This Week in Science

Science  22 Mar 2002:
Vol. 295, Issue 5563, pp. 2173
  1. In Brevia

    In a magnetic field, a suspension of superparamagnetic particles self-organizes into a quasi-regular array of columns, allowing the rapid electrophoretic separation of long (>10 kilobases) duplex DNA, as shown by Doyle et al. (p. 2237).

    CREDIT: DOYLE ET AL.
  2. Ringing True

    Measurements of tree ring widths provide robust records of growing season temperature variations occurring from year to year, but have been criticized as being unreliable over periods of centuries. This concern arises from the fact that some trees experience a trend toward thinner rings with age, an effect which is difficult to distinguish from a cooling trend. Esper et al. (p. 2250; see the Perspective by Briffa and Osborn) demonstrate that multicentennial climate variability can be recovered from long tree-ring chronologies if the proper analysis methods are used. The tree-ring record for the past 1200 years from sites distributed across the Northern Hemisphere extratropics was related to Northern Hemisphere temperatures on multidecadal and longer time scales, and it shows strong evidence for a large-scale “Medieval Warm Period” and Little Ice Age in the extratropics, features not readily apparent in some other tree-ring-based chronologies.

  3. The Changing Colors of Cuprates

    Figuring out just what the “glue” is that binds the electrons into pairs in high temperature superconducting cuprates has been a goal since their appearance nearly 15 years ago. Molegraaf et al. (p. 2239; see the Perspective by Hirsch) report on sensitive optical measurements of the in-plane conductivity of the cuprate superconductor BSCCO and reveal that a transfer of the spectral weight from higher energy to lower energy occurs as the temperature is decreased and the sample becomes superconducting. In contrast to conventional superconductors where the pairing mechanism involves a reduction in the potential energy mediated by the electron-phonon interaction, the “glue” in the cuprate case appears to arise from a reduction in the kinetic energy.

  4. Solvent Effects in Substitution Reactions

    Many common organic reactions are of the SN2 type—a nucleophile (such as a halide ion) displaces another nucleophile at a carbon atom, leading to a substitution at the tetrahedral carbon. The other three substituents around the carbon atom must “invert” like an umbrella, and it has been argued that bulkier substituents should slow down the reaction. However, many competing effects, such as differing heats of reactions and extent of solvation, make it difficult to examine this question directly. Regan et al. (p. 2245; see the Perspective by Farrar) looked at the isotopic substitution reaction of chloride in alkylchloronitriles, which react sufficiently fast to be followed in an ion cyclotron resonance spectrometer. In the gas phase, the difference in barrier height for methyl- versus tert-butyl-substituted molecules was less than 2 kilocalories per mole (kcal/mol). These results, along with Monte Carlo simulations, allow the authors to conclude that most of the “steric” effect seen in solution (with barriers of 5 to 7 kcal/mol) is actually caused by differences in solvation.

    CREDIT: REGAN ET AL.
  5. Uranium-Seeking Noble Gases

    Ices of noble gases are often used to trap and isolate highly reactive molecules because they presumably are “inert.” Once again, the noble gases show unexpected reactivity. In their investigations of the “insertion” product of the reaction of CO with a uranium atom to form the linear CUO species, Li et al. (p. 2242) found that vibrational spectra were much different in the heavier and more reactive noble gases, such as argon (Ar), compared to the spectra for CUO in neon. Theoretical calculations show that Ar can bind to CUO and change the ground state from the singlet state to the triplet, and that multiple Ar atoms may bind to a single CUO.

  6. The Last Wave

    When a sound wave travels through a heterogeneous medium, the wave can be scattered multiple times and can generate small wavetrains at the end of its signal, called the coda. Snieder et al. (p. 2253) have developed and experimentally tested a coda wave interferometer that would allow seismologists to detect changes in a medium as a function of time. This technique could be used to monitor volcanoes, where the movement of magma, as well as the growth and propagation of fractures, changes the character of the medium on a time scale measurable with a seismic interferometer.

  7. No End to Decay

    The information needed to make a protein is contained in messenger RNAs (mRNAs), which are decoded by the macromolecular machine called the ribosome. Termination codons in the mRNA tell the ribosome when to stop adding amino acids, and premature termination codons (PTCs) can lead to truncated proteins that in many instances are deleterious to a cell's health (see the Perspective by Maquat). The quality-control mechanism that deals with PTCs is called nonsense-mediated decay (NMD). Frischmeyer et al. (p. 2258) now describe a phenomenon called nonstop decay (NSD) in which mRNAs lacking any termination codons are degraded, and van Hoof et al. (p. 2262) provide evidence that this is accomplished by the exosome, a cytoplasmic complex of ribonucleases that is recruited to ribosomes that have stalled at the end of these aberrant mRNAs.

  8. Taking the Measure of Amyloid

    A cardinal feature of Alzheimer's disease (AD) is the deposition in brain tissue of amyloid β (Aβ) peptide. Amyloid plaques seem to appear years before cognitive impairment becomes apparent. A means of measuring amyloid plaque burden in the brain would provide a valuable biomarker and might enable therapeutic intervention prior to neuronal loss. Working in a mouse model of AD, DeMattos et al. (p. 2264) demonstrate that administering an Aβ antibody to mice resulted in an efflux of Aβ from the brain into the plasma that was correlated with the amyloid plaque burden in the hippocampus and cortex. Developing a humanized monoclonal antibody may lead to production of a diagnostic test that could quantify amyloid burden in the brains of both preclinical and clinical AD patients.

  9. Dead, But Not Forsaken

    DNA from remains of dead organisms, depending on its age, can be used for a variety of investigations from phylogenetic to forensic (see the news story by Pennisi). Techniques for isolating and sequencing ancient DNA offer the opportunity to assess rates of evolution and to map prehistoric population genetic structure, but hitherto a lack of samples of sufficient size, spanning significant periods of time, has precluded such work. Excavations from Arctic and Antarctic ice-bound subfossil remains are opening the door to reconstructions of new levels of complexity and detail. Barnes et al. (p. 2267) use sequences from frozen remains of brown bears in Alaska and Siberia to examine patterns of population genetic structure in relation to fluctuating Pleistocene climates. Lambert et al. (p. 2270) sequence mitochondrial DNA from Adélie penguin bones in Antarctica, revealing evolutionary rates at least twice as great as those inferred from more conventional phylogenetic analysis.

  10. How Animals Eat

    In animals and plants, there is an inverse relationship between body size and population density, but consistent patterns in this relationship are masked by ecological factors such as variation in quantity and type of resources. Carbone and Gittleman (p. 2273; see the Perspective by Marquet) investigate the variation in the size/density relationship within and between species of carnivorous mammal, and reveal an underlying rule: Regardless of body mass or population density, a given mass of prey will support a given mass of predator. They show that scaling laws can predict population density across more than three orders of magnitude in body size.

  11. Presynaptic Calcium Influx

    Neurotransmitter release is triggered by calcium influx through presynaptic voltage-dependent calcium channels. Modulation of presynaptic calcium currents causes a robust alteration in synaptic efficacy. Tsujimoto et al. (p. 2276) investigated activity-dependent facilitation of P/Q-type calcium currents at the giant nerve terminals of the calyx of Held and found that calcium current upregulation is mediated by the calcium binding protein NCS-1, a homolog of the Drosophila frequenin protein.

    CREDIT: TSUJIMOTO ET AL.
  12. Absolute Versus Relative Success

    Despite the recent lackluster performance of the U.S. stock markets, some individuals and institutions have claimed relative success by losing less money than the averages. Gehring and Willoughby (p. 2279; see the news story by Miller) report the discovery of a neural processing event that distinguishes between true losses and relative losses. In a gambling task, subjects selected small or large wagers and were then informed of the gain or loss outcomes on all possible wagers. A signal arising from the medial frontal cortex near the anterior cingulate region appeared 200 to 300 milliseconds after results were posted, and this brain potential correlated with losses of all sizes, even in optimal outcomes in which a greater loss had been avoided. Furthermore, the magnitude of this potential increased in trials where the subject had incurred recent losses.

  13. A Little Magnetism Goes a Long Way

    Normally, nuclear magnetic resonance (NMR) studies require the use of a high magnetic field to separate atomic resonances according to their local field environment, that is, their chemical shift. McDermott et al. (p. 2247; see the news story by Service) note, however, that NMR lines scale linearly with the applied field, and so, at very low fields, very high signal-to-noise ratios can be achieved and scalar or J couplings, rather than chemical shifts, can be used to characterize which covalent bonds are present. They present data collected with a SQUID (superconducting quantum interference device) detector for molecules in microtesla magnetic fields. Such an approach would be useful for detection environments in which applying large magnetic fields to samples is impractical or inconvenient.

  14. Fighting for Breath

    Tuberculosis is a resurgent disease of worldwide concern, and a better understanding of immune responses to mycobacteria would support improved methods of vaccination. Using a macaque model of tuberculosis infection, Shen et al. (p. 2255) observed that a specific population of γδ T cells could respond vigorously to BCG (Bacille Calmette-Guérin), a nonvirulent mycobacterium currently used as a vaccine for tuberculosis. These γδ T cells expressed a specific receptor chain combination (Vγ2Vδ2+) that predominates in both humans and primates. Subsequent reinfection with BCG could induce an even stronger response, characterized by memory T cell expansion. A similar effect was elicited in BCG-vaccinated macaques, after infection with Mycobacterium tuberculosis, and this correlated with survival of the macaques, suggesting that these γδ T cells may help mediate protective immunity to tuberculosis.

  15. Neuronal Plasticity Promoted by Glia

    Tumor necrosis factor-α (TNF-α), otherwise known as a proinflammatory cytokine, turns out to be active on a continual basis in the nervous system. Beattie et al. (p. 2282), studied synaptic function in cultured hippocampal neurons and hippocampal brain slices and show that TNF-α promotes expression of a neurotransmitter receptor on the cell surfaces at synapses. TNF-α is required continuously, suggesting that it may contribute to rapid adjustment of receptor levels at a synapse. TNF-α is supplied by affiliated glial cells, once again reminding us that the glial cells, previously thought to be rather passive support partners, are important behind-the-scenes participants in neuronal function.

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