Noteworthy Chemistry

October 24, 2011

A single luminophore emits three colors of visible light in response to external stimuli. Stimulus-responsive luminescent materials have potential applications in sensory, memory, security, and display systems. These materials usually have two states, “on” and “off”. Luminescence switching among multiple colors has rarely been achieved.

Y. Sagara and T. Kato* at the University of Tokyo developed a luminescent liquid crystal (LC) system that emits three luminescent colors. The colors can be switched from one to another by mechanical and thermal stimuli.

The LC consists of equimolar amounts of a wedge-shaped dendrimer and a dumbbell-like dendrimer that has a luminophoric core of 9,10-bis‍(phenylethynyl)‍anthracene. When irradiated at 365 nm, this mixture forms a thermotropic micellar cubic phase that emits red-orange light. Mechanically shearing the cubic phase at 90 °C causes a phase transition to a columnar phase and changes the emission color to green.

When the mixture in the micellar cubic or columnar phase is mechanically sheared at room temperature, it forms an unidentified mesomorphic phase that emits yellow light. Heating either of the shearing-induced phases to 145 °C liquefies the crystals. Cooling the isotropic liquid to room temperature regenerates the micellar cubic phase. (Angew. Chem., Int. Ed. 2011, 50, 9128–9132; Ben Zhong Tang)

Use a noncovalent strategy for multiscale protein fiber assembly. C. Meier* and M. E. Welland at the University of Cambridge (UK) used protein and interfacial assembly and wet-spinning to generate hierarchical amyloid protein nanofibers (PNFs) derived from the hen egg white lysozyme (HEWL). The HEWL-derived protein self-assembles into high-modulus nanofibrils 2.6 ± 0.7 nm in diam and >10 µm long.

Wet-spinning a low-pH PNF solution in contact with a solution of an anionic polyelectrolyte gellan gum produces a thin interfacial layer via electrostatic attraction. Continuous withdrawal of this interfacially assembled film yields macroscopic fibers with diameters that depend on the balance between charge density and RG, the ratio of gellan gum to PNF concentration. The wet-spun fibers can be tuned between 50 and 70 µm by varying RG.

The authors observed an internal lamellar microstructure with the β-sheet PNF nanofibers aligned along the long axis. They describe the hierarchical organization of the fiber structure with aligned PNF nanofibers assembled via noncovalent interactions and bonded electrostatically to the amorphous gellan.

Tensile strength, extensibility, and elastic modulus of the wet-spun amyloid nanofibers can be modulated by changing concentration and pH. At pH 4 and RG 0.13, for example, the elastic modulus is 9.9 ± 3.6 GPa; it decreases to 6.8 ± 2.2 GPa when RG is lowered to 0.10. These macrofibers display properties superior to most protein-based materials.

The authors demonstrated the drug delivery potential of the fibers, with release profiles modulated by pH-dependent gellan–PNF interactions. They also investigated biomineralization capabilities and found that the mineral brushite (CaHPO4·2H2O) forms preferentially along the PNF–gellan macrofiber axis and has a bending modulus of 18.7 ± 0.87 GPa. (Biomacromolecules 2011, 12, 3453–3459; LaShanda Korley)

pH-sensitive nanoparticles provide site-specific drug delivery. The use of polymeric nanoparticles (NPs) as drug-delivery systems is well established, but many particles do not provide local, controlled release at the biological target site. Therapeutic NPs that are sensitive to local pH environments are being studied.

V. HÉroguez and coauthors at the University of Bordeaux 1 (Pessac, France) and the University of Bordeaux Segalen (Bordeaux, France) describe a drug-delivery method based on a copolymer system that contains the antibiotic gentamicin sulfate (GS). The pH sensitivity of the GS–copolymer imine linkage takes advantage of the acidic microenvironment at the infection site. This technique also facilitates drug delivery at the infection site by incorporating carboxylic acid groups that anchor the NPs onto the biomaterial surface.

The copolymer matrix is formed by the ring-opening metathesis polymerization (ROMP) of norbornene (1) with pH-sensitive α-norbornenyl-poly‍(ethylene oxide) (PEO) macromonomers. α,ω-Macromonomer 2 contains terminal carboxylate groups to anchor the NPs, and macromonomer 3 is terminated by GS via a pH-labile linkage.

An important consideration in selecting norbornene and PEO polymers is their biocompatibility with human tissue. The researchers chose GS because of its broad-spectrum efficacy for preventing infection in settings such as orthopedic surgery.

The ROMP technique forms functionalized copolymer 4 as monodispersed NPs that consist of a hydrophobic polynorbornene core and a hydrophilic PEO-based shell; Cy is cyclohexyl. The NP configuration is illustrated by drawing 4 in the figure. The NPs are ≈350 nm in diam, and their GS drug densities are ≈0.3 mmol/g NP. The drug density can be varied by adjusting the macromonomer ratio. Drawing 5 depicts the release of GS; the carboxylate groups are retained to tether the NP at the infection site.

The authors evaluated the in vitro activity of NP–drug combination 4 by using a Staphylococcus epidermidis bacterial strain. The minimum inhibitory concentration (MIC) of the drug was <0.5 μg/mL. They used buffered solutions (pH 4–7) to simulate the acidic pH typically observed at an infection site. No bacterial inhibition occurs at neutral pH, but the MIC decreases significantly at pH 4–5, reflecting the cleavage of the acid-labile link and the release of up to 80% of the GS—after 5–7 days.

In the authors’ system, all of the active molecules are located on the outer shell of the NPs, instead of being partially trapped inside the polymer network. With the pH control built into this system, drugs are released only in response to the acidic environments of infection sites. More acute infections produce lower local pH conditions, which would accelerate drug release.

This NP production method makes it possible to introduce two or more drugs simultaneously, a significant advantage for treating antibiotic-resistant microorganisms. (Macromolecules 2011, 44, 7879–7887; W. Jerry Patterson)

The anion controls graphite intercalation in ionic liquids. Highly oriented pyrolytic graphite (HOPG) undergoes surface changes (blisters or lattice damage) during the electrochemical intercalation–deintercalation of ions in aqueous electrolytes such as H2SO4, HClO4, and HNO3. These changes in the HOPG surface are believed to result from initial intercalation of the electrolyte and water into HOPG layers followed by gas evolution at subsurface active sites.

With the use of electrochemical atomic force microscopy (EC-AFM), P. R. Singh and X. Zeng* at Oakland University (Rochester, MI) observed similar surface changes for HOPG even with anhydrous ionic liquids (ILs) as the electrolytes. Because ILs are composed entirely of ions, surface changes to the HOPG must be assigned primarily to electrochemical intercalation–deintercalation of anions from the IL.

During electrochemical intercalation–deintercalation experiments at potentials of +0.8 V or less versus a silver quasi–reference electrode, the authors observed reversible surface changes for HOPG in the IL 1-butyl-3-methylimidazolium hydrogen sulfate ([bmim][HSO4]), which contains the small, readily intercalated HSO4 ion. Conversely, these changes were absent in 1-butyl-3-methylimidazolium bis‍(trifluoromethylsulfonyl)‍imide ([bmim][NTf2]), which contains the large, weakly coordinating superacid-derived NTf2. The authors demonstrate that the electrochemically mediated intercalation–deintercalation of IL ions into HOPG layers can be controlled by the choice of the IL anion.

At potentials of +1.2 V or greater, the HOPG surface degrades and generates carbon nanoparticles (NPs) in either IL. When [bmim][HSO4] is the IL, the NPs are denser and smaller (15–30 nm) than particles formed in [bmim][NTf2] (30–80 nm). This result confirms the facile intercalation of HSO4 ions into multiple HOPG layers.

The slow intercalation of NTf2 ions into HOPG at +1.2 V (the surface oxidation potential of HOPG) means that the ions penetrate fewer layers than HSO4. This leads to the sequential removal of HOPG layers and exposes a freshly cleaved HOPG surface in each oxidation potential cycle. This result may be useful in electrocatalytic applications that benefit from a fresh electrode surface after a particular reaction step. (J. Phys. Chem. C 2011, 115, 17429–17439; Gary A. Baker)

Difluoromethylate a phenol on the multikilogram scale. J. B. Sperry* and K. Sutherland at Wyeth Research (Pearl River, NY) report a safe, scalable method for converting a phenol to the corresponding difluoromethyl ether. Sodium chlorodifluoroacetate (NaO2CCClF2) is the reagent of choice for the reaction, which works well in DMF solvent at 95 °C.

The authors overcame two impediments to running the reaction on a 7-kg scale. The desired product can react with excess phenol to generate double- and triple-addition side products, and the reaction produces ≈1500 L of CO2 byproduct. Adding a phenol solution and 2.0–2.5 equiv NaO2CCClF2 to a hot slurry of K2CO3 in DMF controls CO2 evolution and minimizes over-reaction. The authors obtained an isolated yield of 99% on the 7-kg scale. (Org. Process Res. Dev. 2011, 15, 721–725; Will Watson)

Here’s a better way to measure metabolic stability. Most first-pass drug metabolism is mediated by cytochrome P450 enzymes. The rate at which drug candidates react with enzymic catalysts—metabolic stability—is important in drug optimization.

Current methods for evaluating metabolic stability require liquid chromatography and mass spectrometry. D. S. Clark and coauthors at Rensselaer Polytechnic Institute (Troy, NY) and the University of California, Berkeley, developed a measurement method based on fluorescence.

Reactions catalyzed by cytochrome P450 enzymes generate numerous side products (upper figure; RH is a substrate). This and the high reactivity of H2O2 and superoxide (O2·) make it difficult to quantify reaction rates.

The authors’ system—called MesaPlate (Metabolizing Enzyme Stability Assay Plate)—uses catalase (Cat) and superoxide dismutase (SOD) to convert H2O2 and O2· to water, which simplifies the overall rate equation (lower figure). In this system, the oxidation rate is based only on the depletion rates of NADPH and oxygen, which are simultaneously quantified by measuring their fluorescence.

The authors used their method to determine the catalytic constants kcat and KM and the metabolic stability (–rRH) of several drugs and cytochrome isoforms. The results are comparable with values derived by using conventional chromatography methods. The technique can also measure the effect of inhibitors cytochrome such as amitriptyline.

The method is suitable for high-throughput evaluation of metabolic stability and identifying cytochrome inhibitors in the early stages of drug development. It can also be used with natural mutants of P450 to study subpopulations and specific individuals. (J. Am. Chem. Soc. 2011, 133, 14476–14479; JosÉ C. Barros)

Graduate students who teach improve their research skills. A common belief among academic faculty and graduate students is that performing research promotes teaching skills, but that the reverse is not necessarily true. Therefore, research advisers normally ask graduate students to concentrate on research, and the students try to avoid or reduce teaching responsibilities.

D. F. Feldon and coauthors at the University of Virginia (Charlottesville), Cincinnati Children’s Hospital and Medical Center, the University of South Carolina (Columbia), the University of Texas–Austin, and Zayed University (Abu Dhabi, United Arab Emirates) challenge this belief. They spent a school year studying the relationship between teaching load and research proficiency.

The authors invited 95 graduate students in science, technology, engineering, and mathematics to participate in their study. They divided the students into two groups: one composed of students who had research and teaching responsibilities and the other in which the students performed only research. At the beginning of the year, the participants were asked to write a research proposal related to their field, complete with hypotheses, experimental designs, and data analysis. They were encouraged to revise their proposals throughout the year, and they handed in the final versions at the end of the year.

Statistical evaluation of the differences between the preliminary and final proposals showed that the graduate students who had research and teaching responsibilities improved their research skills significantly more than the research-only group. Members of the research–teaching group formulated more reasonable hypotheses and designed better experiments. The authors conclude that teaching and research experiences benefit each other. (Science 2011, 333, 1037−1039; Sally Peng Li)

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