This Week in Science

Science  08 Feb 2008:
Vol. 319, Issue 5864, pp. 693
  1. Quantum Phase via Geometry


    The phase of a wave function collapses when measurements are made, so additional information is needed to determine phase, and several methods have been developed based on interference with reference waves. Moon et al. (p. 782) describe a non-interferometric approach to phase-determination based on the isospectrality, which describes pairs of simple polygonal shapes that have the same frequency response—that is, if these shapes were drumheads, they could sound the same and be indistinguishable. The authors used scanning tunneling microscopy to position CO molecules on the Cu(111) surface at cryogenic temperatures to bound isospectral shapes. Despite the imperfect nature of this boundary, the spectral fingerprints of the two-dimensional electronic states in the terahertz range were the same within experimental error. The authors then used this property to extract the wave function phase. Phase extraction should be possible in two-dimensional quantum systems provided that the boundary shapes can be constructed.

  2. Spin Hall Effect of Light

    Hall effects manifest as the transverse movements of carriers of electronic current in the presence of an external field, and recent work has concentrated on the spin Hall effect, in which the effects depend on the spin of the electron and not just its charge. Hosten and Kwiat (p. 787, published online 10 January; see the Perspective by Resch) now report on the observation of an optical version of the spin Hall effect that developed a sensitive metrological technique capable of detecting displacements on the angstrom scale. When light refracts at an air-glass interface, there is an additional displacement of the light that depends on polarization. In this optical system, the polarization of the light interacts with a refractive index gradient in a manner analogous to how electronic spins are affected by electric fields.

  3. Moving with the Heat


    Most materials have a positive coefficient of thermal expansion—they expand when heated—but there are exceptions, such as cubic zirconium tungstate, which will contract over a wide temperature range. Silver(I) hexacyanocobaltate(III) is a framework material that has highly underconstrained Co-CN-Ag-NC-Co linkages. Goodwin et al. (p. 794) find that over a wide temperature range, this material can either contract or expand along orthogonal lattice directions with a coefficient an order of magnitude greater than that of most materials. They attribute these large movements to the framework flexing like a hinged lattice, of the sort commonly seen in garden fencing.

  4. Every Step You Take

    As we walk, we expend energy not only in pushing off with our planted leg but also when our other leg decelerates as it makes contact with the ground. Donelan et al. (p. 807) have developed a mechanical device to harvest some of the expended energy in this deceleration step in much the same fashion as hybrid automobiles utilize regenerative braking. The device is light, fastens at the knee, and produces about 5 watts of power, potentially enough power to charge portable medical devices.

  5. Bonds, Shaken and Sliced

    Soon after the development of narrow-frequency laser sources, chemists attempted to use these sources to excite specific chemical bonds. Unfortunately, the deposited energy usually spread around the rest of the molecular framework too rapidly for the chosen bond to break. More recently, studies have shown that if collisions with other molecules or catalytic surfaces occur soon enough after vibrational excitation, reaction efficiencies can be selectively enhanced. Killelea et al. (p. 790; see the Perspective by Mullins and Sitz) now take this approach a step further to show that by exciting the C-H stretch in the CHD3 isotopomer of methane just prior to collision with a nickel surface, they can achieve a 30:1 ratio of C-H to C-D bond cleavage, relative to a 1:3 ratio in a thermally equilibrated sample. Quantification of this selective scission required a technically demanding mass-resolved detection scheme of thermally desorbed products.

  6. Sounding Out Earth's Core

    Earth's solid inner core is predominantly a phase of iron at high pressures. One important clue for determining the properties of the core is that sound waves passing through Earth's solid inner core propagate fastest along the north-south direction, which suggests that there is a preferred alignment of iron crystals. Belonoshko et al. (p. 797) present numerical calculations which show that the body-centered cubic form of iron is strongly anisotropic to seismic waves and can match the observed 12% anisotropy, whereas the hexagonal close-packed form, previously thought to make up the inner core, is not.

  7. Mixed-Up Microflora

    The relationship between an animal host and the complex mixture of microbes it carries in its gut is a delicate one, and the exact role the host immune system plays in maintaining commensal homeostasis remains unclear. Ryu et al. (p. 777, see the Perspective by Silverman and Paquette; published online 24 January) examined the expression of antimicrobial proteins in the gut of the fruit fly, Drosophila melanogaster. Although the key immune transcriptional regulator nuclear factor kappa B (NF-κB) was chronically activated in the flies by indigenous gut microflora, only a subset of NF-κB-regulated antimicrobial genes was actually expressed because of transcriptional repression exerted by the intestinal homeobox gene Caudal. Disruption of Caudal expression resulted in the expression of a different subset of antimicrobial peptides, as well as a dramatic change in the composition of the intestinal microflora that led to the apoptosis of intestinal epithelial cells and loss of host viability.

  8. What a Tangled Food Web We Weave

    Both direct and indirect effects can mediate bottom-up influences on the diversity and complexity of a food web. Bukovinszky et al. (p. 804) compared the effects of two related plants (domestic Brussels sprouts and feral Brassica) on their aphid herbivores, the aphids' parasitoid wasps, and the wasps' secondary parasitoids to examine how resource quality affects food web structure and complexity. In this multilevel trophic system, differences in the resource base cascaded through the system, via an array of direct and indirect effects, and led to substantial differences in the structure and complexity of the resulting food webs.

  9. A Growing Role for Centrosomes


    Genetic analyses of individuals with extreme forms of short stature can provide insights into the biological mechanisms regulating human growth. Rauch et al. (p. 816, published online 3 January; see the Perspective by Delaval and Doxsey) have identified a mutant gene responsible for microcephalic osteodysplastic primordial dwarfism type II (MOPD II). Adults with this rare inherited condition reach an average height of 100 centimeters, and although their brain is comparable in size to that of a three-month-old baby, they are of nearnormal intelligence. The culprit gene is PCNT, which encodes pericentrin, a centrosomal protein implicated in mitotic spindle anchoring and chromosome separation during cell division. Although the precise mechanisms by which the cellular phenotype produces the size phenotype remains to be determined, it is intriguing that other inherited forms of microcephaly (disorders characterized by small brain size) have likewise been genetically linked to centrosomal and mitotic spindle genes.

  10. Tipping the Balance in T Cell Decisions

    In the thymus, key developmental decisions are made that have far-reaching consequences for the immune system, perhaps most notable the commitment of thymocytes to becoming CD4 (helper) or CD8 (cytotoxic) T cells. The transcriptional factor Th-POK commits thymocytes that still express both CD4 and CD8 co-receptors to becoming CD4 T cells, but how is the alternate developmental option generated? Setoguchi et al. (p. 822) find that cells that would otherwise be destined to become CD8+ T cells can be redirected to the CD4 lineage by loss of members of another transcription factor family, Runx. It seems that under normal circumstances, the Runx complex represses Th-POK expression and allows CD8 T cells to emerge.

  11. The Heme Balancing Act

    Heme, a component of several hemoproteins, is required in aerobic cells for oxygen transport and storage (hemoglobin and myoglobin), electron transfer and drug metabolism (cytochromes), and signal transduction (nitric oxide synthases). However, free heme is toxic, so its intracellular concentration must be carefully regulated. Keel et al. (p. 825) have generated mice lacking the heme export protein FLVCR (feline leukemia virus, subgroup C, receptor), and show that this factor is required for terminal red blood cell development. They suggest that heme toxicity may be a common pathophysiology in some erythroid disorders where free-heme-balance is perturbed. Additionally, FLVCR functions in the recycling of heme-iron from senescent red cells, and heme-iron trafficking via FLVCR is involved in systemic iron homeostasis.

  12. Uneven Heat Gain

    Recent global warming will not be, and has not been, uniform across all regions. In order to predict more accurately what the impacts of warming will be, the spatial pattern of temperature change needs to be understood. Lozier et al. (p. 800, published online 3 January) examined how the North Atlantic has gained heat during the past 50 years. The total amount of heat gained within the basin is approximately what would be expected, but the tropics and subtropics have warmed while the sub-polar ocean has cooled. These results are interpreted with the help of an ocean general circulation model to be mostly the result of wind and buoyancy forcing associated with the North Atlantic Oscillation. Although the spatial pattern of temperature change in the North Atlantic does not directly reflect global warming, it may be the indirect result of warming as transmitted through large-scale changes in atmospheric circulation.

  13. Nanoscale 3D Imaging

    “Super-resolution” fluorescence microscopy methods are capable of breaking the diffraction limit of resolution in conventional light microscopy of 200 to 300 nanometers (nm). These techniques have improved two-dimensional (2D) image resolution, but achieving 3D nanoscale resolution has remained a challenge. Huang et al. (p. 810, published online 3 January) have used an optical astigmatism method to develop 3D stochastic optical reconstruction microscopy (STORM). They examined intracellular clathrin-coated pits and microtubules with an image resolution of 20 to 30 nm in the lateral dimensions and 50 to 60 nm in the axial dimension in fixed cells.

  14. Specifying Gene Expression

    The efficiency of gene transcription by RNA polymerase II is modulated by the composition of promoter chromatin, including the presence of nucleosome-binding proteins. Using a genomic approach, Krishnakumar et al. (p. 819) show that poly(ADP-ribose) polymerase-1 (PARP-1) and the linker histone H1, two nucleosome-binding proteins that compete for binding to nucleosomes in vitro, exhibit a reciprocal pattern of binding at many promoters in vivo. At positively regulated promoters, PARP-1 is bound but H1 is not. Thus, promoter localization patterns of nucleosome-binding factors can specify global gene expression outcomes in vivo.

  15. Relatedness and Reproductive Success

    It appears that marrying your cousin may not be such a bad idea, provided there is sufficient distance in the degree of your relatedness. Helgason et al. (p. 813) analyzed recorded genealogies of 10 generations of Icelanders to test the effect of relatedness on reproductive success. Marriages between third and fourth cousins produced more children and more grandchildren than marriages between more highly related or less related people. Icelandic society has urbanized during the past 200 years with rural populations moving into urban areas, and the relatedness of married couples has diminished, which has been accompanied by a drop in reproductive success.