September 15, 2008

Volume 86, Number 37

pp. 34-35

Science & Technology Concentrates

Revising C–H Oxidation

Shipping costs for natural gas limit its widespread feasibility as a fuel and are driving research toward selectively oxidizing methane—the primary compound in natural gas—to a more cheaply transported liquid, such as methanol. A proof-of-concept experiment from Roy A. Periana of Scripps Florida, William A. Goddard III of California Institute of Technology, and coworkers suggests that an alternative to traditional methane functionalization reactions might facilitate that conversion (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200802575). Established catalysts for functionalizing a methane C–H bond generate electrophilic intermediates and require acidic conditions that make it expensive to isolate methanol, Periana says. The team instead sought to functionalize nucleophilic intermediates, a strategy that's less established. The researchers formed a nucleophilic methyl intermediate by activating a methyl rhenium species with a base. By adding a second metal species, osmium tetroxide, they converted the nucleophile to methanol in quantitative yield at room temperature. Theory based on quantum mechanics indicates that the reaction proceeds through a cyclic transition state (shown). "This is an imaginative new direction in oxidation chemistry," says organometallic chemist James M. Mayer of the University of Washington.

Software Uncovers Sample Prep Protein Modifications

A new algorithm for analyzing protein mass spectrometry data has uncovered four types of previously undescribed protein modifications introduced during sample preparation (J. Proteome Res., DOI: 10.1021/pr800456q). The discovery could help prevent misassignments in protein identification due to confusion between the inadvertent modifications and cell-derived modifications. Yingming Zhao and colleagues of the University of Texas Southwestern Medical Center developed new software, called PTMap, to pinpoint sites at which proteins are modified after translation. The team used PTMap with HPLC/MS/MS to identify modifications occurring in the proteins bovine serum albumin and histone H₄ during storage, gel staining and destaining, and other types of protein preparations. The researchers detected esterification of aspartate and glutamate residues by glycerol and ethylation of those same residues. They also observed modifications to cysteine and lysine, although the specifics remain unclear. Zhao and coworkers suggest that esterification and ethylation might be prevented by avoiding acidic buffers rich in glycerol and ethanol during protein processing. They further advocate thorough assignment of all MS peaks to ensure accurate protein identification.

STM Reveals Bimetallic Nanoclusters

High-resolution scanning tunneling microscopy (HRSTM) has afforded researchers an atomic-scale view of a triangle-shaped bimetallic catalyst deposited on a silica substrate for the first time (J. Phys. Chem. C, DOI: 10.1021/jp8068464). The ability to spy the metal clusters with such precision should aid researchers in understanding how catalyst precursors are converted into highly dispersed catalysts and in determining the structure of the active catalysts on oxide supports. Fan Yang and D. Wayne Goodman of Texas A&M University, along with Eszter Trufan and Richard D. Adams of the University of South Carolina, Columbia, first prepared Ru₃(CO)₉(SnPh₂)₃ (Ph is phenyl), a catalyst with a planar Ru₃Sn₃ cluster core. The team then deposited the catalyst on an ultrathin film of silica on a molybdenum surface and heated the material under vacuum to drive off the CO and phenyl ligands. Using HRSTM, the researchers were able to see tiny triangle-shaped Ru₃Sn₃ clusters oriented along silicon chains all over the substrate surface. At the highest resolution they could discern the six individual metal atoms, confirming that the clusters had retained their integrity during processing.

Ancient Mechanism Aids Modern Enzyme

The enzyme inosine monophosphate dehydrogenase (IMPDH) catalyzes a critical two-part sequence in the synthesis of guanine nucleotides in prokaryotes and eukaryotes. First it dehydrogenates IMP to form an intermediate that is bound to a protein cysteine (Cys) residue, then the enzyme hydrolyzes the intermediate to form xanthosine monophosphate (XMP, reaction shown, R = ribose 5′-monophosphate). Now, a group led by Lizbeth Hedstrom of Brandeis University and Wei Yang of Florida State University has used a combination of computational simulations and experiments to study IMPDH's hydrolase activity and found that the enzyme likely works by transferring a proton from water to a neutral arginine residue (PLoS Biol., DOI: 10.1371/journal.pbio.0060206). When the arginine is protonated, however, the group found that the most likely mechanism involves a proton relay in which a threonine residue abstracts the proton from water while transferring its own proton to a nearby glutamate residue. The fact that the threonine and glutamate residues are conserved in a related enzyme, guanosine monophosphate reductase, led the group to propose that the threonine pathway was used in an ancestral enzyme and has been preserved in prokaryotes to satisfy nucleotide demand during proliferation.

Gentler Distillation Yields Better Gin

A superior gin results when the liquor is produced under distillation conditions that are milder than those used for the conventional method, according to Derek M. Greer and colleagues at Bacardi-Martini Product Development, in Jacksonville, Fla., and Clemson University (J. Agric. Food Chem., DOI: 10.1021/jf801308d). To compare the two methods, the researchers created a "model" gin made with spirits derived from wheat and flavored with dried juniper berries, coriander seed, angelica root, and dried lemon peel. They found that gin produced through their high-vacuum, low-temperature distillation (0.1 mm Hg, –15 °C) retains the natural flavor of the botanicals better than other distillation methods, which are carried out at atmospheric or slightly lower pressure and require heating to 50–80 °C. Also, degradation products including α-pinene and α-phellandrene are 10 times higher in the traditional gin. The new type of gin is "cleaner, and the flavor more crisp and refreshing," says Greer, who is now with Sensient Flavors & Fragrances, in Indianapolis. Bacardi is testing the technique for commercial-scale production, using a special blend of ingredients that create a gin that Greer says is "quite extraordinary, superb really."

Making Borosilicate Nanoparticles Is Now Possible

A team of researchers at the Swiss Federal Institute of Technology, Lausanne, reports the first synthesis and characterization of borosilicate nanoparticles (Nat. Nanotechnol., DOI: 10.1038/nnano.2008.262). Borosilicate glass is inert, and as such it can withstand high temperatures and harsh chemical conditions, unlike polymers or silica used to make nanoparticles. Until now, borosilicate microparticles could be formed from a glass melt, but an unstable boron oxide precursor made the fabrication of nanoparticles impossible. Virendra K. Parashar, Martin A. M. Gijs, and colleagues got around this problem by first preparing a borosilicate gel from tetraethylorthosilicate and trimethoxyboroxine, using formic acid as a catalyst and dichloromethane and 2-propanol as solvents. Exposing a droplet of the gel to water without prior contact with air incites a dynamic reaction that immediately forms the solid nanoparticles. The researchers characterized the particles, which range from 100 to 500 nm in size, by microscopy techniques and elucidated the exothermic phase-separation mechanism that forms the nanoparticles via NMR studies. The researchers say the availability of these particles will broaden the potential of nanoparticles for chemical uses, including applications in filtration membranes, optics, and medicine.

Antioxidant Treatment For Chemobrain

Battling cancer can be hellish, not only as a result of the direct effects of the malignancy but also because some patients suffer significant memory loss and attention problems. Now, Gregory W. Konat of West Virginia University and colleagues have demonstrated that chemotherapy can cause these cognitive difficulties, known as chemofog or chemobrain, and that they can be treated (Metab. Brain Dis. 2008, 23, 325). First, the researchers treated healthy rats either with saline or with adriamycin and cyclophosphamide, which are used to treat breast cancer. They then used an electric shock to train the rats to avoid a darkened compartment in an enclosure. When the rats were subsequently returned to the enclosure, those that received the chemotherapy were much more prone to enter the dangerous, dark compartment. This "profound dysfunction of short-term memory" was prevented by coadministering the antioxidant N-acetylcysteine during chemotherapy, according to the authors. The results suggest that oxidative stress caused by chemotherapy, expressed in the form of cell-damaging free radicals, harms the brain. Previous studies indicate antioxidant supplements won't interfere with the efficacy of chemotherapy, the researchers note.

Photocatalysis With Visible Light

To marshal their reagents into action, chemists often turn to the energizing power of light. Most organic compounds, however, only react when they absorb high-frequency ultraviolet light, and the specialized photoreactors needed for such syntheses allow only a few photocatalytic reactions to make it beyond the lab to large-scale operations. Recently, chemists have turned to the popular ruthenium bipyridine complex, Ru(bpy)₃²⁺, to harness energy from lower energy visible wavelengths and inject electrons into reactions with organic molecules (C&EN, Sept. 8, page 10). Tehshik P. Yoon of the University of Wisconsin, Madison, and coworkers have now applied this strategy to [2+2] cycloadditions of enones (J. Am. Chem. Soc., DOI: 10.1021/ja805387f). The researchers found they could get aryl bis(enones) to undergo intramolecular cyclization (shown) with Ru(bpy)₃²⁺, a few key additives, and a standard floodlight. The reaction proceeds in good yield, with excellent diastereoselectivity. An intermolecular dimerization of untethered aryl enones also worked well. "This approach to effecting visible light photocatalysis has enormous potential for the development of new reaction protocols with reduced environmental impact," the researchers note.

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