Issue Date: February 23, 2004
Making two C-C bonds and four stereocenters in one fell swoop
A new catalytic domino reaction combines two acyclic starting materials to yield optically active cyclohexanones with up to four stereogenic centers [Angew. Chem. Int. Ed., published online Feb. 11, http://www3.interscience.wiley.com/cgi-bin/abstract/107614801]. Domino reactions permit multiple reaction steps to be accomplished in one reaction "pot." Karl A. Jørgensen's group at Aarhus University, in Denmark, devised a domino reaction in which a known phenylalanine-derived imidazolidine catalyst is used to accelerate both a Michael reaction (which adds an acyclic β-ketoester to an acyclic unsaturated ketone) and a subsequent aldol reaction (which cyclizes the intermediate made in the first step). "I find it stunning that in one very simple reaction, Jørgensen and coworkers are able to form two C-C bonds and set the configuration of four stereocenters in a highly enantio- and diastereoselective process," comments Carlos F. Barbas III of Scripps Research Institute.
Biosilica method aids enzyme efficiency
Taking a cue from marine diatoms that use silaffin polypeptides to initiate the growth of silica cell walls, a team of Air Force researchers has shown that the peptide repeat unit can be used to catalyze precipitation of silica under mild conditions in a novel technique to immobilize enzymes [Nat. Biotechnol., 22, 211 (2004)]. Led by Jim C. Spain at Tyndall Air Force Base in Florida, the team found that the peptide facilitates formation of 500-nm fused spherical silica particles within seconds of being added to a silicic acid solution. When butyrylcholinesterase is included in the reaction, the enzyme is trapped within the particles and retains 90% of its initial activity--a significant improvement over the 10% activity retained using a sol-gel technique. The immobilized enzymes have greater stability than those prepared by other methods, the researchers report, and the silica is amenable to packed-bed or fluidized-bed reaction systems, which they tested using a standard cholinesterase conversion.
New form of mad cow disease detected
Italian researchers have discovered a new form of mad cow disease (bovine spongiform encephalopathy, or BSE). Salvatore Monaco of the University of Verona, Italy, and colleagues uncovered three lines of evidence suggesting that the condition is a distinct form of BSE [Proc. Natl. Acad. Sci. USA, published online Feb. 17, http://www.pnas.org/cgi/doi/10.1073/pnas.0305777101]. In brains from two cows, the researchers found amyloid plaques, which are not seen in typical BSE cases. Also, the distribution of misfolded prions in the two brains was entirely different from that in typical BSE cases. In addition, the protease-resistant fragment of the prions had a lower molecular mass than did fragments of prions causing typical BSE. The pattern of amyloid plaques in the cattle brains was similar to that in humans with sporadic Creutzfeldt-Jakob disease (CJD). The new form of BSE can be detected with the rapid tests now used in Europe and Japan. The authors say their findings suggest that some cases of sporadic CJD could be caused by meat from infected cattle.
Cross-links strengthen nanotube ropes
Although a lone single-walled carbon nanotube (SWNT) strongly resists deforming under stress, that strength doesn't translate to macroscopic fibers made from SWNTs. Van der Waals interactions are all that hold bundles of the tubes together, and those tubes easily slide past each other in response to an outside force. László Forró at the Swiss Federal Institute of Technology, Lausanne, and colleagues discovered that they can increase the strength of SWNT bundles 30-fold by using moderate electron-beam irradiation inside a transmission electron microscope [Nat. Mater., published online Feb. 15, http://dx.doi.org/10.1038/nmat1076]. Irradiation covalently cross-links the tubes, thereby eliminating slippage. The technique could lead to superstrong nanotube-based structures. The researchers are not certain about the precise structure of the cross-links. But because the tubes are too far apart to be bridged by a single sp3 bond, Forró and coworkers speculate that atoms other than the tubes' carbons form the link.
Amino acids act as asymmetric catalysts
Meteorites carrying a preponderance of L-amino acids may have been the original source of nature's handedness. Could these chiral compounds have transferred their asymmetry to other prebiotic molecules, such as sugars? This possibility has now been examined by Sandra Pizzarello, a chemistry professor at Arizona State University, Tempe, and Arthur L. Weber of the SETI Institute, Moffet Field, Calif. They simulated a prebiotic sugar synthesis by reacting glycolaldehyde and formaldehyde in aqueous buffer in the presence of an amino acid catalyst in various enantiomeric excesses [Science, 303, 1151 (2004)]. Two catalysts were tested: isovaline, the most abundant chiral amino acid in meteorites, and alanine, a common amino acid. The reaction produced threose and erythrose, both with enantiomeric excess. The L-amino acids catalyzed the aldol condensation of the aldehyde, affecting the chirality of the sugar products. It is especially likely that isovaline played a role in directing handedness, Pizzarello says, because isovaline does not racemize in water, as do most amino acids and sugars.
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