Noteworthy Chemistry

September 22, 2014

Make copolymer microbeads for white-light emission. White-light–emitting materials, especially in the solid state, are highly desirable in chemistry research and for lighting devices. There are many strategies for making these materials, but polymer-based white-light emitters are of particular interest because of their high modularity and their flexibility in design and preparation.

S. L. Sonawane and S. K. Asha* at CSIR-National Chemical Laboratory (Pune, India), the Academy of Scientific and Innovative Research (New Delhi), and CSIR-Network Institutes of Solar Energy (New Delhi) developed a polymer-based route to solid-state white-light and multicolor emission from doped polystyrene microbeads.

The researchers chose two classic fluorescent cores, perylenebismide-tetraethylene glycol (PBITEG) and oligo(p-phenylenevinylene) (OPV), and functionalized them into acrylate cross-linkers. The beads made from the copolymer of styrene and PBITEG emitted orange-red light; styrene–OPV copolymer beads gave blue emission. By tuning the PBITEG/OPV ratio in polystyrene polymer formulations, the authors obtained microbeads that emitted almost pure white light (see figure).

White light emitted from the proper combination of copolymer microbeads

This strategy for achieving solid-state white-light emission cleverly solves the fluorescence quenching problem that occurs in similar polymers by introducing fluorophores into polymer matrices as cross-linkers. In the form of highly processible polymer beads, they can be easily integrated into various devices and applications. (J. Phys. Chem. B DOI: 10.1021/jp504718m; Xin Su)

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A duplicated gene gives vaccinia virus resistance to rifampin. Poxviruses are large, enveloped, double-stranded DNA viruses that replicate in the cytoplasm of infected cells. The model poxvirus is vaccinia, which was used to eradicate smallpox. Vaccinia proteins D13 and A17 interact with each other and are essential for morphogenesis during viral assembly.

Rifampin (also called rifampicin) is a drug that prevents poxvirus replication by inhibiting viral assembly. All previously known rifampin-resistant vaccinia isolates have point mutations in the D13 open reading frame (ORF), which suggests that the drug inhibits vaccinia assembly by interacting directly with D13.

B. Moss and colleagues at the National Institutes of Health (Bethesda, MD, and Hamilton, MT) screened for rifampin-resistant viruses. They found that whereas the majority of drug-resistant isolates were altered in the D13 ORF, one resistant virus was unchanged in the D13 ORF but had a partial duplication of the gene encoding A17. This caused the expression of a second, truncated copy of the A17 protein (A17T), which, as the authors predicted, was missing 69 amino acids, including a transmembrane segment. A17T was still able to bind D13 and to be incorporated into viral membranes.

The authors found that even partially duplicating A17 in vaccinia virus conferred rifampin resistance, although the drug tolerance came at the fitness cost of a lower viral yield. When the resistant viral isolate was grown without rifampin, viral loss of drug resistance correlated with its loss of A17T expression.

This finding is the first evidence of a new rifampin-resistance mechanism for poxviruses: not a point mutation, but a double read of the A17 ORF. It supports other instances of poxviruses protecting against antiviral actions by duplicating their genes. Gene duplication may let DNA viruses with high fidelity polymerases quickly protect themselves against environmental changes. (J. Virol. DOI: 10.1128/JVI.00618-14; Abigail Druck Shudofsky)

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This molecule changes emission color when mercury is around. Mercury, in the form of Hg2+, is a neurological toxin that is widely distributed in the environment by natural processes and human activities. Optical techniques for detecting mercury in the field and in living systems have the advantages of being simple, noninvasive, and sensitive. However, few existing fluorescence probes for Hg2+ are compatible with aqueous systems.

A. Misra and co-workers at Banaras Hindu University (Varanasi, India) designed and synthesized a simple molecular fluorescence probe that consists of a central anthracene molecule attached to two benzhydryl groups by piperazine bridges. This photoinduced electron transfer (PET) probe is highly selective for Hg2+, and it can detect concentrations as low as 2 ppb, the US Environmental Protection Agency’s “maximum tolerable level” in drinking water.

A positive response to Hg2+ is easily visible as the color of the probe changes from a fluorescent blue to blue-green. Other common cations do not cause this color change (see figure). 

Hg2+ response to probe molecule

The authors tested their probe on crude-water samples, cellulose paper strips, protein media, and HeLa (“immortal” cancer) cells. The probe molecule exhibits high cell permeability, low toxicity, and good sensitivity in vivo and in protein media.

The probe molecule can be reset to its original state by introducing ethylenediaminetetraacetic acid (EDTA), which binds Hg2+ more strongly than does the probe molecule. This allows the probe molecule to be used for as many as 10 cycles. Concentrations can be estimated by comparison with a standard calibration curve.

The probe can also serve as a logic gate, which may be useful for making molecular switches and other electronic devices. Adding Hg2+ to the probe molecule switches its fluorescence from “OFF” to “ON”. Other ions can be added to produce logic circuits with “INHIBIT”, “TRANSFER”, and “OR” gates. Adding a base such as hydroxide of phosphate resets the circuit so that it can be reused. (Anal. Chem. DOI: 10.1021/ac501780z; Nancy McGuire)

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When others fail, use butyronitrile as a solvent. U. Santhosh and coauthors at AstraZeneca (Bangalore, India; Macclesfield, UK; and Gaithersburg, MD) and Sandoz (Kundl, Austria) describe the development of scalable processes for making 3-hydroxyisoxazole and for its subsequent coupling with 5-hydroxymethyloxazolidinone. They also performed a significant amount of hazard assessment associated with the stability of the compounds involved.

The authors developed a process for making 3-hydroxyisoxazole from ethyl propiolate and hydroxylamine in aqueous tetrahydrofuran that uses sodium hydroxide as the base. Several extraction solvents were screened; dichloromethane (CH2Cl2) gave the best balance of impurity profile and product solubility. But attempts to remove water and ethanol from the CH2Cl2 solution were unsuccessful; even small quantities of ethanol or water had a detrimental effect on the subsequent coupling step.

A search for solvents that form azeotropes with water and ethanol identified butyronitrile as the best option. Extraction with butyronitrile provided 3-hydroxyisoxazole as an 8–12% solution that was used directly in the next step. (Org. Process Res. Dev. DOI: 10.1021/op500063g; Will Watson)

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Use graphene oxide to produce strong chemiluminescence. Graphene oxide (GO) is a useful graphene derivative, but its exact composition and structure have not been resolved. This hydrophilic material has an sp2-planar structure (i.e., nanosheets) and contains several functional groups. It has applications in molecular recognition, photoluminescence, adsorbance, and catalysis.

Z. Zhang and coauthors at the University of Science & Technology of China (Hefei), the Chinese Academy of Sciences (Hefei), and the Institute of Materials Research and Engineering (Singapore) report the use of GO to initiate chemiluminescence. They prepared one GO variant (GO-1) by the Hummers method: Graphite is oxidized with P2O5 and K2S2O8, then the product is exfoliated and further oxidized to GO nanosheets with H2SO4 and KMnO4. Excess KMnO4 is removed by adding H2O2. Production of a second form, GO-2, excluded the H2O2 step. Both variants then underwent several purification steps.

The GOs were similar in appearance, but only GO-1 could oxidize iodide ion to molecular iodine. Adding KI and starch formed a blue complex that indicates the presence of I2.

The authors used the oxidizing properties of GO-1 to trigger the intense blue chemiluminescence of luminol. The observed chemiluminescence was greater than that obtained by using Fenton’s reagent (FeSO4–H2O2) or horseradish peroxidase. The authors attribute this result to the presence of several  π-conjugated carbon radicals. After luminescence, the spent GO-1 can be re-treated with oxidants and reused.

This study provides insight into the structure of GOs and new uses for them. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201405295; José C. Barros)

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