October 21, 2002
Volume 80, Number 42
CENEAR 80 42 p. 41
ISSN 0009-2347


Taste receptor responds to only one class of bitter molecules

At least 24 human taste receptors are believed to mediate the perception of bitterness. Wolfgang Meyerhof, a molecular geneticist at the German Institute of Human Nutrition, Potsdam-Rehbrücke, and coworkers now show that one of these receptors, known as TAS2R16, recognizes only one class of bitter compounds [Nat. Genet., published online Oct. 15, http://dx.doi.org/10.1038/ng1014]. When expressed in cell cultures and presented with compounds that are closely related structurally, TAS2R16 responds specifically to -glucopyranosides, such as salicin (shown), a bitter compound from willow bark that has long been known as an analgesic. A -glycosidic bond between glucose and a large hydrophobic group, such as phenyl, benzyl, or naphthyl, appears to be required for receptor binding. Thus, TAS2R16 doesn't respond to phenyl--D-galactopyranoside, phenyl--D-glucopyranoside, or methyl--D-glucopyranoside. The finding opens opportunities to modify human perception of bitter-tasting substances.



Ladderane lipid membranes

Lipids containing a ladderane--a chain of fused cyclobutane rings--make up the bulk of dense membranes in certain unusual bacteria. Jaap S. Sinninghe Damsté of the Royal Netherlands Institute for Sea Research and coworkers discovered the compounds in anammox bacteria, which anaerobically oxidize ammonia to dinitrogen [Nature, 419, 708 (2002)]. The staircaselike motif of cis-fused cyclobutanes has never before been seen in nature. The most abundant lipid in the bacteria is the methyl ester of a C20 fatty acid with five fused rings (shown). Other lipids contain three fused cyclobutane rings attached to a cyclohexane. Both ladderane moieties occur in alcohols and glycerol ethers as well as in fatty acids. The lipids are found in a membrane that surrounds the compartment within the bacterial cell where ammonia is oxidized. Their presence makes the membrane exceptionally dense and resistant to diffusion, helping to contain toxic intermediates such as hydrazine and hydroxylamine, the researchers suggest.



Metabolites flip natural riboswitches

Scientists at Yale University have discovered what could be a molecular fossil left over from a time when RNA ruled the world: an RNA "riboswitch" that senses binding of a small-molecule metabolite to regulate protein expression. Cells usually use protein middlemen to monitor metabolite levels and signal the necessary adjustments to protein expression. But biochemist Ronald R. Breaker and his coworkers at Yale have found that, in Escherichia coli, messenger RNA encoding a transport protein for cobalamin proteins can bind coenzyme B-12 (59-deoxy-59-adenosylcobalamin), as well as mRNAs that code for proteins involved in thiamine biosynthesis can bind thiamine and thiamine pyrophosphate [Chem. Biol., 9, 1 (2002) and Nature, published online Oct. 16, http://dx.doi.org/10.1038/nature01145]. These metabolites manage to control gene expression without the protein middleman by preventing the cell's translational machinery from accessing the mRNA. Scientists had already used in vitro evolution to coax RNA to perform such tricks--but until now, no natural riboswitches had been found. All three evolutionarily distinct branches of life--archaea, bacteria, and eukaryota--boast such riboswitches, Breaker says, suggesting that they truly are ancient.


Root cause of polyethylene abrasive wear

The number of effective physical cross-links per chain in polyethylene is a simple but important factor that governs abrasion resistance in machine parts and prosthetic devices made from the plastic, according to a study at the Swiss Federal Institute of Technology (ETH), in Zurich [Macromolecules, 35, 8467 (2002)]. Theo A. Tervoort and coworkers developed a predictive model for abrasive wear based on tests of various polyethylene samples. They found that polyethylene grades with lower crystallinity and polydispersities and containing no low-molecular-weight chains have a greater number of cross-links and provide better wear resistance. The results have implications for polyethylene processing: Ultra-high-molecular-weight grades with molar masses near 2,000 kg per mol used in structural plastics must be processed by demanding compaction, sintering, and machining. The ETH study, though, reveals that grades of 100–500 kg per mol with similar abrasion resistance can be melt-processed by more versatile extrusion or injection-molding. ETH plans to develop the discovery through a venture with its spin-off company Omlidon Technologies and Celanese's Ticona polymers business.



Iodine cations oxidize CH4

A solution of iodine in oleum has been found to be an efficient catalyst for low-temperature methane oxidation. Roy A. Periana, associate professor of chemistry at Loker Hydrocarbon Institute at the University of Southern California, and coworkers show that I2 dissolved in sulfuric acid containing SO3 generates a stable active species that catalyzes the functionalization of methane to methyl bisulfate, a compound that is readily converted to methyl alcohol by reaction with water [Chem. Comm., 2002, 2376]. "As far as we know, this is one of the simplest catalyst systems reported to date for converting methane to a methyl product below 200 °C," Periana tells C&EN. The methane conversion and product selectivity--more than 40% and 95%, respectively--are among the highest ever reported, according to Periana. And unlike other methane oxidation systems, expensive oxidants such as hydrogen peroxide are not required. The team proposes that electrophilic I2+ or I+ is the active catalyst that activates the C–H bond. The work could stimulate the design of new catalysts for methane oxidation.


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