SEPARATIONS
Magnetic fields could fish out enantiomers Spin-state effect could lead to new way to run chiral separations
C R E D I T: RO N N A A M A N ( D I AGRA M ) ; IN TE R N AT I O N A L ATO MI C E N ERGY AGE N CY/C& E N (SCR E E N G RAB )
Because of polarization and CISS, electrons with 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 H have an intrinsic property called spin. In the case of HO C NH2 2 electrons, the particles are either spin up or spin D-Alanine 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). 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 different spins (small arrows) have concentrated at the of the CISS effect, if bottom faces of two enantiomers (blue coils). Near a the molecule is chiral, surface magnetized in the upward direction (big arrows), electrons with one spin the enantiomer on the left is more likely to adsorb state are more favored to because the electrons on its face have opposite spins move than those in the from those on the surface. other state. That causes eight times as well as the d-alanine partia concentration of electrons in one spin cles when the magnetic field pointed up state where the molecule interacts with from surface, and the d-alanine nanopartithe surface. cles adsorbed four times as well when the If the surface is magnetized, the spin field pointed down. Paltiel says their group states of the material’s electrons will align has tried other chiral molecules and have parallel to the magnetic field. Electrons yet to find enantiomers that don’t show with like spin states reH preferential adsorption. pel each other. A chiral This preferential adsorption could help molecule approaching H2N CO2H separate chiral molecules, the scientists the surface will either say. HPLC separations employ columns be attracted or repelled L-Alanine filled with a chiral material that adsorbs depending on the spin one enantiomer over the other. “Our state of the electrons that concentrated at idea is people will be able to replace [the the end facing the surface. As a result, the columns] they have now in HPLC with a researchers say, one enantiomer will prefmagnetic column,” Naaman says. erentially adsorb to a magnetized surface, A lot of work needs to be done to show while the other chiral molecule will not. this effect can efficiently resolve racemic They demonstrated the effect with silimixtures, “but it could prove to be very con dioxide nanoparticles decorated with important,” says David Waldeck, a physical enantiomers of a polyalanine oligomer chemist at the University of Pittsburgh, and a magnetized, gold-coated surface. who was not involved with the research Through scanning electron microscope but has collaborated with Naaman on images, the team observed that the l-alCISS.—SAM LEMONICK anine oligomer nanoparticles adsorbed
NUCLEAR CHEMISTRY
Tracking nuclear materials from near and far Monitoring the use of nuclear materials is an issue of global concern, especially amid news that the U.S. is leaving the Joint Comprehensive Plan of Action (commonly known as the Iran nuclear deal), an agreement intended to limit Iran’s nuclear program. Since the same materials needed to build nuclear weapons also show up in power plants, how can international regulators ensure nuclear materials are used only for peaceful purposes? Chemists are helping develop techniques to spot illicit nuclear activity from afar and track it when it slips out of regulatory control. For example, a massive antineutrino detector called Watchman could be a powerful tool to keep tabs on distant nuclear operations—if scientists can get the chemistry right. Learn more at cenm.ag/nuclearmaterials.—KERRI JANSEN MAY 14, 2018 | CEN.ACS.ORG | C&EN
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