This Week in Science

Science  20 Oct 2000:
Vol. 290, Issue 5491, pp. 405
  1. Watching the Moment Go By

    A magnetic field exerts a torque on a magnetic moment that sends the corresponding spins into precessional motion. Although this effect of magnetic fields on magnetization is exploited in magnetic memory devices, the precise dynamics of the magnetization rotation are not clearly understood. Acremann et al. (p. 492; see the Perspective by Miltat and Thiaville) introduce a technique for taking three-dimensional submicrometer-sized snapshots of the magnetization vector with picosecond resolution to build a picture of the intricate motion that the magnetic moment undergoes when subjected to a magnetic field.

  2. Robust Coherence

    The viable development of quantum computers will depend on the implementation of procedures to overcome the problem of decoherence, where the superposition of the quantum states is lost due to disturbances from the environment. Recent theoretical work has suggested that the existence of decoherence-free subspaces can be created—a particular subset of quantum states can be chosen that will be robust to certain perturbations and not decohere. By subjecting the quantum system, in this case a pair of entangled photons, to collective decoherence, Kwiat et al. (p. 498) present experimental evidence for the existence of such decoherent-free states. The use of decoherence-free subspaces can help reduce the burden of quantum error-correction schemes in quantum information processing.

  3. Silicon Over Carbon in a Doubles Match

    Carbon readily forms double bonds, but silicon double bonds can usually be stabilized only if they are protected with bulky constituents. Iwamoto et al. (p. 504) show that silicon double-bond chemistry can in some cases complement that of carbon. Although spiropentadiene, a double-ring carbon compound, has not yet been isolated, the authors show that the silicon analog is stable. Thus, silicon double-bond chemistry may not be as limited as previously assumed.

  4. A Stickier Silicon Surface

    The adsorption of H2 on the (100)-2 × 1 surface of silicon (which forms rows of tilted silicon dimers) is one of the best studied surface reactions, in part because of its technological importance, but also because of the puzzles it presents. Energetics suggests that there should be no significant barrier to adsorption, and that H2 should stick readily at room temperature, but direct adsorption studies instead show a very low sticking coefficient (<10−13) and a barrier of 0.7 electron volts. Buehler and Boland (p. 506) have prepared a bare surface in which the dimers are untilted and horizontal, and found that this preparation increased the sticking coefficient by a factor of 109. Moreover, the sticking coefficient did not change with temperature. These results suggest that finding an untilted dimer is the main barrier to adsorption, and that this surface may be a good model for the transition state for H2 adsorption.

  5. Sacrificial Cells

    Induction of cell death is part and parcel of infection by most pathogenic viruses and bacteria. In some cases, the bacteria themselves can induce apoptotic cell death directly, although how this process influences the course of infection is not clear. Grassmé et al. (p. 527) offer strong evidence that it is imperative for host survival that certain types of cells should die in response to bacterial infection. Pseudomonas aeruginosa—a pulmonary bacterium responsible for pneumonia and sepsis in susceptible individuals—induces extensive apoptosis of lung epithelial cells. This cell death was in fact critical for the survival of mice that had been infected with the P. aeruginosa and depended on the interaction of the cell death receptor Fas (CD95) with its ligand. However, both Fas ligand as well as Fas had to be expressed on the epithelial cells, rather than on lymphocytes as would have been predicted.

  6. Tiny Clouds on Titan

    Titan, a relatively large moon of Saturn, is dominated by a thick nitrogen-rich atmosphere. Why such a thick atmosphere should form on a small moon so far from the sun has been unclear. Griffith et al. (p. 509; see the Perspective by Lorenz) provide high-resolution near-infrared spectra of Titan which indicate the formation of small clouds on a daily basis. Titan may have weather patterns similar to Earth, with convection allowing the condensation of methane into clouds that quickly dissipate and that may shower rain toward Titan's deeply hidden surface.

  7. It's So Easy to Slip

    Landslides have been increasingly recognized as critical in controlling the elevation and morphology of mountains. Not all landslides move catastrophically downhill—some creep or slide more slowly—and soil porosity may determine this marked difference. Iverson et al. (p. 513) examined landslide behavior in a series of experiments on a large flume where they could monitor dilation and contraction of soil. Small changes in soil porosity produced large changes in slip rate.

  8. Exerting Control over HIV

    A vaccine response against human immunodeficiency virus (HIV) can be amplified so that it protects rhesus macaques from a pathogenic variety of the virus. Barouch et al. (p. 486; see the Perspective by Shen and Siliciano) administered a DNA vaccine containing the viral genes gag and env in the presence of a fusion protein of interleukin-2 and immunoglobulin. They observed a strong cytotoxic T lymphocyte response, low to undetectable virus levels, and no disease or death by 140 days after challenge with a chimeric simian-human form of the virus.

  9. Picky About a Mate

    Direct experimental evidence that natural selection may reinforce mate recognition and help drive reproductive isolation (and ultimately speciation) in animal populations is the subject of two reports (see the Perspective by Barton). Higgie et al. (p. 519), using two species of Australian fruit fly that overlap in their geographical ranges, show that reproductive character displacement of pheromones in these natural populations of fruit flies is a consequence of natural selection on mate recognition. Hendry et al. (p. 516) report significant reproductive isolation after only 13 generations of divergent selection in sockeye salmon breeding in two very different habitats—streams and lake shores. This result indicates that divergent selection can precipitate reproductive isolation at a very early stage in an adaptive radiation and suggests that speciation might be amenable to experimental study in species with short generation times.

  10. Guiding Glycerol

    Aquaporins are a family of membrane channel proteins that are selective for water or other small molecules, but exclude ions. The high sequence conservation throughout the aquaporin family is indicative of a common fold. Fu et al (p. 481) have now determined the structure of an aquaporin family member, Escherichia coli glycerol facilitator (GlpF), with three glycerol molecules in the channel, at 2.2 angstrom resolution. Glycerol molecules pass through a narrow “selectivity filter” and line up in single file in an amphipathic channel. The structure provides insight into why glycerol and linear carbohydrates can pass through the channel and exclude water and ions.

  11. Root Cause of an Invasion

    The invasive success of foreign plant species is often attributed to the lack of natural enemies in their new milieu. Callaway and Aschehoug (p. 521; see the news story by Jensen) show that Centaurea diffusa, a European plant that is highly competitive against grassland species in the western United States, is much less so against closely related grasses from its own native range. Experiments with activated charcoal and labeled phosphorus reveal strong interaction effects and suggest a role of root exudates in interspecific competition. These results have implications not only for understanding the processes by which species become invasive, but also for the evolution of plant communities in general.

  12. Somatic Mutations

    Triplex-forming oligonucleotides (TFOs) —short, single-stranded segments of DNA that can bind to specific sites in double-stranded genomic DNA—can induce mutations at their binding sites. TFOs are potentially useful reagents for making mutations in somatic cells (all except the germ cells), but their utility has been limited by the inability to demonstrate their effectiveness in whole animals. Vasquez et al. (p. 530) now show that administering mice with intraperitoneal injections of a TFO (not chemically linked to mutagenic agents) is sufficient to increase the mutation rate at a specific marker site in the genome. Although the mechanism is not yet fully understood and the efficiency still low, the result provides proof-of-principal that somatic tissues can be targeted for mutagenesis by nucleic acids.

  13. Directly Connecting Learning and LTP

    Learning a complex motor task in rats has now been shown to increase synaptic efficacy in motor cortex. Rioult-Pedotti et al. (p. 533) also found that this increase is accompanied by an occlusion in long-term potentiation (LTP) and a simultaneous increase in the capacity for long-term depression in the same synapses. Their findings indicate that the learning-produced synaptic potentiation reflects a shift in efficacy within the normal range for modifications of excitability. These data provide the strongest evidence to date that LTP and learning-related changes in synaptic efficacy share a common mechanism.

  14. An Orderly Breakup of Incoherent Light

    Under the right conditions, a laser beam can be seen to spontaneously break up into filaments and eventually disintegrate as it propagates through a nonlinear medium. This modulation instability (MI), in which small coherent perturbations build up as the light is scattered and interference and reinforcement of these small perturbations leading to large instabilities, has generally been considered to be a property of coherent light sources. Kip et al. (p. 495) now show that, once the nonlinearity of the medium exceeds a threshold, MI can also occur with partially incoherent light. The authors suggest that the rich pattern formations observed here may also be related to those found in other physical systems that are weakly correlated.

  15. Battle of the Bands

    Band theory can describe the electronic properties of periodic crystal lattices, but what happens when translational invariance of the periodic potential is removed? Voit et al. (p. 501) report that in an artificially fabricated crystal lattice comprised of two incommensurate periodic potentials, photoemission experiments reveal the band structure signatures of both materials. The model calculations they introduce to describe the development of dispersive band structure of the system as a whole also show the underlying band structure of the two competing periodic potentials.

  16. Different by a Hairbreadth

    Members of the tumor necrosis factor (TNF) family, best known for their roles in immunity and inflammation, are now shown to function in the morphogenesis of hair follicles in the epidermis. Yan et al. (p. 523) report the identification of two TNF receptor family members as binding partners for two products of the EDA gene. Mutations in this gene cause the disorder anhidrotic ectodermal dysplasia (EDA) in which patients lose hair, sweat glands, and teeth. The EDA gene produces two membrane-bound proteins with TNF-like domains that differ by an insert of only two amino acids. This small difference, however, is enough to cause essentially all or none binding to their respective receptors. EDA-A1, the longer isoform, binds exclusively to the receptor called EDAR whereas the shorter EDA-A2 binds exclusively to the related receptor called XEDAR (for X-linked ectodysplasin-A2 receptor). The two receptor-morphogen pairs appear to have distinct functions in the development of hair follicles.

  17. Indirect Aerosol Forcing

    Crowley (Research Articles, 14 July, p. 270), comparing historical temperature reconstructions with predictions of an energy balance climate model, found that whereas natural forcing by variations in solar irradiance and volcanic emissions can explain most pre-1850 variability in Northern Hemisphere temperatures, trends in the last half of the 20th century cannot be accounted for without including the effects of anthropogenic greenhouse gases. In a comment, Penner and Rotstayn note that Crowley's calculations did not account for the indirect forcing effects of aerosols, an omission that, in the view of Penner and Rotstayn, “could lead to large systematic errors.” Crowley responds that “not all studies…estimate as large an effect” from indirect aerosol forcing as do Penner and Rotstayn, and that if the forcing were as large as they maintain, late-20th-century warming could be simulated only by factoring in additional terms that provide positive feedbacks. “Although it is important to understand the role of indirect aerosol forcing,” he argues, “the principle of parsimony” on which his study rests “still has merit in attempting to explain the temperature record of the last 1000 years.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/290/5491/407a

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