Physical chemistry

Handedness detected by microwaves

Traditional methods for detecting and identifying the handedness of molecules — their chirality — have been based on the same theoretical concept. A technique has been reported that departs from this paradigm. See Letter p.475

The property of handedness known as chirality is one of the most subtle and yet profound aspects of our world. Of particular relevance to chemistry and biology is the fact that molecular structures can be chiral: as with our left and right hands, chiral molecules have otherwise equivalent mirror-image isomers of the opposite chirality1. Such isomers are called enantiomers, and the ability to distinguish between them is called enantiomeric detection. On page 475 of this issue, Patterson et al.2 describe a method that adds to the chemist's toolbox of techniques for detecting and identifying chirality in molecules3. They report that, when microwave radiation associated with transitions between rotational states is emitted by molecules of opposite chirality, the radiation is exactly out of phase, thus providing a clear signal of molecular chirality.

The building blocks of life are 'homochiral': naturally occurring amino acids, proteins, sugars and nucleic acids exist as only one chiral form. If these building blocks had mixed chirality, molecular chaos would ensue and life would not be possible. Nature's lead is being followed by scientists, who have learned that single-enantiomer drugs are more efficient at binding to biological targets, and have fewer side effects, than the equivalent racemic drugs (which contain an equal mixture of opposite enantiomers). In fact, our bodies recognize drugs of the opposite chirality as different molecules, even though, apart from their chirality, they have the same structure. This is because the mirror symmetry of enantiomers is broken by the homochiral biochemistry of the human body, in the same way that only a right hand fits comfortably into a right-handed glove.

The mirror symmetry of enantiomers can also be broken by circularly polarized electromagnetic radiation. Circular polarization occurs when the electromagnetic field of radiation rotates either clockwise or anticlockwise, once per wavelength, as the beam propagates. Two classical forms of 'chiroptical' spectroscopy have been developed on the basis of this symmetry breaking. The older form is optical rotation4, which measures the difference in refractive index for left-circularly polarized (LCP) and right-circularly polarized (RCP) radiation as it passes through chiral materials. The other is circular dichroism (CD), which measures the difference in the absorption of LCP and RCP radiation. Both techniques typically use ultraviolet or visible light and involve electronic transitions in molecules. Vibrational circular dichroism (VCD, which uses infrared radiation) and Raman optical activity (ROA, which mainly uses visible radiation) have also been added to the list of chiroptical spectroscopy techniques5,6,7 and are based on the vibrational transitions of molecules.

Using these four techniques, two fundamental measures of chirality can be determined: enantiomeric excess, which is the excess of one enantiomer over the other in a sample; and absolute configuration, the specific handedness of an enantiomer. Every chiral drug substance approved for sale by the US Food and Drug Administration must have a known absolute configuration and a specified level of enantiomeric excess usually greater than 99% (see go.nature.com/ujg17l).

Patterson and colleagues' use of microwave radiation to detect chirality is surprising, because microwave CD would be expected to be too small for detection by modern spectrometers. All previously measured forms of chiroptical intensity are inversely proportional to the wavelength of the probing radiation. Electronic CD in the ultraviolet–visible region is typically about 100 times larger than VCD in the longer-wavelength infrared region. Microwave CD should be at least 100 times smaller than VCD. This size argument derives from the mechanism underpinning traditional chiroptical spectroscopy: optical activity arises from the interference between the electric-dipole transition moment and the weak magnetic-dipole transition moment (or, in some cases, the weak electric-quadrupole transition moment as well) that is detected when a chiral molecule is irradiated with alternating LCP and RCP light. The optical activity can be positive or negative depending on whether the electric- and magnetic-dipole transition moments point into the same or opposite halves of a sphere centred on the molecule.

The authors' method does not arise by this interference mechanism. Instead, the authors detect chirality by applying two orthogonally polarized, microwave-timescale electric fields (oriented in the x and z directions) to a sample. This causes the molecules in the sample to emit microwave radiation polarized along a third orthogonal direction (y), the intensity of which is proportional to the product of each molecule's three orthogonal rotational electric-dipole moments (μx, μy and μz). This product is independent of the molecule's orientation, but is sensitive to the handedness of the directions of the three rotational dipole moments. It therefore changes sign if any two of the moments are interchanged or, equivalently, if the molecule is exchanged for its opposite enantiomer, and so is a new measure of true chirality8(Fig. 1).

Figure 1: A new mechanism for chiroptical spectroscopy.
figure1

In Patterson and colleagues' method2 for identifying molecular chirality, the measured quantity depends on the handedness of three mutually orthogonal electric-dipole rotational transition moments, μx, μy and μz, which are associated with the three rotational degrees of freedom of a molecule. When microwave radiation interacts with these moments, energy transfer changes the rotational state of the molecule, generating a spectroscopic signal. The moments are vectors, and can be represented by a scalar triple product of μx ·μy × μz. This product can be regarded as a dipole volume, V, that is equal to the product of the magnitudes of the three vectors. The sign of V depends on the order of the vectors. If any two vectors are interchanged (a process equivalent to a mirror reflection), the sign changes. Because V changes sign under spatial inversion (mirror reflection) and is even under time-reversal symmetry, it is a measure of true chirality8.

Patterson and colleagues' approach has several advantages over existing chiroptical spectroscopy techniques. Because it does not depend on a weak magnetic-dipole transition moment, the chiral signal is nearly as large as that of the applied microwaves. Furthermore, the method requires extremely cold, gaseous molecules, which exhibit sharp, narrow lines in their microwave spectra. Molecules of interest can therefore be resolved in the presence of other interfering molecules. Moreover, measurement times can be as fast as tens of seconds; measurements for traditional chiroptical spectroscopy techniques take minutes to hours.

Of course, a few drawbacks remain to be ironed out. Any molecule investigated must be sufficiently volatile to be sampled in the gas phase, potentially placing an upper limit on the size of molecules that can be analysed. And for large molecules that have high conformational freedom, it may be difficult to identify the conformation associated with the microwave line being analysed. Additionally, enantiomeric excess measured using the technique is accurate to only about 5%, and so further development is needed to reach a more desirable level of accuracy.

Finally, the determination of absolute configuration for a sample of unknown chirality has yet to be demonstrated. The scalar triple product of μx·μy×μz will be positive or negative depending on the enantiomer in the sample. But how can one say which enantiomer produces a positive product and which a negative one? This information might be obtained by determining whether the chiral microwave emission of a particular enantiomer has the same phase as the driving field, or the opposite one. However, this information would still need to be connected to the absolute configuration of a chiral molecule, most probably by carrying out a quantum-mechanical calculation to determine the signs of the dipole-moment components. Most other chiroptical spectroscopy techniques require such a calculation to connect measured spectra to the molecule's absolute configuration.

If Patterson and co-workers' method can be widely applied to determine the enantiomeric excess and absolute configuration of previously unassigned chiral molecules, particularly those of pharmaceutical interest, then their paper will be extremely important. But even if its applicability is at first limited (either for sampling reasons or by instrumentation), the unexpected demonstration of a conceptually new form of chiroptical spectroscopy makes this work a landmark in the 200-year-old history of optical activity in chemistry.

References

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    Wagnière, G. H. in Comprehensive Chiroptical Spectroscopy Vol. 1 (eds Berova, N., Polavarapu, P. L., Nakanishi, K. & Woody, R. W.) 3–34 (Wiley, 2012).

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    Barron, L. D. Molecular Light Scattering and Optical Activity 2nd edn (Cambridge Univ. Press, 2004).

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Correspondence to Laurence A. Nafie.

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Nafie, L. Handedness detected by microwaves. Nature 497, 446–448 (2013). https://doi.org/10.1038/497446b

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