April 23, 2007

Volume 85, Number 17

p. 39

Science & Technology Concentrates

Building barriers in roots

Hongchang Cui

When root cells soak up nutrients from the soil, a single layer of cells called the endodermis (indicated by white arrows) acts as the quality-control barrier. This cellular layer forms a tight seal, somewhat analogous to the blood-brain barrier, that permits transport of ions from the soil into the plant but "prevents potentially toxic compounds from reaching the plant vasculature," says Philip N. Benfey, a biologist at Duke University. Benfey and colleagues have just figured out how this tissue roadblock is formed and maintained (Science 2007, 316, 421). A protein transcription activator called SHORTROOT (SHR, shown in green) is made in the center of a root and transported outward from cell to cell until it reaches the endodermis. There SHR meets its match: a protein called SCARECROW that sequesters SHR and prevents its further movement. Stuck in the endodermis, SHR interacts with other transcription factors to activate genes that uniquely define the endodermis. The work may provide molecular insight on how tissue boundaries are formed in multicellular organisms.

Rhenium diboride stands up to diamond

Jonathan B. Levine

Rhenium diboride powder and arc-melted ReB2

With its milquetoast metallic appearance, rhenium diboride (ReB₂) probably won't usurp the place of diamond as a girl's best friend. But the superhard material could replace diamond in certain industrial applications, thanks to its ultra-incompressible nature and its relatively simple synthesis. Sarah H. Tolbert, Richard B. Kaner, and coworkers at UCLA prepared the metallic form of ReB₂ in bulk quantities under ambient pressure via an arc-melting procedure (Science 2007, 316, 436). The material's average measured hardness is comparable to that of other superhard materials, such as cubic BN or B₆O, which must be made at extreme pressures and temperatures. It falls short of being as hard as diamond, but Tolbert and Kaner believe their measurement represents only a lower limit of ReB₂'s hardness. Rhenium diboride can scratch the surface of natural diamond, however, indicating that it's one of the hardest materials known.

Zeptoliter pipette delivers nanodroplets

Peter and Eli Sutter/BNL

Nanometer-sized droplets of liquid crystallize differently than their larger counterparts, according to a new study that challenges conventional theories of phase transformations and may lead to new techniques for manipulating microscopic droplets (Nat. Mater., DOI: 10.1038/nmat1894). Using high-temperature transmission electron microscopy methods, Brookhaven National Laboratory scientists Eli A. Sutter and Peter W. Sutter show that a gold-alloy-tipped germanium nanowire that is encapsulated in a carbon film can function as a pipette and dispense zeptoliter (10^-21 L) metal droplets (13-zL droplet shown). Heating the nanowire and quickly boring a tiny hole into the film with an electron beam causes spherical droplets of Au₇₂Ge₂₈ alloy to be delivered from the tip. A key finding of the study is that the nanodroplets crystallize by way of a surface-faceting process and not via the classic mechanism based on internal nucleation.

Soil bacteria expand antibiotic arsenal

Screening of natural product extracts has unearthed a new antibiotic that could help in the fight against drug-resistant bacteria. Jun Wang, Sheo B. Singh, and colleagues at Merck discovered the antibiotic, which they named platencin (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0700746104). The compound is produced by a strain of Streptomyces platensis that was found in a soil sample collected in Spain. Platencin (shown) resembles the antibiotic platensimycin, which is produced by a different strain of S. platensis that was found in a South African soil sample. The Merck team discovered platensimycin last year by screening a library of 250,000 natural product extracts (C&EN, May 22, 2006, page 7). Both compounds work by striking at the fatty acid synthesis pathway in bacteria. The new antibiotic inhibits two enzymes in that pathway, whereas platensimycin targets just one enzyme. The Merck researchers report that platencin combats a broad range of bacteria, including drug-resistant strains of Staphylococcus aureus. It also has shown potent efficacy in mice without any observed toxicity.

Faster route to NMR chemical shift data

A new technique makes it easier and faster to determine a type of fundamental nuclear magnetic resonance spectroscopy value that is rich with information about molecules. Known as chemical shift anisotropy (CSA) tensors, these values previously have been difficult to obtain. CSA tensors measure variations in NMR chemical shifts that occur when a functional group in a protein or other compound changes orientation with respect to a fixed magnetic field. Chemists use CSA data to refine protein structures, elucidate enzyme mechanisms, and validate theoretical calculations of atomic properties. Traditionally, researchers could measure only one CSA tensor at a time, and recent advanced techniques boosted this to about 10 tensors per experiment. Now, Chad M. Rienstra and coworkers at the University of Illinois, Urbana-Champaign, have devised a way to obtain about 100 CSA tensors at a time (J. Am. Chem. Soc. 2007, 129, 5318). They report using the technique to determine about 95% of the CSA tensors in a small protein and have since achieved similar results with larger proteins.

Chemical & Engineering News

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