Noteworthy Chemistry

July 28, 2014

Dihydrofolate reductase inhibitors may combat tuberculosis. Mycobacterium tuberculosis (MTB), the bacterium that causes tuberculosis, historically is a leading cause of death and still causes more fatalities worldwide than any other bacterial pathogen. Tuberculosis is treatable, but effective multidrug therapy takes 6 months, is poorly tolerated, and is incompatible with many other medications. New drugs are needed for easier and better treatment regimens and to counter resistant MTB strains.

Antifolates interrupt the production of reduced folate cofactors, which are one-carbon donors in essential cellular processes such as the synthesis of methionine and purine. One drug target in the folate pathway is dihydrofolate reductase (DHFR), which is present in bacteria, parasites, and mammals.

The MTB DHFR enzyme is sensitive to the inhibitor methotrexate (1 in the figure), but whole-cell MTB resists this drug. D. R. Sherman and colleagues at the University of Washington (Seattle), Seattle Biomedical Research Institute, Texas A&M University (College Station), Weill Medical College of Cornell University (New York City), and Rutgers University–New Jersey Medical School (Newark) synthesized methotrexate analogues that have activity against MTB. They found that the dimethyl (2) and diethyl (3) esters of methotrexate inhibit DHFR and are potent against whole-cell MTB. 

Methotrexate and two analogues that are more potent DHFR inhibitors]

The authors also discovered that five circulating MTB strains are susceptible to 2, which can be used synergistically with other antimycobacterials. It is possible that the alkylated derivatives are active against whole-cell MTB, whereas methotrexate is not because it is more lipophilic and can break through the hydrophobic mycobacterial cell wall.

When the MTB folate pathway is disrupted, the authors found that the significant interference is with reactions that produce and use S-adenosylmethionine (SAM). The amino acid SAM is an essential donor of reactive methyl groups; depleting it interferes with cellular processes that include DNA methylation and polyamine biosynthesis. A better understanding of folate pathway inhibition will allow rational drug design and may lead to a much-needed alternative tuberculosis treatment. (Chem. Biol. DOI: 10.1016/j.chembiol.2014.04.009; Abigail Druck Shudofsky)

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Initiate and inhibit photopolymerization simultaneously. Temporal and spatial control of the kinetics of photoinduced polymerization and the subsequent gelation of the polymer product are necessary for making complex three-dimensional (3-D) objects, such as those used in holography and photolithography. Few techniques can modulate the onset of or gel formation in a polymerization reaction, making it difficult to manufacture intricate architectures over large areas.

X. Xie, Y. Wei, and coauthors at Huazhong University of Science and Technology and the National Anti-counterfeit Engineering Research Center (Wuhan, China), Tsinghua University (Beijing), the University of Colorado (Boulder), and the University of Sydney (Australia) developed a monochromatic visible-light “photoinitibitor” system that functions as an initiator and an inhibitor when photoirradiated. The system consists of a sensitizer, 3,3′-carbonylbis(7-diethylaminocoumarin), and a co-initiator, N-phenylglycine.

The functions of the photoinitibitor can be manipulated by varying its concentration. The photoinitibitor decreases the polymerization rate, delays gelation, and dramatically amplifies the gelation time difference between the constructive and destructive interference regions of an exposed holographic pattern. These effects enhance the photopolymerization-induced phase separation of liquid crystal–acrylate resins and help form holographic polymer-dispersed liquid-crystal gratings.

By using the photoinitibitor, the authors easily prepared colored 3-D images as large as 70 mm × 80 mm that can be seen by the naked eye. This single-step, cost-effective process is promising for large-scale industrial applications such as optical displays and anti-counterfeiting devices. (J. Am. Chem. Soc. DOI: 10.1021/ja502366r; Ben Zhong Tang)

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Self-folding, nanosized claws grab and hold single cells. A tool for plucking individual cells from solution would facilitate surgery on small blood vessels and analyses of abnormal cell populations. Until now, self-folding "nanogrippers" were too large to select single cells and had limited ability to fold, which prevented them from closing completely around a cell.

D. H. Gracias and coauthors at Johns Hopkins University (Baltimore) and the US Army Research Laboratory (Adelphi, MD) found a reliable way to produce self-folding anchored or free-floating grippers that can enclose single cells completely. They assembled the grippers from silicon monoxide and silica (SiO and SiO2)—biocompatible materials that dissolve in biological fluids. The grippers are optically transparent, which helps to view the entrapped cells. The gaps between the gripper arms allow nutrients, waste, and other biochemicals to flow in and out.

Prestressed SiO2–SiO bilayers are patterned into four-pointed stars, and rigid SiO plates are deposited onto the arms (see figure). When the flat stars are exposed to culture media or biological fluids, the stress in the exposed portions of the bilayer is released, causing the arms to fold upward. Finite element models show that the stress is tunable by using various materials, film thicknesses, and deposition conditions.

Preparation of single-cell grippers and their use on substrates or as untethered tools

The authors used grippers anchored to a substrate to capture single live mouse fibroblasts from a culture media solution. The captured cells remained viable, and the grippers remained anchored to the substrate.

Grippers that were partially released from the substrate entrapped single red blood cells, detached from the substrate, and carried the enclosed cells as free-floating assemblies. This system may be useful for in vivo studies.

The authors speculate that magnetic elements could guide the grippers through the circulatory system. Patterned biomarkers could allow the grippers to target specific cells in vivo. (Nano Lett. DOI: 10.1021/nl500136a; Nancy McGuire)

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Oxidant and solvent changes ready a reaction for scale-up. B. Zheng, R. J. Fox, and co-workers at Bristol-Myers Squibb (New Brunswick, NJ) describe the development of a scalable process for preparing di-tert-butyl potassium phosphate and di-tert-butyl potassium chloromethylphosphate. The second step of the synthesis is the oxidation of di-tert-butyl phosphite to di-tert-butyl phosphate.

The authors initially ran the reaction with stoichiometric amounts of iodine and pyridine in tetrahydrofuran solvent with 5 equiv water; greater amounts of water gave lower yields. The reaction mixture was then azeotropically distilled to produce the pyridinium salt, which was treated with sodium or potassium methoxide in methanol to generate sodium or potassium di-tert-butyl phosphate.

Drawbacks to using iodine, which is a controlled substance, as the oxidant are that large amounts of it are consumed, it does not appear in the product, and significant amounts of pyridinium hydroiodide waste are generated. The authors developed a second-generation process that is catalytic in iodine and uses hydrogen peroxide (H2O2) as the stoichiometric oxidant. The revised process needed a change of solvent because aqueous H2O2 causes iodine to disproportionate, which lowers the yield.

The authors chose a two-phase system with toluene solvent and added acetonitrile to ensure sufficient water in the organic phase. Monobasic potassium phosphate is added to moderate the pH during the reaction. Potassium hydroxide is added at the end of the reaction to increase the pH to 11.

The product di-tert-butyl potassium phosphate partitions into the aqueous phase; it is salted out with potassium carbonate and then extracted into acetonitrile. Azeotropic distillation causes the di-tert-butyl potassium phosphate to crystallize from the acetonitrile solution. (Org. Process Res. Dev. DOI: 10.1021/op500066f; Will Watson)

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Use iron catalysts to hydrogenate esters to alcohols. Ester hydrogenation is an important industrial process because long-chain alcohols are precursors for manufacturing surfactants, plasticizers, and solvents. Current processes for this transformation require heterogeneous catalysts such as copper chromite or homogeneous catalysts such as ruthenium or osmium complexes.

H. Guan and coauthors at the University of Cincinnati and Procter & Gamble (Cincinnati) made iron-PNP pincer complexes and tested them as catalysts for reducing model ester methyl benzoate. (PNP represents phosphorus, nitrogen, and phosphorus atoms in the pincer ligand.) They found that the best conditions—150 psig hydrogen pressure and 3 mol% iron complex 1 (see figure) in toluene solvent at 115 ºC for 3 h—gave a quantitative yield of benzyl alcohol as determined by gas chromatography and nuclear magnetic resonance spectroscopy. 

Iron-PNP pincer catalyst

A catalyst with another PNP ligand gave no product, and others needed an alkoxide base additive to be effective. The procedure with catalyst 1 was expanded to several substituted aromatic and aliphatic esters. The reaction with aliphatic substrates needed 230 psig hydrogen pressure to be effective.

A commercial sample derived from coconut oil (a mixture of C10, C12, C14, and C16 methyl esters) was hydrogenated neat with 1 mol% of the iron complex and 750 psig hydrogen at 135 ºC to produce an alcohol mixture in 98% yield. This iron-based catalytic system works well on industrial oils, but it is not yet comparable with the best ruthenium catalysts (J. Am. Chem. Soc. DOI: 10.1021/ja504034q; José C. Barros)

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Filter high–molecular weight solutes with nanopaper. The traditional definition of filtration is a physical process that separates a solid from a liquid; but organic solvent nanofiltration (OSN) has emerged as a technique that isolates nanosized solutes from solutions. OSN relies on nanopores in filtration media, such as nanofiltration membranes, to separate large solutes (several hundred to thousands of daltons molecular weight) from small solvent molecules.

Nanofibrillated cellulose (NFC), an inexpensive material that has enhanced mechanical and chemical properties, is an ideal component of nanofiltration membranes for OSN because its pore diameters are in the 1–100 nm range. A. Bismarck and coauthors at Imperial College London, University College London, the University of Vienna, and VTT Technical Research Center of Finland (Espoo) report a simple, rapid process to make OSN “nanopaper” from NFC that uses papermaking techniques.

The authors chose two types of NFCs to make nanopaper: TEMPO-oxidized NFC (NFC-O) and NFC-K processed from kraft birch pulp. [TEMPO is the stable free radical (2,2,6,6-tetramethylpiperidin-1-yl)oxy.] The materials have 5–30 nm and 50–100 nm fiber diameters, respectively. In the papermaking process, the fibers are suspended in water, filtered, dried, and hot pressed. Adding multivalent metal ions such as Mg2+ and Al3+ facilitates the flocculation process, especially for the smaller NFC-O, by increasing the ionic strength of the suspension.

Nanopaper can be produced with 10–70 g/m2 grammage (area density), which is proportional to its thickness. The permeability of solvents through nanopaper increases with increasing solvent hydrophobicity. Increasing grammage decreases permeability, and larger NFC fiber diameters give higher permeability values.

The authors evaluated the performance of the nanopaper by measuring its molecular weight cut-off, a quantity that refers to the lowest Mw of a solute when 90% of it is retained by the membrane. They show that both types of nanopaper effectively filter nanosized polymer solutes. The measurements also indicated that the pore sizes of nanopapers may be tunable by altering the size of the original NFC material. (Chem. Commun. DOI: 10.1039/c4cc00467a; Xin Su

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