Science Concentrates METAL-ORGANIC FRAMEWORKS
▸ Synthetic melanin makes colorchanging skin An artificial skin made with synthetic melanin nanoparticles would make chameleons green with envy were the lizards capable of such a petty emotion. The synthetic skin changes color faster than a chameleon’s, provided the humidity of the air surrounding the engineered film changes quickly enough (Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.6b02127). Although the skin can switch its hue quicker than chameleons and other color-changers in nature, it was actually inspired by them, say the skin’s developers, led by Ali Dhinojwala of the University of Akron; Nathan C. Gianneschi of the University of California, San Diego; and Matthew D. Shawkey of the University of Ghent. Iridescent bird feathers, such as those found on tree swallows, reversibly change color with changes in humidity. Scientists believe this is a result of the swelling or shrinking of feather fibers as they take on or lose moisture, respectively. The artificial skin works faster but in much the same way, with stacks of nanoparticles made from polydopamine, a synthetic melanin, playing the role of the keratin structures. Films of the nanoparticles could therefore provide the basis for rapid and easy-to-read humidity sensors, the researchers say.—MATT DAVENPORT
NEUROSCIENCE
▸ Receptor for prion protein identified Prion proteins are infamous for going rogue. In the brain, misfolded versions of the proteins can convert normally folded prions into forms that cause neurodegenerative diseases such as mad cow disease and fatal familial insomnia. But researchers know little about the everyday function of prions. A team of scientists now report that the proteins bind to a specific cell receptor to help maintain the electrical insulation on nerve cells outside of the brain (Nature 2016, DOI: 10.1038/ nature19312). In 2010, Adriano Aguzzi of the University of Zurich and colleagues reported that mice engineered to lack the prion protein gene lost this lipid-and-protein coating,
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C&EN | CEN.ACS.ORG | AUGUST 22, 2016
Structures solved by locking molecules in place Chemists have long relied on X-ray crystallography to unambiguously determine a molecule’s structure. But in some cases, it can be devilishly difficult to grow the crystals required for this technique. A few years ago, chemists introduced the crystalline sponge method, in which a metal-organic framework, or MOF, is soaked like a sponge with a solution of a compound. Weak interactions between the MOF and the compound hold it inside the MOF, allowing the compound’s structure to be determined by X-ray diffraction. But the method is not perfect and is often used as a last resort. Omar M. Yaghi, Seungkyu Lee, and Eugene A. Kapustin of the University of California, Berkeley, have now devised a way to make this kind of structure determination more reliable. The UC Berkeley chemists lock the molecules they are studying in place by coCovalently linking valently linking them to the aluminum (+)-jasmonate to a chiral MOF atoms of a chiral framework known as (yellow) allowed researchers MOF-520 (Science 2016, DOI: 10.1126/ to determine its absolute science.aaf9135). This ensures the stereochemistry. molecules align within the MOF, making it easier to solve structures via X-ray analysis. The team demonstrated the technique works with primary alcohols, phenols, vicinal diols, and carboxylic acids. Using the chiral MOF makes it possible to determine absolute stereochemistry, as the researchers demonstrate with jasmonate, a compound for which no crystal structure had been reported previously.—BETHANY HALFORD
which is called myelin. Now, through a series of experiments, Aguzzi’s team demonstrates that a 28-amino-acid sequence on one end of the prion protein binds to and activates a receptor called Gpr126 on Schwann cells, which are responsible for maintaining myelin sheaths. Once activated, these receptors trigger cellular signaling critical for the myelination process. When the scien-
tists deleted the gene for the receptor from cultured Schwann cells, the prion protein no longer activated this signaling. These findings still don’t explain what prion proteins do inside the brain because Schwann cells are present only in the peripheral nervous system. Aguzzi says his team is actively looking for a brain receptor that binds the prion protein.—MICHAEL TORRICE
In the peripheral nervous system, prion proteins could help Schwann cells maintain neurons’ myelin coatings.
INORGANIC CHEMISTRY
▸ Pnictogen-silicon additions to the inorganic benzene family The discovery of borazine (B3N3H6) as the first inorganic analog of benzene 90 years ago opened the eyes of chemists to the possibility of creating noncarbon aromatic com-
CREDIT: OMAR YAGHI/UC BERKELEY (MOLECULAR MODEL); QUASAR JAROSZ/WIKIPEDIA (NEURON)
MATERIALS
pounds. Although researchers have explored many possible combinations of group 13 to group 15 elements in pursuit of additional examples, only a handful have been found.
P
N N Si
Si
P
N N
P
Si
N
N
A triphosphatrisilabenzene Building on that work, a team led by Manfred Scheer of the University of Regensburg has now added two new molecules to the collection by synthesizing aromatic phosphorus-silicon and arsenic-silicon benzene analogs, along with their related antiaromatic cyclobutadiene analogs (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b07389). The researchers formed the triphospha- and triarsatrisilabenzenes by a metathesis reaction between phosphorus- or arsenic-containing zirconium cyclopentadienyl complexes and a bulky chlorosilylene. Structural studies show that the benzene analogs have slightly distorted ring shapes with P-Si and As-Si bond lengths intermediate between single and double bonds, as expected for conjugated aromatic rings. Computational studies verify the aromatic character of the compounds. These new benzene analogs “give an insight into the chemistry of inorganic aromatic systems and aromaticity in general—150 years after Kekulé’s first report on aromatic compounds,” Scheer and his colleagues write.—STEVE RITTER
ORIGINS OF LIFE
CREDIT: NAT. NANOTECHNOL. (MICROGRAPH)
▸ Ribozyme amplifies RNA The ability to produce many types of RNA molecules without the help of protein enzymes would have been necessary if early forms of life were based on RNA, which is a popular hypothesis. Gerald F. Joyce and David P. Horning of Scripps Research Institute, in La Jolla, Calif., have now identified a ribozyme—an RNA-based enzyme—that can replicate a wide variety of RNA molecules and can even catalyze an all-RNA version of the polymerase chain reaction (PCR) used to amplify genetic material (Proc.
MATERIALS
Stretchy, see-through touch panel made from hydrogel Touch panels of the future will be a lot like Olympic gymnasts—flexible, stretchy, and resilient. With these properties, touch panels can hug the human form like a leotard, letting us wear electronic devices rather than stick them in our pockets, purses, or briefcases. To date, however, most touch panels have been built with materials that resist stretching, such as indium tin oxide and carbon nanotubes. Researchers at Seoul National University now report a transparent touch panel that can be stretched to 10 times as large as its original area without losing functionality (Science 2016, DOI: 10.1126/science.aaf8810). The superstretchy, see-through device is made of a polyacrylamide hydrogel containing lithium chloride salts. Because of the high water content of the hydrogel, the salt dissolves and acts as an ionic conductor. The scientists who developed the technology, led by Jeong-Yun Sun, made an arm-hugging touch panel and used it for writing and playing video games. Perhaps by the time the 2020 Olympics roll around, we’ll be able to watch gymnasts write notes to their fans during downtime with touch panels embedded in their leotards.—BETHANY HALFORD
Natl. Acad. Sci. USA 2016, DOI: 10.1073/ pnas.1610103113). The ribozyme was singled out after 24 rounds of in vitro evolution starting from an engineered form of the class I polymerase ribozyme. In each round of evolution, ribozymes were selected for their ability to synthesize ligand-binding RNA molecules called aptamers from complementary RNA templates. The evolved ribozyme synthesizes RNA molecules about 100 times as fast as the starting ribozyme and is able to synthesize sequences that stymie the original one. The evolved ribozyme synthesized a variety of highly structured RNA molecules with different functions, including aptamers, other ribozymes, and transfer RNA, the kind of RNA used in protein synthesis. The evolved enzyme is even able to exponentially amplify short RNA templates, albeit at low yields, in an RNA-only version of PCR.—CELIA ARNAUD
NANOMATERIALS
▸ Potent photocatalyst for disinfecting water A small amount of inexpensive molybdenum disulfide can function as a potent photocatalyst, killing nearly all bacteria in water samples within minutes, according to a study (Nat. Nanotechnol. 2016, DOI: 10.1038/nnano.2016.138). Using sunlight and a light-activated catalyst is a simple, low-cost way to rid water of harmful pathogens. But the process is slow, taking up to 48
hours, because typical catalysts used in this application respond primarily to ultraviolet light, which accounts for just 4% of sunlight’s energy. So Yi Cui of Stanford University and colleagues designed a catalyst that efficiently harvests visible light, which
Nanometer-thin domains of vertically aligned MoS2 sheets function as a potent bacteria-killing photocatalyst. represents roughly 50% of solar energy. The team grew vertically aligned sheets of MoS2 three to 10 molecular layers thick and analyzed their properties. Exposing the catalyst in water to visible light stimulates electronic excitations that generate bacteria-killing reactive oxygen species, such as O2•−, singlet oxygen (1O2), OH•, and hydrogen peroxide, along the edges of the sheets. Control tests show that visible-light disinfection using this nanostructured form of MoS2 kills more than 99.999% of bacteria within 20 minutes, far outperforming TiO2, which is a common photocatalyst, and other forms of MoS2.—MITCH JACOBY AUGUST 22, 2016 | CEN.ACS.ORG | C&EN
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