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Skunky beer culprit identified
An analog of a compound found in skunk glands has been shown to be responsible for the offensive taste and skunky odor of beer that has been exposed to light. Chemistry professor Malcolm D. E. Forbes at the University of North Carolina, Chapel Hill; pharmaceutical sciences professor Denis De Keukeleire at the University of Ghent, in Belgium; and coworkers used laser flash photolysis and time-resolved electron paramagnetic resonance spectroscopy to investigate the photodegradation of a group of light-sensitive compounds known as isohumulones that are found in hops [Chem. Eur. J., 7, 4553 (2001)]. Hops help flavor beer, inhibit bacterial growth, and maintain the foam in the head of the beer. Forbes's team showed that isohumulones decompose in the presence of visible light to form radicals, which are trapped in beer by sulfur-containing species such as sulfur-rich proteins derived from barley malt. The final product of the process is the "skunky thiol"--3-methylbut-2-ene-1-thiol (shown). "This molecule has an extremely low taste and smell threshold in humans, just a few parts per trillion," Forbes says. "Historically, beer has been stored in brown or green bottles to protect hop-derived compounds from light. Understanding mechanisms behind changes in beer tastes is important because the world beer industry is hoping to save money by storing, shipping, and selling beer in less expensive clear glass."
Metal particles assemble into microwires
When an aqueous suspension of metallic nanoparticles is placed in an alternating current field generated between two planar electrodes, the particles quickly assemble into conducting microwires that grow from one electrode to the other [Science, 294, 1082 (2001)]. The wires, which are micrometers in diameter and millimeters in length, can grow faster than 50 mm per second, according to chemical engineers Eric W. Kaler of the University of Delaware and Orlin D. Velev of North Carolina State University and their coworkers. The method provides a quick and simple way to create electrical connections in watery environments under ambient conditions, they point out. The connections are self-repairing--that is, if the current is increased until it burns a gap in the wire, new nanoparticles immediately start assembling at the wire's tip, restoring the electrical connection. When mixed suspensions of metallic particles and polystyrene latex microspheres are placed in the alternating current field, latex-coated microwires are produced. The researchers believe such structures hold promise for wet electronic and bioelectronic circuits, including chemical sensors.
Reverse-Michael drives resolution
The first dynamic resolution of a racemate driven by a flurry of Michael and reverse-Michael additions has been reported by chemists at Bristol-Myers Squibb [J. Am. Chem. Soc., 123, 11075 (2001)]. Dynamic resolution is the coupling of racemization with resolution. Process research scientists Jaan A. Pesti, Jianguo Yin, Lin-hua Zhang, and Luigi Anzalone use a lipase to hydrolyze the (R)-enantiomer of a propyl oxazolineacetate thioester to the free acid. The (S)-thioester racemizes by continuous ring-opening reverse-Michael and Michael additions. The end result is conversion of all of the thioester to (R)-acid. The researchers credit organic chemistry professor Dale G. Drueckhammer of the State University of New York, Stony Brook, with the idea to use a thioester to render -protons more acidic in the process.
Complexation alters arene chemistry
Reactions of arenes with carbenes, which are frequently controlled by rhodium catalysts, yield many important products useful for fine synthesis. UCLA chemistry professor Craig A. Merlic and colleagues now find that when arenes are complexed with chromium, their reactivities with carbenes are greatly altered, a discovery that opens up possibilities for new synthetic strategies [J. Am. Chem. Soc., 123, 11101 (2001)]. The group studied a number of different modes of reactivity in rhodium-catalyzed reactions of complexed arenes. For example, arenes complexed with chromium tricarbonyl are completely protected from cyclopropanation in the Buchner reaction. Additionally, because the group found that CH insertion is still possible on the complexed arenes, they can be used, along with a chiral rhodium catalyst, to produce planar chiral compounds with high enantioselectivity.
Simple enamines such as vinylamine are hard to make, and their chemistry remains for the most part unexplored. Now, researchers have found a new way to make enamines and their copolymers [J. Am. Chem. Soc., 123, 11083 (2001)]. Primary vinylamines are unstable relative to formation of imines, which tend to react further, so the work by North Carolina State University polymer chemistry professor Bruce M. Novak and University of Massachusetts grad student Jeffrey T. Cafmeyer opens routes to new thermoplastic resins with pendant amino groups. For example, using carbonyltris(triphenylphosphine)ruthenium(I) hydride catalyst, the chemists isomerize allylamine to 1-aminopropene. This compound has a half-life of 40 hours if the reaction vessel is lined with polytetrafluoroethylene, stainless steel, or silylated glass. Upon addition of acrylonitrile and a free-radical initiator of polymerization, the researchers produce the copolymer shown (bottom). The group gets analogous results with 3-methylaminopropene.
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