Issue Date: April 21, 2008
Chemistry Gumbo A La New Orleans
More than 13,000 chemists swept into New Orleans for the American Chemical Society national meeting earlier this month, eager for a taste of the latest advances in chemical research and of the city's famous cuisine.
We can't reproduce the sights and sounds of New Orleans itself, but here's a small sampling of the research that scientists served one another.
Alligators To The Rescue
Alligators may be dangerous out on the bayou, but their blood could provide a new source of antimicrobial compounds. Researchers at Louisiana State University and at McNeese State University, in Lake Charles, La., are analyzing alligator blood to identify peptides and proteins with antimicrobial activity. They presented preliminary results in a poster session sponsored by the Division of Analytical Chemistry.
In previous studies, biochemist Mark E. Merchant of McNeese State found that ex- tracts from alligator blood kill a variety of bacteria and yeast, including “some incredibly nasty strains of antibiotic-resistant bacteria” such as methicillin-resistant Staphylococcus aureus, the notorious cul- prit in many hospital-acquired infections. These blood samples contain peptides that are part of the alligator’s innate immune system.
Merchant has teamed up with mass spectrometrist Kermit K. Murray and grad student Lancia N. F. Darville at Louisiana State to perform proteomic analyses of al- ligator serum and white blood cells to iden- tify those peptides. “Mark was convinced there would be just a few peptides in each of the samples,” Murray said. “We did some one-dimensional separations and found there was quite a bit of stuff in there.”
The instrumental part of the proteomic analysis is fairly straightforward, Murray said, but the identification part is not. “One of the problems with the alligator proteome is that there are very few proteins in the databases,” he said. “We have to feel around a bit in the early going to see what we have in the samples and to try to sequence the peptides.”
Studies of antimicrobial peptides in other species give them clues of what to expect, according to Darville. “There is literature out there as to what these kinds of peptides should look like. They’re arginine- and lysine-rich,” she said. “We’re expecting that alligator antimicrobial peptides should be very similar in terms of sequence to those that are already in the literature.”
To identify the peptides and proteins that are responsible for the antimicrobial activity, the team is analyzing blood taken from alligators before and after their immune systems are challenged. Merchant collects blood from an alligator, injects that alligator with bacterial lipopolysac- charide to simulate an infection, and then takes another blood sample 24 hours later. Such differential analysis could lead to a better understanding of alligators’ im- mune systems and to specific candidates for antimicrobials. “I will isolate and characterize each peptide on an individual basis at first and then investigate the pos- sibility of synergistic activity,” Merchant says.—CELIA ARNAUD
Chemical Breakdown Helps Plants Grow
Weed killers, also called herbicides, are central to modern farming. But they’re only helpful if they selectively destroy weeds without harming crops. Researchers have uncovered new details about how an agrochemical called a safener protects rice from herbicide damage, a finding that could help in designing better weed-control strategies.
Kathryn M. Evans, a postdoctoral researcher working in the labs of biologist Robert Edwards and organic chemist Patrick G. Steel at the University of Dur- ham, England, presented the results to the Division of Biological Chemistry.
Commercial farmers protect crops such as wheat, rice, and corn by spraying with specific combinations of safeners and herbicides. Safeners selectively boost the activity of herbicide-detoxifying en- zymes called glutathione S-transferases (GSTs) in crop plants, but the chemistry behind this is unclear. The Durham team reported that in the case of fenclorim, a safener used with rice, one of its metabo- lites may provide long-term protection from herbicides.
To determine how plants metabolize fenclorim, the team added it to cell cul- tures from a model plant and then looked for metabolites with high-performance liquid chromatography (HPLC) and mass spectrometry. They identified metabolites by synthesizing likely fenclorim break- down products and using them as HPLC reference standards. “This allowed us
to put together a full metabolic pathway for fenclorim,” marking the first time researchers have obtained that level of infor- mation for any safener, Evans said.
To see which metabolites produced a protective response, the team applied fen- clorim or one of several metabolites to her- bicide-treated rice seedlings and measured GST activity. Whereas most metabolites did not activate GSTs as much as fenclo- rim, one called CMTP afforded comparable protection.
The team concluded that as fenclorim breaks down and its safening effect fades, CMTP begins to appear, reactivating the defense. What’s more, plants seem to metabolize CMTP more slowly than fen- clorim, so it may be responsible for longer- lasting protection, Evans said.
This research “may lead to new uses for existing herbicides in crops that were pre- viously not tolerant to a particular herbi- cide,” comments weed science expert Dean E. Riechers at the University of Illinois, Urbana-Champaign. “This is particularly important, as weed resistance to herbicides has become an increasing problem in ag- riculture in recent years.”—CARMEN DRAHL
Illuminating Hydrogen Peroxide In Cells
Hydrogen peroxide is overproduced in cells as part of chronic inflammatory diseases such as heart disease and arthritis, so detecting the molecule in the body is of keen interest for diagnosing and tracking medical conditions. Researchers are also interested in the role of H2O2 in cellular signal transduction. Now, a three-part chemiluminescent system may illuminate the location of H2O2 in cells and tissues.
Detecting H2O2 in vivo is not easy. Itscombined low concentration and low reactivity relative to other oxygen species such as superoxide make it difficult to find a sensitive, specific sensor. In a presentation before the Division of Physical Chemistry, Niren Murthy of Georgia Institute of Tech- nology described H2O2-sensing nanoparticles composed of a peroxalate-containing polymer and a fluorescent dye such as pentacene (Nat. Mater. 2007, 6, 765).
In solution, H2O2 reacts with the peroxalate component of the polymer to produce dioxetanedione. The dioxetanedione then chemically excites the dye, leading to chemiluminescence of the nanoparticles. The reaction is specific for H2O2; superoxide or the hydroxyl radical will not produce the dioxetanedione.
Experiments with the particles in vitro indicated they can detect H2O2 concentrations down to 250 nM. Murthy and co- workers were also able to detect hydrogen peroxide produced in the peritoneal cav- ity of mice after an injection of a lipopoly-saccharide, which induces an immune response.
“This is an exciting development that may overcome problems with the in vivo detection of H2O2,” says Henry J. Forman of the University of California, Merced. He adds that for detecting disease pathology, “the specificity for H2O2 and limit of detection in whole animals appear to be very suitable.” The sensitivity might not yet be low enough to detect smaller amounts of H2O2 produced in physiological signal transduction, he cautioned.
Murthy noted that the nanoparticles, about 500 nm in diameter, are probably too large to be of use clinically, so the group is now working
on adapting the approach to smaller delivery systems. One of those strategies involves incorporating a fluorescent dye and diphenyl oxalate into a polyethylene glycol micelle about 35 nm in diameter.
Although the group hasn’t yet ex- perimented with putting the micelles into mice, lab tests demonstrate that the micelles are even more sensitive than the nanoparticles and can reveal H2O2 concentrations as low as 50 nM. Another approach involves covalently attaching the peroxalate ester to a commercially available dye, naphthofluorescein, to make naph- thofluorescein oxalate. The compound is unstable in aqueous solution, so Murthy and colleagues are now trying to improve its stability and H2O2 sensitivity.—JYLLIAN KEMSLEY
Copying Sea Cucumber's Cool Trick
Sea cucumbers can rapidly alter the stiff- ness of their skin in response to environmental cues. Inspired by these creatures, polymer scientists at Case Western Reserve University, in Cleveland, have mimicked this bio- mechanical switch- ability in an artificial system. Team member Jeffrey R. Capa-dona reported their findings to the Divi- sion of Polymer Chemistry. The researchers also published the work last month in the journal Science (2008, 319, 1370).
The stiffness of a sea cucumber’s skin is governed by interactions between collagen fibers. The team, led by Christoph Weder and Stuart J. Rowan, achieves the same effect by embedding cellulose nanofibers, or “whiskers,” in a rubbery polymer matrix. So far, they have tried two matrices—an ethylene oxide- epichlorohydrin copolymer and poly(vinyl acetate).
The surface of the cellulose fibers is dotted with hydroxyl groups. In a network of these nanofibers, wherever the nano-fibers intersect, hydrogen bonds form, Weder said. “The fibers want to stick to each other like there’s no tomorrow.”
The hydrogen bond network makes the material rigid, but that network can be disrupted by adding water. The material doesn’t simply soak up water and swell like a sponge. Instead, the water also forms hydrogen bonds with the nanofibers and decouples the whiskers. As a result, the composite material becomes flexible.
Marek W. Urban, professor of polymer science at the University of Southern Mis- sissippi, said the work “demonstrates how interplay of different chemical species and their manipulation will lead to tremendous responses.”
Brent S. Sumerlin, assistant professor of chemistry at Southern Methodist University, added that the team has managed to “elegantly reproduce an inherently complex natural phenomenon with straightforward polymer science.”
Weder and Rowan are collaborating with Dustin J. Tyler, a biomedical engineer at Case Western, to fashion the material into microelectrodes that can be implanted in the brain for use as part of an artificial nervous system. Such implants must be stiff enough to be inserted into the brain, but once there, they need to be soft enough not to damage the surrounding tissue. The brain’s aqueous environment is enough to trigger the switch from rigid to flexible.
The team hopes to devise other switchable materials that use different cues, such as light or electrical impulses, to trigger the stiff-to-flexible transition.
Rowan envisions a broad range of applications for such materials, from biomedi- cal devices such as stents to protective clothing and even toys. “You could mold it into whatever shape you want and then make it rigid,” he said. “You can imagine kids loving that sort of thing. I know I would have when I was younger.”—CELIA ARNAUD
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