Issue Date: November 29, 2004
A clever new chemical trick that renders specific neurons sensitive to light may help neuroscientists tease apart how individual neurons are linked into complex neural networks [Nat. Neurosci., published online Nov. 21, http://dx.doi.org/10.1038/nn1356]. Richard H. Kramer, Dirk Trauner, and coworkers at the University of California, Berkeley, introduced a reactive cysteine handle near the pore of a common K+ channel. The researchers then expressed this modified channel in rat neurons and tagged its reactive cysteine with a photoisomerizable azobenzene tether tipped with a quaternary ammonium ion, a moiety known to plug K+ channels' ion-conducting pore. Long-wavelength light causes the azobenzene to adopt an extended trans configuration that allows the ammonium ion to plug the pore (left). But short-wavelength light generates the shorter cis configuration and lets K+ rush out of the neuron (right), rendering the cell inactive. These photosensitive channels allow rapid, precise, and reversible control over specific neurons, making them valuable tools for dissecting complex neural networks in cell culture, Trauner says.
The catalog of nonnatural amino acids that Peter G. Schultz and his colleagues at Scripps Research Institute have been able to incorporate into peptides and proteins stands at more than 30. Now, Schultz, Feng Tian, and Meng-Lin Tsao show that they can combine their previously developed methods of incorporating these nonnatural amino acids into proteins with phage display (a method of generating fusion proteins on the surface of a virus) as a general approach to create libraries of peptides [J. Am. Chem. Soc., 126, 15962 (2004)]. Using a transfer RNA that corresponds to the base sequence TAG and a mutant tRNA synthetase that attaches the desired nonnatural amino acid to that tRNA, they show that they can incorporate five different nonnatural amino acids into the displayed peptides. The researchers believe that their method can be used to make peptide libraries featuring the other nonnatural amino acids as well.
A novel amorphous semiconducting oxide can outperform commonly used materials in flexible thin-film electronics applications, researchers in Japan have demonstrated. Research aimed at developing so-called electronic paper and related display technologies has focused primarily on hydrogenated amorphous silicon and various types of organic semiconductors as the active channel materials in flexible, transparent thin-film transistors. But the low charge-carrier mobilities characteristic of those materials have limited their use in applications. Now, Hideo Hosono, Kenji Nomura, and their coworkers at Tokyo Institute of Technology have shown that thin-film transistors featuring amorphous indium-gallium-zinc oxide channels can exhibit charge-carrier mobilities more than an order of magnitude larger than the more commonly studied materials [Nature, 432, 488 (2004)]. The group reports that the devices, which were fabricated using pulsed laser deposition methods, remain stable even after repeated bending and curling.
Spirodiepoxides, with their dual-epoxide-ringed, three-carbon units, are attractive potential building blocks for creating organic molecules, but chemists had not used them in a total synthesis until now. Lawrence J. Williams and colleagues at Rutgers University, Piscataway, N.J., have used a spirodiepoxide (top) to synthesize the powerful proteasome inhibitor epoxomicin (bottom; portion derived from the spirodiepoxide is shown in red) [J. Am. Chem. Soc., 126, 15348 (2004)]. Like nucleophilic epoxide opening--the ubiquitous technique for creating complex molecules--Williams' group's highly concise sequence also involves the action of nucleophiles, which open the spirodiepoxide rings. This process yields molecules with multiple functional groups and tailored stereochemistry. The route is also likely to be useful for producing new epoxomicin analogs with improved activity and selectivity, the authors say.
Single oxygen atoms are good ligands for early transition metals because oxygen is a strong -electron donor and its donated electron pair can delocalize into vacant d orbitals on the metal. But oxo complexes of metals farther across the periodic table are rare or don't exist because electron-electron repulsion between the increased number of d electrons and oxygen's donated electrons creates instability. Craig L. Hill of Emory University, Atlanta, and coworkers have shown how this electron push back can be mitigated by synthesizing a platinum oxo complex containing polyoxometalate ligands that have delocalized molecular orbitals to accommodate some of the additional electron density [Science, published online Nov. 25, http://dx.doi.org/10.1126/science.1104696]. The complex, K7Na9[Pt(O)(H2O)(PW9O34)2] 21.5H2O, has a platinum(IV) center with six d electrons and is surrounded by six oxygen atoms, including the Pt5O oxo ligand. Oxo intermediates could be key for O2 activation at platinum surfaces, Hill says, including processes in catalytic converters, fuel cells, and organic oxidations.
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