Science Concentrates

Lessons from lotus leaves

A surface of micrometer-sized hills and valleys dotted with waxy nanoparticles gives the lotus leaf its superhydrophobic self-cleaning properties. Water droplets bead up and roll off the rough surface, taking dirt and debris with them. Using a simple, water-based process, researchers from MIT have created a polyelectrolyte multilayer coating that mimics the leaf's tidy topography [Nano Lett., published online May 18, http://dx.doi.org/10.1021/nl049463j]. The group, led by Robert E. Cohen and Michael F. Rubner, first creates micrometer-sized pores in a polyelectrolyte surface (shown) via multiple low-pH treatments. They then add nanoscale texture by depositing silica nanoparticles onto the material, followed by a semifluorinated silane coating. The material retains its superhydrophobic character even after being immersed in water for a week. By eliminating the semifluorinated silane coating step, the team can make the material superhydrophilic.

Alternating lasers analyze single molecules

A new fluorescence spectroscopy method uses FRET (fluorescence resonance energy transfer) with alternating lasers to analyze macromolecular structure and interactions simultaneously at the single-molecule level [Proc. Natl. Acad. Sci. USA, published online June 2, http://www.pnas.org/cgi/doi/<br > 10.1073/pnas.0401690101]. Chemistry professor Shimon Weiss and his colleagues at UCLA use two lasers to switch rapidly between donor and acceptor fluorophore excitations used in FRET. One laser excites the donor directly and can excite the acceptor if the two are within FRET range. Fluorescence measurements obtained with this laser provide a ratio that reveals the FRET efficiency, which reports the distance between donor and acceptor fluorophores. The other laser excites the acceptor fluorophore but not the donor. This yields a second distance-independent fluorescence ratio that is sensitive to the stoichiometry and relative brightness of the fluorophores. The latter ratio can be used to monitor oligomerization and changes in the local environment. Weiss and his colleagues combine the two ratios to monitor protein-DNA interactions.

Osmabenzenes made easy

Interest in metallabenzenes--in which a metal atom formally replaces a CH group in benzene--lies in their aromaticity and ability to mediate organic reactions. A team led by Guochen Jia at Hong Kong Uni versity of Science & Technology has recently developed a convenient way to prepare osmabenzenes (shown; Ph = phenyl). By treating dichloro- or dibromotris(triphenylphosphine)osmium with readily accessible 1,4-pentadiyne-3-ol, Jia's team has made the first air-stable phosphonium salts of metallabenzenes [J. Am. Chem. Soc., 126, 6862 (2004)]. The researchers suggest that the reaction is initiated by coordination of one terminal carbon-carbon triple bond of the dialkyne to osmium, replacing one of the triphenylphospine ligands. Successive nucleophilic attacks on the two carbon-carbon triple bonds by two triphenylphosphines followed by dissociation of the hydroxyl group in the original diyne lead to formation of an osmabenzene phosphonium salt.

Pore reveals how it moves fat

An X-ray study of the membrane protein that bacteria use to take up fatty acids from their environment sheds light on how this transporter works. Cells use fatty acids as an energy source and to make membrane components and signaling molecules. Although fatty acids can cross membranes spontaneously, many organisms have developed proteins to transport them. But it remains unclear how these transporters work. Now, a team including Howard Hughes Medical Institute investigator Tom A. Rapoport and postdoc Bert van den Berg of Harvard Medical School reports crystal structures of the model fatty acid transporter FadL from Escherichia coli at 2.6-Å and 2.8-Å resolution [Science, 304, 1506 (2004)]. They find that FadL is a barrel-like protein spanning the membrane with a hydrophobic fatty-acid-binding pocket near its extracellular entrance. No channel connects this pocket to the inside of the cell; instead, a hatch plugs the barrel. The team suggests that spontaneous conformational changes in the hatch open a passageway for fatty acids to diffuse into the cell.

Diborane loses its color for F^–

By taking advantage of the known ability of borane Lewis acids to bind small anions, postdoctoral researcher Stéphane Solé and chemistry professor François P. Gabbaï of Texas A&M University have designed and synthesized a charge-neutral diborane ligand that has a greater affinity for fluoride ion than other borane receptors. The diborane also loses its bright yellow color when it forms the chelate complex shown, making it the first colorimetric diborane fluoride sensor [Chem. Commun., 2004, 1284]. The researchers studied the ligand and the fluoride complex by spectroscopy, X-ray crystallography, and density functional theory calculations, noting shifts in spectral data and boron-fluorine bond formation that coincide with the loss of color. These changes don't occur when the diborane is treated with chloride, bromide, or iodide ions, they note, indicating that fluoride complexation is a function of the small size of the ligand's binding pocket. Fluoride ion sensors are of interest for the study and treatment of osteoporosis, for dental care, and even for detecting fluoride as a by-product of sarin nerve agent, they note.