Science Concentrates POLYMERS
The spontaneously forming oxide film that stabilizes millimeter-sized gallium droplets (left) offers a simple way to prepare ultrathin crystalline films of gallium sulfide (micrograph, right).
Polyethylene (PE) and isotactic polypropylene (iPP) account for nearly twothirds of the world’s plastic—adding up to $200 billion in annual sales worldwide. When melted together, PE and iPP are immiscible and form a brittle material. So they tend to get recycled into lower-value products because of the expense associated with sorting them from one another. Now, researchers led by Cornell University’s Geoffrey W. Coates, Anne M. LaPointe, and James Eagan, along with the University m
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2-D MATERIALS
Liquid metals yield large 2-D semiconductor films The curious properties of gallium and related liquid metals, “especially the tendency to spontaneously form thin oxide skins in air,” have led researchers in recent years to use these materials for chemical patterning and for making stretchy and self-healing electronics. Now, a team of researchers in Australia and the U.S. has exploited that property to prepare these materials as large, ultrathin patterned semiconductor films via methods that are compatible with electronics industry manufacturing (Nat. Commun. 2017, DOI: 10.1038/ ncomms14482). Due to their potential use in microelectronic devices, metal sulfides and other semiconducting chalcogenide compounds keep grabbing attention—especially when researchers report methods for preparing them as atomically thin films. But many of those methods require high temperatures or yield tiny defective flakes, rendering the methods incompatible with the semiconductor industry. Torben Daeneke and Kourosh Kalantar-Zadeh of RMIT University, Melbourne, and coworkers show that those problems can be avoided by using liquid metals. The team deposited gallium on a wafer-sized substrate that had been patterned with a fluorinated compound and then converted the gallium oxide to a high-quality 1.5-nm-thick film of gallium sulfide. The team used a similar method to make large In2S3 films and showed that the materials can be used to build transistors.—MITCH JACOBY
p
PE-block-iPP-block-PE-block-iPP Tetrablock copolymer of Minnesota’s Frank S. Bates, have come up with a way to turn a mix of PE and iPP into a more valuable plastic (Science 2017, DOI: 10.1126/science.aah5744). They found that adding a small amount—about 1% by weight—of a tetrablock copolymer made from PE and iPP will transform a blend of PE and iPP into a mechanically tough material. The copolymer essentially acts like glue and holds the two immiscible plastics together. The researchers make the tetrablock copolymer using a pyridylamidohafnium catalyst, which they previously reported in a patent (WO2008112133A2). Although development work still needs to be done, the researchers hope the discovery will affect the way plastics are recycled.—
BETHANY HALFORD
O H N
H N
NH
O
S Metal chelation
NH
O S
DRUG DISCOVERY
O
S Holomycin
HS HS Reduced form of holomycin
Zn2+
O
Zn S S
O HN
H N NH
S O NH
O Reduced-holomycin/ metal complex
▸ Mode of action for unusual antibiotics found
Holomycin’s mechanism of action is metal chelation and removal of metals such as Zn2+ from some metalloenzymes.
Dithiolopyrrolones (DTPs) are disulfide-containing natural products with broad-spectrum antimicrobial activity. Studies have shown that DTPs inhibit several essential cellular processes, but their molecular mechanism has remained elusive. Bo Li of the University of North Carolina, Chapel Hill, and coworkers now find that a DTP called holomycin exerts its antibiotic activity by sequestering intra-
cellular metal ions, particularly Zn2+ (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/ pnas.1612810114). This metal chelation activity inhibits some metalloenzymes— potentially interfering with cellular glucose utilization, RNA synthesis, and respiration—and disrupts cells’ internal metal balance. The researchers believe holomycin’s mechanism will prove valid for other DTPs. But they note that it is nevertheless a unique antibiotic mode of action, mak-
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C&EN | CEN.ACS.ORG | FEBRUARY 27, 2017
Metalloenzyme
ing DTPs potentially useful for treating multi-drug-resistant bacterial infections. After holomycin enters cells, its cyclic disulfide is reduced by an unknown mechanism, and the reduced holomycin is the species that actually chelates metals. Holomycin is moderately toxic, so it may need to be modified to reduce toxicity for possible future antibiotic use. The researchers now plan to “investigate the specificity of DTPs for metalloenzymes in bacterial and human
CREDIT: NAT. COMMUN. (GALLIUM/MICROGRAPH); ADAPTED FROM P NAS (ENZYME SCHEME)
C&EN Global Enterp 2017.95:10-11. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/23/18. For personal use only.
▸ Blending immiscible plastics
proteomes and develop ways to improve target selectivity,” Li says.—STU BORMAN
PROCESS CHEMISTRY
INORGANIC CHEMISTRY
Plucking more sulfur from diesel fuel
▸ f-Block elements prefer trans donor ligands
CREDIT: ANGEW. CHEM. INT. ED. (SOFT MACHINES); NAT. COMMUN. (BALL AND STICK STRUCTURE)
In square planar or octahedral inorganic complexes, strong electron donor ligands typically prefer to bind in a cis rather than trans orientation relative to each other. But chemists have observed the opposite in a few actinide complexes in which strong donor ligands prefer a trans orientation and seem to reinforce each other. A new study suggests that situation may be more
Lanthanide and actinide complexes with trans donor ligands, such as in the bis(carbene) structure shown here, may be more common than realized (hydrogens omitted for clarity).
An international industry-academic research team has reported a new approach to removing sulfur from diesel fuel that could improve vehicle performance and reduce KOSi homogeneous hydrodesulfurization the environmental impact of using fossil R1 R1 R2 fuels. Gasoline and Potassium tert-butoxide, S triethylsilane diesel contain residual amounts of natural S organosulfur comR1, R2 = H, alkyl pounds that can foul catalytic converters and generate polluting sulfur dioxide. Refineries employ heterogeneous catalysts such as cobalt-doped molybdenum sulfide that use hydrogen at high temperature and pressure to strip out as much sulfur as possible from the fuel. But the most recalcitrant compounds, in particular alkylated dibenzothiophenes, are mostly left untouched. Researchers led by BP’s John W. Shabaker, UCLA’s Kendall N. Houk, and Caltech’s Robert H. Grubbs have devised a noncatalytic solution-based process that employs potassium tert-butoxide and alkylsilanes under mild conditions to create silyl radicals that cleave the C–S bond of the heterocyclic compounds, with the silicon whisking away the sulfur. The researchers show that the method, which they call KOSi, can reduce sulfur in diesel fuel to about 2 ppm, well below the current target of 15 ppm (Nat. Energy 2017, DOI: 10.1038/nenergy.2017.8). The researchers note that their proof-ofconcept process will need to be adapted to function as a continuous refinery process, but they envision it could become a “polishing treatment” to supplement current desulfurization processes.—STEVE RITTER
R2
MATERIALS
common than realized (Nat. Commun. 2017, DOI: 10.1038/ncomms14137). Previously, researchers had observed trans orientations only in linear, high-valent actinyl complexes with hard ligands, such as UO22+. In the new work, a team led by Stephen T. Liddle of the University of Manchester and Andrew Kerridge of Lancaster University synthesized cerium(IV), uranium(IV), and thorium(IV) bis(carbene) complexes with linear C=M=C cores and surprisingly short C=M bonds. The effect likely occurs because lanthanide 5p and actinide 6p orbitals can transfer electrons to the 4f and 5f orbitals, respectively, creating electron holes that may be filled via electron donation from trans ligands more readily than from cis ligands. The results suggest that the phenomenon may play a broader role than realized in the structure and reactivity of f-block complexes.—
ing light-sensitive azobenzene moieties. By shining ultraviolet and visible light onto specific portions of the liquid crystal, the team created alternating stripes in the film such that half contained a well-aligned polymer, while the remaining stripes were more disordered. Then the researchers cut the striped There’s no shortage of soft robots or actufilm into ribbons and stuck two together, ators able to move themselves and other back-to-back. When illuminated with anothcargo, but these malleable machines tend er dose of UV light, the alternating polymer to work slowly, says Nathalie Katsonis of stripes expanded at different rates, creating the University of Twente. Researchers led a strain that built up in the paired ribbons. by Katsonis and Stephen P. Fletcher of the The ribbons bent, twisted, and ultimately University of Oxford want speedier energy split apart, rapidly releasing their pent-up delivery and decided to turn to liquid crystal chemistry. The researchers began with a thin energy in a manner similar to the bursting seed pods of orchids and other plants (Anfilm of an elastomeric liquid crystal featurgew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201611325). Katsonis believes this work and future efforts will help elucidate the molecular underpinnings of the complex, macroscopic motions seen in nature, but they could also help researchers make more powerful soft robots and By joining strips with periodic polymer properties, microfluidic systems.—MATT researchers created a device that strains itself until
JYLLIAN KEMSLEY
it bursts under UV light, causing the strips to curl.
▸ More power to soft machines
DAVENPORT
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