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Mass spectrometry sorts mirror-image molecules

Mass spectrometry sorts mirror-image molecules

15 February 2024

Giving chiral molecules a spin can separate the left-handed variety from the right-handed one.

When it comes to the machinery of life, handedness matters. Many of the biomolecules that enable our existence are chiral: They take two forms, called enantiomers, that are mirror images of each other yet structurally distinct, with only one enantiomer usable by organisms. Nearly all life’s necessary amino acids, for example, are left-handed, because of the direction in which they rotate the polarization of incident light. The primary ingredients of medicines are often chiral too—one enantiomer tends to be responsible for the desired therapeutic effect, whereas the other is inactive or, in some cases, potentially harmful.

The enantiomers of binaphthyl triflate.
The enantiomers of binaphthyl triflate. Carbon atoms are shown in gray, hydrogen in white, fluorine in green, and sulfur in yellow. Credit: X. Zhou et al., Science 383, 612 (2024)

Given the importance of chirality in biology and medicine, researchers have dedicated tremendous effort toward developing ways to distinguish and separate such exceedingly similar yet not-identical molecules (see Physics Today, July 2018, page 14). Widely used techniques, including chromatography, rely on initiating interactions between the enantiomer under study and other chiral species. Now researchers in the lab of Zheng Ouyang at Tsinghua University in Beijing have demonstrated a relatively simple way to perform chiral sorting with mass spectrometry—a technique that is usually employed to sort molecules by mass.

The researchers started by injecting a sample of a chiral species into a mass spectrometer, where it was vaporized and ionized. Rather than proceeding with the typical sorting by mass-to-charge ratio—a fruitless exercise for molecules with identical mass and charge—they sent the molecules into a built-in ion trap, applied an electric field, and coaxed the ionized molecules to rotate like propellers.

Crucially, the researchers manipulated the field so that both enantiomers would spin in the same direction. That rotation, combined with the enantiomers’ mirror-image structures, ensured that the enantiomers would plow through the ambient gas molecules within the trap differently and thus experience differing amounts of drag. The paths of the left-handed molecules steadily diverged from those of the right-handed species, to the point that researchers could steer one of the enantiomers out of the ion trap.

Two rotating propellers with mirror-image structures move in diverging paths.
A pair of rotating enantiomers, represented in the diagram by red and blue propellers with mirror-image structures, move in diverging paths as they are subjected to an electric field in an ion trap. Credit: X. Zhou et al., Science 383, 612 (2024)

Ouyang and colleagues performed their initial trials with binaphthyl triflates, chiral molecules with an ideal structure for testing the propeller method of enantiomer separation. They found that they could isolate the enantiomers from each other and, using the high-resolution detection capabilities of the spectrometer, precisely determine the ratio of enantiomer concentrations in the original sample. Subsequently, the researchers had similar success sorting more-familiar chiral substances such as glucose and the amino acids leucine and tryptophan. The technique could make chiral sorting easier for drug discovery or synthesis. (X. Zhou et al., Science 383, 612, 2024.)

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