Editors' Choice

Science  04 Oct 2013:
Vol. 342, Issue 6154, pp. 15
  1. Materials Science

    3D Lowers T

    1. Marc S. Lavine

    Most solid oxide fuel cells operate at temperatures above 800°C, necessitating robust supporting materials for insulation and rendering on/off cycling inefficient. Lower-temperature operation has tended to lead to a loss of catalytic activity. An et al. combined several approaches to create a cell that performs well at temperatures as low as 450°C. At the core is a three-dimensional (3D) membrane electrode assembly (MEA) structure, which increases the surface area for the catalytic reactions. On both the top and bottom surfaces, a layer of porous platinum was deposited, further enhancing the reaction area. The addition of yttria-doped ceria at the cathode interface of the MEA below the top platinum surface helped reduce activation losses by 35%. The open circuit voltage was measured to be close to the thermodynamic limit, showing electronic and chemical isolation of the cathode and anode. The work is still at an early stage, though, as the performance of the MEA dropped by about 30% during the first hour of operation, possibly due to changes in structure, but did not show further changes after another 2 hours of operation.

    Nano Lett. 13, 4551 (2013).

  2. Chemistry

    Telling O Where to Go

    1. Jake Yeston

    The abundance of aliphatic C-H bonds in organic molecules poses an enticing, yet maddening challenge to synthetic chemists. On the one hand, direct oxidation protocols are prospectively applicable to an immense range of substrates; on the other hand, genuinely useful methods must achieve selectivity among numerous sites in a given substrate that differ only subtly. Gormisky and White tackle this challenge through the use of a pair of complementary ligands on an iron catalyst that activates peroxide for C-H oxidation. Elaborating on a previously reported ligand, they introduce bulky bis(trifluoromethyl) phenyl groups that roughly halve the conical angle-bounding substrate approach to the metal center. As a result, this bulkier catalyst favors oxidation at sterically unhindered sites on a substrate, whereas the previous catalyst manifested selectivity governed by the substrate's inherent reactivity. The authors develop a predictive model for the site preferences of each ligand based on steric and electronic factors, and they validate it through site-selective oxidations of such complex molecules as artemisinin (where they can select between two of nine possible sites) and the pentacyclic triacetoxytricalysiolide.

    J. Am. Chem. Soc. 135, 10.1021/ja407388y (2013).

  3. Virology

    Experimental Intervention

    1. Caroline Ash

    We know that some viruses can rapidly spread among wild birds, as witnessed by pandemic influenza viruses. Realizing their close similarity to mammalian retroviruses, Niewiadomska and Gifford have been sleuthing among the genomes of reticuloendotheliosis viruses (REV) of birds. Sequences of these viruses have surprisingly also turned up within the genomes of a couple of large DNA viruses: fowlpox and gallid herpesvirus 2 (Marek's disease). Phylogenetically, REV history stretches back beyond the origins of echidnas in the mammalian lineage, more than 8 million years ago, but they only turned up in birds during the 1930s. How? Via human intervention. Further sleuthing in the literature led to just one archived sample of duck infectious anemia virus, which traced back to a series of experiments on a bird malaria isolate collected in Southeast Asia. There was a lot of interest in this newly discovered parasite, and samples were disseminated to five different laboratories. Unfortunately, the malaria samples had somehow become contaminated with REVs, possibly during serial passage using mammalian blood or possibly by contact with the bêtes noirs for pathogen spillovers—bats. The REV was then able to integrate into bona fide bird viruses, and thence into vaccine strains, and ineluctably became one of the hazards modern poultry have to face.

    PLOS Biol. 11, e1001642 (2013).

  4. Biomedicine

    Skin Treatments

    1. Barbara R. Jasny

    Epidermolysis bullosa is the term used to describe a group of inherited disorders that are characterized by severe blistering and skin fragility. Two recent papers show the beginnings of new therapeutic approaches. Dystrophic epidermolysis bullosa is associated with mutations in type VII collagen that result in blistering, deformities, and aggressive squamous cell carcinoma. Woodley et al. demonstrated that intravenously injected, recombinant type VII collagen was able to home to the region of wounds and restore expression for several weeks in two animal models. Junctional epidermolysis bullosa, in which the causative mutation may occur in the type XVII collagen gene, is corrected in some cells by a secondary mutation, leading to revertant mosaicism. Revertant keratinocytes should have significant potential as an autologous therapeutic agent. Gostynski et al. found that although colony-forming potential was high, revertant cells divided more slowly than wild-type cells. However, revertant cells were capable of engraftment and skin regeneration in a humanized mouse model if the cells were first grown as part of a skin equivalent on a plasma-based scaffold.

    J. Invest. Dermatol. 133, 1910; 10.1038/jid.2013.308 (2013).

  5. Materials Science

    Doping by Diffusion

    1. Phil Szuromi

    The electronic properties of bulk semiconductors can be improved by doping, the deliberate introduction of impurity atoms that increase the number or mobility of charge carriers. Usually, just doping the surface of a bulk semiconductor is sufficient, but for semiconductor nanocrystals, the entire particle needs to be doped to achieve the desired properties. Vlaskin et al. report on the doping of CdSe nanocrystals with Mn ions, a process that has proven especially difficult at the high levels desired to impart magnetic properties. Hot-injection synthesis of Mn-doped CdSe nanocrystals (where the particles rapidly form in solution from precursors) results in particles that are depleted in Cd2+ at the surface. The authors were able to incorporate Mn2+ ions uniformly by starting with CdSe seed nanocrystals, which were injected as a solution along with selenium into a solution of manganese acetate at ∼300°C for times varying from seconds to several hours. This thermodynamically controlled process, which uniformly increased the size of the seeds, resulted in diffusion doping of the entire crystal without requiring Cd2+ ion ejection and allowed doping levels of Mn2+ as high as 20% to be achieved.

    J. Am. Chem. Soc. 135, 10.1021/ja4072207 (2013).

  6. Plant Science

    Dessicated Dispersal

    1. Pamela J. Hines

    Equisetum plants (horsetail) have a lineage dating back to the Paleozoic, and these unusual vascular plants reproduce with spores. Marmottant et al. have now taken a closer look at how the spores get around. The spores have four long legs that, in humid conditions, are wrapped closely around the spore body, but as the relative humidity decreases, the legs straighten out. The change in shape is driven by the two-layer construction of the legs, with one layer having a greater tendency than the other to change volume in response to moisture, similar to the change in shape of old bimetallic thermostats in response to changes in temperature. The process is reversible, with legs furling and unfurling as the humidity goes up and down. In the naturally moist habitat that Equisetum frequents, a shaft of bright sunshine or a dry breeze can effectively change the local humidity surrounding a spore. As the legs move, so moves the spore. Occasionally the legs get stuck on each other, and, when they get unstuck, the release of elastic energy can propel the spore into a rather large leap. Whether crawling or leaping, repeated cycles of motility would increase the dispersion of these otherwise sedate plants.

    Proc. R. Soc. B 280, 10.1098/rspb.2013.1465 (2013).

  7. Microbiology

    Parasite Palmitoylation

    1. Stella M. Hurtley

    The molecular mechanisms involved in the active invasion processes used by apicomplexan parasites such as Toxoplasma gondii to enter host cells are not well understood. Compounds that inhibit—or in some cases enhance—the infectivity of T. gondii parasites were recently identified in a chemical-genetic screen. How such small-molecule invasion enhancers might work, however, remains obscure. Child et al. have now been able to identify the molecular target of one class of small-molecule enhancers: a Toxoplasma enzyme, palmitoyl protein thioesterase-1 (TgPPT1). Inhibiting this thioesterase enhanced the invasive capacity of tachyzoites by increasing parasite motility and promoting secretion from micronemes, the parasite's invasion-associated organelles. TgPPT1 acts as a depalmitoylase, removing fatty acids that have been attached to proteins post-translationally. Thus, reversible palmitoylation within the parasite appears to play a key role in the invasion of host cells by T. gondii.

    Nat. Chem. Biol. 9, 651 (2013).

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