SCIENCE & TECHNOLOGY
CONCENTRATES Photoswitch molecule flashes on, off A flashy new molecule rapidly switches back and forth b e tween fluorescent and nonfluorescent isomers [Nature, 420,759 (2002)}. The molecular photoswitch was designed and synthesized
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by chemistry professor Masahiro Irie of Kyushu University, Fukouka,Japan, and coworkers. A fluorescent bis(phenylediynyl)anthracene unit (at right in structure shown) is linked via an adamantyl spacer to a photochromic unit (left side) that can isomerize between a closed-ring form and an open-ring form in which the bond shown in red is broken. The closed-ring isomer does not fluoresce. When irradiated with visible light, the molecule isomerizes, switching to a fluorescent state as the ring opens. UV light switches the molecule back to the closed-ring isomer. "This is the first observation of photoswitching of synthetic single molecules and also the first step to ultra-high-density 'single-molecule, optical memory," Irie tells C&EN.
Silkworms spin collagen cocoons Transgenic silkworms that produce cocoons containing recombinant human collagen have been developed by MasahiroTomita, Katsutoshi %shizato, and other scientists involved in the HkoshimaTissue Regeneration Project, a collaborative effort ofjapan Science & Technology Corp. [Nat. Biotechnol, published online Dec. 16, http://dx.doi.org/10.1038/ nbt771]. T h e study demonstrates the viability of transgenic silkworms as a tool for producing useful proteins in bulk, the researchers say Collagen has many medical applications, including tissue engineering and drug delivery materials. It is currently derived from cow skin, a source that can cause allergic reactions. In the study, the scientists constructed a D N A sequence that produced a fusion protein con24
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taining an ordinary silk protein, a fluorescent marker, and a shortened form of the precursor molecule for human type III collagen. When this D N A was incorporated into a vector and injected into silkworms, the silkworms synthesized the fusion protein in their silk glands and secreted it into cocoons. One chromatographic step separated the fusion protein from the other cocoon proteins.
Quantum dots make LEDs shine brighter Scientists have designed a hybrid organic-inorganic lightemitting diode (LED) that far outperforms previous such devices. Vladimir Bulovic, assistant professor of electrical engineering and computer science, chemistry professor Moungi G. Bawendi, and colleagues at
M I T sandwiched a monolayer of cadmium selenide quantum dots between organic thin films [Nature, 4 2 0 , 8 0 0 (2002)}. Quantum dots offer potentially sharp emission spectra and extremely high quantum efficiency in contrast to the broader spectra of standard organic LEDs. Previous attempts to harness quantum dots' potential by mixing them into conducting polymers have resulted in LEDs with low emission efficiency But Bulovic's sandwich structure, based on a design that's been used in traditional organic LEDs, puts the emitting quantum-dot "fill- ^ ing" between organic electron-donor and -acceptor "bread." T h e resulting ** LED is 25 times more efficient than other quantum-dot LEDs, is easily reproducible, and has an emission spectrum with an "almost perfect Gaussian profile," according to aNa- *" ture commentary
First silicon-tin alkene analog Group U alkene analogs, R 2 E=ER 2 , where E = Si, Ge, Sn, or Pb and R is a bulky substituent, have played a central role in the study of multiple bonding in the heavier main-
group elements. Many homonuclear dimetallenes of this type are now known, but analogs containing two different group 14 elements are still uncommon. Chemistryprofessor Akira Sekiguchi and colleagues at the University of Tsukuba, in Japan, have been working to remedy this situation, and now report the isolation of the first silastannene—a compound with a Si=Sn bond [J. Am. Chem. Soc., 124,14822 (2002)}. The silastannene (shown, hydrogens omitted) was prepared by coupling bis{di-te/?-butyl
(methyl)silyl]dilithiosilane ^ with dichlorobis(2,4,6-triisopropylphenyl)stannane. T h e crystal structure reveals aSi=Sn bond length of 2.42 A—much longer than C = C bonds and intermediate between typical Si=Si and Sn=Sn bond lengths. The Si=Sn bond is highly reactive, forming addition products with phenol or benzenethiol at room temperature.
SCIENCE & TECHNOLOGY ROUNDUP • Last year, Stephen T. Davis and colleagues at Glaxo Wellcome claimed they had synthesized compounds that could limit chemotherapy-induced hair loss (C&EN, Jan. 8,2001, page 25). The researchers have been unable to reproduce their results and have retracted the paper{5bb/aU98,2327(2002)]. • Maxine F. Singer, a biochemist who has headed the Carnegie Institution of Washington since 1988, will retire on Dec.
31. Richard A. Meserve, chairman of the Nuclear Regulatory Commission, will succeed Singer as president of the private nonprofit research institution. • Using time-lapse microscopy, Chicago researchers have obtained the first images of HIVinfecting living cells \J. Cell Biol, 159,441 (2002)}. Still images and movies from the study are available at http://www.uic. edu/depts/paff/newsbureau/hiv photos.html. HTTP://PUBS. ACS.ORG/CEN
