This Week in Science

Science  20 Jun 2003:
Vol. 300, Issue 5627, pp. 1844
  1. Split for Its Spin

    Sources of spin-polarized particles can be useful for enhancing signals in nuclear magnetic resonance experiments and in future “spintronics” applications. Rakitzis et al. (p. 1936) report that dissociation of gas-phase HCl in a supersonic molecular beam with circularly polarized light creates H atoms with an average polarization of 72%, based on an analysis of the angular momentum of the Cl atom by-product. In the “long” time limit (less than 1 nanosecond), both the proton and electron share the polarization, and the H atoms can be ionized to create spin-enriched protons and electrons.

  2. A Long Series of Star Quakes

    Some massive main-sequence stars pulsate with periods of days to months, and long-term data sets are needed in order to identify the modes of oscillations. Aerts et al. (p. 1926; see the Perspective by Kawaler) studied the pulsations of HD 129929 for a period of about 21 years. They determined its age and metallicity and found evidence for mixing between its convective core and outer layers. This analysis will help refine models of stellar interiors and their evolution, which for massive stars is of particular interest because many will ultimately explode in type II supernovae.

  3. Putting Noise to Work

    The determination of absolute temperature in the lab is usually done with secondary standards calibrated against primary thermometers. The primary thermometers available in standards labs tend to operate over limited temperature ranges and can be difficult to use. Spietz et al. (p. 1929) report a primary thermometer based on the electrical noise of a tunnel junction. Their device operates from millikelvin temperatures up to room temperature, is self-calibrating, and should be as simple to use as secondary standards.

  4. Not-So-Silent Slipping and Sliding

    Recent studies have determined that the Cascadia subduction zone can slip without causing an earthquake and that the Japanese fore-arc subduction zone has experienced repeated deep-seismic tremor related to water migration in resonating conduits from the dehydrating slab. Rogers and Dragert (p. 1942; see the Perspective by Melbourne and Webb) have now determined that six slip events in the past 7 years in the Cascadia subduction zone are correlated with deep seismic tremor. Thus, these slip events do have a characteristic seismic signal, and there may be a correlation between slip and water release from the downgoing slab.

  5. Running Hot and Cold

    The oxidation of CO on the surfaces of platinum single crystals at very low pressures displays rich patterns of oscillations and traveling waves. At such low pressures, very little heat is generated and the experiments are essentially isothermal. Cirak et al. (p. 1932) have examined in detail the effects of reaction heat by working at higher pressures and with an ultrathin sample (0.2 micrometer thick) using a sensitive infrared camera. The heat of reaction is sufficient to cause localized surface buckling, but also causes the reactants to desorb, thus halting the reaction. The surface then cools, the crystal unbuckles, and cycle starts all over again in a span of 10 seconds. The authors separate out the thermal behavior in laser heating experiments, and present a detailed model that captures this complex behavior.

  6. Metal-Less Musings from the Main Belt

    M-type asteroids have spectral features that suggest a metal-rich composition, and they are thought to derive from the metallic cores of larger differentiated parent bodies and to be a source of iron meteorites. Margot and Brown (p. 1939) used adaptive optics on the Keck II telescope to discover a small irregular satellite orbiting the asteroid 22 Kalliope, which has been classed as M-type. The orbital parameters of the satellite indicate that 22 Kalliope has a bulk density of 2.37 grams per cubic centimeter. This derived value is too low for a metal-rich asteroid but would be consistent with a silicate-rich body that has 30% porosity. Thus, not all M-types are made of metal. This finding could change models of the evolution of the main belt and of sources for the iron meteorites.

  7. An Open and Closed Case

    Potassium channels can be opened or closed by many different signals, which allows them to function in diverse physiological processes. Kuo et al. (p. 1922) have determined the complete structure of an inwardly rectifying bacterial potassium channel, KirBac1.1, in the closed state at a resolution of 3.65 angstroms. A comparison with previously reported structures of the open state shows how ion flow is prevented in the closed state. In addition, it gives an indication of how conformational changes in the intracellular domain and the transmembrane region are coupled to achieve gating.

  8. Sorting Out Kith and Kin

    In most species with cooperative breeding, helpers are offspring that remain on the natal territory. Is the resultant kin favoritism the result of deliberate kin selection by individuals or some other effect, such as not leaving the neighborhood? Two reports present experimental results addressing this issue (see the Perspective by Dickinson and Koenig). Sinervo and Clobert (p. 1949) describe detailed genetic evidence for the existence of cooperation between unrelated male lizards that share both a high degree of genetic similarity but, more importantly, a similar throat color allele. The authors present evidence of the adaptive value of such male-male associations. Baglione et al. (p. 1947), in a study with carrion crows, using DNA microsatellites, found that immigrants and resident breeders of the same sex are highly genetically related and that crows actively chose to associate with relatives. Cooperation between relatives is more common in this case than expected and cannot be explained by limited dispersal.

  9. Stability at the Brain and Synapse Levels

    Different people have different temperaments, moods, and behavior profiles that tend to remain constant over a lifetime. Are these temperamental distinctions also reflected in basic brain differences? Schwartz et al. (p. 1952) studied the well-established temperamental differences of inhibited versus uninhibited behavior to novelty. They found that people classified in terms of these temperamental categories at the age of 2 showed differential fMRI responses to unfamiliar faces in the brain structure of the amygdala when tested as adults. How stable is a synapse that has just undergone long-term potentiation (LTP) or long-term depression (LTD) in real life? Zhuo et al. (p. 1953; see the Perspective by Chiu and Weliky) induced synaptic plasticity in the retinotectal system of the frog Xenopus. However, in a time window of about 20 minutes, subsequent spontaneous or uncorrelated activity could diminish or even reverse previously induced LTP or LTD. Stable and long-lasting changes in synaptic strength could only be induced if the induction protocol was repeated several times in a spaced fashion, with intervals of several minutes between the plasticity induction events.

  10. Watching CO Get Swept Away

    Time-resolved protein crystallography has allowed structural intermediates to be followed on nanosecond time scales. However, conformational intermediates in reaction mechanisms may form on even shorter time scales. Schotte et al. (p. 1944) used x-ray crystallography to obtain structural data for a myoglobin mutant protein at picosecond time resolution as it evolves from the carboxy to the deoxy state after flash photolysis. Correlated side-chain motions sweep CO away from its primary docking site on a subnanosecond time scale. This work extends the time resolution of crystallography into a time domain accessible by molecular dynamics simulations.

  11. Pulled Aside

    Asymmetric divisions are crucial for generating cell diversity in metazoan organisms and require precise coupling between polarity cues and the positioning of the mitotic spindle. Colombo et al. (p. 1957) report the discovery of two GoLoco proteins required for asymmetric spindle positioning in Caenorhabditis elegans embryos. By studying the interactions between these proteins and G proteins of the mitotic apparatus, they demonstrate the requirement for G protein signaling to generate asymmetric pulling forces on the mitotic spindle. These forces arise through the uneven distribution of the GoLoco proteins under the control of the anterior-posterior polarity components PAR-2 and PAR-3.

  12. Modeling the Spread of SARS

    The early transmission dynamics of severe acute respiratory syndrome (SARS) has been modeled by two research groups (see the Perspective by Dye and Gay). A crucial epidemiological parameter is R0, the basic reproductive rate, which is a measure of how many secondary cases a primary case generates under uncontrolled conditions. Lipsitch et al. (p. 1966) modeled the epidemiology of SARS using a comparative approach that does not overstretch the limited data and offer a cautious interpretation of the impact of control measures. SARS is sufficiently transmissible, as judged by estimates of R0 around 2, to cause a large epidemic if unchecked, but not so transmissible as to be uncontrollable with public health measures. Riley et al. (p. 1961) used a spatial, dynamic model of transmission of the causative agent of SARS in an attempt to replicate the Hong Kong outbreak. In Hong Kong, the epidemic appears to have been brought under control, apart from the risk of a super-spreader event rekindling the epidemic. This outcome was the result of reduced contact with infectious persons, rapid hospitalization of symptomatic people, and restrictions on longer range travel.

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