Microwaves Distinguish Mirror-Image Compounds

Many organic compounds can be made as pairs of chemically distinct mirror images that function differently in biological systems. Because the physical properties of these enantiomers—called left- and right-handed chiral compounds—are identical in most respects, they are difficult to tell apart.

Methods to measure chirality include optical rotation, vibrational circular dichroism, and Raman optical activity. Now, microwave spectroscopy joins that list. Microwave spectroscopy measures the energy of transitions between rotational states. The method, reported in Nature last week, is more sensitive and more selective for specific compounds in mixtures with other compounds than other methods of measuring chirality.

The work is “a major breakthrough,” says Brooks H. Pate, a microwave spectroscopy expert at the University of Virginia, who was not part of the study. “If the implementation is as general as suggested, it will significantly impact physical chemistry, analytical chemistry, and possibly such fields as metabolomics that need methods to look at the chemical properties of small-molecule metabolites to infer biochemical pathways and to use as markers for disease detection.”

Physicist John M. Doyle of Harvard University, one of the method’s developers, sees it as a huge boon to pharmaceutical analysis. “Drugs now have to be patented with a specific chirality,” he says, and this needs to be determined when they’re developed and manufactured.

The technique, reported by Doyle and David Patterson—also of Harvard—and Melanie Schnell of the Max Planck Institute for Nuclear Physics, in Heidelberg, Germany, works by manipulating molecules’ electric dipole moments. These indicate the magnitude and direction of molecules’ charge separation in each of the three spatial dimensions (Nature 2013, DOI: 10.1038/nature12150).

The method involves applying two microwave-frequency electric fields to a sample of cold, gas-phase molecules, such as 2-propanediol. The two fields, which are polarized in different directions, interact with a molecule’s dipoles in a way that makes the molecule emit microwaves polarized in yet another direction.

Two of the dipole moments on one enantiomer have equal magnitudes and point in the same direction as their counterparts on the other enantiomer. The third pair of dipole moments is the distinguishing feature between the two enantiomers: They have equal magnitude but point in opposite directions.

Because those dipole moments point in opposite directions, the enantiomers emit radiation 180° out of phase with one another. “The left-handed one will oscillate starting as a positive value, and the right-handed one will start off with a negative value,” Doyle says. This phase property is thus used to distinguish absolute chirality. In addition, the frequency of emitted microwaves is used to identify chemical identity, and the magnitude is a measure of relative enantiomer concentration.

Beginning with gas-phase molecules at room temperature, the technique requires rapid cooling to 7 K via collisions with a cryogenic buffer gas. “By cooling the molecules, you get much larger signals,” Doyle says. Those enhanced signals improve the method’s sensitivity.

The method should work well with mixtures of multiple compounds because the peaks in the microwave spectrum are narrow and well separated. “No two molecules are going to have the same three frequencies of these rotational transitions, so it’s extremely species selective,” Doyle says.

In an accompanying commentary, Laurence A. Nafie, an emeritus chemistry professor at Syracuse University, writes that “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.”