This Week in Science

Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 13
  1. Critical Aspects of the Mott Transition

    Critical phenomena associated with the Mott metal-insulator transition (MIT) play a role in the high-temperature superconductivity of cuprates and the colossal magnetoresistance of manganites. The MIT observed in V2O3 is the classical paradigm of the Mott transition, but despite numerous studies, detailed and clean experiments have been difficult. Limelette et al. (p. 89; see the Perspective by Kotliar) now report electrical conductivity measurements of Cr-doped V2O3 as a function of temperature and pressure in a parameter range that spans the critical endpoint for the correlation-driven MIT. The results may provide a universal way to describe the MIT in a variety of correlated-electron systems.

  2. Magnetic Monopoles in Momentum Space

    Evidence for the existence of magnetic monopoles in real space, the magnetic counterpart to positive and negative charges that has long been proposed in order to provide a natural symmetry in physics, has remained elusive because of the high energies needed to approach their very heavy mass (>1016 giga-electron volts). In the meantime, a low-energy solid-state system may provide some insight into the behavior of magnetic monopoles. Fang et al. (p. 92) present experimental studies and first-principles calculations of the magnetic and transport properties of the ferromagnetic metal strontium ruthenate. The authors explain the unconventional behavior exhibited by this material by invoking magnetic monopoles in the crystal's momentum space.

  3. Building a Metal-Molecule Interface

    One of the critical issues in molecular electronics is the nature of the contact between the molecule and a metal surface. Nazin et al. (p. 77; see the Perspective by Kummel) have explored how the electronic structure of copper phthalocyanine (CuPc) evolves as it is bonded to two gold chains varying in length from one to six atoms. The CuPc molecule and the gold atoms were adsorbed onto a NiAl(110) surface and assembled into “CuPc@2Aun” structures with a scanning tunneling microscope tip, which was also used to obtain electronic spectra. The molecular orbitals of these “extended molecules” split and shifted as the gold chains were extended.

  4. Soft X-rays in the Water Window

    The generation of coherent soft x-rays in the water window (wavelengths near 4 nanometers), a region transparent to water but absorbing to carbon, is the desired goal in developing table-top coherent x-ray sources for applications in biological imaging. Ionizing gases with high-intensity pulses of infrared light can result in difficult-to-control soft x-ray emission through high-harmonic generation. Gibson et al. (p. 95) show that the control problems can be circumvented by using a spatially modulated waveguide fiber filled with neon gas. In particular, they demonstrate enhanced generation of soft x-rays around 4.4 nanometers whereby the spatially periodic waveguide enables quasi-phase matching of the x-rays generated at higher harmonics.

  5. One Big Stretch Is Better

    The conversion of methane by metal catalysts in reactions such as steam reforming, which generates hydrogen, is of considerable industrial interest. However, there are still several gaps in our fundamental understanding of these reactions, including the role of bond vibrations in activating methane reactions. Some dynamical theories suggest that different vibrational excited states of the same energy will react differently, but statistical theories suggest that such effects will be minor. Beck et al. (p. 98; see the Perspective by Luntz) prepared molecular beams of methane with either a doubly excited carbon-hydrogen (C-H) stretch or two singly excited C-H stretches. The former species proved to about five times more reactive on a nickel (100) surface at the lowest kinetic energies studied.

  6. Protective Endocannabinoids

    Neurons need to protect themselves against the risk of excessive activity that can lead to neurotoxicity. Protective mechanisms likely exist to provide on-demand defense in the case of unusually high neuronal-spiking activity. Marsicano et al. (p. 84; see the Perspective by Mechoulam and Lichtman) created conditional mouse mutants missing the cannabinoid receptor type 1 (CB1) in pyramidal cells but not in interneurons of the forebrain. Protection against seizures induced by the excitotoxin kainic acid was exerted via CB1 receptors in glutamatergic but not in GABAergic neurons. The seizures enhanced production of anandamide—an endogenous cannabinoid—in wild type but not in mutant mice. Thus, the activation of the endogenous cannabinoid system is an on-demand, early, and necessary step for physiological protection against excitotoxicity.

  7. Square Self-Assembly

    The ability to engineer designed protein networks would allow functional proteins such as enzymes or membrane channels to be placed at defined positions. Toward this goal, Ringler and Schulz (p. 106) have engineered protein building blocks that assemble into a regular quadratic lattice. A C4-symmetric tetrameric aldolase, which resembles a flattened cube, was derivatized with biotin at each of its four side faces. This step created rigid four-way connecters that could be linked by stiff streptavidin rods. Spacers with switchable conformations were used to create lattices with a changeable mesh sizes.

  8. The Last Step Toward Integrin Activation

    Integrin affinity for components of the extracellular matrix controls its activation. Integrin activation in turn is associated with numerous cell adhesion and migration events. Although many intracellular signaling pathways have been implicated in controlling the integrin-extracellular matrix interaction, the final step that results in the conformational change needed to kick integrins into activation mode has not been clear. Tadokoro et al. (p. 103) show that for several different classes of integrins and signaling pathways, the association of a cytoskeletal protein called talin is the final common step for integrin activation.

  9. Girders, Beams, and Bolts

    Cells are linked into continuous sheets by intercellular connections of two types, desmosomes and adherens junctions, whose structural cores are composed of members of the cadherin family of proteins. Desmosomes provide strong yet supple support in skin and heart, and He et al. (p. 109) have taken a look at the organization of cadherin molecules within mouse skin desmosomes using electron tomography. They obtained a map of the intercellular bridges at ∼30-angstrom resolution and then fitted the previously determined crystal structure of cadherin within the overall structural envelope. The molecular interface between cadherins from different cells was remarkably flexible in orientation, and the primary points of contact were restricted to the distal one or two domains of the five-domain cadherin.

  10. Mixed Messages Between Neurons in ALS

    Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that begins in mid-life and is characterized by the rapid death of motor neurons. The rare familial form of the human disease has been linked to mutations in the gene for Cu-Zn superoxide dismutase (SOD1). Clement et al. (p. 113) created chimeric mice whose brains contained a mixture of cells possessing either normal or mutant SOD1. The survival of mutant motor neurons in these mice could be extended with help from their normal nonneuronal cell neighbors. The reverse was also true: Adjacent nonneuronal cells carrying the SOD1 mutation hastened the death of normal motor neurons in chimeric animals. Thus, multiple cell types can interact to contribute to the pathology, or to protect from the pathology, of ALS.

  11. Dwarfing Maize

    Dwarf versions of rice and wheat have boosted agricultural outputs, in part by devoting more of the plant's growth to the grain than to the stalk and also by improving the plant's ability to survive wind and rain damage. In maize, mutation of the gene brachytic2 (br2) results in dwarfing, where the lower internodes of the stalk are shortened but no other parts of the plant are affected. Multani et al. (p. 81; see the Perspective by Salamini) show that in br2 plants a reduction in cell size is accompanied by a two-to threefold increase in cell number to yield a plant stalk with enhanced mechanical strength. The br2 gene encodes a protein that is involved in polar auxin transport. A sorghum ortholog of maize br2, dwarf3, produces a dwarf mutant that is also of considerable agronomic interest.

  12. Intrinsic Pain Control

    T-type Ca2+ channels play a role in pain-enhancing pathways, both at the level of peripheral pain receptors, and the spinal cord dorsal horn neurons. However, the function of these channels in supraspinal processing of pain signals, and the specific subtype of channel involved, has not been investigated. Kim et al. (p. 117) combined pharmacological experiments and T-type Ca2+ channel-ablated mutant mice to show that thalamic T-type Ca2+ channels attenuate central visceral pain responses and thus play an analgesic role in the thalamus.

  13. How Plants Make NO

    Nitric oxide (NO) regulates plant growth and development as well as pathogen defense, and plants appear to generate this signaling molecule both enzymatically and by noncatalyzed mechanisms. Guo et al. (p. 100) have identified a plant enzyme with NO synthase (NOS) activity that is required for growth and response to hormone. Although its protein sequence is not similar to animal NOS isoforms, the enzyme acts on the same substrate and requires many of the same cofactors. Plants may thus use specific NO-generating strategies to achieve different physiological outcomes.

  14. Monitoring Consequences

    The anterior cingulate cortex (ACC) plays a central role in monitoring response conflict, errors, and reinforcement. Several hypotheses have been put forward as to what precisely is monitored in the ACC. Ito et al. (p. 120) directly tested these hypotheses against each other, in a countermanded-response saccade (direction of gaze) paradigm in monkeys, using single unit recording. Neural responses were related to errors and reinforcement but not to motor response conflict per se. These results are in agreement with the hypothesis that the ACC monitors the consequences of our actions.