Science Concentrates

Frost flowers waft bromine

Using satellite imaging, researchers have pinpointed one likely contributor to ozone depletion in the lower atmosphere near the poles: frost flowers. Temporary fields of frost flowers on top of new ice appear every year as polar ice grows. These delicate clusters of ice crystals (shown) form only in the below-freezing layer of concentrated brine slush and saturated water vapor above new sea ice. The flowers hold three times the concentration of bromide ions in seawater. A group led by Lars Kaleschke at the Institute of Environmental Physics at the University of Bremen, in Germany, found a correlation between satellite measurements of both tropospheric BrO and likely frost flower coverage [Geophys. Res. Lett., 31, L16114 (2004)]. The researchers suspect the flowers contribute to the well-known bromine explosion that happens at polar sunrise and causes a cascade of atmospheric reactions leading to ozone depletion in the troposphere.

  Novel riboswitch discovered

Most riboswitches--noncoding regions of messenger RNA that regulate gene expression upon binding a small molecule--consist of a single RNA domain that binds the target ligand and suppresses gene expression. Ronald R. Breaker and coworkers at Yale University and the University of Washington, Seattle, have discovered an unusual bacterial riboswitch with two glycine-binding domains that controls expression of genes involved in the glycine cleavage system [Science, 306, 275 (2004)]. Measurements show that glycine binding at one RNA site substantially improves the affinity for glycine at the second site, suggesting that glycine binding is cooperative. The new riboswitch is unusual because it is one of only a few known riboswitches that use ligand binding to activate rather than repress gene expression. Also, the level of cooperativity between the two RNA domains is comparable with that observed between some protein domains, indicating that both RNA and proteins can form sophisticated receptors.

Tightest binding of NO

Clostridium botulinum has a highly sensitive alarm system for the molecule that kills it. Texas researchers have crystallized the bacterial heme domain that detects nitric oxide at femtomolar concentrations--the highest known affinity for NO, says C. S. Raman of the University of Texas Medical School [Science, published online Oct. 7, http://dx.doi.org/10.1126/<br > science.1103596]. The C. botulinum heme protein is fused to a chemotaxis protein that may spur the bacteria to move further away from its poison. The heme sensor is unique because its sensitivity for NO partly depends on an amino acid side chain (tyrosine, shown) that links through a hydrogen bond to NO's oxygen. Other bacteria and plants have similar NO sensors, Raman says. In fact, Raman's research indicates that the bacterial NO sensor may have been co-opted early on by mammals, which eventually began to use NO for signaling. Mammalian binding is not as sensitive, however, and Raman believes the bacterial protein could be put to use in bioengineered NO sensors.

Oxaborolides: furan analogs

Thiophene, pyrrole, and furan are well-known aromatic heterocycles that have myriad uses in chemistry. Readily forming stable -coordinated transition-metal complexes isn't one of them, however. Arthur J. Ashe III and coworkers at the University of Michigan have devised a strategy to modify these five-membered rings by replacing a CH group with the isoelectronic BH^- group; the resulting borolides are better suited electronically to function as ligands. Ashe and coworkers previously prepared borolide analogs of thiophene and pyrrole and have shown the anions to be good ligands to form zirconium complexes that have high activity for olefin polymerizations. Now, they report the synthesis of the furan analog, 1,2-oxaborolide, and show that it also is amenable to forming transition-metal complexes [Organometallics, 23, 5088 (2004)]. The team developed two methods to make alkali-metal oxaborolides: either by ring-closing a vinylborane or by reaction of an oxastannole with a boron reagent. The 1,2-oxaborolide anion functions as a surrogate for cyclopentadienyl ligands to form a ruthenium complex (shown, Ph = phenyl) and other complexes.

Risks of in utero solvent exposure

Children whose mothers were exposed to organic solvents in the workplace while pregnant tend to score lower on certain IQ, memory, and motor function tests and exhibit increased hyperactivity, according to researchers at Toronto's Hospital for Sick Children and the University of Toronto [Arch. Pediatr. Adolesc. Med., 158, 956 (2004)]. Gideon Koren and colleagues studied the children of 32 women who were occupationally exposed to at least one of 78 organic solvents for at least eight weeks of their pregnancies. The women, roughly a third of whom worked as laboratory technicians, chemists, or chemical technicians, were recruited while pregnant and reported using protective measures such as ventilation, masks, and gloves. Koren and colleagues tested the women's children at ages three to nine and compared them with a carefully matched control group. Although the results were subtle--none of the parents perceived that their children had problems--Koren says the exposed children performed less well than the control group on cognitive function tests. Koren calls for further studies to address specific solvents as well as timing and levels of exposure.