Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
Nanoscale Molecular Tweezers The ability to control changes in molecular shape has potential applications in biological processes and development of dynamic devices. For instance, photo-isomerization of retinal allows it to participate in vision-related neuron signaling. Likewise, conformational changes induced by ion gradients and concentration help control reactivity and substrate affinity. Nanomechanical devices potentially offer greater diversity, smaller size, and better control than biomolecules. Ion binding or protonation may cause nanomechanical motion, which results in different physico-chemical properties. A team of researchers led by Jean-Marie Lehn at the Laboratoire de Chimie Supramoléculaire at the Université Louis Pasteur have recently designed and developed two types of receptors to switch between U and W shapes upon coordination of soft metal cations, acting in the manner of mechanical tweezers. Both receptor types are disubstituted heterocyclic triads that allow facile monitoring for bound substrate since the charge-transfer complexes are colored. The pyridine–pyrimidine–pyridine (py–pym–py) sequence displays a natural U shape, due to preference of related heterobiaryls for transoid
conformations, as confirmed by X-ray crystallography. When copper(I) is added to the py–pym–py receptor, it adopts a W conformation, monitored by changes in 1H NMR chemical shift. Even more exciting, py–pym–py receptors bound to substrate molecules release the substrate upon addition of cuprous ions to the solution, as the receptor moves from a U conformation to a W shape. Similar results were obtained with the second class of molecular tweezers. Replacing the pym unit of the earlier receptor with a py group leads to a change in the native conformation. These py–py–py receptors adopt a W conformation that switches to a U shape upon addition of zinc(II) ions. Conformational changes were confirmed by X-ray crystallography and 1H NMR. Once py–py–py adopts the U shape, it can then bind substrates between the branches of the “tweezer”. The confirmed molecular structure showed the expected U shape, with the zinc cation complexed in a distorted octahedral geometry. W(CONHAnt)2, a py–py–py type receptor, demonstrated an ability to be involved in – stacking interactions. Following ZnII complexation, the electron-deprived central pyridine is inserted between two electron-rich anthracene substituents on another receptor molecule. This stacking pattern leads to formation of a one-dimensional polymer.
Representation of metal ion binding induced molecular shape switching in the py–py–py sequence (top) and in the py–pym–py sequence (bottom).
(a) Structures of designed receptors and representations of structural changes induced by ion binding. (b) Substrates studied.
Crystal structure of the U-shaped receptor [ZnII 傺W(CONHAnt)2]⭈2CF3SO3: CPK (left) and ball-andstick (right) representations of face and side views, respectively, of the isolated receptor.
Reprinted with permission from J. Am. Chem. Soc. 2004, 126, 6637–6647. Copyright 2004 American Chemical Society.
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Chemical Education Today
Reports from Other Journals photo: Peter Shang-Tzen Chang
The py–pym–py sequence is an example of a multistate supramolecular switching device. Continuing work in France by Nobel prize winner Jean-Marie Lehn’s team will maximize the potential of such complex nanomechanical devices.
More Information 1. Petitjean, A.; Khoury, R.; Kyritsakas, N.; Lehn, J.-M. Dynamic Devices. Shape Switching and Substrate Binding in Ion-Controlled Nanomechanical Molecular Tweezers. J. Am. Chem. Soc. 2004, 126, 6637–6647. 2. More information on Nobel laureate Jean-Marie Lehn, including an autobiography, is available at http://www.almaz.com/ nobel/chemistry/1987b.html (accessed Sep 2004). 3. Examples of other research areas of supramolecular chemistry are available at http://pubs.acs.org/cen/coverstory/8050/ 8050chemhighlights11.html (accessed Sep 2004). 4. This Journal offers many resources to instructors desiring to incorporate supramolecular chemistry into their courses. See González-Gaitano, G.; Tardajos, G. Chemical Equilibrium in Supramolecular Systems as Studied by NMR Spectrometry. J. Chem. Educ. 2004, 81, 270–274; Hernández-Benito, J.; García-Santos, M. P.; O’Brien, E.; Calle, E.; Casado, J. A Practical Integrated Approach to Supramolecular Chemistry III. Thermodynamics of Inclusion Phenomena. J. Chem. Educ. 2004, 81, 540–544; or Varnek, A.; Dietrich, B.; Wipff, G.; Lehn, J.-M.; Boldyreva, E. Supramolecular Chemistry: Computer-Assisted Instruction in Undergraduate and Graduate Chemistry Courses. J. Chem. Educ. 2000, 77, 222–227.
Cinnamon as Pesticide? Great effort is used to control mosquito larvae since mosquitos are the most important means of transmission of malaria, yellow fever, dengue fever, and dengue hemorrhagic fever. Larvae control efforts include the constant application of organophosphates and insect growth regulators, such as methoprene. While these agents are effective, their continued use raises questions concerning environmental impact and the development of resistance. For these reasons, recent research has focused on the need to develop new strategies for mosquito control and researchers are looking at essential oils from plants indigenous to mosquito territory. Approximately 250 species from the genus Cinnamomum, mostly trees, are found in Asia and Australia. One particular species, Cinnamomum cassia, is an important commerce item: oil from its bark is used in food and beverages. The oil has also demonstrated antimicrobial activity. Its main components are eugenol and cinnamaldehyde, a safe flavoring agent. Cinnamomum osmophloeum kaneh. grows in Taiwanese forests found at 400–1500 m above sea level. C. osmophloeum is classified into nine types based on the chemical composition of essential oil collected from the plant’s leaves. Researchers have previously shown that essential oil from the cinnamaldehyde type serves as a wonderful inhibitor of termites, mites, mildew, bacteria, and fungi. Now researchers have extended that work and investigated the mosquito larvicidal properties of essential oils from C. osmophloeum. Shang-Tzen Chang and colleagues in Taiwan collected leaves from C. osmophloeum and isolated the essential oils by hydrodistillation. Analysis by GC–MS allowed the identifi1692
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Leaves from C. osmophloeum, from which essential oils demonstrated larvicidal activity against A. aegypti, the yellow fever mosquito.
LC50 and LC90 Values of Compounds in C. osmophloeum Essential Oil against Yellow Fever Mosquito Larvae, (g/mL)
Compounds
24 h
cinnamaldehyde cinnamyl acetate benzaldehyde camphor benzenepropanal eugenol bornyl acetate -caryophyllene caryophyllene oxide anethole linalool
48 h
LC50
LC90
LC50
LC90
29 33 >50 >50 >50 33 >50 >50 >50 42 >50
48 >50 >50 >50 >50 >50 >50 >50 >50 >50 >50
21 26 33 >50 >50 13 48 34 >50 16 >50
42 48 >50 >50 >50 37 >50 >50 >50 38 >50
Table reprinted with permission from J. Agric. Food Chem. 2004, 53, 4395. Copyright 2004 American Chemical Society.
cation and quantification of major components. To test the larvicidal activity of the essential oils and pure components, mosquito larvae were exposed to the chemical or mixture dissolved in DMSO, and mortality was recorded 24 and 48 hours after administration. The cinnamaldehyde type and the cinnamaldehyde/cinnamyl acetate types were the most effective in this assay, resulting in 100% mortality after 24 hours at a concentration of 100 g/mL. Further investigation tested the activity of pure components identified in the active essential oils using the same method. Cinnamaldehyde, cinnamyl acetate, eugenol, and anethole showed the strongest activity, with LC50 values of 29, 33, 33, and 42 g/mL, respectively. While all four compounds demonstrated great effectiveness against the mosquito larvae, the latter three are present in minute amounts, and clearly the larvicidal activity of the essential oils is due to cinnamaldehyde. Investigators then looked at structure–activity relationships by determining the larvicidal capabilities of compounds related to cinnamaldehyde (cinnamic acid, cinnamyl alcohol,
Vol. 81 No. 12 December 2004
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Reports from Other Journals cinnamyl acetate). While all displayed activity, cinnamaldehyde was the most effective. Researchers then conducted the same assay on a series of aldehydes to determine whether the activity was due to the presence of an aldehyde moiety: 4-hydroxybenzaldehyde, benzenepropanal, and benzaldehyde all had LC50 values of 50 g/mL. Clearly, the scientists have demonstrated that cinnamaldehyde is a natural and effective larvacide against mosquitos.
More Information 1. Cheng, S.-S.; Liu, J.-Y.; Tsai, K.-H.; Chang, S.-T. Chemical Composition and Mosquito Larvicidal Activity of Essential Oils from Leaves of Different Cinnamomum osmophloeum Provenances. J. Agric. Food Chem. 2004, 52, 4395–4400. 2. An undergraduate lab where students can isolate cinnamaldehyde is available. Taber, D.; Weiss, A. Cinnamaldehyde by Steam Distillation of Cinnamon. J. Chem. Educ. 1998, 75, 633.
Recently Identified Dietary Sources of Antioxidants Artichokes and beans may not be at the top of your list of favorite foods, but when it comes to antioxidants, these veggies earn a coveted place. According to a new USDA study, which researchers say is the largest, most comprehensive analysis to date of the antioxidant content of commonly consumed foods, they are among a variety of foods found to contain surprisingly high levels of antioxidants, which are thought to fight cancer, heart disease, and Alzheimer’s disease. In addition to confirming the well-publicized high antioxidant ranking of such foods as cranberries and blueberries, researchers from the USDA found that russet potatoes, pecans, and even cinnamon are all excellent, although lesser-known, sources of antioxidants. “The bottom line is the same: eat more fruits and veggies,” says Ronald L. Prior, a chemist and nutritionist with the USDA’s Arkansas Children’s Nutrition Center in Little Rock, AR, and lead author of the study. “This study confirms that those foods are full of benefits, particularly those with higher levels of antioxidants. Nuts and spices are also good sources.” The new study is more complete and accurate than previous USDA antioxidant data and includes more foods than the previous study, the researchers say. They analyzed antioxidant levels in over 100 different foods, utilizing the oxygen radical absorbance capacity (ORAC) assay with fluorescein as a fluorescent probe and AAPH as a peroxyl radical generator. The study also employed randomly methylated cyclodextrin as a solubility enhancer, which allowed the measurement of antioxidant capacity of both hydrophobic and hydrophilic components of a sample. The USDA team also measured the total phenolic content of food samples,
since phenolic compounds are credited with a major portion of antioxidant capacity in many plants. Among the fruits, vegetables, and nuts analyzed, each food was measured for antioxidant concentration as well as antioxidant capacity per serving size. Cranberries, blueberries, and blackberries ranked highest among the fruits studied. Beans, artichokes, and russet potatoes were tops among the vegetables. Pecans and walnuts ranked highest in the nut category. Although spices are generally consumed in small amounts, many are high in antioxidants. On the basis of antioxidant concentration, ground cloves, ground cinnamon and oregano were the highest among the spices studied. Prior says that the data should prove useful for those seeking to include more antioxidants in their diet. But he cautions that the total antioxidant capacity of the foods does not necessarily reflect their potential health benefit, which depends on how they are absorbed and utilized in the body. Researchers are still trying to better understand this process, he adds. Most study samples were obtained directly from U.S. markets. Factors such as drought, pests, genetics, and processing that may affect antioxidant capacity were not evaluated, since the purpose of the study was to provide data on foods that are being consumed in the U.S. The results from the study will be used to establish an antioxidant database that will be available on the USDA Nutrient’s Data Laboratory Web site at http://www.nal.usda.gov/fnic/foodcomp (accessed Sep 2004). Currently, there are no government guidelines for consumers on how many and what kind of antioxidants to consume in their daily diet. A major barrier to providing such guidelines is a lack of consensus among nutrition researchers on uniform antioxidant measurements. For now, USDA officials continue to encourage consumers to eat a variety of fruits and vegetables for better health.
More Information 1. Wu, X.; Beecher, G.; Holden, J.; Haytowitz, D.; Gebhardt, S.; Prior, R.; Chang, S.-T. Lipophilic and Hydrophilic Antioxidant Capacities of Common Foods in the United States. J. Agric. Food Chem. 2004, 52, 4026–4037. 2. More information on antioxidants in food is available: Donnelly, T. The Origins of the Use of Antioxidants in Foods. J. Chem. Educ. 1996, 73, 158–161; Beaver, B. Motivating Students in Sophomore Organic Chemistry by Examining Nature’s Way— Why Are Vitamins E and C Such Good Antioxidants? J. Chem. Educ. 1999, 76, 1108–1112.
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P. O. Box 7486, Winston-Salem, NC 27109;
[email protected].
Structures for a number of the molecules discussed in the section on Nanoscale Molecular Tweezers are available in fully manipulable Chime format as JCE Featured Molecules in JCE Online (see page 1818).
Featured Molecules
an interactive modeling feature, Only@JCE Online http://www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules
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