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Synthesis

Science Concentrates

November 8, 2004 | A version of this story appeared in Volume 82, Issue 45

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Credit: © NATURE 2004
Credit: © NATURE 2004

Walking on water

What keeps the aquatic insect known as a water strider afloat? Conventional wisdom holds that hydrophobic wax on the surface of the insect's legs is responsible. But Xuefeng Gao and Lei Jiang of the chemistry institute at the Chinese Academy of Sciences, in Beijing, dismiss that notion [Nature, 432, 36 (2004)]. Using a scanning electron microscope, they have determined that the water strider's legs are covered in tiny overlapping hairs about 50 µm long (top). These microsetae, which give the legs a feathered appearance, are marked with nanoscale grooves (bottom). "Available air is trapped in spaces in the microsetae and nanogrooves to form a cushion at the leg-water interface that prevents the legs from being wetted," according to the researchers. This leg structure allows the critters to stand or run on the surface of water, or even to bounce to avoid being drowned by raindrops. Jiang speculates that the discovery could aid in the development of nonsinking boats and nonwettable shoes that would allow people to walk on water.

New computer memory gets the lead out

Using "strain engineering," a team of materials scientists has enhanced the ferroelectric properties of barium titanate thin films so they can be used to make a new type of random access memory device known as FeRAM [Science, 306, 1005 (2004)]. These devices, which rely on ionic polarization, or ferroelectricity, to function, generally use bismuth- and lead-containing transition-metal oxides. These materials have volatile constituents, however, and the toxicity of lead is an issue. Barium titanate's ferroelectric properties have not been sufficient for FeRAM fabrication, until now. Kyoung Jin Choi and Chang-Beom Eom of the University of Wisconsin, Madison, and coworkers deposited BaTiO3 on top of a GdScO3 or DyScO3 substrate and analyzed the films by variable-temperature X-ray diffraction. A difference in crystal lattice parameters between the BaTiO3 layer and the substrate induces high strain in the top layer, altering its physical properties. This strain markedly improves the BaTiO3 ferroelectric temperature range and increases its ferroelectric polarization so that the material has nearly the same properties as unstrained lead-based films.

Fine-tuning cis-platinum binding

The cancer drug cisplatin binds DNA at preferred sites such as double guanines, and the resulting DNA complex kills growing cancer cells. Unfortunately, cisplatin binds all DNA and also kills normal cells. In an initial step toward fine-tuning the binding of molecules similar to cisplatin, Jacqueline K. Barton and Anne Petitjean at Caltech created a two-headed molecule [J. Am. Chem. Soc., 126, 14728 (2004)]. At one end is the cis-platinum moiety. At the other is a rhodium-based intercalator that inserts itself into mismatched DNA. When the two ends are tethered together, the binding preference of the metallointercalator dominates. This specific two-headed species may not be a good drug, Barton says. However, a similar strategic combination could be useful in helping to direct cis-platinum therapy toward mutated DNA more likely to lead to cancer.

From benzene to picolinic acid

Japanese researchers have come up with a practical method to prepare picolinic acid-containing aromatic compounds. Such compounds are rare in nature and difficult to prepare by conventional synthesis. If readily available, they could be used as starting materials for pharmaceuticals, agrochemicals, and other useful compounds. The team, led by Kazutoshi Shindo at Japan Women's University, Tokyo, incorporated the genes for biphenyl catabolism into Escherichia coli. When the reengineered bacteria are grown in the presence of aromatic compounds containing two benzene rings, the bacteria transform one of the rings into picolinic acid (pyridine-2-carboxylic acid) [J. Am. Chem. Soc., 126, 15042 (2004)]. For example, the compound shown is the product from 7-hydroxyflavanone. The researchers speculate that the expected cleavage products of the catabolic enzymes are unstable in the culture medium and are quickly converted to picolinic acid by incorporating ammonia and undergoing ring closure. Using 11 different starting materials, the team achieved bioconversion yields ranging from 13% for 3-phenylindanone to 84% for 7-hydroxyflavanone.

Size matters in ion swap

New ionic nanomaterials can be made with a simple cation-exchange reaction at ambient temperature and pressure, according to a report from scientists at the University of California, Berkeley [Science, 306, 1009 (2004)]. A. Paul Alivisatos and colleagues found they could easily change the composition of nanocrystals, such as CdSe nanorods, CdS hollow nanospheres, and CdTe tetrapods, by exchanging the Cd2+ ions with Ag+, Cu2+, or Pb2+. In the bulk, some of these ion-exchange reactions won't work under the same conditions because of kinetic hindrance. The researchers say their method presents a versatile route for making new materials without having to develop a new synthetic method for each individual nanostructure. Alivisatos' team also identified a critical size--about 5 nm thick--above which the nanocrystals maintain their original morphologies during the cation exchange. If a nanocrystal is smaller than that critical size, the group found that it will evolve toward its equilibrium shape during the reaction. This result, the researchers say, "demonstrates that inorganic nanocrystals may be far more chemically dynamic than previously realized."

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