This Week in Science

Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 288
  1. Catching a Carbene >>


    Divalent carbon fragments, or carbenes, vary widely in their stability, depending on their substituents. Some, such as N-heterocyclic carbenes, are independently isolatable. Others can be isolated in coordination complexes with metals. Kornecki et al. (p. 351, published online 12 September) synthesized a carbene coordinated to a rhodium dimer representative of an elusive class of short-lived intermediates long postulated to underlie a series of cyclopropanation and C-H insertion reactions.

  2. A Heady Find

    In the past two decades, excavations at the archaeological site at Dmanisi, Georgia, have revealed hominin fossils from the earliest Pleistocene, soon after the genus Homo first dispersed beyond Africa. Lordkipanidze et al. (p. 326; see the cover) now describe a fossil cranium from the site. Combined with mandibular remains that had been found earlier, this find completes the first entire hominin skull from this period.

  3. Testing Black Holes

    Gravitational waves, predicted by General Relativity, are expected to be produced when very massive bodies, such as black holes, merge together. Shannon et al. (p. 334) used data from the Parkes Pulsar Timing Array project to estimate the gravitational wave background produced by pairs of supermassive black holes (those with masses between 106 and 1011 that of the Sun) in merging galaxies. The results can be used to test models of the supermassive black hole population.

  4. Unrestricted Travel in Solar Cells

    In the past 2 years, organolead halide perovskites have emerged as a promising class of light-harvesting media in experimental solar cells, but the physical basis for their efficiency has been unclear (see the Perspective by Hodes). Two studies now show, using a variety of time-resolved absorption and emission spectroscopic techniques, that these materials manifest relatively long diffusion paths for charge carriers energized by light absorption. Xing et al. (p. 344) independently assessed (negative) electron and (positive) hole diffusion lengths and found them well-matched to one another to the ~100-nanometer optical absorption depth. Stranks et al. (p. 341) uncovered a 10-fold greater diffusion length in a chloride-doped material, which correlates with the material's particularly efficient overall performance. Both studies highlight effective carrier diffusion as a fruitful parameter for further optimization.

  5. A Good Read

    Theory of Mind is the human capacity to comprehend that other people hold beliefs and desires and that these may differ from one's own beliefs and desires. The currently predominant view is that literary fiction—often described as narratives that focus on in-depth portrayals of subjects' inner feelings and thoughts—can be linked to theory of mind processes, especially those that are involved in the understanding or simulation of the affective characteristics of the subjects. Kidd and Castano (p. 377, published online 3 October) provide experimental evidence that reading passages of literary fiction, in comparison to nonfiction or popular fiction, does indeed enhance the reader's performance on theory of mind tasks.

  6. Taking Out the Trash


    The purpose of sleep remains mysterious. Using state-of-the-art in vivo two-photon imaging to directly compare two arousal states in the same mouse, Xie et al. (p. 373; see the Perspective by Herculano-Houzel) found that metabolic waste products of neural activity were cleared out of the sleeping brain at a faster rate than during the awake state. This finding suggests a mechanistic explanation for how sleep serves a restorative function, in addition to its well-described effects on memory consolidation.

  7. Return of the Steroid

    Trace levels of organic contaminants enter aquatic ecosystems from a variety of sources, including runoff of from agricultural lands. When these compounds and their metabolites break down, it is generally assumed that they become inert and pose less ecological risk. Qu et al. (p. 347, published online 26 September) tracked the sunlight-mediated transformation of metabolites of trenbolone acetate (TBA)—a common growth-promoting steroid given to beef cattle—across a number of conditions in the laboratory and in the field. When the degradation products were exposed to dark conditions following photodegradation, they surprisingly reverted back to TBA metabolites, including analog steroidal compounds similar to TBA with unknown biological effects.

  8. Changing the Code

    Easily and efficiently expanding the genetic code could provide tools to genome engineers with broad applications in medicine, energy, agriculture, and environmental safety. Lajoie et al. (p. 357) replaced all known UAG stop codons with synonymous UAA stop codons in Escherichia coli MG1655, as well as release factor 1 (RF1; terminates translation at UAG), thereby eliminating natural UAG translation function without impairing fitness. This made it possible to reassign UAG as a dedicated codon to genetically encode nonstandard amino acids while avoiding deleterious incorporation at native UAG positions. The engineered E. coli incorporated nonstandard amino acids into its proteins and showed enhanced resistance to bacteriophage T7. In a second paper, Lajoie et al. (p. 361) demonstrated the recoding of 13 codons in 42 highly expressed essential genes in E. coli. Codon usage was malleable, but synonymous codons occasionally were nonequivalent in unpredictable ways.

  9. Seeing the Trees in the Forest


    Despite botanical exploration over two centuries, knowledge of the species composition and quantitative distribution of the trees of the Amazonian forest has remained decidedly patchy. Ter Steege et al. (p. 1243092) report the results from a network of 1170 tree plots arrayed across the Amazon Basin and Guiana Shield, in which the species of all trees with stem diameter >10 centimeters were identified. The tree flora comprised a total of about 16,000 species. However, just 227 very common Amazonian species accounted for half of the trees in the Amazon—the world's most diverse forest.

  10. Misaligned Planets

    Stars with multiple coplanar planets have not been seen to show misalignments between the equatorial plane of the star and the orbital plane of the planets—a diagnostic of the dynamical history of planetary systems. Huber et al. (p. 331) analyzed the Kepler 56 planetary system, which contains a giant-sized and an intermediate-sized planet. The planets have orbits that are close to coplanar, but the planetary orbits are misaligned with the stellar equator. A third companion in a wide orbit, which could be another star or a planet, could explain the misaligned configuration.

  11. Taking the Strain

    When heavily deformed through compressive or torsional loading, crystalline metals will generate an increasing density of defects or dislocations that effectively strengthens the metal against further deformation. However, at some stage, the fine-grained structure that forms saturates. Liu et al. (p. 337; see the Perspective by Ramtani), show that combining the application of a very-high-rate shear deformation with high strain gradients to the surface layer of a pure sample of nickel can overcome this saturation. Instead of a three-dimensional fine-grained structure, a top layer with a two-dimensional layered structure occupied the first 80 micrometers. In addition to being stronger, this layered nickel structure was also more thermally stable.

  12. No Methane to Be Found

    On Earth, atmospheric methane is mostly produced biologically. Atmospheric methane has also been detected on Mars, but these reports have been controversial. Based on data from the Sample Analysis at Mars instrument suite on the Curiosity rover, which arrived at the surface of Mars in August 2012, Webster et al. (p. 355, published online 19 September) report no methane, with an upper limit of only 1.3 parts per billion by volume, about 6 times lower than previous measurements.

  13. Invade and Adapt

    The mechanisms by which plant and animal species spread into new habitats have become an increasing focus of ecological research, particularly in the context of climate change and species invasions. Colautti and Barrett (p. 364) examined the ecological consequence of local adaptation evolving rapidly along a 1000-kilometer climatic gradient in purple loosestrife (Lythrum salicaria), one of the most notorious invasive plant species in North America. These invasive populations have evolved to become locally adapted within 50 to 100 years with important ecological consequences—increasing reproductive output by more than an order of magnitude.

  14. Breaking Frog Defenses

    The first line of immune defense against most fungal infections consists of innate immune effector cells, including macrophages and neutrophils. However, Fites et al. (p. 366) have found that the fungus currently decimating the world's amphibia, Batrachochytrium dendrobatidis, is readily engulfed by these cells, but that this does not effectively control the infection. The fungus releases cell-wall components that induce lymphocyte apoptosis and inhibit the proliferation of other nonlymphoid cell types, disarming lymphocyte-mediated responses to infection.

  15. Capturing Binding Location and Speed

    Transcription factor–binding sites in chromatin can be mapped by the chromatin immunoprecipitation (ChIP) assay, which analyzes formaldehyde-fixed chromatin fragments obtained from cells. However, the standard ChIP assay does not provide information about how stable the inter-actions are. Other approaches, including live-cell imaging, can reveal aspects of the dynamic behavior of transcription factors but are limited either in location precision or time resolution. Poorey et al. (p. 369, published online 3 October) developed a model to explain how the ChIP signal relates to formaldehyde cross-linking time, and they developed a method to measure chromatin site–specific binding dynamics with high temporal resolution.

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