June 23, 2008

Volume 86, Number 25

pp. 28-29

Science & Technology Concentrates

One Pot, Six Chiral Centers

By using an amine-based catalyst, researchers at Aarhus University, in Denmark, have sculpted six chiral centers into a molecule in one fell swoop (Chem. Commun., DOI: 10.1039/b806418k). Beginning with an unsaturated aldehyde and a tricarbonyl compound, neither of which contain stereocenters, Karl Anker Jørgensen and colleagues developed reaction conditions that form just one bicyclic product out of 64 possible stereoisomers. The team chose a proline derivative featuring a silicon-protected hydroxyl group as the catalyst and added benzoic acid and the cyclic amine base piperidine to help carry out the transformation. The mechanism of the one-pot reaction is not yet clear, but the process generates four carbon-carbon bonds and six chiral centers with excellent selectivity, making a fused ring system found in compounds that show antitumor activity. The method works with a range of aldehydes (one example shown), but aromatic aldehydes gave the best yields. The reaction is "very impressive," says catalysis expert Miguel A. Yus at the University of Alicante, in Spain, noting that the Aarhus team can make products on a gram scale and purify them by crystallization.

Gravity-purified MOFs

Metal-organic frameworks (MOFs) can now be quickly and easily purified via a simple solvent-based method that takes advantage of minor density differences between the porous materials and any impurities, according to a study by Northwestern University's Omar K. Farha, Karen L. Mulfort, Alison M. Thorsness, and Joseph T. Hupp (J. Am. Chem. Soc., DOI: 10.1021/ja803097e). The technique could substantially benefit researchers involved in the discovery and production of MOFs, Hupp says. MOFs are highly porous, low-density materials made from metal ions or clusters joined by organic linker groups. Their large internal surface areas make them amenable for chemical separations, selective catalysis, and storing gases such as H₂ and CO₂. MOFs are typically synthesized in one-pot reactions, but chemists must painstakingly discover reaction conditions that result in ultrapure products because purifying the solids is not feasible by typical methods such as recrystallization. Hupp's team, taking advantage of a known solvent-based separation concept, started by floating MOFs on the surface of CH₂BrCl, a high-density solvent. By adding miscible cosolvents the researchers lowered the density of the solvent system to a point where the densest solid material—either the product or the impurities—sank and the remaining solid kept floating, thereby effectively separating the mixtures.

Chaperonin's Iris-Like 'lid'

By twirling rather than flapping is how a "lid" on the barrel-shaped eukaryotic chaperonin called TRiC closes and opens when proteins enter or depart its interior (Nat. Struct. Mol. Biol., DOI: 10.1038/nsmb.1436). TRiC is a large host complex that provides a protected environment for guest proteins to use as a kind of private dressing room in which to fold properly. After a guest enters, TRiC's lid closes to confine the protein and give it time to fold. Judith Frydman of Stanford University; Wah Chiu of Baylor College of Medicine, Houston; Andrej Sali of the University of California, San Francisco; and coworkers used single-particle electron cryomicroscopy and protein modeling to determine the molecular mechanism of the lid's motion, which is powered by adenosine triphosphate (ATP). Researchers had previously speculated that the lid closed and opened like a flap, but the new findings surprisingly indicate that it closes and opens rotationally, like the iris of a camera lens, and that part of this rotational motion is translated into the interior of the chaperonin. The work also suggests how evolution allowed TRiC to diverge from prokaryotic chaperonins like GroEL, which has a separate detachable lid.

Cantilever Array Weighs Single Cells

A new array of microfabricated silicon cantilevers gives researchers a way to track the growth of individual adherent cells (Lab Chip, DOI: 10.1039/b803601b). Common methods for characterizing single cells, such as flow cytometry, require that cells be suspended in solution. But many so-called adherent cells, such as fibroblast and epithelial cells, must be attached to a surface to grow. Rashid Bashir of the University of Illinois, Urbana-Champaign, and coworkers describe arrays of silicon cantilevers that can determine the mass of individual adherent cells without detaching them from the surface. The researchers calculate cell mass by measuring cell-growth-induced changes in each cantilever's resonance frequency, which is inversely proportional to the square root of its mass. Cells on the cantilever surface change the device's effective mass and thus its resonance frequency. The researchers cultured cancer cells on the cantilevers and calculated the cells' mass from resonance frequency shifts of the cantilevers. Because the cells don't need to be detached from the surface, the technology can be used for time-course studies of individual cells.

Colorful Magnetic Resonance Imaging

Microengineered magnetic particles could bring color to traditionally gray-scale magnetic resonance imaging (MRI), U.S. government scientists report (Nature 2008, 453, 1058). Gary Zabow of NIST and NIH and coworkers created various magnetic microstructures with geometries that produce distinct radio-frequency signatures. Those signatures can be converted into a rainbow of optical colors on a computer screen. Traditional MRI contrast agents are chemically synthesized metal complexes with paramagnetic properties. These agents alter the magnetic field surrounding hydrogen nuclei in water, but not in a way that easily enables distinguishing between different types of tissues. The researchers designed nickel particles to have an open, double-disk sandwich structure, which they created with conventional microfabrication techniques. Each particle type is slightly different—for example, having a thicker disk or a wider gap—to create a customized magnetic field that effectively yields a specific color that can be associated with particular tissues. The researchers say much work must be done before these sensitive and tunable particles can be routinely used in people.

Light-Driven Pulleys Turn Plastic Motor

Tomiki Ikeda at the Tokyo Institute of Technology and colleagues have developed the first plastic motor powered only by light (Angew. Chem. Int. Ed. 2008, 47, 4986). The researchers used an organic-dye-based material to convert light energy directly into mechanical work without the aid of batteries or electric wires. To test whether the material could turn a homemade, millimeter-sized two-wheel pulley system, the researchers made a polyethylene belt and coated it with the material, which is a cross-linked liquid-crystalline elastomer that contains photochromic azobenzene dyes. The material reversibly expands or contracts when illuminated, depending on the wavelength of the light. Simultaneously applying ultraviolet and visible light to different points on the belt changed the belt's shape and rotated the pulley wheels; shutting off the light stopped the motion. Other photochromic polymers can bend or expand, but they can't undergo that type of continuous, complex three-dimensional movement, the researchers note. Ikeda suggests that the material could be used at various dimensions, ranging from nanoscale motors to the plastic wheels of a passenger car.

Five-Membered Chemical Combo Gels

A novel multicomponent combination of chemicals that causes organic solvents to irreversibly gel up may allow scientists to begin tuning or designing other similar mixtures (J. Am. Chem. Soc., DOI: 10.1021/ja8002777). Organogelators, of which there are many types, are usually formed by one or two low-molecular-weight compounds, often discovered by accident, that cause a solution to gel as it cools. Organogels have numerous uses as functional soft materials, ranging from liquid crystals to sensors and even cosmetics. David Díaz Díaz (now at Dow Chemical in Horgen, Switzerland) and colleagues at the University of La Laguna, in Tenerife, Spain, also happened upon their organogelator system serendipitously while studying the tartaric acid-based racemic resolution of (±)-trans-1,2-diaminocyclohexane. In contrast with other organogelators, the new system is a combination of five readily available small molecules: L-tartaric acid, the diaminocyclohexane, methanol, hydrochloric acid, and water. A low concentration of this mixture rapidly and irreversibly gelatinizes numerous cold organic solvents when the solution is warmed. The discovery "opens the door for the design of new low-cost and efficient liquid multicomponent organogelators," the researchers write.

Nanotube Membranes Desalinate Water

Arrays of densely packed, vertically aligned carbon nanotubes can serve as membranes to filter ions out of water while allowing the water to flow significantly faster than through conventional filters, according to a research team led by Francesco Fornasiero, Aleksandr Noy, and Olgica Bakajin of Lawrence Livermore National Laboratory (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0710437105). These nanotube membranes offer a promising technology for desalination, the researchers say. The team created the filtration membranes by embedding nanotubes in a silicon nitride matrix and then uncapping the ends of the tubes by an etching process that also introduced carboxylate groups around tube entrances. The carboxylate groups form a ring of negative charges through which ions must pass to enter the tubes. When the researchers filtered electrolyte solutions, the nanotube membranes rejected as much as 90% of K₃Fe(CN)₆ and 50% of KCl while maintaining a high flow per unit area. Additional experiments varying the pH of the solutions as well as filtering different salts demonstrated that ion exclusion stems primarily from electrostatic rather than steric effects. In addition to desalination applications, the nanotubes could be useful as models for biological membrane pores, the authors suggest.

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