Research Advances: Floating Metals; Oxalate Fights Phytotoxin; A

Chemical Education Today

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Research Advances by Angela G. King

Floating Metals Metals that float on water? A relatively new group of materials—metal foams—do exactly that, defying the conventional notion of metals as heavy and solid. Metal foams are mere wisps, consisting of more than 80% air. Having very high surface areas and continuous open-cell porosity, they are finding applications in catalysis, surface enhanced Raman scattering (SERS), heat transfer, insulation, and other areas. Bryce C. Tappan and colleagues at the Los Alamos National Laboratory report the development of a new and simpler technique for making metal foams. Technology for making metal foams has been limited mainly to aluminum and a handful of other metals in the past. The old technology produces either relatively heavy, dense foams or low-density foams with very large cell sizes. The new Los Alamos technique produces “unprecedented ultralow-density” foams with very small cell sizes from transition metals that could not be foamed in the past. The research team caught a glimpse of the possibilities while studying the decomposition behavior of high-nitrogen

N Figure 1. Ammonium tris(bi(tetrazolato)amine)ferrate(III) complex (Fe-BTA). Structure provided by A. King.

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transition metal complexes. In those experiments, nanostructured monolithic foams were formed that displayed extraordinarily low densities (as low at 0.011g/cm3) and surface areas as high as 270 m2/g! Ammonium tris(bi(tetrazolato)amine)ferrate(III) complex (Fe-BTA) combusts in different styles. As a loose powder in an atmosphere of air, it will ignite and burn vigorously, giving of orange sparks. However, combusting cylindrical pellets of Fe-BTA in a nitrogen atmosphere at various pressures results in formation of the metal foam monolith. Researchers suggest that under N2, the high-nitrogen ligand acts as a blowing agent while liberating decomposition gases at the same time as the metal center is reduced. As the metal centers are liberated from the ligands, they must rapidly find new binding sites to fulfill bonding requirements. BTA ligands can be used with the same approach to yield ultra-low density foams of copper, cobalt, and silver.

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Energy dispersive spectra (EDS) showed that in addition to the metals, the foams contained varying amounts of carbon and nitrogen. Heat treatment of 800 ⬚C under argon or 500 ⬚C under H2 was sufficient to remove most impurities. Some of the metal foams are lighter than Styrofoam. “This new technique shows promise for being a flexible, general approach to the formation of a wide range of new nanoporous metals not currently accessible by state-of-the-art nanoscience”, researchers stated. The research team is now working to optimize Ag foam production and expand the variety of metals employed.

More Information 1. Tappan, B. C.; Huynh, M. H.; Hiskey, M. A.; Chavez, D. E.; Luther, E. P.; Mang, J. T.; Son; S. F. UltralowDensity Nanostructured Metal Foams: Combustion Synthesis, Morphology, and Composition. J. Am. Chem. Soc. 2006, 128, 6589–6594.

Figure 3. Scanning electron micrographs, 1 ␮m scale, of (a) low-pressure iron foam showing a pore structure of approximately 1␮m; (b) high-pressure iron foam showing a pore structure of roughly 20–100 nm. Reprinted with permission from J. Am. Chem. Soc. 2006, 128, 6589–6594. Copyright 2006 American Chemical Society.

Figure 2. Photograph of iron foam next to unburned pellet of the Fe-BTA complex. Reprinted with permission from J. Am. Chem. Soc. 2006, 128, 6589–6594. Copyright 2006 American Chemical Society.

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Reports from Other Journals 2. This Journal has published details on the science of foams. See Hansen, Lee D.; McCarlie, V. Wallace. From Foam Rubber to Volcanoes: The Physical Chemistry of Foam Formation. J. Chem. Educ. 2004, 81, 1581–1584. 3. Technology White Papers on nanoporous materials are provided online at http://www.clubofamsterdam.com/contentarticles/ 01%20Nanotechnology/Nanoporous%20Materials.pdf#search =’nanoporous%20materials%20metallic’ (accessed Sep 2006).

Oxalate Fights Phytotoxin Centaurea maculosa Lam., more commonly known as spotted knapweed, is an invasive destructive weed in North America that can almost extinguish the growth of native grassland species. C. maculosa secretes the flavonoid phytotoxin (±)-catechin from its roots and earlier evidence suggests that over time plants can evolve traits that help resist catechin’s harmful effects. This may in part explain why grasses from the native habitat of C. maculosa, Eurasia, display a greater tolerance to catechin than closely related grass species native to North America. While this tolerance may evolve over time, some native North American plant species, including Gaillardia grandiflora and Lupinus sericeus Pursh, are more resistant to C. maculosa. The harmful effects of catechin follow the generation of reactive organic species, which increase cytoplasmic Ca2⫹ and lower the pH of the cytoplasm, ultimately resulting in cell death. The addition of an antioxidant, such as ascorbic acid, stops the cascade. Plants resistant to heavy metals have been shown to employ organic acids, such as citrate and oxalate, as agents to chelate the toxic metals. Now a research team based at Colorado State University has shown that C. maculosa resistant plants employ a similar strategy when exposed to spotted knapweed. Researchers, led by Jorge Vivanco, established that root exudates of G. grandiflora contributed to its catechin-resistance by growing five-day old A. thaliana plants in growth

OH OH HO

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Figure 4. D-catechin, a phytotoxin released from spotted knapweed. Structure provided by A. King.

medium spiked with 200 ␮g/mL (±)-catechin (control group) or growth medium containing 200 ␮g/mL (±)-catechin and G. grandiflora root exudates. Using lactophenol tryphan blue, root mortality was observed, and results demonstrated that plants grown in the medium containing G. grandiflora root extract had a greater survival rate. At that point researchers were certain that some component of the root extract conferred resistance to A. thaliana. To determine whether the active component was an organic acid, seedlings of A. thaliana and G. grandiflora were grown on solid pH indicator medium (90 mg bromcresol purple/L). (±)-Catechin-sensitive A. thaliana plants showed very little medium acidification while (±)-catechin-resistant G. grandiflora secreted acids from roots, with maximum acidification occurring around lateral roots within 48 hours of being placed on the medium. Placing a filter paper saturated with (±)-catechin at the tip of the primary root increased acidification and pH changes in the medium could be visually observed. Researchers used existing procedures to collect root exudates from plant species both resistant and sensitive to (±)-catechin, as well as exudates from the same plants grown with 200 ␮g/mL (±)-catechin in the medium to elicit increased secretion. The researchers identified

Figure 5. (a) Five day-old seedlings of A. thaliana grown in media containing G. grandiflora root exudates and (±)-catechin (top), MS medium (middle), or MS medium amended with (±)-catechin (bottom). After 5 days, the roots of (b) untreated controls and (c) seedlings treated with catechin but grown in Gaillardia root exudates remained viable, but (d) roots of Arabidopsis treated with catechin alone were dead, as indicated by staining with the vitality stain lactophenol tryphan blue. (e) G. grandiflora grown on a pH indicator medium containing bromcresol purple caused dramatic acidification of the medium within 48 h. (f) When root tips were exposed to a filter disk saturated with catechin, the time required for visible acidification of the medium was reduced to 1 h. Reprinted with kind permission of Springer Science and Business Media from Weir, Tiffany L.; Bais, Harsh Pal; Stull, Valerie J.; Callaway, Ragan M.; Thelen, Giles C.; Ridenour, Wendy M.; Bhamidi, Suresh; Stermitz, Frank R.; Vivanco, Jorge M. Oxalate Contributes to the Resistance of Gaillardia grandiflora and Lupinus sericeus to a Phytotoxin Produced by Centaurea maculosa. Planta 2006, 223, 789; Figure 1.

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Figure 6. 1 mM Na3VO4, anion channel blocker, was added to the resistant plant G. grandiflora to inhibit the secretion of oxalate. Plants were then treated with 400 ␮g/mL (±)-catechin, a level typically insufficient to cause mortality. Plants treated with catechin and Na3VO4 were significantly smaller than control plants treated with methanol or plants treated with Na3VO4 or catechin alone. Asterisks indicate means significantly different from the control. Reprinted with kind permission of Springer Science and Business Media from Weir, Tiffany L.; Bais, Harsh Pal; Stull, Valerie J.; Callaway, Ragan M.; Thelen, Giles C.; Ridenour, Wendy M.; Bhamidi, Suresh; Stermitz, Frank R.; Vivanco, Jorge M. Oxalate Contributes to the Resistance of Gaillardia grandiflora and Lupinus sericeus to a Phytotoxin Produced by Centaurea maculosa. Planta 2006, 223, 790; Figure 2c.

the acidic components through a combination of HPLC and mass spectrometry. They found substantial amounts of oxalate in the root secretions of (±)-catechin-resistant G. grandiflora and L. sericius, and the amounts of oxalate in the secretions showed significant increase if the plants were treated during culture with (±)-catechin. Conversely, (±)-catechin-sensitive A. thaliana, P. spicata, and F. idahoensis secreted only small amounts of oxalate, and the levels did not increase when the plants were cultured in (±)-catechin-laced medium. To confirm that oxalate is secreted to protect plants from spotted knapweed’s exudates of (±)-catechin, researchers prepared 10–200 ␮M solutions of oxalate and administered them to A. thaliana alone or simultaneously with (±)-catechin solutions. Results showed that 50–100 ␮M solutions of oxalate protected the susceptible plants from the rhizotoxicity of (±)-catechin without significant ill effects. Lower oxalate concentrations produced plants that were viable but significantly smaller than controls. A. thaliana plants treated with oxalate were protected from catechin-induced damage and showed no significant decreases in biomass when compared to controls. However, plants of the catechin-resistant species G. grandiflora succumbed to (±)-catechin treatment when sodium orthovanadate (Na3VO4) was also administered. Na3VO4 is an anion channel blocker that inhibits the secretion of organic acid. The observed decrease in biomass when Na3VO4 is administered further implicates oxalate as a protective secretion. Researchers also gathered evidence that oxalate protects against (±)-catechin by acting as an antioxidant. Nitroblue tetrazolium is a reducing dye that indicates the presence of O2 radicals. A. thaliana plants that were treated with 100 ␮g/mL (±)-catechin displayed blue coloration in roots and leaves upon staining with 0.1% w/v nitroblue tetrazolium solutions. Untreated plants or plants treated with oxalate showed no staining. Working in the field, scientists have gathered data suggesting that the effects of oxalate secretion may influence ecological interactions between other plants as well. Areas that contain oxalate-secreting plants such as L. sericius are more likely to also contain native grasses that are not (±)-catechinresistant than grassland areas that do not contain plants rewww.JCE.DivCHED.org

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sistant to spotted knapweed. Such work indicates that during invasion by aggressive plants, the ensuing chemical warfare (both attack and counter attack) may involve plant–plant interactions more complex than previously imagined.

More Information 1. Weir, Tiffany L.; Bais, Harsh Pal; Stull, Valerie J.; Callaway, Ragan M.; Thelen, Giles C.; Ridenour, Wendy M.; Bhamidi, Suresh; Stermitz, Frank R.; Vivanco, Jorge M. Oxalate Contributes to the Resistance of Gaillardia grandiflora and Lupinus sericeus to a Phytotoxin Produced by Centaurea maculosa. Planta 2006, 223, 785–795. 2. Perry, Laura G.; Thelen, Giles C.; Ridenour, Wendy M.; Weir, Tiffany L.; Callaway, Ragan M.; Paschke, Mark W.; Vivanco, Jorge M. Dual Role for an Allelochemical: (±)-Catechin from Centaurea maculosa Root Exudates Regulates Conspecific Seedling Establishment. J. Ecol. 2005, 93, 1126–1135. 3. Find more information on Jorge Vivanco’s research at http:// lamar.colostate.edu/~jvivanco/research.htm (accessed Sep 2006). 4. Two courses based on plant chemistry have been described in this Journal. See J. Chem. Educ. 2005, 82, 1787–1790 and J. Chem. Educ. 2002, 79, 976–979. 5. The American Journal of Clinical Nutrition reports that (±)catechin can be found in human plasma after consumption of red wine. See http://www.ajcn.org/cgi/content/full/71/1/103 (accessed Sep 2006). 6. Microscopy images of catchin hydrate from tea plants are available online at http://micro.magnet.fsu.edu/phytochemicals/pages/ catechinhydrate.html (accessed Sep 2006).

A Taste of Success in the Search for an Electronic Tongue Efforts are underway to develop electronic devices that mimic the human senses of taste and smell and have useful applications. An electronic tongue, for instance, could be used in quality control in the beverage industry to ensure that each batch of soda pop or beer is uniform in flavor. Medical applications include analyzing blood and other biological fluids. From fruit flies to humans, characteristic odors and flavors are perceived through chemosensory strategies in which


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Figure 7. (A) Ligands and indicators used to construct sensor array. Absorbance spectra for (B) 1, Cu(OTf)2, CCR and Val; (C) 3, Cu(OTf) 2 , CAS and Val; (D) N,N’ tetramethyl-ethylenediamine, Cu(OTf)2, CAS and Val. (E) Colorimetric output for 1, Cu(OTf)2, CAS and amino acid. Reprinted with permission from J. Am. Chem. Soc. 2006, 128, 5652–5653. Copyright 2006 American Chemical Society.

receptor proteins recognize analytes to provide differential interaction patterns. Trying to signal the same information electronically has posed a challenge to scientists. One version of an electronic tongue uses “taste buds” consisting of chemically coated beads that change color upon encountering flavor molecules that are sweet, sour, bitter, or salty. In a slightly different format, Eric V. Anslyn and colleagues at The University of Texas at Austin have now taken a major step toward giving electronic tongues the fuller range of gustatory prowess of the human tongue. When most left-handed amino acids land on the human tongue, taste buds register a bitter taste. Right-handed amino acids commonly taste sweet. This chiral differentiation is a key component of human taste. In recent work, Anslyn’s group developed the first technique with this human-like taste discrimination by employing indicator displacement assays (IDAs). IDAs were used to create sensors specific for single enantiomers of hydrophobic ␣-amino acids. Previously, ␣amino acids had been used in IDAs because of their affinity for metal ions. Anslyn and his research team investigated the use of a series of Cu(II) complexes with bidentate ligands (1– 3) as receptors with salycylate-derived chromophores (PCV, CCR, CAS) serving as indicators. The indicators were selected because there is a large red shift in their absorbance spectra upon metal coordination. Using different combinations and concentrations of ligands and receptors, researchers created a library of IDAs. Four natural and one unnatural amino acids were analyzed under 21 different conditions, with absorbance spectra measured for each analyte/IDA combination. Principal component analysis was performed to identify diagnostic patterns. 1582


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The results displayed good resolution. It was even possible to separate amino acids differing by only one methylene group. These results indicate great promise in simultaneously distinguishing structurally similar and enantiomeric substrates.

More Information 1. Folmer-Andersen, J. Frantz; Kitamura, Masanori; Anslyn, Eric V. Pattern-Based Discrimination of Enantiomeric and Structurally Similar Amino Acids: An Optical Mimic of the Mammalian Taste Response. J. Am. Chem. Soc. 2006, 128, 5652–5653. 2. Several articles pertaining to the chemistry of taste have appeared previously in this Journal. See Elder, David P. Pharmaceutical Applications of Ion-Exchange Resins. J. Chem. Educ. 2005, 82, 575; Rohrig, Brian. Fizzy Drinks: Stoichiometry You Can Taste. J. Chem. Educ. 2000, 77, 1608A; Richman, Robert M. Detection of Catalysis by Taste. J. Chem. Educ. 1998, 75, 315; Kuangchih, Tseng; Hua-zhong, He. Structural Theories Applied to Taste Chemistry. J. Chem. Educ. 1987, 64, 1003; Guild, Walter, Jr. Theory of Sweet Taste. J. Chem. Educ. 1972, 49, 171; Ferguson, Lloyd N.; Lawrence, Aetius R. The Physicochemical Aspects of the Sense of Taste. J. Chem. Educ. 1958, 35, 436. 3. Anslyn’s research is described at this Web site: http:// www.cm.utexas.edu/directory/eric_anslyn/ (accessed Sep 2006). 4. Ault, Addison. The Monosodium Glutamate Story: The Commercial Production of MSG and Other Amino Acids. J. Chem. Educ. 2004, 81, 347.

Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109; [email protected].


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Research Advances: Floating Metals; Oxalate Fights Phytotoxin; A

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