This Week in Science

Science  02 Mar 2001:
Vol. 291, Issue 5509, pp. 1659
  1. Vortex Magnetism

    The most familiar characteristics of superconductors are their ability to maintain a current without dissipation, and to exclude small magnetic fields (the Meissner effect). However, type-II superconductors, which include the cuprate family of high-temperature (high-Tc) superconductors, can sustain a supercurrent in large magnetic fields. In such materials, nonsuperconducting vortices containing a single quantum of magnetic flux penetrate the superconductor and are dispersed throughout the superconducting fluid. The spin dynamics of the vortices are thought to play a role in the properties displayed by the high-Tc superconductors, but little is actually known of the magnetic properties of the vortices. Lake et al. (p. 1759) used neutron scattering to look directly at the magnetic structure of the vortices of an optimally doped superconductor and provide evidence that the spins within the vortices show a strong tendency to order antiferromagnetically.

  2. Mix and Unmatch

    Combinatorial chemistry creates large libraries of compounds—and also large separation problems. A convenient way to keep track of different reactions is to attach one of the reactants to a solid support and use the support to separate molecules after the reactions are finished. However, in many cases, the solid support can limit the type and speed of reactions that can be performed. Luo et al. (p. 1766) show that by tagging reactants with fluorinated species of different chain length, mixtures of compounds can undergo reactions in a single pot and then be sorted quickly by fluorous chromatography.

  3. Reversible Crystal Steps and Wrinkles

    Photochromic materials, which undergo reversible reactions that can be driven back and forth with two different wavelengths of light, can be of use in data storage and memories. However, in many of these materials, thermal reactions can drive one state to the other, which limits the material's utility. Irie et al. (p. 1769; see the Perspective by Scheffer and Scott) recently reported on a compound that photochemically forms and breaks an intramolecular ring reversibly but that does not undergo thermal reactions. They now present atomic-force microscopy studies of the surfaces of single crystals of this compound before and after ultraviolet (UV) and visible irradiation. After a short induction period, UV irradiation changed the colorless crystal to blue and produced surface steps on one surface and valleys on another. These effects could be reversed by bleaching with visible light. Comparison with x-ray diffraction studies show that the crystal expands and contracts in steps of one unit cell, which suggests possible applications as a nanoscale actuator.

  4. Holding Its Charge

    Materials that can retain trapped electrical charge or polarization, known as electrets, could provide a convenient route to nanofabrication or data storage if they could be patterned readily on a fine scale. Jacobs and Whitesides (p. 1763) show that flexible rubber stamps coated with a metallic conductor can, upon contact and application of a voltage pulse through the layers, induce a charge pattern in poly(methyl methacrylate) (PMMA) film on a conductive support. Areas up to a square centimeter could be patterned at a resolution of 150 nanometers in less than 20 seconds, and particles could be assembled on the charged surfaces. Such charge transfer was incomplete when stiffer substrates supported the patterned electrode.

  5. Super-Intense Superchron

    Geodynamo simulations suggest that during long periods of time when Earth's magnetic field was not reversing (a superchron), the magnetic dipole was stronger. Previously measured paleointensities of the magnetic dipole for the last 165 million years do not corroborate this relation. Tarduno et al. (p. 1779; see the Perspective by Banerjee) measured the paleointensities of plagioclase crystals for a series of lava flows that erupted from the Rajmahal Traps, India, within a 3-million-year period during the Cretaceous Normal Polarity Superchron. These measurements yielded a much higher paleointensity than previous measurements on whole rocks. Observations and simulations can now be coupled, thus clearing the way for understanding how the geodynamo works when the field is stable and when it flips.

  6. The First Chondrules?

    Chondrules are millimeter-sized rounded fragments of glass and crystals found in the most primitive meteorites called chondrites. They may have formed by flash-heating events in the solar nebula. Krot et al. (p. 1776) studied two atypical chondrites that contain zoned iron-nickel metal grains. The petrographic and chemical characteristics suggest that they formed in one large region that vaporized the materials and created the chondrites by gas-liquid fractionation. The unaltered nature of the chondrites and the large heating region required for their formation indicate that they formed early in the lifetime of the solar nebula.

  7. Sex Antiseptic

    Sexually transmitted diseases are the bane of many organisms, yet little is known of the host responses in the male genital tract when encountering pathogens during sex. Li et al. (p. 1783) have discovered an antimicrobial peptide that is secreted specifically from a region of the epididymis of male rats and that has primate homologs. This peptide, with sequence similarity with defensins, may not only protect against invading microorganisms such as gonococci and treponemes but may also aid the maturation of sperm during normal development.

  8. Annals of Mammals

    The relationships among the major groups of mammals have long been of great interest—and debate. Liu et al. (p. 1786; see the Perspective by Springer and de Jong) present a meta-analysis of a large body of molecular and morphological phylogenetic data to produce a “supertree” of the family-level relationships among the placental mammals. Encouragingly, they find a substantial degree of congruence between molecules and morphology in the sense of placement of families within orders, which argues convincingly for the continued importance of the use of both kinds of data.

  9. Equipping Bacterial Antibiotic Laboratories

    Drug discovery largely has been a process of screening natural products from microorganisms and plants, identifying a promising lead, and then tinkering with the chemical structure in the laboratory to improve pharmacokinetics and diminish side effects. Recently, there have been efforts to shift the tinkering stage back into the natural producers in order to co-opt the unparalleled skill with which enzymes achieve stereospecificity. Pfeifer et al. (p. 1790; see the news story by Ferber) have taken an alternative approach by transferring the polyketide biosynthetic genes from the producer of erythromycin, Saccharopolyspora erythraea, to the bacterial workhorse, Escherichia coli. This strain of E. coli yields the core of erythromycin in quantities comparable to that of the parent.

  10. Eat Your Spinach

    Fans of “Popeye” cartoons will know the importance of dietary iron. However, how the iron that we eat actually gets out of the gut for absorption into the body has been a bit of a mystery. The ferric form of iron is quite insoluble and must be converted to the ferrous form to become bioavailable. McKie et al. (p. 1755) have identified a protein in the gut that appears to be the key to iron absorption. The protein, duodenal cytochrome b, can reduce dietary iron into a form that can be transported across the gut wall.

  11. How to Grab a Membrane

    After endocytosis, endosomes fuse with one another. One targeting factor appears to be the production of phosphatidylinositol 3-phosphate in membranes; this lipid head group is then recognized by so-called FYVE domains found on some endosomal fusion proteins. Kutateladze and Overduin (p. 1793) present an analysis of the structure of the FYVE domain of early endosome antigen 1 in complex with phosphatidylinositol 3-phosphate and suggest a mechanism by which these interactions may promote endosome recognition.

  12. Life Without Telomerase

    The integrity of eukaryotic genomes has been linked to the function of telomeres, the DNA sequences that cap the ends of chromosomes. Studying the mustard weed Arabidopsis thaliana, Riha et al. (p. 1797) examined the fate of plants deficient in telomerase, the enzyme that maintains telomeres. The mutant plants survived for 10 generations, although the plants in the later generations showed increasing evidence of cytogenetic damage. Such toleration of telomere dysfunction contrasts sharply with what has been observed in animals and reflects the remarkable plasticity of plant development and genome organization.

  13. Of Digits and Distractors

    Visual attention and working memory both have been studied for many years. De Fockert et al. (p. 1803; see the news story by Wickelgren) present both behavioral and functional brain imaging data to show that working memory load in the prefrontal cortex regulates the level of processing in posterior visual cortices. In particular, greater activation of the prefrontal cortex reduces the ability to discriminate between relevant and distractor stimuli and actually leads to poorer task performance.

  14. Micrometeorites in the Stratosphere

    Micrometer-sized extraterrestrial particles rarely make it to the surface of Earth. Instead, they are ablated and vaporized in the atmosphere, which adds uncertainty to estimates of the mass flux of extraterrestrial material. Cziczo et al. (p. 1772) measured the spectra of sulfuric acid and water particles that were frozen in the upper stratosphere and coagulated micrometeoric material before it could be vaporized. Comparison of the stratospheric spectra with laboratory spectra of simulated meteoritic aerosols yielded a mass flux of 8 to 38 gigagrams per year, which is low compared to most previous estimates.

  15. Talking of Sphingosine

    Attention has focused recently on cross-talk between signaling pathways in mammalian cells. Indeed, activation of G protein-coupled receptors (GPCR) can result in activation of growth factor receptor tyrosine kinases even when the ligand for the latter is not present. Hobson et al. (p. 1800) now show that cell motility induced by stimulation of the platelet-derived growth factor receptor (PDGFR, a receptor tyrosine kinase) requires signaling through the receptor EDG-1 (a GPCR). In this case, stimulation of the PDGF receptor causes increased production of sphingosine-1-phosphate, which acts in an autocrine or paracrine manner as an activating ligand for EDG-1. In mouse embryo fibroblasts lacking sphinosine kinase or EDG-1, chemotaxis of the cells toward PDGF was inhibited.