This Week in Science

Science  17 Oct 2003:
Vol. 302, Issue 5644, pp. 353
  1. An Orbital Approach to Plasmon Resonances

    Hollow metallic nanospheres display a distinctive optical response, a plasmon resonance, that arises from deformations of its electron gas. These resonances are sensitive to the inner and outer shell radii, and it is desirable to tune these responses for complex geometries. Prodan et al. (p. 419) show that the optical response of dielectric-metal core-shell structures can be viewed as a hybridization of elementary plasmons similar to that of hybridization of atomic orbitals that creates molecular orbitals. They fabricated four-shell nanoparticles consisting of a dielectric silica core, a gold shell, a silica spacer, and a second metal shell, and show that their theory can match the experimentally measured infrared spectra.

  2. Conducting Salts

    Ionic liquids (ILs) could be used as electrolytes in rechargeable lithium batteries and in fuel cells, but the low fluidity and dielectric constants of these pure salts limit their conductivity when compared with simple aqueous salt solutions. Xu and Angell (p. 422) now show that protic ILs (ones that can transfer a proton between anions and cations) that have low glass transition temperatures can have conductivities rivaling those of concentrated aqueous solutions.

  3. Halfway to Diamond

    In diamond, carbon forms sp3 bonds in a three-dimensional structure, whereas in graphite, carbon sheets form through sp2 bonds. A variety of enigmatic structures between these two have been hinted at when graphite is compressed at ambient temperatures. Mao et al. (p. 425) used near K-edge spectroscopy to resolve this transition. At pressures around 17 gigapascals, half of the weak bonds between the graphite layers convert to diamondlike bonds to create a material hard enough to indent diamond.

  4. Factoring in the Unknowns

    How do you estimate how many fish are in a river or pond after catching just a few, or predict word usage in a text from a few snippets? Such problems are examples of those that come up repeatedly in analyzing and managing large systems—from linguistics to ecology to genomes—where the available data represent a small sample of the whole. One useful predictor was first derived by Good and Turing in trying to crack the Enigma code during World War II, and a number of modifications have emerged for various applications. Orlitsky et al. (p. 427) develop a measure for quantifying these approaches and searches and formulate a general approach that is asymptotically optimal by this measure (probabilities converge to unity as the data set increases).

  5. The Seas of Titan

    Titan, the largest satellite of Saturn, has an unusually thick and dynamic atmosphere similar to Earth's. Titan's haze and clouds make it difficult to observe the surface, but previous work suggests that an ocean of hydrocarbon-laden soup or hydrocarbon-rich cryovolcanism must exist to explain methane cloud formation. Campbell et al. (p. 431; see the Perspective by Lorenz) found evidence for fluid hydrocarbons covering regions as large as 50 kilometers from Arecibo and Green Bank radar observations. Lakes of hydrocarbons embedded in water ice may explain Titan's weather patterns, although much remains to be learned about the source and stability of such lakes in the cold outer reaches of the solar system.

  6. Defective Correction in SCID Patients

    The successful correction of X-linked severe combined immunodeficiency by gene therapy unfortunately also led to the development of a leukemia-like syndrome in two patients. Hacein-Bey-Abina et al. (p. 415; see the Perspective by Williams and Baum) have now characterized the condition and identified the lymphocyte subsets involved and the site of integration of the retroviral vector. In both cases, integration occurred near the promoter of LMO2, a transcription factor responsible for regulating cellular proliferation and differentiation. The resulting up-regulation of LMO2 apparently stimulated unchecked clonal T cell proliferation.

  7. Sexual Deception by Orchids

    In most plant-pollinator relationships, the plant is cross-fertilized while the pollinator gains a food reward. In sexually deceptive orchids, the flower mimics a female insect in shape, color, and odor, and males are deceived into “mating” with the flowers, thus transferring pollen without receiving a reward. Schiestl et al. (p. 437; see the news story by Pennisi) describe an extreme example of this phenomenon in an Australian orchid. The flower produces a volatile compound, 2-ethyl-5-propylcyclohexan-1,3-dione, that is identical in all respects to a pheromone produced by females of its pollinating thynnine wasp. Such dependence on a single compound is highly unusual and may imply limited evolutionary flexibility; nevertheless, the occurrence of more than 300 thynnine-orchid pollination relationships suggests that other highly specific communication systems may occur in nature.

  8. Protein-Protein Interaction Networks

    Existing databases on protein interactions have been combined with other data sources—the latter recording information not on interactions themselves, but on correlated properties, like biological function or classification of genes essential for life—to improve the accuracy of prediction of new protein pairs. Jansen et al. (p. 449) used Bayesian networks to combine existing data sets and assign a probability for the true occurrence of particular protein interactions, instead of using the pre-existing binary values (exists or does not exist) for each interaction. Experimental validation confirmed association of newly predicted protein pairs with apparent functions, for example, in nucleosome organization and DNA replication.

  9. Glycine Receptors On the Move

    Semiconductor quantum dots (QDs) have recently emerged as probes with the potential to revolutionize fluorescence imaging. Dahan et al. (p. 442) have now detected single QD-tagged glycine receptors in living cultured spinal neurons. By combining in vivo monitoring and post hoc electron microscopic analysis, the dynamics of receptors was monitored over time and the entry of a receptor into the synapse was directly observed.

  10. Two Tales of Lipids and Pathogenesis

    In coronary disease, accumulation of lipids along blood vessel walls progresses into characteristic lesions that are hallmarks of the disease process. Lee et al. (p. 453; see the Perspective by Plutzky) show that the inflammatory response associated with atherogenesis may hinge on a nuclear receptor called PPARδ that is expressed in macrophages. PPARδ appears to control the expression of proinflammatory genes in mouse macrophages, and its absence in mice caused a reduction in lesions by about 60%. This nuclear receptor may thus act as a molecular switch of the inflammatory program in macrophages and may represent a key target point for controlling disease progression. The intracellular bacterial pathogen, Listeria monocytogenes, takes up residence within the mammalian host cell, which becomes both a refuge and a source of biochemical support for the invader. O'Riordan et al. (p. 462) identified a metabolic adaptation of Listeria that allows the bacterium to replicate in the cytosol of the host cell. Specifically, the bacteria contain a lipoate protein ligase, LplA1, which has a critical, specific, and nonredundant role in intracellular growth by promoting the activation of pyruvate dehydrogenase. Bacteria lacking the ligase are severely compromised with respect to intracellular growth and virulence in mice.

  11. Environmental Escape of GM Crops

    The potential for unwanted environmental consequences arising from the hybridization of genetically modified (GM) crops has mainly been assessed on small regional scales. Using a multidisciplinary approach, Wilkinson et al. (p. 457) provide a nationwide estimate of the rate of hybridization between rapeseed/canola (Brassica napus) and its wild relative, B. rapa, and present regional data on the areas in the United Kingdom that are most and least prone to hybrid formation. Hybrid formation depends on recipient ecotype identity, with riverside B. rapa populations recruiting hybrids mostly in eastern England but not in Scotland. This information allows for the precautionary designation of “low-risk” sites for the positioning of preliminary field trials. The significant number of potential long-range (>3 kilometers) hybrids suggests that imposition of isolation distances would be ineffectual as the sole means of preventing gene flow.

  12. Separate Signals in Blood Cell Development

    Guanosine triphosphatases (GTPases) of the Ras superfamily regulate broad cellular functions, including activity of the cytoskeleton, apoptosis, gene transcription and intracellular trafficking. The Rho subfamily GTPases, Rac1 and Rac2, have been implicated in several of these activities within hematopoietic stem cell and leukocytes. Using conditional deletion of the Rac1 and Rac2 genes, Gu et al. (p. 445) determined that although both proteins are required to regulate actin assembly, Rac1 specifically controls stem cell proliferation with Rac2 governing cellular migration and production of the superoxide burst in neutrophils. In B cells, Walmsley et al. (p. 459) observed vital cooperation between the two GTPases, with deletion of both alleles resulting in a block in B cell development. This finding corresponded with a signaling defect and failure to express the receptor for the Baff protein, a critical regulator of B cell function and development.

  13. Patagonian Ice Loss

    In the Southern Hemisphere, the icefields of Patagonia are eclipsed in size only by the Antarctic ice sheets. Rignot et al. (p. 434) have combined orbital radar topography, traditional cartographic surveys, and a digital elevation model of ice extent to estimate the volume change of the 63 largest Patagonian icefields by comparing studies made in 1968, 1975, and 1995 to those made in 2000. These mountain glaciers contribute more—in absolute terms as well as per unit area—to sea level rise than their Northern Hemispheric counterparts, and their rate of thinning since 1995 was more than twice as great as that during the previous 25 years.

  14. Inner Workings of the Circadian Clock

    Evolutionary conservation of the master circadian clock that controls processes such as metabolism and behavior has been debated. All but one of the core clock components, a transcriptional regulator called Timeless, have been characterized as essential in both mammals and Drosophila. Barnes et al. (p. 439) now show that a particular isoform of mammalian Timeless is required for regulating the rhythmic activity of neurons that comprise this principal timepiece in the rat brain. As in Drosophila, Timeless associated with another clock component called Period in these neurons. The findings prompt reconsideration of mammalian clock composition and mechanism.