, then we have many springs that are bouncing randomly," says Wenhao Sun, a scientist in Schnell's group who is the first author of the publication. "What we are doing is to make these oscillations between the two enantiomers coherent, and we make them oscillate in one direction, all together – so we know that the left- and right-handedness of the molecule isn't random."

By controlling en masse when the molecules all begin to change from one form to another, the team showed that they could express handedness directly using their advanced control scheme based on microwave pulses. While this does not physically isolate the two forms of the molecule, it makes the compounds all follow the same form in a distinct, wave-like pattern.

"This is a key step in figuring out how to force a molecule to stably have one form of handedness or the other," Sun says. "The next goal will be to develop how to stop the oscillations from happening in a controlled way so that we have one form or the other. Being able to control the handedness of these chiral molecules is highly relevant for further experiments that aim at producing the chiral compounds without using chiral reagents or catalysts, but only with the help of light, reaching a long-standing goal of absolute asymmetric synthesis. We will use our controlled molecules as starting points for high precision spectroscopy experiments aiming at revealing more differences between the enantiomers."

(from DESY News)


Reference:
W. Sun, D.S. Tiknonov, M. Schnell, Direct observation of time-dependent coherent chiral tunneling dynamics, Physical Review Letters (2025), DOI: 10.1103/PhysRevLett.134.123403