This Week in Science

Science  11 Feb 2000:
Vol. 287, Issue 5455, pp. 929
  1. Modeling Magnetic Semiconductors

    The technologically important III-V (GaAs) and II-VI (ZnTe) compound semiconductors, which are usually nonmagnetic can be made ferromagnetic by the addition of small amounts of manganese. In such materials, electronic switching might be controlled through spin interactions as well as charge. So far, the understanding of this ferromagnetism has been limited and rather phenomenological. Dietl et al. (p. 1019) now put the subject on a much stronger footing by proposing a theory that can describe the magnetic properties as a function of the materials' properties and the concentration of magnetic ions introduced into the system. Moreover, by extending the theory to other materials, they suggest a route for increasing the operation temperature of future magnetic devices.

  2. Organic Transistors that Conduct Both Ways

    Charge transport in organic semiconductors is predominantly through hole carriers (p type or p channel); conduction of electrons (n type or n channel) is usually poor. This limitation is a serious obstacle to their widespread application as a cheaper and fully complementary alternative to conventional electronics based on inorganic semiconductors. Schön et al. (p. 1022) now demonstrate ambipolar operation (conduction in both p-channel and n-channel modes), field-effect transistors (FETs) based on pentacene single crystals. Furthermore, the carrier mobilities at room temperature in these organic FETs are comparable to those of hydrogenated amorphous silicon, and the temperature dependence of the mobilities suggests bandlike transport.

  3. Mobile Cellular Signaling

    Microbes seeking food and phagocytic immune cells stalking their prey rely on chemical signals, or chemoattractants, to guide their paths. Chemotactic cells can orient their anterior edge toward a stimulus gradient despite their uniform distribution of cell surface receptors. Five reports focus on the role that phosphatidylinositol 3' kinases (PI3Ks) play in chemotaxis (see the Perspective by Dekker and Segal). Hirsch et al. (p. 1049), Li et al. (p. 1046), and Sasaki et al. (p. 1040) describe the phenotype of mice that lack PI3Kγ, an isoform that is activated in response to G protein- coupled receptors (GPCRs). Defects in these animals show that PI3Kγ is required for a number of functions of neutrophils and T cells in vitro and in vivo. All three groups note that the inflammatory response is disrupted in several ways. Neutrophils from the mutant animals showed impaired migration and respiratory burst in response to stimulation through GPCRs. Sasaki et al. also found T cell activation through the T cell receptor was disrupted. Li et al. also generated mice lacking phospholipase C (PLC)-β2 and PLC-β3. They report that whereas PI3Kγ is required for normal production of immunoglobulin (Ig) containing the λ light chains, the PLC pathway can inhibit chemotaxis and production of IgλL. The results should enhance efforts in the development of pharmaceuticals to manage inflammation. An internal signaling gradient may accommodate such the directional chemotatic response. Jin et al. (p. 1034) show that in highly polarized amoeba cells, membrane-bound G protein subunits were present in a shallow anterior-posterior gradient. Servant et al. (p. 1037) also determined that such an internal signal gradient involves the activities of Rho family of guanosine triphosphatases and PI3K in neutrophil-like cells. Hence, signaling molecules appear to regulate an internal gradient that determines chemotactic sensitivity.

  4. Imaging Antiferromagnets

    Thin layers of magnetic materials are finding more applications in storage media, and additional methods for characterizing their magnetic properties will be required. One particular problem has been determining the orientation of magnetic domains in antiferromagnetic thin films. Scholl et al. (p. 1014) introduce a new technique in which x-ray magnetic linear dichroism, in which the response of x-rays incident on a surface depends on the orientation of the magnetization in that layer, is combined with atom-specific photoemission electron microscopy. They can image antiferromagnetic domains in LaFeO3 thin films with 20-nanometer resolution.

  5. Making Molecules in a Bose-Einstein Condensate

    Diatomic molecules have been formed in a Bose-Einstein condensate of rubidium atoms by using laser fields. Wynar et al. (p. 1016; see the Perspective by Williams and Julienne) took advantage of a stimulated Raman process—a close pair of atoms adsorb a photon at one frequency and emit a photon of slightly higher frequency, which drops them into an excited molecular state. This process selects for a particular rotational and vibrational state that corresponds to the frequency difference. The molecules are formed with almost no translational energy, which results in extremely narrow linewidths that facilitate the measurement of molecular binding energies with high precision.

  6. Keeping Cold

    Improved thermoelectric materials that work well below room temperature are of particular interest for electronics. Such materials, which provide cooling by converting thermal energy into electrical current, could refrigerate superconducting devices or help remove heat from transistors. Promising materials must have an unusual combination of high electrical conductivity, high thermoelectric power, and low thermal conductivity. Chung et al. (p. 1024; see the news story by Cho) report that CsBi4Te6, when appropriately doped, already rivals the low-temperature performance of the currently used Bi2-xSbxTe3-ySey alloys, and several potential doping and alloying systems for CsBi4Te6 have yet to be explored.

  7. Core Consistency

    The melting temperature of iron at the high pressures (about 140 to 330 gigapascals) of the liquid outer core has been difficult to nail down. Experiments cannot directly measure the melting temperature, and extrapolations have led to divergent results—shock wave estimates suggest higher temperatures than do static high-pressure studies. Laio et al. (p. 1027) have combined two theoretical approaches, first-principles calculations and molecular dynamic simulations, to determine the temperature and density of liquid iron. They calculated a melting point of 5400 ± 400 K for iron at the inner core boundary (330 gigapascals) and a density about 6% greater than the density of liquid iron at the outer core boundary (140 gigapascals). These results not only help to reconcile previous experimental results but are also consistent with seismic data.

  8. Avoiding Natural Killer Cells

    Human cytomegalovirus (HCMV) is known to encode a series of proteins that enable it to avoid cytotoxic T cells. However, in doing so, the virus should induce natural killer (NK) cells. Tomasec et al. (p. 1031) found that the amino-terminal region of glycoprotein UL40 of HCMV contains a peptide identical to a cellular molecule that binds to human lymphocyte antigen-E (HLA-E). Binding results in increased synthesis of HLA-E on the surface of infected cells that acts to inhibit NK cells.

  9. Early Effects of Alcohol Exposure

    Exposure of the human fetus to alcohol can result in fetal alcohol syndrome; brain mass is reduced, and subsequent neurobehavorial effects that may occur range from hyperactivity to depression and psychosis. Ikonomidou et al. (p. 1056; see the news story by Barinaga) studied the effects of alcohol administration on brain development in neonatal rats, which is a corresponding stage for widespread synaptogenesis to take place in rodents. They found widespread apoptosis in the forebrain that was caused by blockade of NMDA (N-methyl-D-aspartate) receptors and excessive activation of GABAA receptors. Transient exposure of ethanol led to the deletion of millions of neurons and can explain the reductions in brain mass.

  10. What's Not Thrown Away-Accumulates

    The lack of correlation between genome size and organism complexity or gene number (the C-value paradox) remains one of the major enigmas of molecular evolution. One recent hypothesis to explain this enigma is that differences in genome size may result from persistent differences between organisms in the rate of loss of nonessential DNA. Petrov et al. (p. 1060; see the Perspective by Capy) provide experimental support for this hypothesis by comparing DNA loss in two insect genera with very different genome sizes. Hawaiian crickets of the genus Laupala have a genome size an order of magnitude larger than that of the fruit fly Drosophila and eliminate nonessential DNA at a rate almost 1/40th as fast.

  11. A C. elegans Analysis

    The sequencing of a genome is only the beginning step toward understanding the biology underlying a living organism. Hutter et al. (p. 989) have analyzed extracellular matrix and cellular adhesion proteins by examining the recently completed genome of Caenorhabditis elegans. Their studies suggest ways in which new genes and proteins arose during evolution.

  12. Auger Processes in Quantum Dots

    Excited atoms with empty low-level electronic states may lose energy radiatively (for example, by emitting an x-ray), but this process competes with Auger emission of an excited electron. Similar Auger processes occur in bulk semiconductors but are generally less efficient. Klimov et al. (p. 1011) have studied how Auger processes occur in semiconductor quantum dots, the intermediate case between atoms and bulk solids. The Auger relaxation rates are cubic in carrier concentration, as in bulk solids, but they find that the characteristic Auger constant shows a strong size dependence and varies as cube of the dots' radii, rather than to the sixth power as predicted by bulk models.

  13. Critical Requirements

    Female sexual receptivity in mice and rats is mediated by the progestin receptors. Mani et al. (p. 1053) found that antisense nucleotides to DARPP-32, a phosphoprotein that is a critical intermediate in progesterone signaling, blocked the effects of progesterone on sexual receptivity in rats. Mice carrying a null mutation for DARPP-32 also showed minimal progesterone-facilitated receptivity.

  14. NADH Shuttle and Insulin Secretion

    Eto et al. (Reports, 12 Feb. 1999, p. 981) tested the role of shuttles that transport nicotinamide adenine dinucleotide (NADH) from the cytoplasm into mitochondria in coupling glucose-induced metabolism increases to insulin release from pancreatic β cells of mice. They concluded that inhibition of both the glycerol phosphate shuttle and the malate-aspartate shuttle blocks glucose-induced insulin secretion. Schurr and Payne comment that lactate derived from glycolysis in the cytoplasm could influence mitochondria if the lactate were converted to pyruvate, with concomitant generation of NADH, through the action of lactate dehydrogenase. Eto et al., however, respond that the results of experiments proposed by Schurr and Payne to test this idea do not appear to support a critical role for lactate.

    In a separate comment, MacDonald and Fahien point out that they and others have reported evidence that aminooxyacetate (AOA), an inhibitor of the malate-aspartate shuttle, has a more pronounced effect on rat β cells than that reported by Eto et al. from murine cells. They also find Eto et al.'s conclusion that inhibition of the shuttles does not prevent glycolysis or transport of pyruvate to mitochondria to be “biochemically impossible.” Eto et al. reply that their results may require revision of the “prevailing hypothesis.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5455/931a

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