This Week in Science

Science  30 Oct 1998:
Vol. 282, Issue 5390, pp. 841
  1. Better with a Twist

    The second-order nonlinear optical responses of chiral molecules, such as frequency doubling, can be enhanced by supramolecular ordering. Verbiest et al. (p. 913) show that Langmuir-Blodgett films of substituted helicene molecules form helical fibers. Films of one molecular handedness are better organized and show much stronger second-order nonlinearity (by a factor of 30) than films of racemic mixtures. These helicene molecules would not normally be expected to exhibit high second-order effects, but the formation of chiral helical bundles enhanced certain components of the susceptibility tensor describing optical nonlinearity. The ability to form layers of opposite chirality will be useful for generating thicker films that maintain their nonlinear properties through quasi-phase-matching methods.

  2. Patterns of Dewetting

    Theoretical studies have pointed toward the formation of particular patterns, called spinodal modes, when a liquid film is deposited onto a substrate that is not wetted by the film. However, the experimental observation of these patterns has been difficult because they cannot easily be separated from other surface phenomena, such as the nucleation of dry patches at substrate defects. Herminghaus et al. (p. 916; see the Perspective by Reiter) provide experimental evidence for spinodal dewetting behavior in two systems, liquid crystal and liquid metal films. They can rationalize the observations in terms of the different forces dominant in the two systems.

  3. Shaping Chemical Reactions

    Although in principle it should be possible to use carefully prepared laser pulse sequences to control the outcome of chemical reactions such as photodissociation, in practice it is difficult to anticipate all of the possible outcomes as different states interact on different time scales. Assion et al. (p. 919) circumvent this difficulty by using an evolutionary algorithm to adjust the input laser pulse to a reaction to maximize formation of a particular product. Tailoring of femtopulse sequences allowed either of two different bond-cleaving reactions of an iron complex to be selected.

  4. Opalescent Carbon

    Ordered macroporous materials with three-dimensional periodicities on the scale of optical wavelengths are of interest, for example, as photonic band gap materials (which have band gaps for the propagation of photons at specific frequencies, allowing the control of light through the material). Zakhidov et al. (p. 897; see the cover and the Perspective by van Blaaderen) prepare microporous carbon materials via a synthesis route resembling the geological formation of opal, a gem consisting of small silica spheres with hydrous silica filling the voids between the spheres. They infiltrate crystals made of sintered silica spheres with carbon precursors and subsequently extract the silica. A range of ordered, stable materials with different properties of the wall materials depending on the synthesis conditions can be made that have periodicities at optical wavelengths.

  5. Long-Term Agroecosystem Experiments

    Is world food production sustainable? What influence will global change have on ecosystems? Can historical trends in atmospheric contaminants be determined? Such questions are difficult to answer, in part because the ecological processes involved are slow and short-term studies often fail to detect trends. Rasmussen et al. (p. 893) overview the contribution that a small number of long-term agroecosystem experiments (in some cases dating back to the middle of the last century) have been making in assessing changes in the environment and their influence on agricultural and natural ecosystems.

  6. A Thin Skin on the Inner Core

    Seismic waves travel faster through the inner core in a north-south direction than waves that travel east to west. Song and Helmberger (p. 924) present evidence for a subtle layer in the inner core based on analysis of seismic waves that traverse or reflect off of the inner core beneath the central Pacific Ocean. They use unusually broad pulses from long-period inner-core seismic waves, anomalous reflections from short-period inner-core waves, and a possible triplication of the short-period waves to model a thin isotropic outermost shell of randomly oriented iron grains (about 200 kilometer maximum thickness) that surrounds an anisotropic innermost core of iron grains aligned along a north-south direction. Such a model can be used to test theories about the geodynamo, the cooling of the core, and the structural transition between the inner and outer core.

  7. Isotopic Growth

    Crystals of different isotopes of an element will have different lattice constants, but this effect is extremely small, typically on the order of 1 in 100,000, and is difficult to measure. Previous measurements have been made on isotopically pure single crystals, which can be hard to obtain at the level of perfection required for such measurements. Kazimirov et al. (p. 930) have grown germanium-76 films epitaxially on a normal germanium substrate and measured the small lattice mismatch between them by Bragg diffraction. Good agreement with theory is obtained for the temperature dependence of the effect.

  8. Watching an Enzyme at Work

    To a geneticist, RNA polymerase initiates the process of gene expression by making a single-stranded RNA copy of the double-helical DNA genome. To a biochemist, RNA polymerase strings together single ribonucleotide triphosphates in a sequence dictated by the DNA template. Wang et al. (p. 902) use an optically controlled gauge to measure force and velocity online, revealing how RNA polymerase appears to a physicist: moving at constant speed at times, pausing at other moments, and even moving backward occasionally.

  9. Testing Male Fitness

    Haldane's rule predicts that the fitness loss from hybridization between species will affect the viability and fertility of the heterogametic sex more than the homogametic sex. Presgraves and Orr (p. 952; see the Perspective by Turelli) study two genera of mosquito to test theoretical explanations for Haldane's rule. Analysis of the structure of the Y chromosomes and the effects of hybridization on fertility and viability reveal some of the mechanisms operating behind Haldane's rule.

  10. Planning a Colony

    Filamentous multicellular colonies of the cyanobacterium Anabaena require both photosynthesis and nitrogen fixation, but these processes must occur in different cells. Determination of the fate and organization of these two cell types within the colony represents a simplified case of pattern formation. Yoon and Golden (p. 935; see the Perspective by Haselkorn) have identified a small peptide signal that emanates from one cell type to inhibit differentiation of similar neighbors. Thus, through cell-cell signaling, the nitrogen-fixing cells form a pattern distributed throughout the colony of cyanobacteria.

  11. Hepatitis C Virus Target

    Approximately 70 million people worldwide are chronically infected with hepatitis C virus (HCV), which can result in hepatitis, cirrhosis, and liver cancer. The inability to infect cells with HCV in vitro has made it difficult to study events in the virus life cycle that could provide clues to vaccines and therapeutic strategies. Pileri et al. (p. 938) have found that the viral envelope protein E2 binds to the major extracellular loop of the human protein CD81. Only chimpanzees who had been vaccinated with the viral envelope and resisted subsequent viral infection had antibodies that inhibited the ability of HCV to bind to CD81.

  12. Sticking with It

    Inflammatory sites entice leukocytes to leave the circulation and move into the surrounding tissue. One of the adhesion molecules that participates in this process is CD44. Maiti et al. (p. 941) found that a cytokine commonly produced at such sites, tumor necrosis factor-α, increases the adhesiveness of the CD44 on leukocytes by stimulating the cells to decorate CD44 with sulfate, not by inducing increased production of CD44. Thus, sulfation is a regulated, inducible process that contributes to the increased adhesiveness of leukocytes and that may aid their function during inflammation.

  13. Long Live . . . the Fly

    Understanding the process of aging may help in extending life-span and in maintaining improved quality of life. Lin et al. (p. 943; see the news story by Pennisi) isolated a mutant fly, methuselah, with a 35% increase in average life-span. The mutation was confined to a single gene that also confers improved resistance to stress. The methuselah gene product is related to a set of receptors involved in cellular signal transduction.

  14. Less Aggressive, More Successful

    Why are certain organisms so successful when introduced into a new environment? One possiblity is behavior modification. Holway et al. (p. 949; see the news story by Strauss) identify loss of intraspecific aggression—and a concomitant lack of territoriality—in an introduced species of ant. In its native range, the Argentine ant exhibits pronounced aggression, but seldom do colonies introduced in southern California. In paired lab combinations, nonaggressive colonies have lower mortality and actually cooperate, providing significant advantages over aggressive, territorial combinations.

  15. Wiring Earth's Mantle

    The electrical conductivity of the mantle affects the propagation of the geomagnetic field through the Earth. Xu et al. 922 show that in the lower mantle, ferric ions in perovskite are likely to have a great effect on the overall electrical conductivity. Ferric iron is fractionated preferentially into perovskite, along with aluminum. Consideration of the likely aluminum and ferric iron contents of perovskite yield electrical conductivity variations consistent with geophysical models.

  16. Composition of the K-T Impactor

    The extinction of many species at the Cretaceous-Tertiary (K-T) boundary, about 65 million years ago, has been correlated with a global layer of sedimentary rock enriched in noble metals and high-pressure mineral phases and the identification of the large Chicxulub impact crater off the Yucatan Peninsula. Shukolyukov and Lugmair 927 assessed the composition of the extraterrestrial impactor by measuring the isotopic ratio of chromium in sedimentary rocks above, below, and at the K-T boundary. The isotopic ratio of the K-T layer was distinct from the rocks above and below the boundary and similar to that measured in carbonaceous chondrites.

  17. Remodeling Specialist

    Vascular endothelial growth factors (VEGFs) are key regulators of blood vessel development. Dumont et al. 946 evaluated the specific role of VEGFR-3, a receptor for VEGF-C, by generating mice efficient in the receptor. The mutant embryos showed severe defects in remodeling of large blood vessels such as the aorta, leading to fluid accumulation in the pericardial cavity and death in utero. Thus, the VEGFR-3 signaling pathway is essential for early development of the cardiovascular system.

  18. Bad Vibrations for Quantum Dot Devices

    Devices based on semiconductor quantum dots, which will rely on putting electrons into specific energy states, must contend with spontaneous emission, which can cause unwanted changes in the electron's state. Fujisawa et al. 932 measured dc electron currents in a double quantum dot device and characterized the elastic and inelastic transitions between two energy levels. They show that the inelastic transitions exchange energy with lattice vibrations (acoustic phonons). Such coupling, which could limit the use of quantum dots for coherent processes used in quantum computing, might be eliminated with clever device geometries.

  19. Ebola Virus, Neutrophils, and Antibody Specificity

    Z.-y. Yang et al. reported that the secreted glycoprotein (sGP) of Ebola virus interacted with human neutrophils through the CD16b surface receptor (Reports, 13 Feb., p. 1034).

    T. Maruyama et al. comment that the study “did not exclude the possibility that the rabbit antibosdy used to detect the binding of Ebola sGP” mediated the interaction between virus and cell. They “performed flow cytometry to study the putative interaction of sGP with neutrophils” with human and rabbit monoclonal antibody fragments specific for sGP. They “could not detect any binding of sGP to neutrophils.” They also did not observe absorption of sGP by purified neutrophils. They conclude that “Ebola sGP does not bind FcγRIIIb (CD16) or any other receptor on neutrophils.

    In response, Yang et al. confirmed the inability to detect binding with an antibody fragment. However, they give several reasons for this result, and they performed a new experiment, which suggests that the explanation provided by Maruyama et al. “does not account for all of the available data.” “Additional studies,” they say, “will be required to determine whether CD16 is directly or indirectly responsible for the interaction.” (Yang et al. also provide a correction to the far-left graph of figure 1A of their report.)The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/282/5390/843a

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