This Week in Science

Science  30 Oct 2009:
Vol. 326, Issue 5953, pp. 639
  1. Superconductivity Sliced Thin


    Since the initial characterization of high-temperature cuprate superconductors, an intriguing challenge has been determining the minimum number of copper oxide planes needed to support the superconducting state. The observation of superconductivity at interfaces of metallic oxides and insulators provides a route to addressing this question. Logvenov et al. (p. 699) describe a method for layer-by-layer synthesis of alternating oxides of metal and insulators based on La and Cu. Layers three unit cells thick supported superconductivity with a transition temperature of 32 kelvin. When selected layers were then doped with Zn atoms to suppress superconductivity, the interface superconductivity was shown to arise from a single copper-oxide plane.

  2. Origins of Egalitarianism

    Wealthy contemporary societies exhibit varying extents of economic inequality, with the Nordic countries being relatively egalitarian, whereas there is a much larger gap between top and bottom in the United States. Borgerhoff Mulder et al. (p. 682; see the Perspective by Acemoglu and Robinson) build a bare-bones model describing the intergenerational transmission of three different types of wealth—based on social networks, land and livestock, and physical and cognitive capacity—in four types of small-scale societies in which livelihoods depended primarily on hunting, herding, farming, or horticulture. Parameter estimates from a large-scale analysis of historical and ethnographic data were added to the model to reveal that the four types of societies display distinctive patterns of wealth transmission and that these patterns are associated with different extents of inequality.

  3. Ribosomes Caught in Translation

    To synthesize proteins, the ribosome must select cognate transfer RNAs (tRNAs) based on base-pairing with the messenger RNA (mRNA) template (a process known as decoding), form a peptide bond, and then move the mRNA:tRNA assembly relative to the ribosome (a process known as translocation). Decoding and translocation require protein guanosine triphosphatases (GTPases), and, while high-resolution structures of the ribosome have greatly furthered our understanding of ribosome function, the detailed mechanism of these GTPases during the elongation cycle remains unclear. Two Research Articles now give a clearer view of these steps in bacterial protein synthesis (see the Perspective by Liljas). Schmeing et al. (p. 688, published online 15 October) present the crystal structure of the ribosome bound to Elongation factor-Tu (EF-Tu) and amino-acyl tRNA that gives insight into how EF-Tu contributes to accurate decoding. Gao et al. (p. 694, published online 15 October) describe the crystal structure of the ribosome bound to Elongation factor-G (EF-G) trapped in a posttranslocation state by the antibiotic fusidic acid that gives insight into how EF-G functions in translocation.

  4. Electron Comings and Goings


    Advances in computational chemistry have enabled remarkably detailed calculations of electrons' spatial arrangements in complex molecules. Experimental confirmation of these orbital geometries remains challenging, however, particularly for molecules bound in thin layers to underlying surfaces—a morphology of increasing interest in organic semiconductor development. Puschnig et al. (p. 702, published online 10 September) mapped the trajectories of photoelectrons emitted from polycyclic aromatic molecules deposited on copper and extracted spatial profiles of the orbitals from which these electrons emerged. The method of analysis takes advantage of the straightforward Fourier relation between position and momentum and complements orbital imaging techniques based on scanning tunneling microscopy.

  5. Zinc-Based Bases

    Conventional methods of stripping a proton from a hydrocarbon yield an alkali metal-coordinated carbanion as the preliminary product. In certain cases, however, this preliminary product falls apart before it can be used for further constructive synthetic purposes. Kennedy et al. (p. 706; see the Perspective by Marek) show that in such cases, zinc ions can act as potent stabilizers. Specifically, a bimetallic base incorporating both sodium and zinc ions was used to deprotonate the common cyclic ethers tetrahydrofuran and tetrahydropyran. Zinc coordination to the carbanion inhibited an otherwise rapid ring-opening decomposition pathway. Similarly, a zinc-potassium combination facilitated deprotonation of ethylene to a stabilized product.

  6. Converging on Dynamics

    Electron diffraction is a versatile technique for discerning atomic-level structure, but the data emerge averaged over the micron–scale area sampled by the electrons, and so blur local distinctions in systems that aren't strictly periodic. A recent approach to minimizing this problem has been to focus the electron beam impinging on the sample. Yurtsever and Zewail (p. 708) have now applied convergent focusing to an ultrafast electron diffraction apparatus and were thus able to resolve picosecond structural dynamics in local regions tens of nanometers across. The technique was used to probe heterogeneous temperature changes in laser-heated silicon.

  7. Of Ancient Iron and Oxygen

    Finding clues to understand the early evolution of ocean and atmospheric chemistry and its links to the evolution of life remains a daunting task. Often just a few rock samples provide our only evidence of what conditions on Earth were like long ago. Reinhard et al. (p. 713) combined iron speciation data from a 2.5-billion-year-old shale from Australia with sulfur isotope data from this and nearby formations to conclude that oxygen chemistry predominanty consisted of an anoxic sulfide–rich water column, instead of iron-rich oceans, as previously speculated. Thus, brief pulses of reduced iron from hydrothermal vents may have been responsible for the formation of nearby banded iron formations and may have provided enough buffering to prolong the appearance of atmospheric oxygen generated by the expansion of newly evolved cyanobacteria.

  8. Flu's Tricky Tricks


    After vaccination against influenza A virus, single-point mutations are selected in hemagglutinin (the virus molecule that binds to sialic acid molecules on the surface of host cells) that escape neutralization by polyclonal antibody responses. Hensley et al. (p. 734) have discovered that in mice these mutations increased the virus's avidity for sialic acid. Amino acid substitutions that occur during reiterations of immune escape and avidity modulation can thus drive antigenic variation. This constant evolution of influenza viruses requires us to change vaccine components annually, and, for equine influenza, Park et al. (p. 726) show that as the match between virus and vaccine strains drifts apart with time, the probability of becoming infected and the length of the infectious period increase to the point where outbreaks occur. Nevertheless, even imperfect vaccines may be of benefit to a population because increasing the proportion of vaccinated individuals can supply enough herd immunity to offset a poor antigenic match, especially if used in conjunction with antiviral drugs. For humans, Yang et al. (p. 729, published online 10 September) estimate that the rate of transmission within U.S. households puts influenza A 2009 H1N1 (the current pandemic “swine flu”) in the higher range of transmissibility, compared to past seasonal and pandemic strains. Thus, to achieve mitigation this fall, children should be the first recipients of vaccine, followed by adults—aiming overall for 70% coverage of the population.

  9. All Together Now

    Deciding how to change emissions of polluting gases that affect climate through their radiative forcing properties requires that the quantitative impact of these emissions be understood. Most past calculations of this type have considered only the radiative forcing of the specific emission and its atmospheric lifetime. Shindell et al. (p. 716; see the Perspectives by Arneth et al. and by Parrish and Zhu) use sophisticated atmospheric chemical and climate modeling to determine how gas-aerosol interactions affect the radiative properties of the atmosphere, finding significant departures from the standard method for emissions of methane, carbon monoxide, and nitrogen oxides. These findings should help to optimize strategies for mitigating global warming by reducing anthropogenic emissions.

  10. Iron Sensor

    Intracellular iron is an essential cofactor for many proteins, but can also damage macromolecules, so its levels are carefully controlled. Cellular iron homeostasis is mediated by iron regulatory proteins that regulate the expression of genes involved in iron uptake and storage. However, it is not clear how cells sense iron bioavailability (see the Perspective by Rouault). Using different approaches, Salahudeen et al. (p. 722, published online 17 September) and Vashisht et al. (p. 718, published online 17 September) have identified the F-box protein FBXL5 as a human iron sensor. FBXL5 is part of an E3 ubiquitin ligase complex that regulates the degradation of iron regulatory proteins and thereby cellular iron levels. It contains a hemerythrin domain that binds iron and acts as an iron-dependent regulatory switch, causing the degradation of FBXL5 under low iron conditions. This alternative pathway for the regulation of iron homeostasis has implications for both normal cellular physiology and disease.