This Week in Science

Science  19 Sep 2003:
Vol. 301, Issue 5640, pp. 1625
  1. Relaxation in Water

    One way to probe the rearrangements of the hydrogen bond network in liquid water would be to use ultrafast vibrational spectroscopy. Such rearrangements should lead to changes in the OH vibrational frequency as a function of time. Fecko et al. (p. 1698) have now produced laser pulses short enough [∼50 femtoseconds (fs)] to observe three distinct decay phases of the HOD molecule in D2O. They observed an initial 60-fs decay as well as an oscillation at 170 fs that shows the induced motion is not strongly damped. Modeling of these results indicates that the D2O molecule nearest the OH proton plays the greatest role in determining the observed frequency shifts. These local dynamics are obscured by larger collective motions once 200 fs have passed.

  2. Finding Your Way in the Dark

    One feature of the cold dark matter paradigm is the formation of galaxies from the mergers of dark-matter halos. Evidence for these halos around many spiral galaxies comes from the radial distribution of the velocities of orbiting stars farther and farther from the center of the spiral. For elliptical galaxies, it is harder to determine stellar velocities farther from the center and thus the extent of any dark-matter halos. Romanowsky et al. (p. 1696) have circumvented this problem by determining the velocities of planetary nebulae distributed around three elliptical galaxies. The velocities indicate that there are no dark-matter halos around these elliptical galaxies, a result that leaves open the mechanism for their formation.

  3. Unchaining Secrets of Polymer Flow

    The behavior of polymer chains under flow conditions is very complicated because of the convolution of the response of individual chains and the response of the entangled networks that they form. However, relating the rheological response to the underlying chain motion would aid in polymer processing because the resulting morphology plays a key role in determining the properties of the final product. Bent et al. (p. 1691; see the Perspective by Marucci) obtained neutron scattering and birefringence data for a set of monodisperse polymers inside a specially designed flow cell that allows the polymer to be studied as it flows through a contraction. The authors successfully correlated these results with the current theories on chain motion and observed much slower orientation decays at length scales of the entire chain compared to the chain entanglement lengths.

  4. High-Powered Soliton Propagation in Microstructured Fibers

    Optical solitons, bullets of light that propagate through a material without modification to their intensity or spatial and spectral distribution, arise for particular matching conditions of dispersive and nonlinear optical properties of the medium. They are typically generated when high-intensity laser pulses are launched into optical fibers, and as they are robust to inhomogeneities, they are being investigated for use as information carriers in optical communication networks. Unfortunately, solitons in conventional fibers self-interact and disperse, or change color, as they propagate. Skyrabin et al. (p. 1705) and Ouzounov et al. (p. 1702) show that the dispersive properties can be controlled and the color shift eliminated when a microstructured photonic band-gap fiber is used. Moreover, Ouzounov et al. show that the peak powers delivered by such microstructured fibers can exceed 5 megawatts.

  5. When Rodents Roared?

    Rodents represent one of the most successful orders of mammals. Sánchez-Villagra et al. (p. 1708; see the Perspective by Alexander) describe an exceptionally complete skeleton of the largest fossil rodent, previously known only from isolated teeth. Phoberomys pattersoni lived in northern South America during the Upper Miocene, and is estimated to have weighed about 700 kilograms—the size of a buffalo. Like the largest existing rodent, the 50-kilogram capybara, Phoberomys is thought to have lived and foraged close to water, and possibly to have been semi-aquatic.

  6. Pre-Conquest Amazonia

    There has been much debate about the extent of pre-conquest human habitation in Amazonia, and whether its lowland forests were essentially “pristine” or densely settled and widely used by the pre-Columbian human population. Heckenberger et al. (p. 1710; see news story by Stokstad) describe bridges, causeways, canals, and circular settlement plans in the Upper Xingu region of Brazil that date back to the first millennium A.D. These settlements contained many more structures than exist today in the same area and suggest that there had been a much larger local population. These finds suggest that the Amazonian environment was no obstacle to sociopolitical complexity, defined in terms of landscape effects, active resource management, and population density.

  7. Reconstituting an Ancient Hormone Receptor

    Can the ancestors of present-day proteins be reconstructed to determine what kinds of functionality they might have had? Thornton et al. (p. 1714) have examined a class of steroid hormone receptor proteins previously believed to be restricted to vertebrates. An estrogen receptor gene from a mollusk was identified and characterized, suggesting that this family of proteins in fact predates the origin of bilaterally symmetrical animals. Using phylogenic approaches, the hypothesized 670- to 1100-million-year-old ancestor (Proterozoic) protein was then reconstructed, synthesized, and expressed. The primordial steroid receptor was likely to have been an estrogen-activated protein.

  8. Real-Time Signaling Both Ways

    Integrins are heterodimeric membrane proteins that signal bidirectionally across the cell surface. Their activation is associated with conformational changes in their extracellular domains, but interaction between the subunit intracellular domains is also likely to be important for regulating activation and signaling. Using fluorescence resonance energy transfer in living cells, Kim et al. (p. 1720) demonstrated that integrin activation involves a conformational change in the cytoplasmic domains that allows their separation. Signals transmitted in either direction—from the cell outward, or from outside the cell inward—appeared to be coupled to this mechanism.

  9. Zinc Links in a Receptor-Kinase Complex

    Association of the T cell coreceptors CD-4 and CD-8 with the cytoplasmic kinase Lck regulates T cell activation and maturation. Kim et al. (p. 1725) have explored the requirement for zinc in this interaction and report that the metal ion acts as a clasp that stabilizes a receptor-Lck complex. Peptides corresponding to a short region of Lck and in either receptor's cytoplasmic tail were unordered, but the presence of zinc induced the synergistic mutual folding of the peptides. Protein modifications could unlatch the clasp and modulate receptor-Lck binding.

  10. G Proteins, Plant Growth, and Parasites Invasion

    The function of regulators of G protein signaling (RGSs) is just as the name indicates—in mammals and yeast, these proteins attenuate signals initiated by G protein-coupled receptors (GPCRs) at the cell surface. Chen et al. (p. 1728) have now identified an RGS protein in Arabidopsis that functions together with a heterotrimeric G protein to control root and shoot growth. The plant RGS behaves biochemically like a bona fide family member, yet its sequence predicts a protein that contains a seven-transmembrane-spanning region similar to GPCRs, quite unlike other known RGS proteins. The process of invasion of red blood cells by the malaria parasite Plasmodium falciparum is fundamental to disease progression. Harrison et al. (p. 1734) reveal the requirement for GPCR activation in the host erythrocyte cell membrane to allow successful invasion. It is likely that the activated receptors promote the rearrangement of the rigid erythrocyte subcortical cytoskeleton to allow for the deformation of the membrane required during parasite invasion.

  11. Reducing Short-Term Risks

    A decrease in calorie intake has been linked to an increase in life-span in vertebrates and invertebrates. For example, Drosophila fed a restricted diet live longer as adults. Mair et al. (p. 1731; see the Perspective by Vaupel et al.) analyzed more than 7000 fruit flies and found that enforcing calorie restriction at any stage in their adult life had the same life-prolonging effect by reducing the short-term risk of death. The finding suggests that current nutrition, perhaps more than past dietary conditions, can affect survival.

  12. Classic Conditioning, Timing, and the Cerebellum

    Timing of movements is crucial for the survival of animals, and the nervous system is continuously trying to optimize this timing. A well-studied example is classical conditioning of the eyelid blink response, whose timing is ultimately determined by the interval between the onset of the conditioned stimulus and that of the unconditioned stimulus. Using a novel eyelid movement recording technique and cell-specific mutant mice, Koekkoek et al. (p. 1736; see the Perspective by Linden) show that cerebellar long-term depression is not only necessary for adapting the amplitude of trained movements but also for learning the appropriate timing via an input-specific mechanism.

  13. Life in the Treetops

    The interaction between biodiversity, community structure, and ecosystem function has become a major focus of ecological research. Wardle et al. (p. 1717) assessed decomposer community structure and ecosystem functioning in the suspended soils of insular rainforest treetop epiphytes or “treetop islands.” Island properties (their size and distance from the “mainland”) affected ecosystem-level attributes by altering the community structure of the component organisms. The effects of isolation and substrate (in this case, host tree species) on soil invertebrate diversity and ecosystem processes such as litter decomposition were also examined using descriptive and experimental approaches. These findings are relevant to isolation and fragmentation of other types of ecosystems.