Science Concentrates NANOMATERIALS
▸ Dendrimer molecule spins in its loose crystal
Residue from a droplet of Glenlivet mixed with fluorescent particles shows even deposition (left); dried coffee shows darker rings along the edges (right).
COATINGS
Why whiskey doesn’t have a ring to it The characteristic rings at the edges of a dried drop or splash of coffee come from how the water in the solution evaporates: Water disappears faster from the edge of a drop than the center, and as liquid flows outward to replenish the loss it, carries suspended particles along. Whiskey, in contrast, leaves behind a more uniform evaporation pattern. That’s because whiskey contains alcohol, natural surfactants such as phospholipids, and macromolecules such as lignin and polysaccharides, reports a team led by Howard A. Stone, Hyoungsoo Kim, and François Boulogne of Princeton University (Phys. Rev. Lett. 2016, DOI: 10.1103/physrevlett.116.124501). Inspired by photographer Ernie Button’s images of dried whiskey, the researchers set out to understand the liquid’s evaporation mechanism by using fluorescent markers to track fluid motion. They found that the quick evaporation of alcohol induces chaotic flow that keeps the solution well mixed. Once the alcohol is almost gone, surfactant molecules collect at the edge of the remaining solution, forcing the fluid to flow inward instead of outward. Meanwhile, macromolecules coat the underlying surface, capturing particles to create a relatively even deposition pattern. Understanding whiskey’s drying behavior could provide new insights for preparing uniform coatings, the team says.—JYLLIAN KEMSLEY
EPIGENETICS
▸ Controlling gene expression with light Chemical biologists have reported a method to control gene expression in cells with enzyme inhibitors triggered by visible light (Nat. Chem. Biol. 2016, DOI: 10.1038/ nchembio.2042). The new tool could help scientists study how epigenetics regulate gene expression, as well as lead to possible therapeutics that can be pinpointed to specific bits of tissue. Epigenetic enzymes in cells can control gene expression by adding or removing chemical groups to and from histone proteins that serve as packing material for DNA. One of these chemical mod-
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C&EN | CEN.ACS.ORG | APRIL 4, 2016
ifications involves acetylation of histone lysines. A team led by Ralph Mazitschek and Stephen J. Haggarty of Massachusetts General Hospital designed light-controlled inhibitors of enzymes called histone deacetylases (HDACs), which remove these acetyl groups. The control molecules are based on known HDAC inhibitors and feature an azobenzene group that switches from trans to cis geometry in blue light. This isomerization alters the electronic properties of the molecules, increasing their affinity for a zinc ion in the active site of HDACs. Cultured cancer cells treated with one of the inhibitors, BG14, had similar gene expression profiles as ones treated with a known HDAC inhibitor, but only when the BG14-treated cells were hit with blue light. This activity demonstrat-
PHYS. REV. LETT. (SCOTCH); SHUTTERSTOCK (COFFEE)
Envision a crystalline material and you might think of atoms holding static positions within a regular molecular lattice. A new dendrimeric molecular rotor represents another extreme: In crystalline form, the molecule’s aliphatic framework stays stationary, but its 25 phenyl rings rapidly rotate (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b01398). A team led by Xing Jiang, Miguel A. Garcia-Garibay, and Kendall N. Houk of UCLA constructed the molecule (shown), which has a phenylene core connected on each end to a triphenylpropynyl group. Each of those six phenyl rings in turn connects to another triphenylpropynyl group. The molecule consequently has three types of phenyl rings: one core phenylene, six branch phenylenes, and 18 peripheral phenyls. X-ray, NMR, and molecular dynamics data show that crystals of the compound are loosely packed and each of the three kinds of phenyl rings may rotate independently of the others. Within a particular set of phenyl rings, however, the motion may be coordinated. In a set of three branch phenylenes, for example, one ring may rotate while the other two oscillate, or all three rings may rotate synchronously. The molecule could find use in molecular machines.—JYLLIAN KEMSLEY
ed that the designed molecules function as light-controlled switches for the epigenetic regulators.—MICHAEL TORRICE
IMAGING
▸ Ironclad fluoroemulsion boosts MRI sensitivity Magnetic resonance imaging has become a critical technology in medical diagnostics, and chemists continue to devise new types of MRI contrast agents. In one of the latest examples, a team led by Eric T. Ahrens, Roger Y. Tsien, and Alexander A. Kislukhin of the University of California, San Diego, has created a paramagnetic fluorinated nanoemulsion with embedded metal ions that provides up to a fivefold boost in sensitivity over previously reported 19F MRI cellular detection systems (Nat. Mater. 2016, DOI: 10.1038/nmat4585). The researchers first crafted a metal-binding fluorinated com-
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NANOMATERIALS
Polypeptoid self-assembles into nanotubes A study shows that diblock copolymers made from peptide analogs called peptoids can stack together in a novel way to form crystalline nanotubes. Ronald This illustration N. Zuckermann of Lawrence shows an Berkeley National Laboraamphiphilic tory and coworkers created nanotube formed the new class of nanotubes by self-assembly of by synthesizing diblock polypeptoid blocks copolymers containing same(orange and blue). sized blocks of hydrophilic (ethyleneoxy side-chain) and hydrophobic (decyl side-chain) poly(N-substituted glycines). The peptoid chains form rings that stack vertically, with the hydrophilic and hydrophobic blocks aligning with one another, by a not-yet-understood mechanism, to form nanotubes with amphiphilic interior and exterior surfaces (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1517169113). Self-assembling nanotubes have been prepared before by mechanisms involving hydrogen bonding, electrostatic, or π-π interactions. But the new ones do not form via any of those mechanisms. The researchers hope to learn how they do form, as well as determine their atomic-resolution structure, cross-link them to make them stronger, and functionalize them to assess potential applications. Chemical separations are one possible use. Virgil Percec of the University of Pennsylvania comments that “these unprecedented tubular structures could provide new applications that are not accessible by already known biological and synthetic tubular assemblies.”—STU BORMAN
R POLYMERS RF = perfluoropolyether R = p-anisyl
ZUCKERMANN GROUP (NANOTUBE)
Iron-based nanoemulsion pound by introducing β-diketone groups to the ends of a perfluoropolyether precursor. They then emulsified this fluorinated oil (yellow) in an aqueous solution containing a commercial surfactant. The researchers next added various transition and lanthanide metals, which were bound by the diketonate groups. In a surprise finding, the team observed that iron(III) provided the best MRI sensitivity, surpassing that of gadolinium(III), which typically provides the best image contrast for conventional 1 H MRI. Kislukhin says the intended use of the new nanoemulsion is for labeling therapeutic stem cells and immune cells to track their movement via 19F MRI once the cells are implanted in a patient, which the team demonstrated in mice.—STEVE RITTER
▸ Visible light switches on organocatalyzed polymerization Atom-transfer radical polymerization, or ATRP, has proven to be a popular process for making polymers. The reaction uses a catalyst to reversibly generate a radical from an alkyl halide initiator. This radical then goes on to react with the double bond of a monomer, such as styrene or methyl methacrylate, a reaction that propagates until the monomer supply is exhausted. ATRP offers precise control over polymer chain growth, but the metal catalysts usually used are tough to remove, making most of the synthesized polymers unsuitable for biomedical or electronics applications. In recent years, chemists have developed or-
ganic catalysts for ATRP, but these require ultraviolet light to activate them and aren’t as good as metal catalysts at making certain types of polymers. Garret M. Miyake and colleagues of the University of Colorado, Boulder, now report an organocatalyzed ATRP that’s driven by visible light (Science N 2016, DOI: 10.1126/ science.aaf3935). N Because the reaction runs with visible light, sunlight can mediate the reaction instead of UV lamps, thereby Diaryl cutting costs. Miyake’s dihydrophenazine team used computational studies to develop its diaryl dihydrophenazine catalysts (example shown). These catalysts work as well as their metal catalyst counterparts for ATRP of methyl methacrylate.—BETHANY HALFORD APRIL 4, 2016 | CEN.ACS.ORG | C&EN
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