Issue Date: April 9, 2007
Chicago's Feast Of Chemistry
WHEN THE 233RD American Chemical Society national meeting blew into the Windy City late last month, chemistry seemed to be everywhere, but it wasn't all necessarily connected with the meeting itself. For example, as detailed in C&EN's ACS meeting blog (cen07.wordpress.com), a Chicago restaurant, Alinea, offered a molecular gastronomy experience called the "black truffle explosion," and the Museum of Science & Industry allowed visitors to watch master chef Julia Child making primordial soup.
Of course, the hard science was concentrated in the convention center and hotel meeting rooms, where more than 9,000 papers were presented. Here's a taste of some of the research findings presented, in areas as diverse as insect natural products, superoxide levels in the ocean, electrochromic sunglasses, and organic synthesis with halides.
"LADYBIRD, LADYBIRD, fly away home ..."—but not my home, if you please. Also known as ladybugs and lady beetles, these critters bedevil homeowners by emitting a stinky and lingering odor when disturbed or squashed. The same odor can ruin wine if the bugs settle in a vineyard and are processed along with the grapes.
Iowa State University researchers have now completed an exhaustive study of the chemicals responsible for the characteristic smell of the bugs' noxious emissions. Their findings could lead to new strategies to eliminate the offensive compounds and improve wine quality.
Jacek A. Koziel, an agricultural engineer, took up the project as a change of pace from his usual work analyzing livestock odor (C&EN, Sept. 25, 2006, page 104). "I wanted to work with something different, with a different odor profile, something that many people could easily identify with," he told C&EN. "Since there were hundreds of lady beetles in my old office with leaky windows, I chose them. They released this characteristic odor when I handled them or when, in my frustration, I tried to swat them."
Koziel enlisted his postdoc Lingshuang Cai as well as Iowa State entomologist Matthew E. O'Neal, a ladybug expert, to work on the project. Cai presented the team's findings before the Division of Agrochemicals.
The team studied the multicolored Asian ladybird beetle, which invaded the U.S. early last decade. Homeowners encounter the bugs because they aggregate inside buildings for shelter during winter, Cai noted. Wine growers cross paths with them when the bugs feast on damaged grapes in vineyards.
Because humans' odor detection threshold for the odiferous ladybug compounds is so low, "even a tiny amount in the wine will cause a great sensory impact on human tasting," Cai said. "That's why these compounds are so noxious."
The extent of the resulting wine taint problem is unclear, Koziel noted. But "there have been reports of vineyards going bankrupt because entire vintages were lost due to this problem."
Cai began the project by placing ladybugs in a capped glass vial for a day and then collecting the volatile compounds they released. She used gas chromatography and mass spectrometry to separate and identify individual compounds in the mixture. Then, a panel of human "sniffers" told her which of those compounds were the most important contributors to the ladybug scent. Of the 38 compounds identified, Cai determined that 2,5-dimethyl-3-methoxypyrazine (DMMP), 2-isopropyl-3-methoxypyrazine (IPMP), 2-sec-butyl-3-methoxypyrazine, and 2-isobutyl-3-methoxypyrazine play a major role.
The overall smell is a mixture of nutlike, green bell pepper, potato, and moldy odors. At the concentrations present in ladybug emissions, the mixture is "really stinky," Cai said.
The Iowa State researchers are not the first to assess ladybug emissions. But Cai noted that they were the first to discover that DMMP is an important contributor to the odor. "This study also provided the first unambiguous evidence that IPMP is responsible for the characteristic odor of live" ladybugs, Cai said. Koziel added that "we are the first to observe a correlation between the color of the beetles and the amount of methoxypyrazine found in the air about them." Orange beetles release more of the compounds than do yellow beetles.
The Iowa State team's process also represents a "very sensitive, accurate method for identifying the presence of these compounds," Koziel noted. "This method will help those studying ways to prevent methoxypyrazines from contaminating wine." The group recently published details of its findings online in the Journal of Chromatography A (DOI: 10.1016/j.chroma.2007.02.044).
SUPEROXIDE (O2-) may not be top of mind as a major underwater electron source, but this reducing agent may play a larger role in ocean energetics than was previously thought, according to new research.
New direct measurements of superoxide concentrations at 18 locations in the Pacific Ocean near Australia and Central and South America suggest that existing superoxide levels far exceed previous estimates-sometimes by as much as an order of magnitude, according to Andrew Rose, a postdoc at both Woods Hole Oceanographic Institution and the University of New South Wales in Sydney, Australia. Rose presented his findings before the Division of Geochemistry.
"It's exciting to find that superoxide has a lifetime on the order of minutes such that it can build up to a concentration significant enough to potentially play a role in the ocean's metal redox chemistry," said Bettina Voelker, an ocean chemist at the Colorado School of Mines.
Furthermore, Rose found superoxide at depths far beyond light's penetration, which challenges the conventional wisdom that photooxidation is the primary source of superoxide in the ocean. Microorganisms may be producing the missing quantities of superoxide to "cope with life in a highly oxidized environment," Rose told C&EN.
For example, most underwater microbes need iron for their metabolism but cannot readily absorb the organically complexed Fe3+ dominant in the ocean. Rose argued that these organisms make superoxide to reduce nearby iron to more readily absorbable Fe2+, which they can then use.
"Iron has been shown to be a limiting nutrient for microorganisms and phytoplankton," Rose explained. "These organisms are deliberately producing superoxide in their environment by passing electrons produced in metabolism across the membrane." This energetic expenditure pays off by reducing metals into a more biologically useful state, he added.
Superoxide may also be important in the cycling of ocean copper, manganese, and chromium, according to William J. Cooper, an ocean chemist at the University of California, Irvine.
Rose measured levels of superoxide by using a chemiluminescence reagent called methyl Cypridina luciferin analog that glows at an intensity proportional to superoxide concentration. He checked not only the absolute concentration of superoxide at different ocean locations but also the ability of cyanobacteria, diatoms, and algae to produce it. Next, Rose will test a greater diversity of creatures for their ability to make superoxide.
This "incredibly sensitive" chemiluminescence method also will help the broader research community figure out the relative contributions of photochemical and "dark" sources of superoxide in the ocean, Voelker said.
HAVE YOU EVER wanted to make your sunglasses darker or lighter with the touch of a button? Soon you may be able to.
Chemical engineer Chunye Xu and colleagues at the University of Washington, Seattle, have been working on "smart" sunglasses that can lighten or darken on command. The group's prototype sunglasses are made with a polymer that changes its shade with some help from a watch battery. Graduate student Chao Ma presented them before the Division of Polymeric Materials: Science & Engineering.
Sunlight can vary considerably during outdoor activities such as motorcycling or skiing. Traditional sunglass lenses, created with tinted glass or polycarbonate materials, have a fixed color state. Lenses made with photochromic materials adjust to changing light, but rather slowly. The electrochromic polymer in Xu's prototype enables the lenses to lighten or darken in literally one second with mere milliwatts of power. The button that engages the battery is located on the sidearm of the eyewear.
The researchers found that the lenses, in the colored state, block up to 99% of a wavelength of yellow light that can particularly aggravate human eyes.
Electrochromic polymers are not new, but this application stands out, commented Guoqiang Li, an optics researcher at the University of Arizona. Other similar materials require significantly more voltage, respond more slowly, and cannot maintain the color change without additional power, and this prototype can, he said. "This is a great advantage."
But Claes-Göran Granqvist, a physics professor at Uppsala University, in Sweden, who also works with Uppsala-based electrochromic technology firm ChromoGenics, cautioned that the prototype lenses would be heavy and cumbersome.
Xu described the prototype lenses as having several layers sandwiched between two pieces of indium tin oxide-coated glass. The working layer is a polydioxepine that turns blue when voltage is applied. A layer of vanadium oxide stores ions, and the transparent electrolytic gel layer facilitates ion transport.
The researchers also have created polymers that darken to red or green, but they have not yet made prototype sunglasses from these materials. Xu said her group is also refining the circuitry of the sunglasses to make it more compact, and they are working with a company to examine the possibility of incorporating vision correction to make the smart glasses even smarter.
a new halide addition reaction represents a previously unrecognized way by which halides can be incorporated into biomolecules.
The reaction was discovered by chemistry professors Charles L. Perrin and Joseph M. O'Connor and graduate student Betsy L. Rodgers at the University of California, San Diego. They reported their findings before the Division of Organic Chemistry and in a publication (J. Am. Chem. Soc., DOI: 10.1021/ja070023e).
Halogens add to organic molecules by nucleophilic substitution reactions at saturated carbons. Halogens also add biosynthetically to electron-rich aromatic rings by electrophilic substitution reactions catalyzed by haloperoxidases or halogenases.
Perrin, O'Connor, and Rodgers have now expanded the repertoire of halide additions with the discovery of a reaction in which a nucleophilic halide anion can be attached to an aromatic ring.
They got the idea for the reaction when oceanography professor William Fenical of UC San Diego and coworkers isolated two pairs of chlorinated marine natural products, the sporolides and cyanosporasides, and suggested that they were derived from an enediyne precursor that cycloaromatizes to a p-benzyne biradical. But the mechanism by which chlorine was incorporated was unknown.
The team has now reproduced such a process experimentally. By heating a model enediyne in the presence of chloride, bromide, or iodide and a weak acid, they were able to show that the enediyne cyclizes to a p-benzyne biradical, which quickly adds halide and is then protonated. This mechanism differs from the usual radical chemistry of p-benzynes, and it can account for the biosynthesis of the sporolides and cyanosporasides. A halogen atom and hydrogen have not previously been introduced into a substrate in lab experiments, according to the researchers.
The reaction has important implications for incorporating halides into aromatics and biomolecules. "We believe these results will open a new chapter in the widely followed chemistry of enediynes and p-benzynes," Perrin said.
Fenical said the group's discovery "is truly important in fundamental organic chemistry and will significantly expand understanding of the reactivity of enediynes."
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