Noteworthy Chemistry

July 14, 2014

Boost light emission from solutions with anion coordination. Conventional luminophores with high emission efficiencies often become weakly luminescent when their molecules aggregate. A new group of luminogens behaves in the opposite way: Their individual molecules are nonemissive, but they become highly luminescent when they aggregate.

This phenomenon, aggregation-induced emission (AIE), occurs when portions of molecules cannot rotate (restriction of intramolecular rotation, or RIR). B. Wu and coauthors at Northwest University (Xi’an) and Lanzhou University (both in China) used the RIR mechanism to boost light emission from a luminogen in solution with the recently developed technique of anion coordination.

The authors functionalized tetraphenylethylene (TPE), a quintessential AIE luminogen, with four bisurea groups. Bisureas are excellent chelating units for tetrahedral anions such as phosphate. Chelation proceeds through complementary hydrogen bonding interactions. When TPE–bisurea ligand 1 is coordinated with monohydrogen phosphate (HPO42–) ions, anionic complex 2 (see figure) is formed. 

TPE–bisurea ligand structure and its coordination complex with HPO42–

Anion coordination changes the originally nonemissive ligand into a strong emitter in solution and in the solid state. On the basis of their experimental data, especially the crystal structure of 2, the authors conclude that luminescence is turned on by the rigidification of the TPE core by the sandwiched coordination between the phosphate anions and the bisurea arms. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201402169; Ben Zhong Tang)

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Not all silver nanoparticles come out in the wash. Soldiers who swelter inside armored vehicles and athletes who work up a sweat in the gym may rely on T-shirts and socks that contain silver nanoparticles (AgNPs) to keep body odor at bay. But the AgNP antibacterial agents that are being used increasingly in consumer products are causing concern about what happens when these particles are released into the environment (see cartoon). 

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Cartoon illustrating silver release from clothing

J. Hedberg and co-workers at the KTH Royal Institute of Technology (Stockholm) monitored changes in ionic release and particle properties when samples of uncoated AgNPs were exposed to standard “sweat” solutions (NaCl, urea, and lactic acid), laundry detergent (alcohol ethoxylates, dodecylbenzenesulfonate, and pH buffer), and fresh water (NaHCO3, KCl, CaCl2•H2O, and MgSO4•7H2O). They exposed some of their samples to each solution in sequence; others were exposed to only one solution. They also compared laundry detergents with and without zeolites.

The authors found that, in all cases, the amount of silver released was <1% for the single-solution tests and <0.5% of the total amount for the sequential tests. All AgNP samples released comparable amounts of silver into the first solution to which they were exposed, regardless of the solution makeup. The AgNPs exposed to zeolite-containing detergent solutions were an exception: They released far less silver. In particular, samples exposed to artificial sweat, detergent, and fresh water in that order (simulating body contact, washing, and release into the environment) released substantially more silver to the first solution than to the last two.

The authors found evidence that soluble silver on the nanoparticle surfaces was released during exposure to the first solution. The remaining particles were less likely to release silver on the second and third exposures. (Environ. Sci. Technol. DOI: 10.1021/es500234y; Nancy McGuire)

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It’s all about the solvent and the base in dicyclopropylamine synthesis. B. Mudryk, B. Zheng, and co-workers at Bristol-Myers Squibb (New Brunswick, NJ) describe an efficient three-step synthesis of dicyclopropylamine from cyclopropylamine, 4-nitrobenzenesulfonyl chloride (nosyl chloride), and cyclopropaneboronic acid (CPBA). In the first step, cyclopropylamine is protected by treating it with nosyl chloride to produce one component for the subsequent Chan–Lam coupling with CPBA.

The original reaction was run in dichloromethane (CH2Cl2) solvent and Hunig’s base (i-Pr2NEt). The authors used an aqueous workup that required adding a second solvent (e.g., 2-methyltetrahydrofuran) because the nosylate is poorly soluble in CH2Cl2. An improved protocol uses ethyl acetate as the solvent, an aqueous wash to remove i-Pr2NEtHCl, a solvent swap to n-heptane, and finally crystallization.

An even better option is using toluene as the solvent and triethylamine (Et3N) as the base. In this case, the product crystallizes from the reaction mixture; adding water to dissolve Et3NHCl leads to a three-phase mixture that filters rapidly. Et3N is preferred to i-Pr2NEt because Et3NHCl is more easily purged to the aqueous phase during isolation. (Org. Process Res. Dev. DOI: 10.1021/op500031z; Will Watson)

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How does fullerene C60 dissolve in water? Buckminsterfullerene C60, a nanocarbon that may have many biological applications, is inherently hydrophobic because of its structure. It has been widely reported, however, that C60 can exist in water as a stable colloid. The properties of fullerene aqueous colloid solutions (C60FAS) depend on the way they are prepared (e.g., extended mixing, sonication, or solvent exchange).

Two main hypotheses have been proposed to explain the stability of C60 in aqueous solutions: hydrogen-bonding by a water shell around the molecules and hydroxylation of C60 by water. M. P. Evstigneev and coauthors at Taras Shevchenko National University of Kyiv (Ukraine), the Joint Institute for Nuclear Research (Moscow), Belgorod State University (Russia), the Institute of Physics of the National Academy of Sciences of Ukraine (Kyiv), and Ilmenau University of Technology (Germany) identified the mechanism for the stabilization of C60 in water in two C60FAS preparations. [The authors use “stabilization” and “solubility” of C60 in water interchangeably.—Ed.]

The authors prepared C60FAS samples from high-purity (>99.99%) C60 in two ways:

  • ultrasonication of C60 powder and water 
  • solvent exchange between water and a C60–toluene solution under ultrasonic conditions.

Atomic force microscopy and small-angle neutron scattering experiments showed that in both cases C60 forms 0.7–90-nm polydispersed aggregates with long-term stability. More importantly, both C60FAS samples exhibited strong C–O vibrations in Fourier transfer infrared spectra, which indicates that hydroxylation of the fullerene cage (see figure) occurs during sample preparation.

Hydroxylated fullerene molecule

This study presents compelling support for the hydroxylation solubilization mechanism, which would stabilize individual C60 molecules and clusters. In addition, it sheds light on how to improve the bioavailability of molecules in the fullerene family for biomedical applications. (Langmuir DOI: 10.1021/la404976k; Xin Su)

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How does HIV infection cause T-cell death? People with untreated HIV infections have significantly lower numbers of T cells than before they were infected. A major cause of T-cell depletion during HIV infection is thought to be apoptosis (programmed cell death), but no one is sure how the HIV-1 virus triggers apoptosis. It has been suggested that the HIV-1 protease plays a role in this because it can cleave some cellular proteins, but no connection has been found between protease activity and apoptosis. Until now, that is.

M. Rumlová and collaborators at the Academy of Sciences of the Czech Republic (Prague) and the Institute of Chemical Technology Prague show that the HIV-1 protease does induce cell death. They found that once the virus infects a cell, the protease heads straight to the mitochondria, where it causes a decrease in the organelle’s membrane potential, a typical indicator of the onset of apoptosis. The viral protease can also activate cellular proteins such as caspases, which set off signaling cascades that result in cell death.

The authors’ work suggests that the HIV-1 protease is directly involved in making the mitochondrial membrane permeable, a process that occurs in the intrinsic mitochondrial apoptosis pathway (as opposed to the extrinsic, nonmitochondrial apoptotic pathway). This apoptotic pathway is the one commonly used for cell death in humans. The authors speculate that the HIV-1 protease cleaves mitochondrial proteins and thus interferes directly with the integrity of the mitochondrial membrane and initiates apoptosis. (Retrovirology DOI: 10.1186/1742-4690-11-37; Abigail Druck Shudofsky)

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tert-Butyl isocyanide replaces CO in quinazolinone synthesis. Quinazolinones are well-established building blocks in organic and medicinal chemistry. They usually are prepared by the reaction of 2-aminobenzoic acid derivatives with aldehydes or carboxylic acids or by the insertion of carbon monoxide, a one-carbon source, into diamines. S.-J. Ji and co-workers at Soochow University (China) report a different one-pot synthesis of quinazolinones that uses tert-butyl isocyanide (t-BuNC) as a substitute for carbon monoxide.

The authors’ model system uses anthranilamide as the diamine substrate and iodobenzene as the aryl halide reactant. They found that the best reaction conditions are PdCl2 catalyst (5 mol%), 1,3-bis(diphenylphosphino)propane (DPPP, 10 mol%) as the ligand (L), t-BuONa as the base, CaCl2 as a drying agent, and toluene solvent at 145 ºC (see figure). The isolated yield is 93%. They expanded the reaction to several substituted anthranilamides and substituted aryl halides; the yields ranged from 41 to 93%.

Synthesis of quinazolinones and proposed mechanism

The authors propose a mechanism that consists of an oxidative addition of Pd(0) to the halide, insertion of the isocyanide, addition of the anthranilamide with recovery of Pd(0), and cyclization to produce the quinazolinone and eliminate tert-butylamine. This is a simple, direct one-step route to quinazolinones without the need for toxic carbon monoxide. (J. Org. Chem. DOI: 10.1021/jo500636y; José C. Barros)

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