Science Concentrates

September 5, 2005

Volume 83, Number 36

p. 32

• Small molecules bust biofilms
• New approach to organometallic polymers
• Currents smooth thin-film surface
• Boron anchors expandable ferrocene dimer
• Electric field swings tiny pendulum

Small molecules bust biofilms

Two small molecules that block the formation of life-threatening biofilms of gram-negative bacteria have been identified by chemists at the University of Wisconsin, Madison. Biofilms are drug-impervious bacterial communities that underlie a wide variety of chronic infections. Efforts to block biofilm formation with small molecules have focused largely on analogs of N-acyl L-homoserine lactones (AHLs), bacterial signaling molecules that regulate the chemical communication system (known as quorum sensing) that triggers biofilm formation in gram-negative bacteria. Helen E. Blackwell and coworkers at Wisconsin now have developed a facile and flexible solid-phase synthetic route to a wide range of AHL analogs (J. Am. Chem. Soc. 2005, 127, 12762). Two of these analogs (shown) strongly inhibit the formation of Pseudomonas aeruginosa biofilms, the root cause of often-fatal lung infections in cystic fibrosis sufferers. "By facilitating the production of focused combinatorial libraries of AHL analogs, our route should dramatically accelerate the discovery of new molecules that modulate quorum sensing and biofilm formation," Blackwell says.

New approach to organometallic polymers

Conjugated organometallic polymers are materials with potentially desirable electronic and mechanical properties, but synthesizing such polymers generally requires factors such as inert atmospheres and anhydrous conditions, which make the process difficult. Christopher W. Bielawski and coworkers at the University of Texas, Austin, have now developed a synthetic approach to conjugated organometallic polymers that overcomes these restraints (J. Am. Chem. Soc. 2005, 127, 12496). The technique uses N-heterocyclic carbenes to integrate metals into conjugated arene-based organic polymers. Polymerization and metal incorporation are carried out by mild heating of bis(imidazolium) bromides with a palladium or platinum complex in dimethyl sulfoxide and subsequent precipitation of products. In the product structure shown, large circles represent arenes, X is a halogen, M is palladium or platinum, and R is benzyl or butyl. Bielawski and coworkers are currently studying the electronic and physical characteristics of the polymers, noting that they "should open new opportunities in polymer synthesis, conductive polymers, nonlinear optics, and electronic devices."

Currents smooth thin-film surface

Thin diamond films created by a rain of energetic carbon atoms onto a surface are amazingly smooth--a well-documented, but not well-understood, phenomenon. Now, Michael Moseler at the Fraunhofer Institute of Mechanics of Materials in Friedburg, Germany, and colleagues have modeled the process, showing that a carbon atom strikes the surface, burrowing in a few angstroms and pushing aside other atoms. This generates tiny particle currents that push and pull the atoms into valleys or dips, smoothing the surface (Science 2005, 309, 1545). "The induced currents point downhill simply because the displacement into the downhill direction requires smaller forces," Moseler says. Their predictions agree with atomic force microscopy measurements, and the authors say the effect can be generalized to other types of surfaces, such as amorphous silicon.

Boron anchors expandable ferrocene dimer

A diborylated ferrocene dimer (shown) has been synthesized and found to undergo reversible conformational changes promoted by redox chemistry or by complexation with nucleophiles. The ability to expand or contract the structural framework of the molecule at will could make it a useful building block in molecular machines, conclude Krishnan Venkatasubbaiah and Frieder J”kle of Rutgers University, Newark, N.J., and their colleagues (Angew. Chem. Int. Ed. 2005, 44, 5428). The molecule has strong boron-iron electronic interactions, and it can undergo reduction at the diboryl ring or oxidation at a ferrocene unit. The diboryl ring sits slightly tilted in the plane between the ferrocene molecules, but when the neutral compound is oxidized or reduced, the ring flattens in the plane, pushing the iron atoms farther apart. A greater expansion is observed when the neutral molecule is complexed with two equivalents of a substituted pyridine. The pyridine nitrogen atoms coordinate to the boron centers, which causes the ring to tilt even more and, coupled with steric effects, to push the iron atoms even farther apart.

Electric field swings tiny pendulum

Using a single-walled carbon nanotube as both a mechanical support and a torsional spring, scientists have built a tiny, electrically controlled pendulum (Science 2005, 309, 1539). Jannik C. Meyer and Siegmar Roth of Germany's Max Planck Institute for Solid State Research and Matthieu Paillet of France's University of Montpellier II used lithographic and etching techniques to attach a relatively large metal block onto the wall of a carbon nanotube. Applying an electric field to the system causes the nanotube to twist, moving the metal block away from its original position by almost 180ƒ. When the electric field is removed, the object returns to its original position. Meyer's group thinks that the pendulum could be used as a movable micromirror or as an important component in nanomechanical devices that require continuous tilting. The researchers also note that other small perturbances can start the pendulum swinging, making the device a potentially useful component for nanoscale force sensors.

Chemical & Engineering News

ISSN 0009-2347

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