This Week in Science

Science  19 Jan 2001:
Vol. 291, Issue 5503, pp. 393
  1. Ellipsoidal Colloidals

    One approach for making colloidal particles is to use crystals of colloidal particles as templates. The intervening space can be filled with a polymer, and upon removal of the original colloid, used as a template to make particles of other materials that may have interesting magnetic or optical properties but that do not readily form uniform colloidal particles in a particular size range. Jiang et al. (p. 453; see the Perspective by Mallouk) demonstrate this approach in a general way for metals, oxides, and polymers using a polymer template. They also show that the polymer can be deformed uniformly in one direction so that ellipsoidal particles with axial ratios up to 4 can be made.

  2. Gas-Phase Dynamics at the Structural Level

    Ultrafast dynamics of chemical reactions, which can be initiated by femtosecond laser pulses, are usually probed indirectly, such as by vibrational or mass spectroscopy. Direct structural studies of such processes, as can be obtained with x-ray diffraction, are usually limited to crystals or surfaces. Ihee et al. (p. 458) now report on results obtained with an ultrafast electron diffaction apparatus for gas-phase reactions of small molecules. They characterized the stepwise elimination reaction of haloethane and the ring opening of a cyclic hydrocarbon. The latter case shows that heavy atoms are not necessary in order to obtain useful ultrafast electron-diffraction data.

  3. Entanglement in a Quantum-Dot Molecule

    Potential solid-state schemes for quantum computing are of interest because these implementations offer the possibility of scale-up and integration. Bayer et al. (p. 451) describe results on a vertically coupled pair, or molecule, of single quantum dots. Under optical excitation and an applied electric field, a coupled electron-hole pair (exciton) is created on the molecule with just four possible arrangements for the electron-hole pair. When the dots are coupled, the excitation spectrum differs from those for the single dots. Entanglement between these possible exciton states manifests itself as an energy splitting in the excitation spectrum. Moreover, the energy splitting increased to 40 millielectron volts as the dot separation was reduced to 4 nanometers, which indicates that room-temperature operation may be possible.

  4. Biological Insulators

    Insulators or chromatin boundary elements are specialized chromatin structures that regulate gene activity. These structures block the action of transcriptional enhancers or silencers when located between the gene regulatory element and the gene's promoter. Two studies, performed by Cai and Shen (p. 493) and Muravyova et al. (p. 495), used Drosophila genetics to examine the mechanism of action for insulators. Experiments in which insulator copy number and position were altered indicate that insulators are likely to affect enhancer-promoter interactions by stimulating the formation of chromatin loop domains through the interaction of protein-bound insulator complexes. A review by Bell et al. (p. 447) highlight recent research on the structure, function, and regulation of a wide range of insulators.

  5. Little Green Lines on Venus

    Optical emissions from solar irradiation of planetary atmospheres produce distinctive line emissions that can be used to understand the composition and dynamics of the atmospheres. In particular, a common transition of atomic oxygen produces a green line emission that can be used to estimate the density of oxygen atoms. Venus does not have a green line emission based on observations by the Venera and Pioneer missions and model studies which suggested that the atomic oxygen transitions were quenched by collisions with carbon dioxide. Slanger et al. (p. 463) used the high-resolution echelle spectrograph on the Keck I telescope to obtain spectra of the atmosphere of Venus and detected a strong green line emission. Thus, models of the abundance and dynamics of oxygen in the venusian atmosphere will need to be refined.

  6. Depleted Polar Tropospheric Ozone

    When the Arctic sun rises after the long polar night, surface-level ozone can become severely depleted. Laboratory studies have suggested that this depletion results from a catalytic process initiated by bromine release from gas-phase precursors like Br2, HOBr, or BrCl. Foster et al. (p. 471) now report in situ measurements of Br2 and BrCl in ambient air at the surface of the snow in the Canadian Arctic. Increases in the concentrations of these species were tightly correlated with ozone depletions. The absence of Cl2 indicates that BrCl is the dominant source of photolyzable chlorine in the polar regions, contrary to what had often been assumed.

  7. Tracing Horse Pedigrees

    Archaeological evidence suggests that the horse was domesticated about 6000 years ago in Central Asia, with major consequences for human civilization. Was domestication achieved using individuals from one or a small handful of wild populations, or using multiple wild populations over a wide area and time span· Vilà et al. (p. 474; see the news story by Pennisi) used mitochondrial and microsatellite markers to quantify genetic variation in modern horse breeds and from wild horse remains 12,000 to 28,000 years old. These results show that the modern domestic horse population was founded by a diverse array of matrilines, implying multiple ancestry and possibly a period of extensive taming from wild populations. The interaction between humans and horses may have long preceded the first morphological evidence of domestication.

  8. Kept in Line

    In some cooperative animal societies, breeding is distributed unevenly across the adult females, such that only a few dominants reproduce while the remainder are “helpers.” Different models have been advanced to account for this “reproductive skew,” but have proved difficult to test unequivocally. In a 7-year study of meerkats in southern Africa, Clutton-Brock et al. (p. 478; see the Perspective by Koenig and Haydock) find that subordinates will only breed when beyond the control of dominant females. They find no evidence for the alternative hypothesis that dominant females periodically allow subordinates to reproduce in order to retain their services as helpers.

  9. Gauging Rain

    Understanding the patterns and mechanisms of ecosystem responsiveness to climate variability is fundamental to any attempt to predict ecosystem response to climate change. Traditional views of plant production in ecosystems have held that interannual variability of production should correlate directly with variability in precipitation. A long-term study of 14 different ecosystems in the United States, spanning a range of precipitation from 250 to 1400 millimeters per year, indicates that the pattern is more complex. Knapp and Smith (p. 481; see the news story by Kaiser) show that variability among years in production is not associated with variability in precipitation, and that the sites with the highest variability in precipitation do not exhibit the highest variation in production. They propose that the variability in production depends on an interaction between precipitation and growth potential of plants.

  10. New Ideas on an Old Remedy

    The beneficial effects of administering intravenous _ globulin (IVIG) in a range of illnesses have long been recognized, yet at a mechanistic level this treatment has remained poorly defined. One of the major applications for IVIG is in the treatment of immune thrombocytopenia (ITP), a disease in which platelets are excessively removed from the circulation by phagocytic cells through an antibody-dependent pathway. In their study of a mouse model of ITP, Samuelsson et al. (p. 484; see the Perspective by Lin and Kinet) present convincing evidence that the effects of IVIG are mediated by inhibitory Fc receptors. The exposure of the inhibitory Fc receptor pathway in a proven therapeutic setting is likely to set important clinical precedents in the treatment of inflammatory disorders.

  11. Setting Some Clocks Ahead

    Animals have numerous circadian clocks in peripheral organs that are thought to be driven by the light-entrained “master” clock in the hypothalamus. However, Stokkan et al. (p. 490) show that this master-slave relation is not so clear. The authors engineered rats to express the luminous molecule luciferase so that they could easily monitor the animals' clocks in vivo. When the rats were put on an altered feeding schedule, the liver clock—but not the brain clock—was rapidly entrained to the new feeding times.

  12. Starting Off Correctly

    For accurate protein synthesis, the initiator transfer RNA (tRNA) must be base-paired to the messenger RNA start codon in the P site of the 30S ribosomal subunit. In prokaryotes, this is achieved in an initiation complex that also has three initiation factors (IF1, IF2, and IF3) bound. Carter et al. (p. 498) have determined the crystal structure of a complex of IF1 and the 30S subunit. The A site appears to be sterically blocked by IF1, which, together with IF2, may be involved in the correct positioning of initiator tRNA in the P site. Local structural changes caused by IF1 binding induce conformational changes in regions of the 30S subunit that contact the 50S subunit in the intact ribosome.

  13. Sound Velocities in Iron

    Earth's core is thought to be composed mostly of iron and a small amount of lighter elements such as sulfur or oxygen that are needed to account for the seismic velocities of sound waves that traverse the core. Laboratory experiments cannot reach the pressure and temperature conditions of the core, and thus the composition and structure of iron is extrapolated using the equation of state relations and indirect estimates of the sound velocities. Fiquet et al. (p. 468) directly measured the longitudinal velocity of hexagonal close-packed iron (hcp iron) from 19 to 110 gigapascals with a phonon dispersion method in a diamond anvil cell. Their results suggest that a linear extrapolation of the hcp iron data using a Birch law yields longitudinal wave velocities for the inner core that are consistent with seismic data that require elements lighter than iron in the inner core.

  14. Coupling Kaapvaal Crust

    Cratons are thick sections of ancient continental crust that may represent the first pieces of the continents to form. A few have survived to the present, possibly because they are coupled to thick sections of ancient mantle to create a deeply rooted tectosphere that is protected from any extensive heating and alteration initiated by mantle convection. Moser et al. (p. 465) studied the geology of the lower crust of the Kaapvaal craton that was uplifted and exposed by the Vredefort impact event in South Africa. Uranium-lead dating of rocks related to the last major magmatic events at Kaapvaal suggest that the crust was formed and extensively altered up until about 3.11 billion years ago. A short time later (about 3.09 billion years ago), the crust became coupled to the mantle to create the tectosphere.