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Environment

Science Concentrates

May 30, 2005 | A version of this story appeared in Volume 83, Issue 22

Compounds block select kinases

The ATP analogs shown (X = F or Cl) potently and irreversibly inactivate just two of 491 human protein kinases, a new study shows (Science 2005, 308, 1318). Protein kinases catalyze the transfer of a phosphate from ATP; they reg ulate many signaling pathways and are important drug targets. Because the protein kinase active site is highly conserved, an inhibitor designed to target a specific kinase usually inhibits many others as well. For this reason, the highly specific inhibitory activity of the analogs is unusual. At the University of California, San Francisco, Michael S. Cohen, Chao Zhang, Kevan M. Shokat, and Jack Taunton relied on two features of molecular recognition to narrow the specificity of potential kinase inhibitors. One is noncovalent interaction with a threonine residue near the ATP-binding site that allows bulky groups to enter a deep hydrophobic pocket in the binding site. The other is rapid covalent bond formation with a specific cysteine in the active site. The analogs, designed to exploit these interactions simultaneously, inhibit only the kinases RSK1 and RSK2.

How Ca doping boosts the supercurrent

In its polycrystalline form, the high-temperature superconductor YBa2Cu3O7-* can carry more current across grain boundaries if some of the Y3+ ions are replaced with Ca2+ ions. This effect has generally been attributed to the fact that Ca2+ doping introduces excess holes (electron vacancies), which are the basis of the current. Now, an international team has shown that the ionic radius of calcium is more important than its valence in terms of boosting the supercurrent (Nature 2005, 435, 475). Robert F. Klie of Brookhaven National Laboratory and his collaborators find that Ca2+ not only substitutes for Y3+ but also replaces the larger Ba2+ and the smaller Cu2+ in strained grain-boundary regions. The substitution relieves strain and suppresses oxygen vacancies, which when unsuppressed reduce the local hole concentration. The researchers suggest that silver, which has roughly the same ionic radius as yttrium and calcium, might serve as an alternative dopant. They say it might increase the supercurrent across the grain boundary just as calcium does but without calcium's detrimental effects on superconducting properties within the grain. A University of Wisconsin, Madison, team reports related results (Nat. Mater. 2005, 4, 470).

Garlic's heat explained

The sulfur compound allicin (shown) has been identified as the source of garlic's characteristic burning and prickling flavor (Curr. Biol. 2005, 15, 929). Despite garlic's widespread popularity as a food ingredient and an herbal remedy, little is known about the source of its distinctive pungency. Allicin is an unstable compound produced when raw garlic is bruised, cut, or crushed. A team led by Ardem Patapoutian of Scripps Research Institute and the Genomics Institute of Novartis Research Foundation now has shown that the binding of this molecule to the ion channels TRPA1 and TRPV1 gives rise to garlic's distinctive pungency. These channels stud the outside of pain-sensing neurons located in the mouth and tongue. The researchers suggest that garlic loses its pungency when baked because the garlic enzyme responsible for synthesizing allicin is inactivated when heated.

Photocatalyst is driven by visible light

A new sulfide photocatalyst induces water to generate hydrogen in the presence of visible light. Many photocatalysts split water when irradiated with ultraviolet light, but that leaves a large portion of light unused. Akihiko Kudo and coworkers at Tokyo University of Science showed that a solid solution of zinc sulfide, copper indium sulfide, and silver indium sulfide is a highly active photocatalyst when loaded with ruthenium [Angew. Chem. Int. Ed., published online May 6, http://dx.doi.org/10.1002/anie.200500314]. The chemists dispersed the ZnS-CuInS2-AgInS2 photocatalyst powder in an aqueous solution containing potassium sulfite and sodium sulfide and then loaded the ruthenium cocatalyst in situ by photodeposition. When the mixture is irradiated by visible light, the sulfite and sulfide ions act as sacrificial reagents that donate electrons to hydrogen ions, which then form hydrogen gas. They also showed that the ZnS-CuInS2-AgInS2 solid solution absorbs light over a wider wavelength range than do ZnS-AgInS2 or ZnS-CuInS2 solid solutions. These solid solutions could find practical use in photocatalytic hydrogen production if quantum yields and activities can be further improved, they suggest. One of the potential environmental benefits of these systems is the use of unwanted by-products, such as hydrogen sulfide and sulfur dioxide emitted from chemical factories and power plants, as sources of sulfide and sulfite, they add.

A turnoff for the koala palate

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Credit: Ben Moore/James Cook University
The dietary habits of koalas such as this male on Phillip Island are influenced by tree chemistry.
Credit: Ben Moore/James Cook University
The dietary habits of koalas such as this male on Phillip Island are influenced by tree chemistry.

Koalas dine almost exclusively on eucalyptus leaves. When given a choice, wild koalas prefer to dine on larger trees, which provide more food, shelter, and interaction with other koalas, according to two biologists in Australia. The animals also prefer leaves from trees that contain more nitrogen and less of the lipophilic phenolic chemicals known as formylated phloroglucinol compounds (FPCs) (Nature 2005, 435, 488). Nitrogen serves as a measure of nutritional value. FPCs, on the other hand, are thought to serve as a plant defense mechanism that induces nausea in animals that eat too much of the plant. Two of the most common FPCs are sideroxylonal A (shown) and macrocarpal G. William J. Foley of the Australian National University and Ben D. Moore of James Cook University found that the higher the concentration of FPCs in a particular tree and the lower its nitrogen content, the less likely that koalas would dine on its leaves.

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