April 7, 2008

Volume 86, Number 14

pp. 50-51

Science & Technology Concentrates

Step-By-Step Tooth Enamel

Tooth enamel, made from tightly packed hydroxyapatite nanorods, is one of the hardest and most durable biomaterials known. Exactly how it is manufactured in the body is still a mystery. Now, a group led by George H. Nancollas at the State University of New York, Buffalo, has identified specific stages of enamel crystallization and found that the matrix protein amelogenin promotes hydroxyapatite nucleation (J. Phys. Chem. C, DOI: 10.1021/jp077105+). Much past research into enamel crystallization involved extreme conditions that do not reflect those in physiological systems. Nancollas and colleagues took a different tack by reducing experimental hydroxyapatite concentrations to slow down crystallization and investigating hydroxyapatite nucleation at physiological pH and temperature. The researchers found that the first step in their in vitro enamel production was the cooperative formation of composite nanospheres of nanocrystallite apatite and amelogenin. The composite nanoparticles then aggregated into nanorods that are about 50 nm in diameter and 250 nm long. The nanorods, in turn, further assembled to form organized, elongated crystals (shown). The findings indicate that amelogenin may play a critical role in controlling structural growth of enamel crystals, the researchers say.

The Subsurface Counts In Catalysis

Catalysis is generally thought of as a surface phenomenon, yet a new study demonstrates that what happens several atomic layers below the catalyst surface can have a big impact on catalytic reactions. Detre Teschner at the Fritz Haber Institute, in Berlin, and colleagues show that the atoms just beneath the surface of a palladium catalyst help determine if alkyne hydrogenation proceeds selectively to an alkene or proceeds nonselectively all the way to an alkane (Science 2008, 320, 86). The group had previously studied selective partial hydrogenation of alkynes, showing that carbon from 1-pentyne is incorporated into the first three atomic layers of a palladium catalyst and forms a Pd-C phase that affects the hydrogenation activity. Now, with both in situ X-ray photoelectron spectroscopy and gamma activation analysis, the group has monitored the carbon and hydrogen content in palladium during catalysis. They found that when palladium indiscriminately hydrogenates alkynes to alkanes, far fewer carbon atoms are dissolved in the catalyst's subsurface. "This result suggests that not only the surface but also the subsurface region is affected by the chemical potential of the reaction mixture," the authors write.

Manganese Ions Do An Enzyme's Job

To guard against reactive superoxide radicals (O₂???^-), many organisms rely on superoxide dismutases (SODs), which are enzymes that convert superoxide into O₂ and H₂O₂. Some bacteria lack SODs, but they have a high internal concentration of manganese(II) ions, which provide antioxidant protection by a mechanism that is not entirely clear. A team led by Joan Selverstone Valentine at UCLA and Diane E. Cabelli at Brookhaven National Laboratory has shown that, in the presence of phosphate anions, Mn²⁺ catalyzes the destruction of superoxide radicals, but by a mechanism distinct from that of SODs (J. Am. Chem. Soc., DOI: 10.1021/ja710162n). Superoxide is known to rapidly react with Mn²⁺ to form a short-lived MnO₂⁺ adduct. The researchers found that, within a certain range of phosphate concentrations that mimic levels found inside cells, two of the adduct molecules participate in a disproportionation reaction—simultaneous oxidation and reduction—to regenerate Mn²⁺ ions. This process differs from SOD mechanisms, which involve separate reduction and oxidation steps at the enzyme's metal center.

Defluorinating Bioaccumulative PFOS

Perfluorooctane sulfonate (PFOS) has been used as an ingredient in many industrial and consumer products such as surfactants and fire-fighting foams. Some companies stopped manufacturing the persistent and bioaccumulative chemical nearly a decade ago, and little is known about its environmental degradation pathway. Reyes Sierra-Alvarez and colleagues at the University of Arizona, Tucson, have shown for the first time that vitamin B-12, a cobalt-based macrocycle, can catalyze the partial defluorination of PFOS (Environ. Sci. Technol., DOI: 10.1021/es702842q). Although vitamin B-12 might seem like a surprising reagent in this context, the researchers were aware that most known reductive dehalogenase enzymes depend on vitamin B-12 to help cleave halogens. They suggest that microbes that produce vitamin B-12 might be able to break down PFOS in the environment. In lab tests, the researchers monitored PFOS in the presence of vitamin B-12 and a reducing agent, observing fluoride release by monitoring the solutions with an ion-selective electrode and by ¹⁹F NMR spectroscopy. PFOS is a 75:25 mixture of linear and branched isomers, including the branched 6-CF₃-PFOS shown. The team's results indicate that branched PFOS isomers are more prone to degradation than linear isomers, but the scientists have yet to elucidate a mechanism.

Silica And Ice Template Creates Holey Microfibers

With a "brick and mortar" assembly approach, researchers have made porous, hollow carbon/metal oxide microfibers with well-controlled pore structures (J. Am. Chem. Soc., DOI: 10.1021/ja800376t). Such composites of carbon and metal oxide are useful for catalysis, electrochemistry, and electrooptical applications. Although existing techniques produce uniform pore structures in carbon fibers, materials researchers say precise control over the formation of uniform pores in carbon/metal oxide fibers has remained challenging. In the new study, Galen D. Stucky and Qihui Shi at the University of California, Santa Barbara, and colleagues created "bricks" by coating silica spheres first with an oxide of zirconium or of titanium and then with polyacrylonitrile. The spheres were dispersed into an aqueous polyvinyl alcohol (PVA) solution and frozen. With PVA left behind to act as "mortar," the spheres formed fibers when the ice was sublimed away by freeze-drying. The researchers then pyrolyzed the resulting fibers. Dissolving the silica cores with a basic solution finally produced porous, hollow composite fibers (shown). Stucky says this clean, economic, and versatile method is suitable for assembling different core/shell-structured particles into a single fiber, giving the fiber multiple functionalities, among them catalytic or magnetic properties.

GFP Mutant Forms A Red Chromophore

A U.S.-Russian team of scientists has created the first red mutant of the jellyfish green fluorescent protein (GFP), a widely used biological marker (Biochemistry, DOI: 10.1021/bi702130s). Although red fluorescent proteins for cellular labeling are available from other organisms, the ease of expressing GFP in any organism makes a red GFP mutant desirable. The team, led by Vladislav V. Verkhusha of Albert Einstein College of Medicine, in New York City, and Konstantin A. Lukyanov of the Shemiakin-Ovchinnikov Institute of Bioorganic Chemistry, in Moscow, found red GFP mutants after multiple rounds of random mutagenesis of GFP-expressing bacteria and high-throughput screening of the resulting protein variants. The structure of the brightest red fluorescent mutant protein is shown, with the amino acids of the chromophore highlighted. This protein actually emits both red and green light. "This mutant is the first step toward the ideal monomeric red fluorescent protein," Verkhusha says. "We plan to continue our work to make the mutant absolutely red without any green component."

Chemistry Influences Earth's Rotation

The possible existence of a highly conducting mineral layer just above the boundary between Earth's core and mantle has gotten earth scientists a little excited. Such a layer would, by electromagnetic coupling, have a small influence on the exchange of angular momentum between the planet's rotating fluid core and its solid mantle. Theorists say this tiny difference could explain why the length of an Earth day fluctuates by a millisecond or so year-to-year and could contribute ever so slightly to Earth's wobbling axis of rotation. Kei Hirose and Kenji Ohta at Tokyo Institute of Technology and coworkers present a chemical model that provides the first direct conductivity measurements of the proposed mineral (Science 2008, 320, 89). The team prepared samples of perovskite, a common magnesium-iron silicate mineral, and tested its electrical conductivity in a laser-heated diamond anvil cell. At the high temperature and pressure expected at the core-mantle boundary, the scientists observed the mineral undergo a phase transition to form "postperovskite." This transition is accompanied by a shift in the electronic structure of metal atoms, causing the conductivity to jump by two orders of magnitude. It is a postperovskite layer, perhaps 300-km thick, just above the core-mantle boundary that could give rise to Earth's observed rotational fluctuations, the researchers say.

Nanobacteria May Only Be Nanoparticles

Nanobacteria are a putative novel life form first identified in the 1990s and implicated in the origin of life and in a variety of diseases. Now it seems that nanobacteria may simply be calcium carbonate nanoparticles, according to research by Jan Martel of Chang Gung University, in Taiwan, and John Ding-E Young of Rockefeller University (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0711744105). Martel and Young used published methods to "culture" nanobacteria from human blood and compared the resulting nanospheres to calcium compounds likely to precipitate in biological fluids. They found that calcium carbonate nanoparticles (shown) prepared in cell culture media from (NH₄)₂CO₃ and CaCl₂ closely resemble previously reported isolated nanobacteria. Specifically, the CaCO₃ nanoparticles appear to have an amorphous layer at the surface that looks like a membrane, and two adjacent particles seem to be undergoing cell division. The researchers also found that purported nanobacteria-specific antibodies, which are commercially available, may bind to serum albumin and form an insoluble matrix with CaCO₃. Before now, CaCO₃ particles had only been identified in the inner ear, so finding them in other tissues such as blood may have implications for disease pathology.

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