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August 23, 2004 | APPEARED IN VOLUME 82, ISSUE 34

Slow growth key to Pt nanowires

Large quantities of uniform, single-crystal platinum nanowires can be grown using a solution-based process developed by chemists at the University of Washington, Seattle. Platinum nanostructures interest chemists chiefly because of their potential as catalysts, but to grow anisotropic structures, like nanorods or nanotubes, scientists usually employ a template to guide the structure's growth. Younan Xia and colleagues discovered that by using iron salts to slow down the reaction rate during a polyol reduction of H2PtCl6 or K2PtCl6, they can grow platinum nanowires in solution without any templating [J. Am. Chem. Soc., published online Aug. 17,]. During the first few hours of the process, Xia's group notes that the Pt atoms form nanoparticle agglomerates. As the reaction rate slows, the Pt atoms organize into nanowires on the agglomerates' surfaces (shown). The final structures look like sea urchins. Xia's group then strips the nanowires away from the agglomerates using a simple combination of sonication and centrifugation.

Intelligent glass beats the heat

A bright idea in window coatings may bring relief from high temperatures and high air-conditioning bills. Ivan P. Parkin of University College London, and his colleague Troy D. Manning, currently working at the University of Liverpool, in England, developed an intelligent window coating that allows visible light to pass through the glass but will reflect infrared radiation at temperatures above 29 °C [J. Mater. Chem., 14, 2554 (2004)]. The coating is made mostly of vanadium dioxide, a material that scientists knew could switch between absorbing and reflecting IR light, depending upon the electrons' arrangement. Until now, however, this switch occurred around 70 °C--too warm to be practical. By adding 1.9% tungsten to the vanadium dioxide coating, Parkin and Manning were able to lower the thermochromic switching temperature. They also say the process they use to make the smart windows is well suited to high-throughput manufacturing. However, because the coating turns the glass a greenish-yellow hue, the team hopes to tone down the color before trying to bring it to market.

Pyrrolysine biosynthesized directly

Scientists have shown that the 22nd amino acid, pyrrolysine, is biosynthesized as a free amino acid. Like the other 21 genetically encoded amino acids, pyrrolysine (shown) is directly inserted into the growing peptide chain by a dedicated transfer RNA (tRNA). Scientists figured pyrrolysine biosynthesis would parallel biosynthesis of selenocysteine, the 21st amino acid: A lysine would be attached to the pyrrolysine tRNA and then derivatized to give pyrrolysine. But new work by Dieter Söll of Yale University and colleagues suggests otherwise [Proc. Natl. Acad. Sci. USA, published online Aug. 16,]. They chemically synthesized a racemic 4-methyl version of pyrrolysine and showed that it (but not lysine) can be attached directly to the pyrrolysine tRNA by an enzyme they dub pyrrolysyl-tRNA synthetase. These results suggest that pyrrolysine is biosynthesized as a free amino acid and then attached to its tRNA. It should therefore be possible to expand an organism's genetic code to include pyrrolysine simply by introducing the genes for pyrrolysine tRNA and its synthetase as well as exogenous pyrrolysine.

Still surprises in Jupiter's atmosphere

When the Cassini spacecraft swung by Jupiter several years ago, the craft's infrared spectrometer gathered an armful of new data about the gases swirling around Jupiter's stratosphere and their behavior. The bounty includes the discovery of methyl radical and diacetylene in the upper regions of Jupiter's largely hydrogen atmosphere, an international team of scientists reports [Science, published online Aug. 19, 0240]. The authors speculate that these compounds--never before seen in Jupiter's atmosphere--participate in stratospheric photochemistry, with methyl radical forming ethane and methane, and diacetylene helping to form stratospheric haze. Cassini also examined the atmospheric chemistry in the regions of Jupiter affected by the comet Shoemaker-Levy's impact 10 years ago. Two of the major chemical products that resulted, hydrogen cyanide and carbon dioxide, have different dispersion patterns--an unexpected finding that may be related to postimpact chemistry, the authors say.

P takes on nanorod shapes

New structural forms of elemental phosphorus have been revealed in studies carried out by German researchers [Angew. Chem. Int. Ed., 43, 4228 (2004)]. Phosphorus occurs in nature in various phosphate-bearing rocks, and for some 350 years it has been known that pure phosphorus can be prepared by reducing these materials. Several phosphorus allotropes consisting of P4 and other units have been identified, with the white, red, violet, and black color modifications being the main forms. Arno Pfitzner of the University of Regensburg and Hellmut Eckert of the University of Münster and coworkers have now isolated two red-brown forms, one each from solutions of (CuI)8P12 and (CuI)3P12. They prepared the compounds by reacting CuI with red phosphorus. The German team used electron microscopy and NMR to identify the red-brown forms as rod-shaped P12 units (shown), noting that red-brown phosphorus is distinctly different from the amorphous red phosphorus, which is also thought to be polymeric.



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