July 28, 2008

Volume 86, Number 30

pp. 50-51

Science & Technology Concentrates

Light-Triggered Base

By employing a light-responsive molecular shield, Stefan Hecht and coworkers at Humboldt University, in Berlin, have developed an organic base that can be reversibly switched on and off on demand (Angew. Chem. Int. Ed. 2008, 47, 5968). The controllable process could lead to new applications in chemical surface patterning, the researchers suggest. Hecht's team used piperidine, a nitrogen-containing six-membered ring, as the starting point for their reversible base. By adding a substituent to the nitrogen, they effectively locked its basic lone pair of electrons into place. They then incorporated a photoisomerizable azobenzene tether tipped with a bulky aromatic ring. Short-wavelength light (365 nm) causes the azobenzene to adopt a cis form that leaves the nitrogen's basic lone pair sterically accessible (right). Long-wavelength light beyond 400 nm regenerates the trans form, which blocks access to the lone pair, rendering the base inactive (left). The research complements recent work directed by Neil R. Branda of Simon Fraser University, in Burnaby, British Columbia, which used light to tune acidity by altering a Lewis acid's electronic properties (Angew. Chem. Int. Ed. 2008, 47, 5034).

Measuring Mass With A Nanotube

Physicists at the University of California, Berkeley, have devised a nanotube-based mechanical sensor with atomic resolution that has several advantages over traditional mass spectrometers (Nature Nanotech., DOI: 10.1038/nnano.2008.200.) The new nanomechanical device does not destroy samples via ionization and is more sensitive to large molecules such as proteins. In addition, it is small enough to incorporate into an electronic chip. Alex Zettl and Kenneth Jensen at UC Berkeley and coworkers previously designed a radio receiver from a nanotube. To explain their new work, they wrote, "In effect, we broadcast a radio signal to the nanotube and listened for its vibrations." The Berkeley researchers hooked up one end of a double-walled carbon nanotube, 2-nm wide and 254-nm long, to an electrode and left the other end free to vibrate, similar to a diving board. To test the sensor at room temperature, the researchers loaded several gold atoms onto the free end of the nanotube. The mass of the gold lowered the mechanical resonance frequency of the vibrating carbon nanotube. By monitoring this change, the researchers could determine the mass of the attached gold atoms.

Hydrogenase Sites surprisingly Similar

The structure of the iron-based active site in mononuclear [Fe]-hydrogenase reveals unexpected similarities to iron centers in the binuclear [NiFe]- and [FeFe]-hydrogenases, according to a group of researchers led by Seigo Shima at the Max Planck Institute for Terrestrial Microbiology and Ulrich Ermler at the Max Planck Institute of Biophysics, both in Germany (Science 2008, 321, 572). Hydrogenases catalyze the formation and consumption of H₂ and are of interest commercially as replacements for platinum catalysts that produce H₂ fuel. The crystal structure of [Fe]-hydrogenase indicates there are five ligands to the metal: a cysteine (Cys) sulfur, two CO molecules, the nitrogen of a 2-pyridinol cofactor compound, and a ligand the researchers were unable to identify. A solvent molecule occupies the sixth ligation site, but it is too far away to be considered a ligand. The researchers suggest that the sixth site is where H₂ binds. Notably, although the structures evolved independently, iron sites of the [NiFe]- and [FeFe]-hydrogenases also include a cysteine sulfur, two CO molecules, and a cyanide ligand akin to the 2-pyridinol, making the hydrogenases a remarkable example of convergent evolution, the authors say.

What's In A Whiff Of Whiskey?

Aficionados of American Bourbon whiskey may be interested in the results of a new study that identifies the beverage's most odor-active compounds. Researchers have been studying the volatile components of whiskey for more than 40 years, but not all such compounds can be detected by the human olfactory system, note Luigi Poisson and Peter Schieberle of the German Research Center for Food Chemistry, in Garching. To zero in on the key aroma compounds, Poisson and Schieberle used gas chromatography and aroma extract dilution analysis (J. Agric. Food Chem. 2008, 56, 5813). The pair identified more than 40 aroma compounds that contribute to whiskey's fruity, smoky, and vanillalike odor profile, including 13 compounds that hadn't been detected previously. The most active aroma compounds are (E)-β-damascenone, which smells like cooked apple, and γ-nonalactone, which resembles coconut. The researchers suggest that such studies could help whiskey producers modify or improve the aroma of whiskey by changing the recipe or the manufacturing process.

Light-Controlled Nanowires

Researchers in China have prepared hybrid organic-inorganic semiconducting nanowires where electrical conductivity can be switched on and off with light (J. Am. Chem. Soc. 2008, 130, 9198). The study broadens understanding of photoelectrical phenomena on the nanometer scale and may lead to new types of miniature circuits. Yanbing Guo, Yuliang Li, and coworkers at the Chinese Academy of Sciences' Institute of Chemistry, in Beijing, used porous templates to grow nanowires composed of polypyrrole (PPY) and cadmium sulfide. On the basis of single-nanowire microscopy images (top) and elemental maps (bottom), the team reports that the nanowires, which measure 200–400 nm in diameter, grow with an abrupt interface between the CdS and PPY segments. Furthermore, they find that unlike pure CdS or pure PPY nanowires, the hybrid structures' electrical conductivity can be controlled with light. In the dark, the hybrid nanowires are insulators. Under illumination, however, their capacity for carrying electrical current varies strongly with the intensity of the radiation.

Antibiotic Boosts RNA Interference

An FDA-approved antibiotic makes the gene-silencing technique known as RNA interference (RNAi) more effective in the laboratory, according to a new report (Nat. Biotechnol., DOI: 10.1038/nbt.1481). The small-molecule RNAi booster, a fluoroquinolone antimicrobial called enoxacin, may help scientists learn more about how RNAi machinery works. A multi-institution team led by Peng Jin of Emory University discovered this trait of enoxacin by using a cell-based assay that can detect enhancers and inhibitors of gene silencing. RNAi enhancement is not a general property of fluoroquinolones, the authors write, since most other variants had little to no effect on gene silencing. They propose that enoxacin works by facilitating the interaction between specialized RNAs and a part of the protein complex involved in silencing. Outside experts say that enoxacin could theoretically lower the needed doses of therapeutic RNAs, thereby reducing the chance of side effects, but they emphasize that more work is needed to verify that possibility. Emory has licensed the technology to Effigene Pharmaceuticals, an Atlanta-based company cofounded by Jin that focuses on RNAi technology for studying and treating diseases.

Catalysts Under Pressure

A team of researchers has recorded atomic-resolution transmission electron microscopy (TEM) images of catalyst particles (shown) while the solids were exposed to relatively high pressures of reactive gas (1 atm H2) and heated to 500 °C (Ultramicroscopy, DOI: 10.1016/j.ultramic.2008.04.014). The imaging experiment, which was conducted at 100 times greater pressure than in previous TEM studies, may lead to new ways of probing materials that undergo subtle but important structural changes as chemical reactions proceed on their surfaces. Generally, researchers aiming to record atomic-resolution TEM images conduct their experiments under high vacuum and at moderate temperatures because higher pressures and temperatures limit resolution and image quality. The usual imaging conditions, however, differ greatly from typical industrial catalytic reaction conditions, which may alter a catalyst's structure from an inactive to a catalytically active form. To get an up-close view of catalysts under demanding conditions, J. Fredrik Creemer of Delft University of Technology, in the Netherlands; Stig Helveg of catalyst manufacturer Haldor Topsøe, in Denmark; and coworkers designed a TEM-compatible microreactor and used it to probe a Cu/ZnO methanol-synthesis catalyst. While activating the catalyst at high temperature in hydrogen, the team directly observed the growth, structure, and evolution of copper nanocrystals with angstrom resolution on a subsecond timescale.

Flexible Circuits From Carbon Nanotubes

Random networks of single-walled carbon nanotubes can be used to construct high-performance integrated digital circuits on flexible plastic substrates, according to a new study (Nature 2008, 454, 495). The work advances the possibility of developing low-cost electronic displays and other devices that are more flexible, lightweight, and shock resistant than similar devices based on traditional silicon wafers or other rigid substrates. Earlier work in several labs has focused on developing flexible circuitry by using semiconducting small organic molecules and various types of polymers. Compared with electronics based on those materials, the carbon-nanotube circuits—designed and fabricated by John A. Rogers of the University of Illinois, Urbana-Champaign, and coworkers—show superior charge-carrier mobilities, operating voltages, switching speeds, and other electronic properties. George Grüner, a professor of physics and astronomy at UCLA, calls Rogers' work a proof-of-concept experiment. These flexible circuits could become "a paradigm-changing technology," Grüner says.

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