Chemical Education Today
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Research Advances by Angela G. King
Nitric Oxide Synthase Reduces Cr(VI) Chromium’s oxidation states range from 2⫺ to 6⫹, and as the oxidation numbers change so do biological effects. Trace amounts of Cr3⫹ are needed to metabolize glucose and deficiency in Cr3⫹ is associated with health problems. Conversely, Cr(VI) is toxic and associated with cancer and other health problems. Cr(VI) is more readily absorbed than Cr(III) and accumulates in tissue, where it is reduced to Cr(V)/ Cr(IV) and Cr(III). Less than 8 h after ingestion, humans secrete 60% of absorbed Cr(VI) dose through the kidneys. Now researchers have examined the reduction of Cr(VI) by nitric oxide synthase (NOS), an event which could contribute to cytotoxicity. This is of particular importance in cells with high levels of NOS, such as endothelial and brain cells. Scientists used electron paramagnetic resonance (EPR) and absorbance spectroscopy (max = 372 nm) to follow the reduction of Cr(VI) by neuronal NOS. EPR indicated that Cr(V) was generated by the reaction, while absorbance spectroscopy did not indicate the presence of Cr(V), possibly due to low concentration. EPR data also showed that yields of Cr(V) correlate with the rate of Cr(VI) reduction. UV-vis spectroscopy suggested that the Cr(VI) ions are reduced at the flavo-containing carboxy–reductase domain of the enzyme. Further work demonstrated that the metal could be reduced with just NOS’s reductase domain (Rd-NOS) instead of the intact enzyme, although the reaction rate was faster when the natural enzyme was employed. The reduction was also carried out using CPR-NOS, a chimera protein of NADPH-cytochrome P450 reductase and the 33 amino acids of the “tail” C-terminal of nNOS, which gave rates similar to NOS. Additional support for a mechanism in which Cr(VI) is reduced by the flavin domain of NOS came from the fact that the reaction was inhibited by diphenylene iodonium (DPI), a flavoprotein inhibitor (Figure 2, in-
Figure 2. Quantification of Cr(V) formation from the reduction of Cr(VI) by NOS and related proteins calculated from EPR spectra obtained in incubations of Cr(VI) with (䉬), NOS (䉫) reductase domain of NOS, and (䊏) for CPR-NOS in Hepes buffer pH 7.4 at room temperature. Insets: Cr(V) spectrum detected in respective incubations of Cr(VI). Reprinted with permission from Chem. Res. in Toxicol. 2005, 18, 834–843. Copyright 2005 American Chemical Society.
set) and the fact that metal chelators, specifically desferal and diethylene tetraaminopentaacetic acid (DPTA), do not inhibit the reduction of Cr(VI) by NOS. Additionally, the research team found through free radical trapping studies that Cr(VI) stimulates the formation of superoxide from tetrahydrobiopterin-saturated NOS. Further studies measuring oxygen consumption showed that the resulting superoxide reacts with Cr(V) to produce Cr(IV) according to eq 1. Cr(V) + O2·⫺ → Cr(IV) + O2 (1)
Figure 1. UV-vis measurement of Cr(VI) consumption by full-length NOS, reductase domain of NOS, and CPR-nNOS tail chimeric protein. (A) Reactions initiated by addition of NOS incubation mixtures of Cr(VI), NADPH in Hepes buffer, pH 7.4 at room temperature. Inset: Same conditions except reaction initiated by CPR-NOS. (B) Same as A except Rd-NOS was used. Inset: inhibition of NADPH and Cr(VI) consumption by DPI. (C) Cr(VI) consumption was calculated from 372-nm absorbance over time for (䉬) full-length NOS, (䉫) reductase domain of NOS, and (䊏) for CPR-NOS. Reprinted with permission from Chem. Res. in Toxicol. 2005, 18, 834–843. Copyright 2005 American Chemical Society.
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Reports from Other Journals Now that scientists have a grasp on the reduction of Cr(VI) by NOS and the mechanism by which this reduction occurs they can fully investigate the role this reaction plays in biological systems.
More Information 1. Porter, Ryan; Jachymova, Marie; Martasek, Pavel; Kalyanaraman, B.; Vasquez-Vivar, Jeannette. Reductive Activation of Cr(VI) by Nitric Oxide Synthase. Chem. Res. in Toxicol. 2005, 18, 834–843. 2. More information on the Free Radical Research Center at the Medical College of Wisconsin, where part of this research was conducted, can be found at http://www.mcw.edu/display/router.asp? docid=1401 (accessed Sep 2005). 3. Due to the toxicity of chromium, some schools have removed chromium compounds from teaching labs. See Wilcox, Mary F.; Koch, Judith G. A Freshman Laboratory Program without Chromium. J. Chem. Educ. 1993, 70, 488.
The Chemistry of Popcorn: It’s All about “Pop-ability” If you took a survey of life’s small annoyances, surely those unpopped kernels at the bottom of the popcorn bag would rank high on the list. But perhaps not for long. Besides being a nuisance, unpopped kernels, also called “old maids”, can break teeth, destroy fillings, and cause choking. Manufacturers have tried to reduce the number of unpopped kernels through trial-and-error breeding of the better performing corn kernels, but the problem persists, especially in microwave popcorn. Now, science has come to the rescue. “We think the secret to maximizing ‘pop-ability’ is found in the special chemistry of the corn kernel”, says food chemist Bruce Hamaker, director of Purdue University’s Whistler Center for Carbohydrate Research. Hamaker is part of a team of scientists who have identified a key crystalline structure in popcorn that appears to determine its popping quality. The finding could lead to a better microwave popcorn variety. Hamaker and his associates analyzed 14 different genetic varieties of yellow popcorn and compared their microwave popping performance. Using the same experimental conditions, they determined that the number of unpopped kernels ranged from 4% (best) to 47% (worst), depending on the variety. They then analyzed the better-performing kernels to determine which properties contributed to “pop-ability”. They found that the key factor influencing popping quality is the chemical structure of the pericarp, or outer hull, which is composed partly of cellulose (a polymer of glucose). During heating, the corn pericarp acts like a pressure cooker, locking moisture inside the corn kernel. The heated moisture leads to a pressure buildup until the kernel eventually ruptures and pops at around 177 ⬚C and 135 psi, essentially turning the kernel inside out and producing the fluffy white product that we eat. From X-ray diffraction, differential scanning calorimetry, and moisture-loss measurements, scientists determined that “pop-ability” depends a great deal on carbohydrate structures. In the best popping kernels, Hamaker and crystallographer Rengaswami Chandrasekaran have found that the pericarp is composed of a stronger, more highly ordered crystalline 1756
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Figure 3. Differential scanning calorimetry measurements reveal an irreversible, exothermic transition at about 90–105 ⬚C. The enthalpy change for that transition correlates with the percentage of kernels that pop. Error bars indicate standard deviations. Reprinted with permission from Biomacromolecules 2005, 6, 1654– 1660. Copyright 2005 American Chemical Society.
arrangement of the cellulose molecules than the pericarp of the poorer performing varieties. In laboratory studies, the researchers demonstrated that these stronger crystalline structures tend to maximize moisture retention, leading to a more complete rupture and fewer unpopped kernels. “We believe that the amount and location of the cellulose component of the kernel are critical for crystallinity and think that this property can be transferred to corn kernels to improve their popping performance”, Hamaker says. “We’re not sure yet exactly how this will be achieved, but we’re optimistic that enterprising researchers will be able to do this in the near future.” Possible techniques include selective breeding of those kernel varieties that best exhibit this optimal crystalline structure, chemical modification of corn kernels to produce the desired structure, and even genetic engineering of the corn plant. If researchers are successful, Hamaker predicts the new microwave popcorn could be available to consumers in 3 to 5 years. Although the new popcorn will be slightly different chemically from conventional microwave popcorn, mainly because of the presence of more cellulose, it will look and taste just like any other popcorn, he says. Although this study focused on microwave popcorn, 500,000 tons of which is consumed in the U.S. annually, the modified kernels are also likely to show improvements in popping quality when hot oil and hot air popping techniques are used.
More Information 1. Tandjung, Agung S.; Janaswamy, Srinivas; Chandrasekaran, Rengaswami; Aboubacar, Adam; Hamaker, Bruce R. Role of the Pericarp Cellulose Matrix as a Moisture Barrier in Microwaveable Popcorn. Biomacromolecules 2005, 6, 1654–1660. 2. Two labs have been published in this Journal using popcorn to teach techniques involved in the scientific method and experimental error. See Sauls, Frederick C. Why Does Popcorn Pop? An Introduction to the Scientific Method. J. Chem. Educ. 1991, 68, 415 and Kimbrough, Doris R.; Meglen, Robert R. A Simple Laboratory Experiment Using Popcorn To Illustrate Measurement Errors. J. Chem. Educ. 1994, 71, 519.
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Nanotubes May Help Heal Broken Bones
Figure 4. Schematic diagram of the relations between carbon nanotubes and hydroxyapatite crystals in the mineralized bundle. Adapted from Chemistry of Materials 2005, 17, 3235–3241. Copyright 2005 American Chemical Society.
a professor of chemistry at the University of California, Riverside, and lead author of the paper. Single-walled carbon nanotubes are a synthetic form of carbon, similar to naturally occurring graphite or diamond, in which atoms are arranged like a rolled-up tube of chicken wire. They are among the strongest materials known. Bone tissue is a natural composite of collagen fibers and hydroxyapatite crystals. Haddon and coworkers have demonstrated for the first time that nanotubes can mimic the role of collagen as the scaffold for growth of hydroxyapatite in solution and on surfaces. “This research is particularly notable in the sense that it points the way to a possible new direction for carbon nanotube applications, in the medical treatment of broken bones”, says Leonard Interrante, editor of Chemistry of Materials. “This type of research is an example of how chemistry is being used every day, world-wide, to develop materials that will improve peoples’ lives.” The researchers expect that nanotubes will improve the strength and flexibility of artificial bone materials, leading to a new type of bone graft for fractures that may also be used in the treatment of bone-thinning diseases such as osteoporosis. In a typical bone graft, bone or synthetic material is shaped by the surgeon to fit the affected area. Pins or screws then hold the healthy bone to the implanted material. Grafts provide a framework for bones to regenerate and heal, allowing bone cells to weave into the porous structure of the implant, which supports the new tissue as it grows to connect fractured bone segments. The new technique may someday give doctors the ability to inject a solution of nanotubes into a bone fracture and then wait for the new tissue to grow and heal. Simple single-walled carbon nanotubes are not sufficient to do this, since the growth of hydroxyapatite crystals relies on the ability of the scaffold to attract calcium ions and initiate the crystallization process, so researchers carefully designed nanotubes with several chemical groups attached. Some of these groups assist the growth and orientation of hydroxyapatite crystals, while others improve the biocompatibility of nanotubes by increasing their solubility in water. Nanotubes
Figure 5. SEM images of SWNT-CONH(C6H3SO3HNH)n mineralized for seven days. The thicknes is about 2.4 m. Reprinted with permission from Chemistry of Materials 2005, 17, 3235–3241. Copyright 2005 American Chemical Society.
Figure 6. Schematic showing how implanted functionalized singlewalled nanotubes (SWNTs) may heal heal broken bones.
The success of a bone graft depends on the ability of a scaffold to assist the natural healing process. Artificial bone scaffolds have been made from a wide variety of materials, such as polymers or peptide fibers, but they suffer from several drawbacks, including low strength and the potential for rejection in the body. Scientists have recently shown for the first time that carbon nanotubes make an ideal scaffold for the growth of bone tissue. The new technique could change the way doctors treat broken bones, allowing them to simply inject a solution of nanotubes into a fracture to promote healing. Compared with existing scaffolds, “the high mechanical strength, excellent flexibility, and low density of carbon nanotubes make them ideal for the production of lightweight, high-strength materials such as bone”, says Robert Haddon,
schematic Bin Zhao and colleagues
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Figure 7. Illustration of PRINT process compared with traditional lithography in which the affinity of the liquid precursor for the surface results in a scum layer. In PRINT, the nonwetting nature of fluorinated materials and surfaces (shown in green) confines the liquid precursor inside the features of the mold, allowing for the generation of isolated particles.
Figure 8. Manipulation of PEG particle shape and size using PRINT: (A) 200 nm trapezoidal particles; (B) 200 nm ⫻ 800 nm bar particles; (C) 500 nm conical particles that are
