Science Concentrates SAFETY
▸ Mechanism behind ‘killer fog’ identified London’s “Great Smog” of 1952 killed thousands and led the U.K. to enact clean air laws in subsequent years. Severe haze events still present a serious air-quality problem, particularly in China, but even after all these years the chemistry behind how airborne chemicals and particulates combine to produce such haze remains hazy. A key component is aqueous oxidation of sulfur dioxide to sulfate by nitrogen dioxide, says an international team of researchers led by Gehui Wang of the Chinese Academy of Sciences, Renyi Zhang of Texas A&M University, and Mario J. Molina of the University of California, San Diego (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1616540113). Sulfur dioxide and nitrogen dioxide are coproduced during combustion of coal and other fuels. Through lab experiments and field studies, the researchers found significant SO2 oxidation under two conditions: when cloud droplets are available, as in London, or on fine aerosol particles when there is high humidity and enough ammonia around to neutralize the pH, as is the case in haze events in China. The results suggest that NO2 and NH3 emissions must be controlled along with SO2 to reduce severe haze.—JYLLIAN KEMSLEY
TOXICOLOGY
▸ Masked fungal toxins in our food When fungi attack agricultural crops, they don’t just decrease yield, they can also leave behind poisonous compounds on the harvest. Although food safety scientists have long monitored these fungal products, known as mycotoxins, in recent years they’ve discovered that plants can chemically modify the compounds for their own survival, say, by adding a protective sugar group. However, food safety tests don’t currently look for these masked mycotoxins. Furthermore, the masked mycotoxins “may be hydrolyzed in the [human] digestive tract, thereby releasing the parental form and increasing the total exposure to the toxin,” notes a research team led by Sabine E. Kulling of the German Federal Research Institute of Nutrition & Food (J.
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C&EN | CEN.ACS.ORG | NOVEMBER 21, 2016
Toning down emergency flares Many emergency flares rely on a maBurning phosphorus(V) nitride, terial called red phosphorus, one of which is a safer alternative to red the natural forms of the element, to phosphorus in marine flares. produce billowing smoke that draws the rescuers’ attention. Researchers have been seeking an alternative because red phosphorus has multiple safety flaws. For example, it can ignite with a bit of impact or friction; red phosphorus is often added to match heads to help them ignite. Although useful in a match, the compound’s propensity to explode when handled in bulk amounts is a danger to factory workers and during transportation. Furthermore, when the chemical is exposed to moist air—common in marine emergency situations—it can degrade into toxic phosphine, PH3, which causes a range of symptoms in people, including vomiting, breathing difficulty, and pulmonary edema. A team led by Stanisław Cudziło of the Military University of Technology in Warsaw and Ernst-Christian Koch of Lutradyn, an energetic materials company, have come up with a safer alternative: phosphorus(V) nitride, P3N5 (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201609532). Unlike red phosphorus, P3N5 won’t ignite with friction or sudden impact, nor will it degrade in moist air to harmful compounds.—SARAH EVERTS
OH O
ASTROCHEMISTRY O
-O SO 3
OH O HO
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▸ Solid CO2 spotted on a comet
OH
OH An alternariol sulfate glucoside
Alternaria fungi (right) produce a toxin called alternariol, which can be conjugated with sulfate (blue) and glucoside (red) groups, pushing the poison off the radar of food safety monitors. Agric. Food Chem. 2016, DOI: 10.1021/acs. jafc.6b03120). Kulling and her colleagues report that when Alternaria alternata infects tomato cells, the fungus first decorates its toxins—including alternariol (shown)—with a sulfate group and then the tomato’s enzymes add a glucoside. Because monitoring only focuses on the parent toxin and not its metabolites, levels of mycotoxins such as alternariol in food might be much higher than previously thought. “If you don’t look for these conjugates, you have no chance of seeing them,” Kulling tells C&EN.—SARAH EVERTS
Before it ended its mission in September, the European Space Agency’s Rosetta spacecraft, which began orbiting comet 67P/Churyumov-Gerasimenko in 2014, made the first observation of solid CO2 on a comet. The craft also unveiled complex sublimating and freezing patterns taking place in the comet’s water ice and dust. These phenomena result from seasonal and diurnal extremes that a comet experiences during its elliptical orbit around the sun. An international team led by Gianrico Filacchione of the Institute for Space Astrophysics & Planetology examined data from Rosetta’s infrared spectrometer and found a patch of solid CO2 that existed on the comet’s surface when it was in its deep winter. This patch of CO2 ice disappeared within three weeks after reexposure to the sun. CO2 is so volatile that its ice sublimation temperature is a very low –190 °C; therefore, scientists expected it to exist only below the comet’s surface. The Filacchione team’s findings suggest that even more volatile species, such as carbon monoxide and methane, could also exist as surface ice during the comet’s winter (Sci-
CREDIT: COURTESY OF ERNST-CHRISTIAN KOCH (FLARE); J. AGRIC. FOOD CHEM. (FUNGI)
POLLUTION
ence 2016, DOI: 10.1126/science.aag3161). Another team, led by Sonia Fornasier of the Paris Observatory, found that exposed water ice survives on 67P’s surface for a short time and that it is well-mixed with dust, which may help explain why comet cores appear dark even if they are rich in water ice (Science 2016, DOI: 10.1126/ science.aag2671).—ELIZABETH WILSON
NEUROSCIENCE
CREDIT: DANIEL M. ROSENBAUM (RIBBON STRUCTURE); CAFER T. YAVUZ (TEST TUBES); J. AM. CHEM. SOC. (NITRONIUM CATION)
▸ Cannabinoid receptor revealed Two research groups have independently elucidated the first crystal structures of the cannabinoid receptor CB1, a cell-membrane protein involved in a host of appetite, pain-sensation, memory, and othStructure of the er physiological antiobesity drug processes. The taranabant bound to protein’s floppy CB1 (teal). movements between active and inactive states make it difficult to study, so Alexandros Makriyannis of Northeastern University and colleagues designed a strong inhibitor to bind and help immobilize CB1 to get a better glimpse of its structure (Cell 2016, DOI: 10.1016/j. cell.2016.10.004). Meanwhile, a team led by Daniel M. Rosenbaum of the University of Texas Southwestern Medical Center published a CB1 structure with slightly higher resolution, which they stabilized by binding the antiobesity drug taranabant (Nature 2016, DOI: 10.1038/nature20613). Because cannabinoids are quite varied themselves, Ken Mackie of Indiana University says the new papers “finally give us a clear sense of the nooks and crannies in CB1 that cannabinoids can interact with.” Despite some discrepancies, both groups discovered a binding pocket that allows lipophilic cannabinoid molecules to interact with the receptor. “A long-held hypothesis about how cannabinoids enter the receptor has now been proven,” says Patricia Reggio of the University of North Carolina, Greensboro. Reggio adds that the structural work could be a launching pad for designing new therapeutics targeting CB1.—RYAN CROSS
REACTION DYNAMICS
Regioselectivity without a transition state Toluene nitration by nitronium (NO2+) salts yields curious regioselectivity: Although nitration should occur equally at all positions of the ring because the reaction is highly exothermic and the energy barrier is near zero, only 2% of the products are substituted at the meta position. Of various mechanisms proposed to explain this selectivity, none has proven to be satisfactory. That is because the reaction involves no intermediates and no transition states beyond the Before binding to initial encounter of toluene and nitronium, according toluene, NO2+ roams to computational work by Yexenia Nieves-Quinones (red path) above the and Daniel A. Singleton of Texas A&M University (J. aromatic ring until Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b07328). The solvent (not shown) researchers find that after the toluene and nitronium reorganizes to promote encounter each other, they don’t immediately react. product formation. Instead, the nitronium wanders around the area above the aromatic carbons until random fluctuations reorient the counterion and solvent molecules, from stabilizing nitronium to stabilizing the product cation. Once that reorganization occurs, the nitronium faces downhill paths for reacting with any of the carbons—but the paths for ortho or para substitution are steeper and easier to access than for meta or ipso substitution.—JYLLIAN KEMSLEY
MATERIALS
▸ Porous fluoropolymer separates watersoluble organics Porous materials such as zeolites, metal organic frameworks, and nanocarbons are known for their ability to selectively interact with and separate chemical species having similar sizes and functional groups. But the ability to separate charged molecules of various sizes, especially when dissolved in water, has remained a challenge. A team led by Cafer T. Yavuz of Korea Advanced Institute of Science & Technology (KAIST) has now reported a microporous network fluoropolymer that can selectively separate cationic dyes and other charged molecules from mixtures of water-soluble organics (Nat. Commun. 2016, DOI: 10.1038/ncom ms13377). The researchers first prepared an inexpensive new covalent organic polymer, dubbed COP-99, by treating commercially available tetrafluorohydroquinone with potassium carbonate (shown). They found that the material pulls modestly sized charged molecules such as the dye methylene blue out of water, but it isn’t capable of sequestering larger dye molecules
O
OH F
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F OH
K2CO3
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F O
A porous fluoropolymer (dark material in test tubes) selectively removes the dye methylene blue from water (left to right). such as rhodamine B or uncharged molecules such as bisphenol A. The key to the material’s selectivity is its restrictive pore size and the exposed fluorine atoms, Yavuz notes, which both create hydrophobic pores and provide strong electronegative forces attractive only to charged organics. The KAIST team envisions a range of applications, including water treatment to remove artificial dyes, pesticides, and prescription drugs.—STEVE RITTER NOVEMBER 21, 2016 | CEN.ACS.ORG | C&EN
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