This Week in Science

Science  11 Jun 2004:
Vol. 304, Issue 5677, pp. 1561
  1. Ferroelectrics Sliced Thin


    Is there some thickness below which ferroelectric behavior is no longer possible? Fong et al. (p. 1650; see the Perspective by Spaldin) made x-ray diffraction measurements of a series of ultrathin ferroelectric perovskite films. As the film thickness decreased, the transition temperature showed a precipitous decline but was still observable for films as thin as three unit cells.

  2. Catalysis in Flux

    Heterogeneous catalysts often consist of nanoscale metal particles absorbed on a metal oxide support. Such nanoscale particles can differ from bulk metals in both their electronic structure (“quantum confinement” effects) and their surface size (“geometric” effects). A less well studied effect is that of fluctuations—very small particles will absorb relatively few molecules, which should lead to larger statistical fluctuations in their surface coverage. Johánek et al. (p. 1639, published online 6 May 2004) now show that such fluctuations change the reaction kinetics of the oxidation of carbon monoxide over Pd nanoparticles. The reaction over larger nanoparticles (500 nanometers) displays two different regimes of stable kinetics (hence two different rates can be seen at a given CO to O2 pressure ratio, depending on whether the mixture is becoming CO or O2 rich). However, over smaller nanoparticles (∼2 nanometers), the bistable regimes cannot be created because statistical fluctuations in the relative surface concentrations of adsorbed molecules are large compared with the concentration differences that would define the two kinetic regimes.

  3. The Basics of Enzymatic Peroxidation

    The P450 family of enzymes can perform demanding partial oxidation reactions on relatively inert substrates such as cyclohexane, and the question has been posed as to how the Fe-porphyrin centers manage to do this chemistry and not oxidize the rest of the protein. Green et al. (p. 1653) have used extended x-ray absorption spectroscopy to characterize activated chloroperoxidase and its one-electron reduction product (CPO-I and CPO-II, respectively). The bond distances they observe for CPO-II indicate that it is not an oxoiron(IV) species but rather a protonated ferryl species, FeIV-OH. Strong electron donation by a closer-than-expected axial thiol ligand makes the oxygen more basic. This acid-base chemistry allows a lower redox potential to be used in alkane oxidation.

  4. Cycling Sulfur

    The chemical cycle of sulfur is coupled to those of carbon, oxygen, and a number of other key species. The exogenic sulfur cycle is reflected by its isotopic composition, the record of which contains valuable information about marine productivity, nutrient distribution, redox conditions, and volcanic input. Paytan et al. (p. 1663) have reconstructed a record of the isotopic composition of marine sulfur for the last 130 million years using the highly resistant sulfate mineral barite. Several large variations occurred over that interval, which imply changes in the rate of pyrite burial, and suggest that the isotopically light sulfur compositions of much of the Cretaceous reflect increased volcanic and hydrothermal activity.

  5. Ancient Farming on Hawai'i

    What was the impact of agriculture in tropical environments before European contact with indigenous cultures? Vitousek et al. (p. 1665) describe patterns in climate and soil fertility associated with a large, intensive rain-fed agricultural complex in Hawai'i. They reveal the mechanisms that made soils in this environment conducive to agricultural intensification, and show how these mechanisms interacted with the environmental heterogeneity of the Hawaiian Islands to influence the development of different farming patterns—and hence contrasting societies—across the archipelago.

  6. Cellular Regulation of Phospholipid Biosynthesis

    How do cells homeostatically regulate the lipid composition of their membranes? Phospholipid metabolism in yeast is coordinately repressed by the transcription factor Opi1p, which in turn is activated by extracellular inositol. Loewen et al. (p. 1644) now show that there is a pool of phosphatidic acid (PA) on the endoplasmic reticulum (ER) that is highly sensitive to growth conditions, in particular to the presence of exogenous inositol. This PA is directly sensed by the soluble transcriptional repressor Opi1p, which binds to PA in the ER and prevents it from entering the nucleus and altering transcription. When inositol is restored to cells, the Opi1p-associated PA is metabolized, which releases the Opi1p from the ER and allowing its entry into the nucleus.”

  7. Femtotesla Magnetoresistive Sensors


    For measurements of magnetic fields on the scale of femtoteslas (10−15 Tesla), superconducting quantum interference devices (SQUIDs) provide low noise and high sensitivity but operate at liquid helium temperatures. Pannetier et al. (p. 1648) have developed a device that combines a flux-to-field transformer and a magnetoresistive (MR) junction that can operate at 77 kelvin. The transformer, a superconducting loop with a narrow constriction, locally amplifies the field through the loop at the constriction, which in turn is detected by a nearby MR device. Theoretical limits for such MR devices suggest that subfemtotesla detection should be possible.

  8. The Pulse of Southern Climate

    Records from polar and subpolar regions for the past 100,000 years suggest that climate change in the Southern Hemisphere led its Northern Hemisphere counterpart by approximately 1500 to 3000 years, but mid-latitude records are needed to put this difference in a global context. New Zealand is an ideal location to obtain such mid-latitude records, but different studies have reached conflicting conclusions about whether New Zealand's climate resembles that of the North or the South Pole. Carter and Gammon (p. 1659) present a 3.75-million-year-long marine record of sediment discharge from glaciers in the Southern Alps of New Zealand, which shows that the extent of glaciers in this region varied in concert with the climate of Antarctica (as recorded by the Vostok ice core) for at least the past 370,000 years. The older part of the record, beyond the age of the Vostok ice core, shows a glacial history that exhibits both similarities to, and differences from, the climate record inferred from the deep-sea oxygen isotope record.

  9. Muscular Restrictions


    In metazoa, homeoproteins are involved in many different developmental processes. Lee et al. (p. 1675; see the Perspective by Cirillo and Zaret) describe how the Msx family of homeoproteins regulate myogenic gene expression during Xenopus embryogenesis. Msx1 homeoprotein associates with the linker histone H1b, which is involved in the structural assembly of chromatin fibers. Msx1 and H1b repress the myogenic regulator MyoD, and can inhibit myogenic differentiation in cell culture and in Xenopus animal caps.

  10. Chitin Degradation and Asthma

    Chitinases have multiple functions in lower eukaryotes and prokaryotes, including defense against parasites. Although similar functions of mammalian chitinases have yet to be observed, the strong similarity between antiparasitic immunity and asthma in humans led Zhu et al. (p. 1678; see the news story by Couzin) to consider chitinases in asthma pathogenesis. Acidic mammalian chitinase (AMCase) increased both in asthmatic human lung tissue and in a mouse model of induced asthma. AMCase inhibition in the mouse model reduced inflammation by its effects on cytokines involved in the T helper-2 pathway, most specifically interleukin-13. Thus, AMCase and other chitinases may be valuable therapeutic targets in asthma and other allergic conditions.

  11. Shhh, the Dog May Be Listening

    The early learning of words by young children normally proceeds effortlessly and rapidly, and often after hearing words only once. Kaminski et al. (p. 1682; see the Perspective by Bloom) bring evidence of a dog's word-learning capacity to the enduring discussion about whether this the language abilities of human children is specialized or is based on general learning mechanisms. Rico, a 9-year-old border collie, can retrieve 200 objects, identified by spoken name, and can associate new object names with unfamiliar objects reasonably accurately even 4 weeks after a single exposure to the word.

  12. I'm All Thumbs

    Studies of tetrapods have identified several of the factors that participate in vertebrate limb patterning. A major player in limb development, Sonic hedgehog (Shh), has a key function in organizing limb asymmetry, such as the difference between the thumb and index finger. Zákány et al. (p. 1669; see the Perspective by Deschamps) examined the genetic cascade leading to anterior-posterior polarity of the vertebrate limb. Early posterior Hox gene expression triggers Shh activation, which in turn induces a second phase of Hox gene expression in a posterior domain. Thus, Hox genes act both early and late in establishing anterior-posterior asymmetry of the limb by acting upstream and downstream of Shh signaling.

  13. Oxygen Activity in the Mantle

    The amount of oxygen in the mantle influences the composition, rheology, and melting behavior of mantle rocks and may affect crustal and even atmospheric conditions over geologic time scales. Williams et al. (p. 1656) have measured a correlation between iron isotopic abundances and relative oxygen fugacity in spinels from mantle peridotites. This correlation may make it easier to estimate the activity of oxygen in the mantle from iron isotopes.

  14. The Push and Pull of a Calcium Pump

    Pushing an object uphill not only requires energy, but also a pawl to prevent backsliding. In the adenosine triphosphate (ATP)-driven calcium pump from sarcoplasmic reticulum, the experimental sign that the pawl has locked into place is that the pair of Ca2+ ions are trapped within the protein so that they cannot exchange with external calcium. Sørensen et al. (p. 1672) describe the crystal structures of two conformational states of the calcium pump, one with a nonhydrolyzable ATP analog, AMPPCP, and another with both adenosine diphosphate and aluminum fluoride bound. These states are analogs of the reaction intermediates prior to and just after the high-energy phosphate group has been transferred from ATP to the enzyme, respectively. In the former state, the calcium is not occluded, while in the latter, it is. The structural change induced by phosphorylation begins as an overall compaction of the cytoplasmic domain, which pulls on two of the transmembrane helices and shuts the door from the Ca2+ ion-binding site to the cytoplasm. Relaxation of the high-energy conformation then releases the ions into the lumen of the sarcoplasmic reticulum, where they are stored until needed to trigger muscle contraction once again.

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