POLYTHEONAMIDES ARE BORN OF RIBOSOMES
VISIBLE LIGHT PHOTOSWITCH
A deep dive into the DNA of a marine sponge has settled the question of where the giant natural-product toxins called polytheonamides get their start—on ribosomes of the sponge’s symbiotic bacteria, not through a nonribosomal peptide synthetase, as previously believed (Science, DOI: 10.1126/ science.1226121). Researchers had thought a nonribosomal route seemed logical, because polytheonamides contain multiple unusual amino acids. However, a team led by Jörn Piel of the Swiss Federal Institute of Technology, Zurich, and the University of Bonn surmised that the large size of the polytheonamides—48 residues—pointed to a ribosomal origin. They confirmed their hunch by running the polymerase chain reaction on samples of Theonella swinhoei, a sponge that harbors a host of symbiotic microbes. They found a bacterial gene cluster encoding both a peptidic polytheonamide precursor and a suite of enzymes. With mass spectrometry, they have confirmed so far that three of the enzymes contribute to the 48 chemical transformations needed to convert the precursor into polytheonamides, including dehydration and epimerization steps. The study reveals a whole new set of modifications to ribosome-synthesized natural products, says Wilfred A. van der Donk of the University of Illinois, Urbana-Champaign. “I find this work very exciting.”—CD
With applications in molecular machines and protein probes, azobenzene might have one of the most useful cis-trans isomerizations in chemistry. Yin Yang, Russell P. Hughes, and Ivan Aprahamian of Dartmouth College have now used azobenzene’s skill to cut a path to a visible light switch (J. Am. Chem. Soc., DOI: 10.1021/ja306030d). To minimize damage to living samples, researchers have been pushing to make azobenzene’s reversible isomerization occur by visible light instead of the ultraviolet light typically required. Most
PNAS
PIXELATED PLANT The brilliant blue hue of the Pollia condensata fruit does not arise from pigments, but rather from the layered structure of nanoscale cellulose fibrils in its skin’s cell walls, researchers report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1210105109). A team led by Ullrich Steiner of Cambridge University examined a sample of Pollia condensata collected in Ghana in 1974 and preserved at the Royal Botanic Gardens. The scientists were unable to extract any type of pigment from the fruit, so they studied its anatomy to figure out the origin of its unusual metallic blue color. They found that the fruit’s multilayered cell walls act like tiny reflectors, thanks to the helical cellulose The brilliant blue fruit of Pollia condensata.
Visible light interconverts the trans (magenta) and cis (orange) isomers of the Dartmouth team’s azo-BF2 compound.
attempts have been bedeviled by heat-induced cis-trans isomerization or by side reactions. The Dartmouth team extended the conjugation of azobenzene by tacking on a larger aromatic group and then complexed that molecule with the Lewis acid BF2. Shining 570-nm light on the trans isomer converts it to cis, accompanied by a color change from magenta to orange; 450-nm light accomplishes the reverse. The probe undergoes faster cis-trans isomerization in an oxygenated solvent versus a deoxygenated solvent. The researchers are trying to understand and control this oxygen effect and plan to explore the stability of related compounds in water. “There’s a lot of room to play around with the molecule,” Aprahamian says, including modifications to the aromatic skeleton and the Lewis acid.—CD
fibers within them. Each cell reflects a specific color. Blue reflectance dominates, but there are also cells that reflect red or green, giving the fruit a pixelated appearance. Depending on the orientation of the helices, the fruit can reflect both left- and rightcircularly polarized light. “The bright blue coloration of this fruit is more intense than that of any previously described biological material,” the researchers point out.—BH
E ALKENES FROM ALKYNES A practical method for making E (trans) alkenes from alkynes has been developed by chemists in Germany (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201205946). As organic chemistry students are taught, making alkenes with Z (cis) stereochemistry from alkynes is a simple matter of semihydrogenation with a poisoned catalyst. But until WWW.CEN-ONLIN E .ORG
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now, there has been no broadly applicable way to directly turn alkynes into E alkenes. Karin Radkowski, Basker Sundararaju, and Alois Fürstner of the Max Planck Institute for Coal Research report that a rutheniumbased catalyst is capable of performing just such a transformation. The hydrogenation proceeds in good yields and with high Eselectivity. The transformation tolerates a range of functional groups, including esters, amides, carboxylic acids, ketones, primary alcohols, methyl and silyl ethers, and an elimination-prone primary tosylate. Reducible sites, such as a nitro group, an alkyl bromide, the N–O bond of a Weinreb amide, an aromatic nitrile, and a terminal alkene also remain intact under the reaction conditions. Adding silver triflate also improves the reaction. “The new method constitutes the first practical, efficient, functional-grouptolerant, broadly applicable, and highly E-selective semihydrogenation protocol for alkynes,” the researchers note.—BH
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A highly water-repellent silicon-based coating wards off the effects of a violent boiling process that can occur when hot solids contact water, according to a study published in Nature (DOI: 10.1038/nature 11418). The finding may lead to chemical treatments for equipment used with water in high-temperature settings, such as nuclear power reactors. The familiar way in which water droplets dance across the surface of a hot iron or frying pan results from a levitating vapor film that remains stable when the surface temperature is above a critical value. Ivan U. Vakarelski of King Abdullah University of Science & Technology, in Saudi Arabia; Neelesh A. Patankar of Northwestern University; and coworkers used high-speed photography to VIDEO ONLINE monitor boiling
The method combines photoswitchable fluorescent labels with total internal reflection microscopy, which restricts illumination to a thin layer at a glass-water interface. The combination works at micromolar concentrations, whereas other single-molecule imaging methods are limited to nanomolar concentrations. The researchers first label a protein of interest with a photoswitchable protein and allow it to interact with an immobilized substrate. They then illuminate the protein with a wavelength of light that switches the label from one fluorescent form to another. After a few hundred milliseconds, during which unbound proteins diffuse away, they excite the bound proteins with another wavelength and image the resulting fluorescence. To demonstrate the technique, the researchers tagged and imaged flap endonuclease, a protein involved in stitching together the discontinuous lagging strand of DNA during replication. The results suggest that origins of replication are closer together and more frequent than had been measured with other methods.—CHA
NANOPARTICLES SENSE ULTRALOW LEVELS OF TOXIC IONS processes on steel balls chemically treated to make their surfaces vary from hydrophilic to superhydrophobic. In one case, they found that immersing 20-mm hydrophilic balls heated to more than 400 °C in water caused the water to boil at the steel surface gently in a nearly bubble-free manner known as film boiling (shown, left). As the surface temperature cooled to 275 °C, the protective vapor layer collapsed, leading to an explosive transition to the “nucleate boiling” regime (right). A textured superhydrophobic coating completely suppresses the violent transition, they report.—MJ
SINGLE-MOLECULE IMAGING BREAKS CONCENTRATION BARRIER A new fluorescence method extends the concentration range of single-molecule imaging, enabling the technique to now work at physiologically relevant concentrations, Antoine M. van Oijen of the University of Groningen, Johannes C. Walter of Harvard Medical School, and coworkers report (Nat. Methods, DOI: 10.1038/nmeth.2174).
An electronic sensor coated with patterned nanoparticles can detect concentrations of the toxic ion methylmercury down to attomolar (10–18 M) levels in water (Nat. Mater., DOI: 10.1038/nmat3406). This detection limit—about 600 methylmercury cations per mL of solution—is far below what other analytical methods, such as atomic fluorescence spectroscopy, can achieve, according to the research team that designed the sensor. To fabricate the device, Bartosz A. Grzybowski of Northwestern University; Francesco Stellacci of the Swiss Federal Institute of Technology, Lausanne; and colleagues produced gold nanoparticles covered with alternating rows of short and long molecular bristles: n-hexanethiols and n-hexanethiols tipped with polyethylene glycol units. The team cast a film of the particles onto a glass substrate between two electrodes. When dipped into water containing methylmercury, the particles selectively trap the cations among their bristles. After the sensors are dried, the researchers measure this capture via an increase in conductance across the films. The researchers tested their sensors on water from Lake Michigan and on fish collected in Everglades National Park. The methyl-
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Patterned nanoparticle
mercury levels determined from the sensor data agreed within error to published values for the lake and values measured by the U.S. Geological Survey for the fish.—LKW
BRANCHED NANOTUBES INSPIRE NANOCAGES Researchers in Japan have achieved an innovation in nanoarchitecture by preparing the first all-benzene nanocages (Chem. Sci., DOI: 10.1039/c2sc21322b). A team led by Yasutomo Segawa and Kenichiro Itami of Nagoya University in collaboration with Kenji Kamada of the National Institute of Advanced Industrial Science & Technology created a cyclic precursor by cobbling together six cyclohexane-based L-shaped units and two benzene-based three-way units via crosscoupling and homocoupling reactions. The team used acid-mediated aromatization of the cyclohexane rings to reach the final cage structure. The researchers had initially recognized the nanocage structure as a junction unit in branched carbon nanotubes. They were inspired to try to make
Japanese chemists synthesized a carbon nanocage that occurs in branched nanotubes.
it by analogy to carbon nanoring units in straight carbon nanotubes. In addition to the aesthetic appeal, the nanocage could find application in logic gates or transistors and in host-guest chemistry. The researchers note that the high fluorescence quantum yield and large two-photon absorption cross section of the nanocage could be useful in optoelectronic applications.—SE
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