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ENERGY STORAGE
Rechargeable battery runs at –70 ºC
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New battery design could power vehicles in space and the Arctic Rechargeable batteries perform poorly when it’s cold out. Now researchers have designed a new lithium-ion battery that still works at –70 °C. Such batteries could improve the performance of electric cars in winter and help power high-altitude machinery, space stations, and planetary rovers (Joule 2018, DOI: 10.1016/j. joule.2018.01.017). Lithium-ion batteries work poorly in extreme cold because their electrolyte solvents become viscous or even freeze, which hampers the movement of lithium ions between the anode and cathode during charging and use. To make a rechargeable battery that would operate safely and maintain performance in the extreme cold, Yongyao Xia, a
When this battery charges, lithium ions
physical chemist at Fudan University, react with the polyimide anode while selected ethyl acetate as a cold-tolerbis(trifluoromethanesulfonyl)imide (TFSI) ant electrolyte solvent. Ethyl acetate’s anions move into the organic cathode. When the freezing point is –84 °C, and it doesn’t battery discharges, the reaction reverses. In the become viscous when it’s cold. structures, gray is carbon, yellow is oxygen, and orange is nitrogen. Xia’s group combined the solvent with electrodes made of organic of its room-temperature storage capacity. materials instead of the conventional inMaintaining performance over such a organic ones. When the battery charges, wide temperature range is impressive, says the polyimide anode material undergoes Shirley Meng, a materials scientist at the a reaction that allows lithium ions to bind University of California, San Diego. Howto it, while counter anions absorb onto a ever, the Fudan battery works at only 1.2 V, polytriphenylamine cathode. When the which is a relatively low voltage, she says. battery discharges, the reaction goes in Xia says his group is further tailoring reverse, lithium ions get released, and the electrolyte and electrode materials the anions desorb. The resulting organic to improve performance.—KATHERINE battery works from 50 °C down to –70 °C. And at –70 °C, the battery maintains 70% BOURZAC, special to C&EN
BIOCATALYSIS
Evolved heme proteins make prized three-membered rings With some engineering in the lab, a quartet of iron-containing heme proteins from microbes can convert inert alkenes into each possible stereoisomer of cyclopropanes, which are valuable motifs in medicinally active compounds (ACS Cent. Sci. 2018, DOI: 10.1021/acscentsci.7b00548). Previous engineered proteins needed help from an artificial cofactor to complete this feat. This work suggests that heme proteins are perfectly capable of doing this chemistry on their own. Building cyclopropanes with protein catalysts is not new, says Frances H. Arnold, the California Institute of Technology professor who led the work. However, prior heme protein catalysts made by her group and others worked best on relatively reactive alkenes. “These proteins are being commercialized, and our clients want more challenging cyclopropanations,” including transfor-
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C&EN | CEN.ACS.ORG | MARCH 5, 2018
mations of unactivated alkenes, she says. So graduate student Anders M. Knight and colleagues used directed evolution, which simulates natural selection, to find promising candidates. They optimized four heme-containing proteins from bacteria and archaea, each of which R1 R2 produced a different
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R1 = C6H13 R2 = COOCH2CH3
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Each engineered heme protein (colored ribbons) catalyzes formation of a different cyclopropane stereoisomer.
cyclopropane stereoisomer from the unactivated alkene 1-octene. The cyclopropane-making reaction, a carbene transfer, takes place inside Escherichia coli cells and works in the presence of alcohols and other groups that might normally interfere with the reaction. Caltech has filed a provisional patent application on the technology. “This work shows that the diversity of heme proteins in nature, coupled with the power of directed evolution, can be a route to novel stereoselective catalysts,” says John Hartwig of the University of California, Berkeley. His team has carried out this chemistry using proteins with a nonnatural iridium cofactor. So far, the new heme proteins convert terminal alkenes only, but Hartwig thinks with more work, they could convert internal alkenes too. Knight agrees. “These active sites are very tunable,” he says. Arnold adds, “I hope that as we do more difficult target substrates, it’ll push people in industry to give this a try.”—CARMEN DRAHL
C R E D I T: J OU L E
Enzymes serve up diverse cyclopropanes
