Editors' Choice

Science  06 Oct 2006:
Vol. 314, Issue 5796, pp. 19

    Stretchy Clay Composites

    Clays are considered useful reinforcing materials on account of the individual platelets exposed through exfoliation; these platelets offer a large surface area for chemical bonding. However, the synthesis of a composite material by the addition of large amounts of clay to a polymer is problematic because of dispersion or aggregation effects that lead to poor mechanical or optical properties. Haraguchi et al. used gel formation in an aqueous medium to create a composite of hydrophobic poly(2-methoxyethyl acrylate) and hydrophilic hectorite clay. During the polymerization, the clay platelets were excluded from the polymer particles and instead formed a shell around them. Once dried, the clay shells comprised a three-dimensional network, which the authors structurally characterized using transmission electron microscopy.

    A surprising feature of the composites was the ability to undergo huge elongations when subjected to stress. After an initial irreversible necking deformation, subsequent applied large strains were shown to be reversible, with good shape recovery observed on release. The composites were also transparent, with greater than 90% light transmission independent of clay concentration (up to 30 weight %). Unlike many clay composites, these materials did not reswell on exposure to water, nor did they dissolve in organic solvents that could solubilize the pure polymer. — MSL

    Adv. Mater. 18, 2250 (2006).


    Salt Solution

    Ionic liquids (ILs), or more specifically cation-anion pairs that form a stable fluid near room temperature, are playing an increasingly practical role as chemical reaction solvents, electrolytes, and heat-transfer media on scales that range from the laboratory bench to industrial manufacturing processes. Their advantages include miniscule vapor pressure, high polarity, and relative inertness. However, efforts to fine-tune IL properties by structural modification are hindered by a limited understanding of why specific cation-anion combinations melt at such low temperature.

    Krossing et al. have developed a simple predictive framework for calculating the melting point of a given IL with knowledge of the dielectric constant, or conversely estimating dielectric properties from the melting point. Their method involves computing the free energy of fusion using a thermodynamic cycle that adds the lattice energy (required to move ions from the lattice to the gas phase) to the compensating stabilization energy arising from naked ion solvation in a dielectric medium. Enthalpic and entropic parameters are calculated using a combination of quantum chemical methods and volume-based-thermodynamics approximations. The modeling scheme proved highly effective in reproducing experimental data for 14 ionic liquids composed of the most commonly used cations (substituted imidazolium, pyrrolidinium, pyridinium, and ammonium) and anions [BF4, PF6, CF3SO3, and (CF3SO2)2N]. — JSY

    J. Am. Chem. Soc. 128, 10.1021/ja0619612 (2006).


    Caught in the Act

    Real-time intracellular imaging allows detailed visualization of viral entry mechanisms. Arhel et al. have attached a tetracysteine tag to the integrase protein of human immunodeficiency virus-1 (HIV-1). The tagged viruses retain infectivity and can be labeled with a fluorophore, allowing real-time tracking of individual viral DNA-containing complexes in the cytoplasm and nucleus of human cells. Before entering the nucleus, the viral particles exhibited actin- and microtubule-based movement from the periphery toward the nucleus. Their mobility was restrained as they docked with and crossed the nuclear envelope, and the particles were then able to move diffusively once inside the nucleus itself. This type of technology will be important for identifying the itineraries of viruses during infection and for testing potential interventions that would interfere with the establishment of productive infections. — SMH

    Nat. Methods 3, 817 (2006).


    Shallow Chills

    Observations show that the world oceans as a whole have been warming for the past 50 years. This result is an important confirmation of global warming inferences based on surface atmospheric temperature measurements, as the oceans have more than a thousand times the heat capacity of the atmosphere. The rise in ocean heat content has not been spatially or temporally uniform, however, and because most models do not reproduce such unforced variations, their origin remains an open question.

    Lyman et al. have taken advantage of the rapidly expanding network of Argo autonomous profiling floats to present a global temperature data set for the upper 750 m of the world oceans. The study reveals a large cooling since 2003. These data also have implications beyond the pattern and extent of cooling. For instance, because it is unlikely that so much heat was transferred so quickly to the deeper ocean, the measurements indicate that a whole-ocean cooling has occurred, a phenomenon expected to induce a decrease in sea level due to thermal contraction of the water. Sea level rise has not slowed during the time period, however, suggesting that other factors such as increased rates of glacial melting are more than adequate to compensate for the thermal effect on volume. — HJS

    Geophys. Res. Lett. 33, L18604 (2006).


    How to Handle Methane

    The biological oxidation of methane to carbon dioxide proceeds sequentially—through methanol, formaldehyde, and formic acid. Carrying out these reactions under mild conditions demands exquisite control, which generally translates into precisely structured metal centers and diffusion-restricted intermediates. Myronova et al. have isolated an enzyme complex containing the membrane-bound (or particulate) form of methane monooxygenase (pMMO) and methanol dehydrogenase (MDH), and they map its structure using cryoelectron microscopy. They are able to fit three copies of the previously solved crystal structure of pMMO, an enzyme with mononuclear and binuclear copper centers, into the body of a 500-kD assembly; likewise, three copies of the crystal structure of MDH can be fitted into the cap, which is connected to the body via three arms. This supramolecular organization may facilitate the controlled transfer of electrons, which appears to be a common theme among membrane-bound enzymes catalyzing redox chemistry. — GJC

    Biochemistry 45, 10.1021/bi061294p (2006).


    Silencing in Triplicate

    The movement and activity of transposons, also known as jumping genes for their ability to replicate and move throughout the genome, have long fascinated biologists. In some cases, transposon activity is reduced, and more recently it has been demonstrated that this reduction is often due to epigenetic silencing and paramutation: the epigenetic change in expression of one allele effected by another.

    Woodhouse et al. propose that the establishment, maintenance, and inheritance silencing of the MuDR transposon in maize are due to multiple genes. These genes include Mu killer, a transcribed template for RNA interference silencing that initiates silencing; a homolog of the Arabidopsis RDR2 gene, named ZmRDR2/Mediator of paramutation 1, which is required to initiate silencing in the cases where a double-stranded RNA hairpin is lacking and perpetuates silencing through RNA-directed DNA methylation; and two maize homologs of nucleosome assembly protein 1, which maintain heritable silencing, most likely by modification of the chromatin structure. The redundant mechanisms involved in regulating transposon activity demonstrate the importance of controlling the replication of parasitic DNA elements within the host genome, which has been suggested as the raison d'être for epigenetic silencing. — LMZ

    PLoS Biol. 4, e339 (2006).

  7. STKE

    Inflammatory Pores

    As part of the process of infection, pathogenic microbes secrete protein toxins, such as aerolysin, that form pores in the target cell's membrane. Gurcel et al. show that exposure of mammalian cells to aerolysin increased the activity of the transcription factor SREBP-2 (sterol response element-binding protein-2), as assessed by a selective increase in gene expression and an increase in total cellular cholesterol as a consequence of both increased biosynthesis and uptake (the low-density lipoprotein receptor is encoded by a SREBP-responsive gene). In addition, exposure of cells to a K+-selective ionophore triggered SREBP-2 activation, and a decrease in intracellular K+ (caused by leakage through the pores) is known to activate the protease caspase 1 via the assembly of inflammasomes containing IPAF, an intracellular pattern recognition receptor. SREBP-2 was not cleaved directly by caspase 1; instead, caspase 1 activated the SREBP-activating pathway that involves SCAP (an escort protein) and S1P and S2P (the two enzymes responsible for SREBP proteolysis and release). Using various pharmacological and RNA interference approaches, the authors showed that activation of the inflammasome, caspase 1, and the SREBPs was required for cell survival after exposure to aerolysin. — NRG

    Cell 126, 1135 (2006).