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
F
F
F OH
K2CO3
F
F
O
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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