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Science Concentrates

March 15, 2004 | A version of this story appeared in Volume 82, Issue 11

Capsid does double duty

After emptying a viral capsid of its RNA cargo, chemists in California covalently attached a reactive handle on the inner surface of the hollow protein shell (shown in cutaway) [J. Am. Chem. Soc., published online March 9, 1021/ja031790q]. Such a capsid might be an effective anticancer drug shuttle or a protected shell for isolating sensitive catalysts that would otherwise deactivate as dimers, says author Matthew B. Francis, assistant professor of chemistry at the University of California, Berkeley. His group's unusual chemistry targets interior tyrosine residues, allowing for more conventional bioconjugation to be done on exterior cysteine residues. (The virus, bacteriophage MS2, is pockmarked with holes that allow substrates to diffuse into the hollow interior.) Francis and his Berkeley colleagues activated exposed tyrosines with a diazonium coupling reaction, reduced the resulting azo compound to an amino group, and oxidized the amine derivative to an o-iminoquinone. The scientists then demonstrated the handle's performance with a successful hetero Diels-Alder reaction.

First triple bond to lead

Double- and triple-bonded group 14 elements other than carbon have been a hot topic since the first double-bonded silicon compound was observed in the early 1980s. Nearly all members of this class of compounds have been reported so far, but not one featuring a lead triple bond. Now, even that has been accomplished by chemistry professor Alexander C. Filippou and coworkers Holger Rohde and Gregor Schnakenburg of Humboldt University, in Berlin, who report the synthesis of a molybdenum complex containing a Mo2Pb triple bond [Angew. Chem. Int. Ed., published online Feb. 27,]. The triple bond is formed by reacting a molybdenum complex containing phosphine and dinitrogen ligands with a bulky aryl lead bromide in toluene at room temperature. The reaction proceeds with a color change from orange to brown and elimination of N2 gas, resulting in the stable but air-sensitive plumbylidyne complex (shown, R = 2,6-[2,4,6-triisopropylphenyl]phenyl). The crystal structure reveals linear coordination of Pb and a short Mo2Pb triple bond length of 2.55 Å, which is 0.43 Å shorter than Mo2Pb single bonds.

Protein folding trajectory is seen directly

Scientists at Columbia University have obtained the first direct observations of the complete folding trajectory of a protein [Science, 303, 1674 (2004)]. Biological sciences professor Julio M. Fernandez and research assistant professor Hongbin Li tethered a polyubiquitin chain--composed of nine repeats of the small protein ubiquitin strung together--between a surface and the cantilever tip of an atomic force microscope. After mechanically unfolding the polyubiquitin chain by pulling the surface away from the tip, they decreased the stretching force and watched each ubiquitin in the chain fold back up. Since ubiquitin unfolds in well-defined, discrete steps, Fernandez and Li expected that folding would proceed in steps, too. But instead, they found that folding occurs through a series of continuous stages. Fernandez hypothesizes that the stretched peptide first collapses into a more compact form, then slowly forms the contacts required for its native fold. He and Li are now performing the same experiment under different conditions and with mutated ubiquitin to correlate the folding trajectory they observe with specific structural changes in the protein.

Study reveals pedigree of top-tier faculty

A recently published peer-reviewed study examines where chemistry faculty members at top-tier schools received their doctoral and postdoctoral training. Visiting professor of chemistry Valerie J. Kuck and her colleagues at Seton Hall University, South Orange, N.J., examined the academic pedigrees of American-trained male and female faculty members at the National Research Council's top 50 programs in 2001 who earned their Ph.D. degrees between 1979 and 2000 [J. Chem. Ed., 81, 356 (2004)]. Last year, the team announced preliminary results regarding the academic pedigrees of faculty at the top-10 NRC-ranked schools (C&EN, Sept. 29, 2003, page 42). In their current analysis, the team reports that doctoral graduates from the top five NRC-ranked schools hold 43% of the faculty positions at the top 50 schools. Generally, the analysis shows that the top 50 NRC-ranked schools preferentially hired male rather than female graduates from a given university, Kuck says. In addition, she points out, 42% of top-50 faculty members completed their first postdoctoral fellowship at a top-five NRC-ranked school.

Nanotube fibers simply spun

Limitless lengths of carbon nanotube fibers can be spun during chemical vapor deposition (CVD) synthesis of carbon nanotubes, according to a report by researchers from the University of Cambridge [Science, published online March 11,]. While there are other ways of making carbon nanotube fibers, they all involve an initial nanotube synthesis step followed by a post-processing step. Now, materials science professor Alan H. Windle and postdocs Ya-Li Li and Ian A. Kinloch report that they can make the fibers (shown twisted into a rope) in a one-step process. They use a rotating rod to mechanically draw the fibers from the "elastic smoke" of nanotube aerogel formed in the CVD process. By adjusting reagent concentrations, temperature, and hydrogen flow rates, the team can make either single-walled nanotube fibers or multiwalled nanotube fibers. Furthermore, Windle's group contends that this simple production method is both relatively cheap and environmentally friendly.



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