This Week in Science

Science  07 Feb 2003:
Vol. 299, Issue 5608, pp. 777
  1. Thinking Coherently About the Second Law

    The efficiency of heat engines has been limited by the Carnot cycle, which states that the efficiency cannot exceed 1 - Tc/Th, where Tc and Th are the absolute temperatures of the cold and hot reservoirs feeding the engine. Scully et al. (p. 862, see the Perspective by Linke) show how this limiting efficiency can be beaten with a coherent heat source in which the atoms in the heat bath have an associated phase f. Such an engine can even produce net work when Tc = Th. However, the Second Law of Thermodynamics still holds because an outside energy source, such as a microwave field, is needed to create the initial coherence.

  2. A Bigger Gap in Gold Clusters

    The physical and chemical properties of small assemblies of atoms often differ from those of the corresponding bulk materials. For example, whereas bulk gold is relatively unreactive, many small gold clusters can catalyze a variety of reactions. Li et al. (p. 864 prepared small gold clusters by laser vaporization and found that Au20 clusters tend in the other direction. The Au20 clusters have an extremely large energy gap and an electron affinity comparable to that of C60, which suggests that Au20 clusters should be highly stable and unreactive. Density-functional calculations reveal that the Au20 cluster has a tetrahedral structure with a high surface area.

  3. Chemistry via Carbon Tunneling

    Many examples exist of chemical reactions that proceed via tunneling of a hydrogen species, but evidence for a reaction proceeding through tunneling of a much more massive species such as a carbon atom has been largely limited to the example of the automerization of 1,3-cyclobutadience. Zuev et al. (p. 867, see the Perspective by McMahon) now present experimental and theoretical evidence that the photoinduced ring expansion of 1-methylcyclobutylfluorocarbene proceeds via carbon atom tunneling without thermal activation at 8 kelvin. This singlet carbene isomerization proceeds through the lowest vibrational level of the reactant.

  4. Connect the Holes

    Magnetic reconnection in space involves the energetic crossing, breaking, and then reconnection of magnetic field lines that are usually associated with the ejection of high-energy plasma jets from the reconnection region. The pace of the reconnection and the amount of energy released in the jets have been difficult to understand. Drake et al. (p. 873, see the Perspective by Hoshino) simulate the particle-scale processes and find that turbulence in the electric field leads to electron holes (areas of reduced electron density and potential). The development of the holes ensures a rapid pace of reconnection and provides enough energy to explain some of the plasma jets in Earth's magnetosphere that were observed by the Polar spacecraft.

  5. Resolving Lattice Oxygen Atoms

    Oxygen atoms normally scatter too weakly to be resolved by high-resolution transmission electron microscopy. Using a new generation of aberration corrected microscopes, Jia et al. (p. 870, see the Perspective by Spence) developed an imaging mode based on a negative spherical-aberration coefficient that resolved oxygen atoms in the dielectric SrTiO3 and the high-temperature superconductor YBa2Cu3O7. Quantitative differences in intensity among the oxygen columns reveal information concerning the local stoichiometry, which plays a vital role in the properties of such materials.

  6. Can El Niño Take the Heat?

    Could global warming cause a permanent El Niño-Southern Oscillation (ENSO)? The direct observational record of ENSO is too short to answer this question. Huber and Caballero 877 combined results from a climate model and data from a period of extreme global warmth, the Eocene (55 to 35 million years ago), to help understand the stability of the tropics and its role in climate change in a high-temperature world. In simulations with atmospheric CO2 concentrations twice those of preindustrial values, similar to those expected to occur by the end of the 21st century, they calculate polar and deep-ocean temperatures 6º to 13ºC warmer than modern ones, with much less warming in the tropics. ENSO oscillations were robust features even when major changes occurred in greenhouse gas concentrations, deep-ocean temperatures, and paleogeography.

  7. Unmasking Marine Lipids

    Less than 40% of the particulate organic matter below the surface layers of the ocean has been classified, yet the identity of this material is one of the keys for understanding how marine organic matter is recycled. Hwang and Druffel 881 found that the acid-insoluble fraction of particulate organic carbon, a proxy for the uncharacterized component, show 13C and 14C signatures similar to those of the lipid fraction and may consist of lipid-like macromolecules.

  8. The Warmer Side of GM Crops

    Debates on the risk-benefit balance from genetically modified (GM) food crops often have not taken into account the differing circumstances of developing and tropical countries. Qaim and Zilberman 900 analyze the results of field trials in India with cotton that expressed genes from Bacillus thuringiensis (Bt) to provide bollworm resistance. These test plots showed important financial and ecological gains from reduced use of toxic chemical pesticides, but even greater benefit was apparent from increased yield. Both the increased pest pressure of tropical climates and the financial limitations of sub-optimal economic conditions lead to greater yield differentials than would be apparent in more benign conditions.

  9. Chloroplast Synthesis Under the GUN

    The chloroplast, the light-converting energy factory of the plant cell, is a complex assemblage of proteins, of which only a portion are actually encoded within the chloroplast genome. The nucleus and the chloroplast thus must carry on a highly coordinated conversation not only to supply the chloroplast with its requisite components, but also to coordinate chloroplast function with other aspects of the cell's development and physiology. Encoded by the nucleus but resident in the plastid, the GUN4 protein is key to chlorophyll synthesis. Larkin et al. (p. 902 now find that GUN4 functions by controlling the synthesis of a signaling intermediate, Mg-protoporphyrin IX, through its actions on the Mg chelatase enzyme.

  10. Combating Liver Disease

    Protecting the liver, either naturally or therapeutically, is the focus of two reports. Endothelial cells (ECs) were once thought to function primarily in the delivery of nutrients and oxygen to tissues, but recently ECs were shown also secrete factors that induce organ development in vertebrate embryos prior to the establishment of blood flow. Studying mouse models, LeCouter et al. (p. 890, see the Perspective by Davidson and Zon now show that ECs have a protective function in the adult liver that is independent of their role in new blood vessel growth. In response to activation of vascular endothelial growth factor receptor-1 (VEGFR-1), ECs were shown to secrete several proteins, including hepatocyte growth factor, that stimulate hepatocyte proliferation and reduce tissue damage in a liver injury model. In the search for effective drugs in treating chronic cases of hepatitis B virus, it is important to find drug candidates that target novel aspects of virus physiology. Deres et al. (p. 893) describe a compound, Bay 41–4109, that appears to act by inhibiting the maturation of the viral nucleocapsids.

  11. Staying in Circulation

    Protein pharmaceuticals synthesized either by posttranslational or chemical modification of recombinant or natural proteins are heterogeneous, making it difficult to optimize their performance. Kochendoerfer et al. (p. 884) have used total chemical synthesis to prepare synthetic erythropoiesis protein (SEP), a modified protein with a defined covalent structure. SEP has a polypeptide chain similar to the glycoprotein hormone erythropoietin (Epo), but instead of the two to four glycosylation sites in Epo, it has two covalently attached negatively charged polymer moieties. SEP showed in vivo potency in erythropoiesis similar to Epo, but remained in circulation for more than twice as long.

  12. NO, More Mitochondria

    The body can generate additional mitochondria, the energy-producing organelles, in response to a variety of stressors, including cold. Nisoli et al. (p. 896, see the Perspective by Brown) found a role for nitric oxide (NO) in the process. Mitochondrial biogenesis was stimulated by NO in brown adipocytes and in cell lines. Consistent with this result, mice that had been engineered to lack endothelial NO synthase showed increased weight gain and a slower metabolic rate.

  13. Getting Iron Through

    Gram-positive pathogens like Staphylococcus aureus can synthesize heme, but it is more efficient for them to scavenge iron and heme from their hosts. Mazmanian et al. (p. 906) examine the means of heme iron import across the cell wall envelope. A pair of sortases was important to anchor protein factors needed to bind and transfer heme iron from extracellular hemoglobin into the bacterial cytoplasm.

  14. Cancer Research Goes Fishing

    The transparent zebrafish, long used to identify pivotal genes that regulate vertebrate development, may one day be a valuable model for studying the genetic pathways that lead to cancer. An important step toward this goal is described by Langenau et al. (p. 887), who have generated transgenic zebrafish expressing the murine c-Myc oncogene. The transgenic fish developed T cell leukemia that arose in the thymus and disseminated widely, a process that could be directly visualized in the fish with the help of a fluorescent marker. These fish can now be used in genome-wide screens to identify mutations that enhance or suppress cancer development, and may also be valuable in screening small molecules for anticancer activity.

Log in to view full text

Via your Institution

Log in through your institution

Log in through your institution