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Science Concentrates

November 22, 2004 | A version of this story appeared in Volume 82, Issue 47

Credit: © NATURE 2004
Credit: © NATURE 2004

Metabolite binding flips riboswitch

Many bacteria contain RNA sensors that directly control gene expression by binding various small-molecule metabolites. Now, the first structure of one of these so-called riboswitches--which are embedded in the noncoding region of certain messenger RNAs--reveals exactly how metabolite binding influences gene expression [Nature, 432, 411 (2004)]. Robert T. Batey and coworkers at the University of Colorado, Boulder, crystallized hypoxanthine (a purine) bound to the purine-binding portion of a riboswitch that controls the production of bacterial enzymes involved in purine metabolism and transport. Their 1.95-Å X-ray crystal structure of this prototypical riboswitch (shown) reveals that hypoxanthine (red) is almost completely enveloped by the RNA. Binding of hypoxanthine (or guanine) stabilizes the riboswitch's complex fold and triggers the formation of a hairpin-shaped structure in the neighboring RNA that stops the cell's transcription machinery, Batey says. This mechanism allows the riboswitch to single-handedly turn off gene expression in response to an increase in the intracellular concentration of a metabolite and is likely common to all riboswitches, he points out.

Molecules in flow visualized

Atomic force microscopy monitors individual polymer molecules within a monolayer as it spreads over a graphite surface.
Atomic force microscopy monitors individual polymer molecules within a monolayer as it spreads over a graphite surface.

In work that could have an impact on the development of microfluidic devices and nanoscale machines, a research team has monitored the motion of individual molecules in a drop of liquid polymer as it spreads over the surface of a solid substrate. Sergei S. Sheiko of the University of North Carolina, Chapel Hill; Krzysztof Matyjaszewski of Carnegie Mellon University; and coworkers used atomic force microscopy to visualize the flow process of a thin film of polymer molecules on graphite, mica, and silicon [Phys. Rev. Lett., 93, 206103 (2004)]. The liquid studied by the team is a melt of comblike polymer molecules known as molecular "bottle-brushes" comprising a polymethacrylate backbone and bristlelike poly(n-butyl acrylate) side chains. "The drop first spreads by generating a molecularly thin precursor film moving ahead of the macroscopic drop," Sheiko says. "The experimental technique provides an opportunity to explore a new direction in molecular fluidics wherein one can monitor, probe, and manipulate flows one molecule at a time."

Sensor lights up peroxide in living cells

A new selective, cell-permeable optical probe (right) for hydrogen peroxide promises to shed light on this reactive oxygen species' dueling pathological and physiological roles. Oxidative damage caused by H2O2 has been linked to cancer, cardiovascular disorders, and neurodegenerative diseases. Yet this potentially dangerous species plays an important cellular role as a signaling molecule. Christopher J. Chang of the University of California, Berkeley, says his team's H2O2-specific sensor molecule will be useful for studying the interplay of these pathological and physiological roles in living cells [J. Am. Chem. Soc., 126, 15392 (2004)]. Among biological reactive oxygen species, only H2O2 hydrolyzes the boronate protecting groups on Chang's water-soluble, colorless sensor, converting it into visible-light-emitting fluorescein. Having demonstrated that the cell-permeable sensor can be used to detect H2O2 in living mammalian cells, Chang now plans to use it to study both peroxide-mediated signaling and oxidative damage.

Resolution with chiral Se reagent

The chiral selenium compound shown (Tf = triflate) promotes kinetic resolution of racemic allylic alcohols in methanol, according to a study by chemists at the University of Perugia, in Italy. The reagent preferentially reacts with one enantiomer--adding across the double bond--leaving the unreacted allylic alcohol enantiomerically enriched [Org. Lett., published online Nov. 11,]. "To the best of our knowledge, this is the first example of a kinetic resolution process promoted by an organoselenium reagent," write the researchers, led by Marcello Tiecco. The kinetic resolution is not catalytic: Two equivalents of racemic allylic alcohol require one equivalent of the reagent. After column chromatography, the unreacted alcohol is recovered with enantiomeric excesses of 90 to 94%. Chemical treatment of the addition product with triflic acid releases the other enantiomer. The chiral selenium reagent is recovered as the diselenide.

Loading up metals with H2 for fuel cells

Metal hydrides are being explored as potential substrates in future hydrogen storage systems for fuel cells. So far, MHn species with up to nine hydrogen atoms have been made, such as ReH92-. Laura Gagliardi of the University of Palermo, in Italy, and Pekka Pyykkö of the University of Helsinki, in Finland, have now completed molecular-level calculations to show that MHn species with up to 12 hydrogen atoms are possible [J. Am. Chem. Soc., 126, 15014 (2004)]. They started by examining the analogy between hydride ions and gold anions, calculating that the hydrogen analog of the recently observed WAu12 would be stable. Gagliardi and Pyykkö then extended their calculations to MH12 species containing chromium, molybdenum, and other metals. They predict that H2 should bind side-on, M(h-H2), to the metal center for chromium and as a mixture of M(h-H2) and M–H bonds for molybdenum and tungsten. A design target of 6.5 weight % hydrogen has been set for fuel-cell storage, the researchers note. The CrH12 and MoH12 compounds, as well as salts containing TiH122–, VH12, and MnH12+, weigh in at more than 10 wt % hydrogen.


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