This Week in Science

Science  08 Nov 1996:
Vol. 274, Issue 5289, pp. 893
  1. Tighter orbits

    The detection of Jupiter-sized planetsorbiting very close to a central star in other solar systems has sparked a flurry of modeling to explain how a planet could have evolved to such a tight yet apparently stable orbit. Rasio and Ford (p 952) present the results of simulations that start with two Jupiter-sized planets orbiting far from the central star. They find that in some cases one planet will be ejected from the system while the other planet will decrease and circularize its orbit by tidal dissipation.

  2. Soft landings

    One approach for constructing nanoscale structures would be to deposit them onto surfaces from the gas phase. However, the impact with the surface may fragment the deposited structures into smaller species and may damage the surface. Bromann et al. (956) used variable-temperature scanning tunneling microscopy to study the effects of impact energy on the deposition of silver clusters on a platinum surface and looked at ways to minimize destructive effects. Fragmentation and substrate damage occurredat impact energies above 1 electron volt, but clusters with higher impact energies could be adsorbed intact if a buffer layer of argon, an inert gas, was adsorbed first. The buffer layer efficiently transferred kinetic energy from the hot cluster before it could fragment.

  3. Thermally activated optical switches

    Just as the arrangement of atoms in a crystal scatters x-rays, larger colloidal particles in a crystalline array can scatter visible light. Weissmanet al. (959) have developed polymeric colloidal arrays whose periodicity responds to changes in temperature. In one case, the degree of ordering, and hence scattering efficiency, was changed, and in another the periodicity, and hence the scattering wavelength, was changed. Such materials could find use as tunable optical filters.

  4. Inner core convection

    Seismic observations have indicated that sound waves travel faster through the inner core if they travel along a path parallel to the rotation axis of the Earth. The reason for this inner coreanisotropy remains a mystery. Romanowiczet al. (p.963) used compressional wave velocities and free oscillations at the Earth's surface that are sensitive to the structure of the inner core to derive a model for the inner core anisotropy. They suggest that large-scale convection cells may be producing the anisotropy.

  5. Molecular motion

    Imaging single molecules often requiresimmobilizing the molecules in solids or at surfaces or at low temperatures to slow down molecular motion. Application of such methods to biological molecules, however, would require imaging in aqueous solution if the results are to be meaningful. Dicksonet al. (p. 966) used the water-filled pores present in a polyacrylamide gel, whose pore size can be tuned by changing the concentration of the gel, to restrict the Brownian motion of molecules. Laser excitation was performed in a thin plane so that the lateral motion of a molecule could be followed for about 1 second. For example, a dye molecule, nile red, could be imaged in a gel with 2-nanometer pores. Larger molecules, such as fluorescently tagged antibodies, were imaged in the same manner by usingless concentrated gels with larger pores.

  6. Form and function

    The visual systems of the Xenopus tadpole process information even as they continue throughdevelopment. Neurons in the tadpole's optic tectum represent a range of developmental maturation states. Wu et al. (p. 972), after analyzing the synaptic transmission characteristics of these neurons, suggest that function directs concurrent development. Immature neurons can signal with N-methyl-D-aspartate but do not contribute to functional visual processing. With maturity, promoted by feedback signaling from an active synapse, comes the shift to signaling with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid in a manner that contributes to visual processing.

  7. HIV-1 reservoirs

    Human immunodeficiency virus-type 1 is produced and persists in lymphoid tissue, but quantitating the viral burden in these organs is much more difficult than doing so for a blood sample. Haaseet al.(p. 985; see cover) used in situ RNA hybridization and image analysis to analyze HIV-1 load of several presymptomatic individuals, most of whom were receiving antiretroviral therapies. Large, stable pools of virus were found in follicular dendritic cells that were 100 to 10,000 times greater in number than those found in plasma. Such analysis will allow monitoring of the effects of therapies on overall viral burden.

  8. Plant signals

    The hormone cytokinin regulates a varietyof aspects of plant growth and physiology. Kakimoto (p. 982) identified a gene affected in cytokinin-independent mutants ofArabidopsis. The gene CKI1 encodes a protein similar to those in the two-component signal transduction systems of bacteria that mayact in cytokinin recognition or regulation.

  9. Clean capture

    Many studies in cell biologyand biomedicine require the isolation of specific cell populations from heterogeneous tissue sections. Emmert-Bucket al. (p. 998) developed a rapid one-step methodfor this purpose. In laser capture microdissection, a focused laser beam is used to transfer cells of interest within a microscopic field to a thermoplastic film. The cellular material is then readily removed from the film and can be subjected to standard nucleic acid and enzyme assays.

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