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Separations

Magnetic fields could fish out enantiomers

Spin-state effect could lead to new way to run chiral separations on racemic mixtures

by Sam Lemonick
May 11, 2018 | APPEARED IN VOLUME 96, ISSUE 20

 

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Credit: Ron Naaman
Because of polarization and CISS, electrons with different spins (small arrows) have concentrated at the bottom faces of two enantiomers (blue coils). Near a surface magnetized in the upward direction (big arrows), the enantiomer on the left is more likely to adsorb because the electrons on its face have opposite spins from those on the surface.

When chemists need to separate one chiral molecule from a mixture of enantiomers—for instance, when synthesizing potential drug molecules—they often turn to high-performance liquid chromatography (HPLC). A new study suggests an alternative to this approach. The authors report that magnetic fields could separate enantiomers in a racemic mixture (Science 2018, DOI: 10.1126/science.aar4265).

This effect is possible because electrons don’t behave the same way in one enantiomer as they do in another. Electrons and other elementary particles have an intrinsic property called spin. In the case of electrons, the particles are either spin up or spin down. In a chiral molecule, these spin states affect electron motion. Electrons in one spin state will move more easily than those in the other state. Ron Naaman of Weizmann Institute of Science and Yossi Paltiel of Hebrew University of Jerusalem call that phenomenon chiral-induced spin selectivity (CISS).

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Now they’ve shown how CISS could be exploited to separate enantiomers in a racemic mixture. When molecules approach a surface, they become polarized, which involves electrons moving through the molecule. Because of the CISS effect, if the molecule is chiral, electrons with one spin state are more favored to move than those in the other state. That causes a concentration of electrons in one spin state where the molecule interacts with the surface.

If the surface is magnetized, the spin states of the material’s electrons will align parallel to the magnetic field. Electrons with like spin states repel each other. A chiral molecule approaching the surface will either be attracted or repelled depending on the spin state of the electrons that concentrated at the end facing the surface. As a result, the researchers say, one enantiomer will preferentially adsorb to a magnetized surface, while the other chiral molecule will not.

They demonstrated the effect with silicon dioxide nanoparticles decorated with enantiomers of a polyalanine oligomer and a magnetized, gold-coated surface. Through scanning electron microscope images, the team observed that the l-alanine oligomer nanoparticles adsorbed eight times as well as the d-alanine particles when the magnetic field pointed up from the surface, and the d-alanine nanoparticles adsorbed four times as well when the field pointed down. Paltiel says their group has tried other chiral molecules and have yet to find enantiomers that don’t show preferential adsorption.

This preferential adsorption could help separate chiral molecules, the scientists say. HPLC separations employ columns filled with a chiral material that adsorbs one enantiomer over the other. “Our idea is people will be able to replace [the columns] they have now in HPLC with a magnetic column,” Naaman says.

A lot of work needs to be done to show this effect can efficiently resolve racemic mixtures, “but it could prove to be very important,” says David Waldeck, a physical chemist at the University of Pittsburgh, who was not involved with the research but has collaborated with Naaman on CISS.

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Comments
David Mendenhall (Wed May 16 15:44:16 EDT 2018)
After all the negative stories about killing dozens and wounding thousands of Palestinians in the Gaza concentration camp, it is refreshing to read about a positive Israeli contribution to civilization.
Emil Friedman (Wed May 16 19:40:59 EDT 2018)
Gaza is not a "concentration camp". It's a region that chose to be hostile to Israel and that routinely attacks Israel, even though Israel provides it with various forms of aid. It could have chosen peaceful coexistence.
Bill (Wed May 16 20:32:01 EDT 2018)
Yeah, it's pretty much a concentration camp. Of course, Israel doesn't provide the aid, nor would they. The United Stated and other nations provide the money, Israel simply distributes it, as they agreed to, after skimming a bit off the top. I do applaud the nation of Israel's peaceful contributions to science, though.
TG (Mon May 21 08:53:32 EDT 2018)
I believe the author of this note should have included the following condition which is critical to the successful demonstration of the CISS: "Here, we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization."
It means that the substrate must itself be asymmetric (up or down magnetic dipols) to effect preferential interaction with the solute (which by the way is the principle on which every previous separation of enantiomers has worked.)

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