SCIENCE & TECHNOLOGY

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Cyclopentane oligomer binds key lipid A novel class of compounds that recognizes a lipid involved in bacterial infections has been developed by Benjamin L. Miller, assistant professor of chemistry at the University ofRochester, and r^i graduate students Robert ^γ^ D. Hubbard and Scott R. Horner. Simulations reveal that compounds featuring three highly substituted cyclopen-tanes in series present a sur­face that complements that of lipid A, a target of drugs to pre­vent sepsis. Lipid A is part of Upopofysac-charide, which is a key constituent of the cell membrane of gram-negative bacteria. Among several oligocyclopentanes the re­searchers prepared, the compound shown binds lipid Aas strongly as the clinically used cyclic peptide antibiotic polymixin Β \J.Am. Chem. Soc, 123,5810 (2001)}. The new compounds represent a type of structure that so far has not been found in nature nor explored in the literature, Miller says. His lab also is studying use of the compound shown as a sensor for lipid A.

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Organic bilayers self-assemble into icosahedra Although supramo-lecular hollow icosa-hedral structures— shapes composed of 20 equilateral trian­gles—are favored by some viral capsid proteins and a few minerals, they have been difficult to make from synthetic organic components. However, Monique Dubois of the French Atomic Energy Commission (CEA/Saclay), in Gif sur "Wette, and coworkers have discovered conditions under which bilayers of organic surfactants self-as­semble into aggregates with a regular icosahedral shape [Na­ture, 411,672 (2001)}. The re­searchers create the icosahedra by heating and subsequently cooling salt-free solutions con­taining mixed cationic and anionic surfactant bilayers. The

resulting icosahedral structures (shown) are about 1 μηι across and have a molecular mass of about 1010 dahons, making them

larger than any known icosa­hedral protein assemblies.

The structures are stabilized by pores of about 150-Â di­

ameter found at each ver­tex. The pores

make the struc­tures potentially useful for con­trolled drug or DNA release, the researchers note.

Sulfur ylide offers safer route to metal carbenes A new general route to pre­pare metal carbene catalysts that are useful in olefin meta­thesis and other reactions has been developed by organic chemistryprofessor David Mil-stein and coworkers at Weiz-

mann Institute of Science in Rehovot, Israel \J.Am. Chem. Soc, 123,5372 (2001)}. Interest in transition-metal benzylidene complexes of the type M=CHC6H5 has grown in re­cent years because of their catalytic properties. The usual synthesis of these complexes in­volves a diazoalkane as the car­bene precursor, but the in­stability of diazo compounds and the safety issues involved with handling them hamper this method, the researchers note. The new method uses a sulfur ylide { (QH^S-CHQiy as the carbene precursor, which they say is sirnplerarMi safer. The Weizmann group has prepared several rhodium, ruthenium, and osmium carbene com­plexes using the ylide, includ­ing the versatile Grubbs cata­lyst {(PCy^QRu-CHQHs, where Cy=cyclohexyQ.

Planar benzene sails into vacuum chamber Just as logs floating down a river align themselves with the current, planar molecules car­ried into a vacuum chamber in a molecular beam will orient themselves to offer the least resistance to the carrier gas. ŒemistryprofessorVincenzo Aquilanti of the University of Perugia, along with colleagues there and at the University of Trento, used benzene molecules to demonstrate that collisions with smaller, faster moving car­rier gas molecules cause the benzene molecules to align edge-on to the beam [Phys. Rev. Lett., 86,5035 (2001)}. Similar orientation effects had been seen earlier for smaller, inor­ganic molecules, such as 0 2 and CO, but this is the first demon­stration of the effect in planar organic molecules. In their ex­periments, the researchers found

a20 to 30% increase in edge-on benzene molecules over what would be expected from ran-domcdentatioaTTiat'senough, they suggest, to make the tech­nique a useful way to produce intense beams of aligned mole­cules for studies ofsteric effects in gas-phase processes and in surface catalysis.

Stable carbenes with spectator substituent

Stable carbenes—neutral molecules in which a carbon atom is surrounded by only six electrons—are difficult to achieve. Usually an electron-active substituent is needed on either side of the carbene to stabilize it. A team led by Guy Bertrand of Paul Sabatier Uni­versity inToulouse, Ranee, has synthesized stable singlet car­benes containing amino and aryl groups (shown) {Science, 292,1901 (2001)}. The amino group, which has both ir-donor and σ-acceptor electronic char­acter, ensures that the carbene center remains neutral. X-ray data indicate that the aryl group, which contaire(Krifh]0-romethyl groups, does not in-

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22 C&EN / JUNE 1 1 , 2001 H T T P : / / P U B S . A C 5 . O R G / C E N

CONCENTRATES

teract with the carbene lone pair of elections and serves as a spectator. These carbenes can be isolated at room tempera­ture at close to quantitative yields. Because these carbenes don't require two strongjy inter­acting substituents, the num­ber and variety of such carbenes should increase, Bertrand and colleagues write, which will lead to new synthetic developments and applications.