Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King Seeing Is Detecting
Tracking Dragon’s Blood
Chemosensors are molecules used to detect the presence of some species (substrate). Most chemosensors consist of two parts: a signaling moiety and an efficient binding site for the target substrate. Chemosensors demonstrate a change in their interaction with light upon substrate binding. Chemosensors are currently employed in biology, medical analyses, and environmental tests. For instance, environmental hazards associated with Hg2+ are well documented, and the need to rapidly and easily detect its presence is critical. Researchers at ChungAng University in Seoul, Korea, have recently developed a highly selective chemosensor for this toxin to address this need. Moon and colleagues coupled the molecular framework of an 8-hydroxyquinoline moiety, an ionophore that binds metal ions, with the longer wavelength characteristics and high luminescence intensity of boron-dipyrro-methene (BDP moiety, a great signaling group). The result is a chromo- and fluorogenic ionophore with a sensitive fluorescent signaling behavior towards Hg2+ ions. The chemosensor 1 forms a complex with Hg2+ in a stoichiometric ratio of 1:1. An aqueous dioxane solution of the new derivative, an amber-colored solution, changes to a red solution upon the addition of Hg2+ ions. This easily observable change affords rapid “naked-eye detection” of dangerous mercury ions in chemical and biological samples while still in the field. However, before it can be used for biological samples, investigators must ensure that other metal ions present in biological samples—such as Na+, K+, and Ca2+—do not interfere with the Hg2+ detection. Exposing iononophore 1 to Hg2+ along with samples of the above ions in their physiological concentrations showed that the presence of these biologically important metal ions did not interfere with the detection of mercury(II) ions.
For centuries the name “Dragon’s blood” has been used to describe red resins from plants from a variety of species and continents that have been used since ancient times in medicine and art. Dracaena cinnabari, the dragon tree, is thought to be the source of the resin in Roman times. The name derives from a story recorded by Pliny (61–113 A.D.) in which the blood of an elephant and that of a dragonlike basilisk (familiar to Harry Potter fans) are mixed during a struggle. Over time, alternative sources of the red resins emerged, including other Dracaena species and plants from the Croton and Daemonorops genera. Researchers have recently applied Raman spectroscopy to the analysis of well documented dragon’s blood resins from botanical collections to determine the plant source of the resins, detect fake dragon’s blood resins, and investigate the effects of aging on dracorubin and dracorhodin, the main components of Dracaena dragon’s blood resins. Samples originating from Dracaena species can be identified by having the strongest O O absorbance at 1605 cm ᎑1 , a shoulder around 1560 cm ᎑1, and a band at 1170 cm᎑1. It is O O also interesting to note there was no observable change in spectra from Dracaena O samples from 19th and 20th dracorubin centuries. This indicates that spectral deterioration is a function of processing and not O O resin age. Daemonorops resin samples have an intense absorbance at 1600 cm᎑1, doublets at 1510–1540 cm ᎑1 and O 1420–1450 cm ᎑1 , and a less dracorhodin intense band at 1001 cm ᎑1. Additional samples of dragon’s blood from a Croton source must be studied before characteristics are identified. The ability to determine the plant source of dragon’s blood samples is of interest to art restorers, who may need to determine the identity of pigments in order to find accurate and suitable replacements when restoring artifacts and paintings. Additionally, tracing the spread of plants through documented use of their resins is a tool used by archaeologists tracing trade routes. Raman spectroscopy will now let them determine the plant source of dragon’s blood resins used in art and medicine in ancient civilizations.
N
N
OH N
BF3
Structure 1: The structure of the newly designed chemosensor.
More Information 1. Moon, S.; Cha, N.; Kim, Y.; Chang, S.-K. New Hg2-Selective Chromo- and Fluoroionophore Based Upon 8-Hydroxyquinoline. J. Org. Chem. 2004, 69, 181–183. 2. Young, J. A. Mercury(II) Nitrate Monohydrate. J. Chem. Educ. 2003, 80, 1373. 3. Czarnik, A. W. Chemosensors and Chemical Privacy. http:// www.aaas.org/spp/yearbook/2002/ch29.pdf (accessed Apr 2004).
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More Information 1. Edwards, H.; de Oliveira, L.; Prendergast, H. Raman Spectroscopic Analysis of Dragon’s Blood Resins—Basis for Distinguish-
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Chemical Education Today
Reports from Other Journals/Research Advances ing between Dracaena (Convallariaceae), Daemonorops (Palmae) and Croton (Euphorbiaceae). Analyst, 2004, 129, 134–138. 2. Pearson, J.; Prendergast, H. Daemonorops, Dracaena and Other Dragon’s Blood. Economic Botany, 2001, 55, 474–477. 3. McClain, B.; Clark, S.; Gabriel, R.; Ben-Amotz, D. Educational Applications of Infrared and Raman Spectroscopy: A Comparison of Experiment and Theory. J. Chem. Educ. 2000, 77, 654–660.
NH3+ HO HO
O +
H3N O
HO
O
NH3+ OH
+H3N HO HO
H2N O
O
OH
O NH3+
neomycin B
Structure of neomycin B (left); lowest-energy conformation of neomycin B docked to LF (right) [reprinted with permission from J. Am. Chem. Soc. 2004, 126, 4774–4775. Copyright © 2004 American Chemical Society].
Shutting Down Anthrax The toxic effects of an anthrax (Bacillus anthracis) infection are caused by three proteins: protective antigen (PA), edema factor (EF), and lethal factor (LF). Any of these proteins alone is harmless, but pathogenesis results when they act cooperatively. PA forms a self-assembled heptamer that allows the other two proteins to gain entry into mammalian cells. Once within the cell, EF causes fluid accumulation through Ca2+/calmodulin-dependent adenylate cyclase activity. LF is a Zn-dependent endopeptidase and cleaves the N-terminus of the D-domain of MAPKK (mitogenactivated protein kinase kinases), effectively shutting down the host macrophage’s signaling pathway. Bacillus anthracis strains that do not produce LF are less virulent, while a lack of EF does not affect pathogenesis. Therefore to stop the pathogenic effects of anthrax, LF inhibitors are needed. Researchers in California screened a library of ~3,000 compounds in a fluorescent assay for LF inhibition. Neomycin B showed the greatest activity against LF in this assay, with a Ki value of 7.0 nM. Neomycin B belongs to the aminoglycoside family of antibiotics, and in a follow-up assay other aminoglycosides were screened and observed to be competitive inhibitors of LF with a pH-dependent binding. Additionally, as the assay was conducted in higher concentrations of salt, the affinity for LF decreased. Computational docking experiments demonstrated that neomycin B has the potential to bind to the LF active site. In the lowest-energy docked conformation, neomycin is bound to LF in the vicinity of the catalytic Zn(II) and is surrounded by negatively charged residues. These negatively charged residues bind the antibiotic through electrostatic interactions and hydrogen bonds. Aminoglycosides are known as RNA binders, and in fact, exert their antibiotic properties by binding to bacterial RNA, which results in errors in genetic code translation. LF is the first protease known to be inhibited by an aminoglycoside, and researchers were surprised with the results. Aminoglycosides were also assayed as inhibitors of furin, a serine protease with negative residues surrounding its active site, but no inhibition was recorded. Thus neomycin B works against Bacillus anthracis through both DNA binding and selective inhibition of LF. Current work is aimed 1088
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at the development of novel synthetic aminoglycosides with improved antibiotic activity to increase their effectiveness against LF.
More Information 1. Lee, L.; Bower, K.; Liang, F.-S.; Shi, J.; Wu, D.; Sucheck, S.; Vogt, P.; Wong, C.-H. Inhibition of the Proteolytic Activity of Anthrax Lethal Factor by Aminoglycosides. J. Am. Chem. Soc. 2004, 126, 4774–4775. 2. A description of enzyme inhibition and binding is described in Burlingham, B.; Widlanski, T. An Intuitive Look at the Relationship of Ki and IC50: A More General Use for the Dixon Plot. J. Chem. Educ. 2003, 80, 214–218. 3. http://www.biotechjournal.com/Pathways/anthrax.htm (accessed May 2004). 4. Scanning electron micrograph images of Bacillus anthracis can be found at http://www.srs.dl.ac.uk/Annual_Reports/AnRep01_02/ anthrax.htm (accessed May 2004).
A Renewable Source of Hydrogen for Fuel Cells For hydrogen-powered cars to become a reality, the hydrogen either needs to be stored in a lightweight container in the vehicle or produced in the car as needed. Most hydrogen produced today is the result of heating a mixture of natural gas and water, and this can be done with small, mobile units that could be situated in a car. The drawbacks of this method are that natural gas, a fossil fuel, is not a renewable source of energy, and the side product of this process is carbon dioxide, which contributes to global warming when released into the atmosphere. One way to ensure that hydrogen-powered cars are a “green” method of transportation is to produce the hydrogen they use from a source powered by the sun. Ethanol, produced by the fermentation of plant carbohydrates, would qualify as a fuel ultimately derived from the sun, since photosynthesis produces the original biomass. Ethanol, a required additive in gasoline in the U.S., produces 1.34 times more energy when used as a fuel than is needed in its production (planting, harvesting, processing…), and is available at an affordable $1/gallon. Since fermentation is a highly ef-
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ficient slightly endothermic reaction, and the oxidation of C2H5OH to hydrogen is slightly exothermic, calculations reveal that more than 20% of the plant sugar’s energy is lost during the conversion process. But previous techniques for the conversion of ethanol to hydrogen have required an external heat source, the bulk of which makes the process unsuitable for use in hydrogen-powered cars. Partial oxidation of ethanol to carbon monoxide and hydrogen (eq 1) appears to be the most straightforward way of generating hydrogen from ethanol, but this reaction is endothermic and thus will not generate the high temperature needed for rapid reaction unless there is also some complete oxidation of the ethanol to carbon dioxide (eq 2). This exothermic process produces the needed heat but uses the fuel without producing any hydrogen. C2H5OH + 1/2O2 → 2CO + 3H2
(1)
C2H5OH + 3O2 → 2CO2 + 3H2O
(2)
The steam reforming reaction (eq 3) is utilized in most processes that convert ethanol to hydrogen. This reaction is highly endothermic and requires temperatures of 800 °C in order to achieve fast reaction times. C2H5OH + H2O → 2CO + 4H2
(3)
Under the high temperatures required for steam reforming, the water-gas shift (WGS) reaction (eq 4) reaches equilibrium. It is an additional source of hydrogen, but only produces sufficient quantities of hydrogen under low temperatures and if copious amounts of water are present. CO + H2O → CO2 + H2
(4)
The best scenario for producing hydrogen gas is one where partial oxidation with steam reforming (eqs 1 and 3) and the water–gas shift reaction (eq 4) are combined to utilize both the heat generated by completed oxidation and the additional hydrogen produced by the WGS. These reactions can be combined using Hess’s Law to give the exothermic reaction C2H5OH + 2H2O + 1/2O2 → 2CO2 + 5H2
(5)
Now a team of chemical engineers from the University of Minnesota and The University of Patras in Greece have developed a process that will generate hydrogen (according to eq 5) from the oxidation of ethanol simply and efficiently, does not require all the water to be removed after fermentation, and produces its own heat. To maximize hydrogen production and minimize undesired carbon monoxide generation, this exothermic process is carried out in the presence of water and air. In this process H2 and CO selectivities are defined as the fraction of carbon and hydrogen atoms in ethanol that go into CO and H2, respectively. Note that if the hydrogen atoms in water were also converted to hydrogen gas, it could theoretically give the overall process a H2 selectivity >100%. The team obtained the best results using a catalyst of a noble metal (Rh) with a cerium oxide additive deposited from salt solutions onto alumina foams or spheres. www.JCE.DivCHED.org
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Design of the reactor that allows the autothermal generation of hydrogen from ethanol. Reprinted with permission from Science 2004, 303, 993–997. Copyright © 2004 AAAS.
C2H5OH H2 O fuel injector air
25 °C
The process works by injecting an ethanol or ethanol–water mixture into heater a tube with an electronically controlled automotive fuel-injector (shown at right). When the small droplets are 140 °C sprayed in, they land on the walls of the 1 cm catalyst tube, which has been heated to 700 °C insulation ~140 °C. The droplets are rapidly vaporized and mixed with air, and in less than 50 ms, products exit the catalyst and can be analyzed with gas chromatography. The exothermic reaction heats the catalyst to more than 700 °C and continues the autothermal process. The ethanol conversion remained at >95%, and the H2 selectivity peaked at 80% (without the addition of water). Additional hydrogen and less carbon monoxide can be generated if the WGS reaction is allowed to go to completion (eq 4), but this requires a lower temperature. This problem was addressed by devising a two-stage reactor, where a second catalyst (Pt-ceria) was added after the Rhceria partial oxidation catalyst. This increased the H 2 selectivity to 130%. The output stream is approximately 50% hydrogen and water free. This is suitable for direct use in a solid oxide fuel cell, but due to partial oxidation (eq 1), the process produces too much carbon monoxide for use directly in the PEM fuel cells needed to power cars. With extra processing, such as preferential CO oxidation, this obstacle may be overcome.
More Information 1. Deluga, G.; Salge, J.; Schmidt, L.; Verykios, X. Renewable Hydrogen from Ethanol by Autothermal Reforming. Science, 2004, 303, 993–997. 2. Cho, A. Hydrogen From Ethanol Goes Portable. Science, 2004, 303, 942–943; available online at http://www.sciencemag.org/ cgi/content/full/303/5660/942b (accessed May 2004). 3. A general article describing fuel cells and the work of Sossina Haile can be found at http://addis.caltech.edu/publications/ haile_engenious.pdf (accessed May 2004). 4. The Hydrogen and Fuel Cell Letter covers alternative energy news. Monthly feature stories are available free of charge at http://www.hfcletter.com/ (accessed May 2004). 5. The U.S. Department of Energy oversees the Hydrogen, Fuel Cells and Infrastructure Technologies Program. An animated tutorial on fuel cells for students and teachers is available at http:// www.eere.energy.gov/hydrogenandfuelcells/ (accessed May 2004).
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P. O. Box 7486, Winston-Salem, NC 27109;
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