Science Concentrates ENERGY STORAGE
▸ Revised view of how Huntington’s protein misfolds Huntington’s disease, a lethal neurodegenerative condition, is believed to be caused by misfolding of mutated versions of huntingtin protein in which a glutamine-containing sequence is repeated too many times. But how the protein misfolds is still uncertain. Researchers have speculated that the conformation of monomeric huntingtin undergoes a sharp transition when the number of glutamine repeats exceeds 36 or 37, making the domain inflexible, like a rusty hinge. However, mutant huntingtin monomers have been difficult to study because they aggregate rapidly. Hilal A. Lashuel of the Swiss Federal Institute of Technology, Lausanne (EPFL), Rohit V. Pappu of Washington University in St. Louis, Edward A. Lemke of the European Molecular Biology Laboratory, and coworkers have now used protein semisynthesis, single-molecule fluorescence
Huntingtin protein has a tadpolelike head (green and orange) and tail (purple) structure. A new study finds that the head grows larger continuously (right) as the number of glutamine repeats increases. resonance energy transfer spectroscopy, and atomistic computer simulations to structurally characterize huntingtin monomers (J. Am. Chem. Soc. 2017, DOI: 10.1021/ jacs.7b06659). The study reveals that the monomers have a tadpolelike structure with a globular head and flexible tail. Glutamine repeats are in the head, which enlarges gradually as the repeat number increases instead of being part of a sharp structural transition. The researchers propose that this enlargement, rather than a
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C&EN | CEN.ACS.ORG | OCTOBER 2, 2017
Ionic liquid gel gives supercapacitors a boost A simple method for confining an ionic liquid in a methyl cellulose matrix yields a porous, gel-like material that may be useful for fabricating supercapacitors, according to a new study (ACS Appl. Mater. Interfaces 2017, DOI: 10.1021/acsami.7b07479). Supercapacitors are rechargeable electrostatic energy storage devices that are widely used in electric vehicles, medical devices,
Carbon nanofibers a few hundred nanometers wide (left) coated with an ionic liquid electrolyte gel (center) serve as electrodes in long-lasting supercapacitors (right, zoomed-out cross section shows two electrodes on either side of a separator). and consumer electronics. Unlike batteries, which store a relatively large electrical charge and deliver energy slowly, supercapacitors deliver limited quantities of electrical energy in rapid bursts. Broadening the voltage window over which supercapacitors operate could help expand their range of applications. For that reason, researchers have tried replacing the aqueous electrolytes typically used in these devices with nonaqueous substitutes such as ionic liquids. But ionic liquids often cause leakage and corrosion problems. So a team led by Vibha Kalra of Drexel University and Parameswara Rao Chinnam of Temple University devised a method to bypass those problems. The team prepared an ionic liquid solution containing 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and a small amount of methyl cellulose. Then they used that solution, which entraps the ionic liquid in a durable polymeric matrix, as a gelled electrolyte to coat highly porous carbon nanofiber mesh electrodes. From that material, the team made high-performing supercapacitors that retained more than 95% of their initial capacitance, even after 20,000 charging cycles.—MITCH JACOBY
rusty hinge, may cause misfolding.—STU
BORMAN
REAGENTS
▸ Tellurium twist on hypervalent chemistry Hypervalent iodine compounds are versatile reagents for transferring functional groups to organic molecules. For example, these compounds have become popular as electrophilic trifluoromethylating reagents. The fluorinating reagents, first
reported in 2006 by Antonio Togni and his group at the Swiss Federal Institute of Technology (ETH), Zurich, take advantage of iodine(III)’s ability to maintain two lone pairs of electrons while bonding with a CF3 group and a phenyl ring and forming a stabilizing side ring through interactions with an adjacent oxygen or nitrogen phenyl substituent (one example shown). Since 2006, Togni and coworkers have made several modifications to the original compounds by way of changing the fluorinated and phenyl substituent groups. In the latest effort, Togni and Ewa Pietrasiak in his group have gone further to replace iodine itself with its periodic table neigh-
C R E D I T: ACS AP PL . M AT ER . I NT E R FACES ( MI C RO G RA P H ) ; J . A M . C H EM . S O C. (P ROT E I N )
PROTEIN FOLDING
F3C I
O
RF
Te
X R1 R2
Togni’s reagent
Togni’s tellurium analogs RF = CF3, CF2H, C6F5 X = O or N donor R1, R2 = various groups
bor tellurium (Organometallics 2017, DOI: 10.1021/acs.organomet.7b00535). Hypervalent compounds are those that contain a group 13 to 18 element bearing more than eight valence electrons. Because iodine(III) with 10 valence electrons fits that bill, Togni and Pietrasiak thought tellurium(II) compounds with a similar electronic structure might work as fluorinating reagents as well. The researchers made a series of fluorinated tellurium(II) compounds (shown), including ones with CF2H and C6F5 groups that aren’t known for the iodine reagents, and studied their hypervalent nature via structural and spectroscopic data. The team is now testing the tellurium derivatives for their ability to function as perfluoroalkyl transfer reagents.—STEVE RITTER
CATALYSIS
C R E D I T: PAU LO Z A RAGOZ A
▸ Tuning singleatom catalysts with ionic liquids Isolated metal atoms dispersed on the surface of a support material offer unique opportunities in heterogeneous catalysis. For example, this class of single-site catalysts uses precious metals with maximum efficiency. In addition, because of their relative simplicity compared with standard multiatom nanoparticle catalysts, single-atom supported catalysts allow researchers to deduce valuable mechanistic details far more easily. But the single-atom variety is tough to tailor. And although modifying catalyst synthesis methods sometimes improves performance, it’s not always clear why. A team led by Alper Uzun of Koç University and Bruce C. Gates of the University of California, Davis, reports that treating single-atom iridium complexes supported on γ-alumina with 1,3-dialkylimidazolium ionic liquids improves the complexes’ catalytic properties for reasons the team quantifies via high-resolution X-ray absorption spectroscopy (ACS Catal. 2017, DOI: 10.1021/acscatal.7b02429). Specifically, the team prepared atomically dispersed
INFECTIOUS DISEASE
Mutation in Zika virus protein linked to microcephaly Even though Zika virus was first identified in 1947, the infectious disease remained obscure until a 2015 outbreak in Central and South America. That’s when doctors noted a dramatic rise in infants with underdeveloped heads and brains (a condition known as microcephaly) born to women who were infected with the virus when pregnant. Now, researchers have found that mutation of a single amino acid in one of the Zika virus’s structural proteins, known as prM, may be responsible for causing microcephaly by enabling the virus to kill developing brain cells (Science 2017, DOI: 10.1126/science.aam7120). A team led by Cheng-Feng Qin of the Beijing Institute of Microbiology & Epidemiology and Zhiheng Xu of the Chinese Academy of Sciences compared three recent strains of Zika virus with one from 2010 and found the more recent versions to be deleterious or deadly to neonatal and fetal mice, whereas the 2010 virus was not. Biochemical sleuthing led them to discover that a serine residue in prM of the 2010 virus had mutated to an asparagine in the more recent strains. When the researchers made this single mutation to a virus that was otherwise identical to the 2010 strain, it was dramatically more deadly to neonatal mice than the unmutated version. When the researchers modified this asparagine to a serine in a 2016 version of the virus, it proved to be less deadly.—BETHANY HALFORD
then rebounds—the hydroxyl group moves onto the carbon radical and the metal is reduced by one electron. Although plenty of circumstantial evidence suggests this is what happens, observing the rebound step in this reaction has been impossible because it is fast compared with the hydrogen atom removal step. Now, chemists led by David P. Goldberg of Johns Hopkins University have created the first model system in which the rebound reaction can be observed (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b07979). The system features Fe–OH ensconced in a tris(triphenyl) phenyl corrole ligand, which stabilizes the high redox level of the Fe complex and has sufficient steric bulk to prevent dimerization—a problem that often plagues FeOH systems. “In our case, the rebound reaction is best described as a concerted process as opposed to a stepwise process,” Goldberg says. Such mechanistic insight, he adds, could be used to engineer an enzyme or synthetic catalyst This model system to facilitate or inhibit the makes it possible to rebound step and thereby observe the radical influence the efficiency rebound reaction and selectivity of the hy(C is gray, H is white, droxylation.—BETHANY N is blue, O is red,
Ir(CO)2 complexes on alumina, coated them with various ionic liquids, and used them to catalyze partial hydrogenation of 1,3-butadiene, an important industrial process. The team found that the ionic liquids significantly increased selectivity for butenes—in some cases, from roughly 50% to 80%. They attribute the enhancement to electron donation from the ionic liquid to iridium, a process they examined in detail spectroscopically.—MITCH JACOBY
REACTION MECHANISMS
▸ Radical reaction caught on the rebound For more than 40 years, chemists and biologists have used the radical rebound mechanism to describe what happens when enzymes with transition metals, such as cytochrome P450, hydroxylate C–H bonds. Such enzymes first remove a hydrogen atom from a C–H bond using a high-valent metal-oxo species. This produces a carbon radical and a metal-OH intermediate that
Fe is orange).
HALFORD OCTOBER 2, 2017 | CEN.ACS.ORG | C&EN
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