In recognition of chirality: Ultraviolet probe

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  • Published: Apr 1, 2012
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: In recognition of chirality: Ultraviolet probe

Gas probe

Identifying the chirality of a compound in the gas phase using light is difficult because of the low particle densities present. However, collision-free conditions could lead to highly sensitive analytical applications as well as allowing researchers to unravel fundamental aspects of the way light interacts with matter.

The handedness, or chirality, of nature's molecules has fascinated chemists for decades and ever since Louis Pasteur plucked apart the left-and right-handed enantiomers of tartaric acid salts with tweezers and magnifying glass there has been an urge to find ways to understand the origins of chirality and how to manipulate it to the chemist's own ends. Distinguishing between enantiomers is, of course, not simply a matter of aesthetics. Biomolecules and the pharmaceuticals that we synthesise to interact with them and treat disease, for instance, are sensitive to the chirality of the components taking part in the action. There are various techniques available to chemists, but the gas phase recognition of chirality offers numerous advantages, such as collision-free conditions.

Synchrotron radiation, the polarised and highly energetic photons generated by a particle accelerator, can be used to eject electrons from chiral molecules of interest. By tracking the trajectories - the photoelectron angular distribution (PAD) - of these particular missiles researchers can extract the chirality of the molecules from the system. However, synchrotron radiation is sourced using rather specialist instrumentation so a team in Germany has turned to laser light instead and demonstrated that their experiments also work with a compact laser system for chiral recognition.

Tricky analysis by turns

The trick, the team explains, was to replace the individual high-energy photons from the synchrotron with three laser photons that excite the molecule through intermediate levels until it releases an electron. This technique is known in short as REMPI, Resonance-Enhanced Multi-Photon Ionization. "It is thus possible to eject electrons with less energetic but more intense light," explains team leader Thomas Baumert of the University of Kassel. He is working with Christian Lux, Matthias Wollenhaupt, Tom Bolze, Qingqing Liang, Jens Koehler and Cristian Sarpe.

The only real prerequisite of swapping synchrotrons for lasers is that the laser light must be circularly polarized for the approach to work so that the photons "follow" a helical oscillation. The team explains that molecules in the gas phase are randomly oriented and thus encounter the polarised laser light from all possible angles, this also means that the ejected electrons travel in all possible directions. By determining the electron PAD, however, the team can see what effect the chirality of the molecules being investigated has. Linear polarized light gives a symmetrical PAD. "However, when the electrons are ejected by circularly polarized light, we find a distinct asymmetry to the angles at which the free electrons are found in relation to the laser beam," explains Baumert. This asymmetry is inverted if left circularly polarized light is used instead of right, this is the photoelectron circular dichroism effect. "We observe the same effect when we keep the circular polarization the same but change from the right handed to the left handed structure of the chiral molecule being observed," he adds.

The team demonstrated proof of principle with the well-known chiral molecule, camphor, the smelly component of mothballs and the structurally similar fenchone, which is found in absinthe and the essential oil of fennel.

Taking a technique to wider laboratories

"This circular dichroism effect has previously only been observed with synchrotron radiation," Baumert points out. "In contrast, our procedure uses a compact laser system, so that this method is not limited to basic laboratory research but, because of the magnitude of the observed effects, may also find its way into analysis."

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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