Chirality plays an important role in pharmaceutical compounds, and methods for detecting and quantifying chirality remains
challenging. Researchers at Harvard University and the Center for Free-Electron Laser Science (CFEL) and the Max Planck Institute
in Germany recently reported on the use of enantiomer-specific detection of chiral molecules by microwave spectroscopy.
The researchers pointed out that the spectroscopic methods of circular dichroism and vibrational circular dichroism are commonly
used in analyzing chiral molecules. These methods, however, have limitations in electric dipole approximation as the resultant
weak effects produce weak signals and require high sample densities (1). In contrast, nonlinear techniques probing electric-dipole-allowed
effects have been used for sensitive chiral analyses of liquid samples. Influenced by these methods, the researchers carried
out nonlinear resonant phase-sensitive microwave spectroscopy of gas-phase samples in the presence of an adiabatically switched
nonresonant orthogonal electric field. They used this technique to map the enantiomer-dependent sign of an electric dipole
Rabi frequency onto the phase of emitted microwave radiation (1). They describe theoretically how this results in a sensitive
and species-selective method for determining the chirality of cold gas-phase molecules. They implemented the approach experimentally
to distinguish between the S and R enantiomers of 1,2-propanediol and their racemic mixture. They reported that this technique produced a large and definitive
signature of chirality and has the potential to determine the chirality of multiple species in a mixture.
"We can soon measure mixtures of different compounds and determine the enantiomer ratios of each," said Melanie Schnell, co-author
of the study in a CFEL release. In a next step, the researchers plan to apply the technique in a broadband spectrometer at
CFEL that could then measure the enantiomer ratios in mixtures of substances. In the longer run, the method opens the door
to develop a technique for separating enantiomers.
1. D. Patterson, Melanie Schnell, and John M. Doyle, Nature, 497 (7450), 475-477, 2013.