Editors' Choice

Science  27 Apr 2007:
Vol. 316, Issue 5824, pp. 516

    Reducing Together

    1. Phil D. Szuromi

    Lanthanide intermetallics, which display the large magnetic anisotropies needed for high-field permanent magnets, are usually synthesized by reaction-diffusion processes that require the removal of components previously introduced to accelerate these transformations. For example, the synthesis of the ternary material Nd2Fe14B with CaH2 necessitates the removal of the calcium ions. Kim et al. report the aqueous synthesis of this intermetallic by sodium borohydride reduction of the metallic chlorides to form an easily isolable amorphous nanoparticulate product, which they characterized by electron microscopy and a range of diffraction techniques. The authors argue that electrostatic coupling of the Nd(III) ion with an initially formed Fe-B alloy helps to overcome the high reduction potential of the lanthanide ion to the corresponding metal. Heating of the product converts these soft magnets into a ferromagnetic material with higher coercivity. — PDS

    J. Am. Chem. Soc. 129, 10.1021/ja0706347 (2007).


    Improving the Balance Sheet

    1. Gilbert J. Chin

    Plants incorporate (fix) CO2 into hexoses (sugars) by coupling it to the five-carbon compound ribulose-1,5-bisphosphate in a reaction that is catalyzed by the enzyme rubisco. Unfortunately, a competitive and apparently unavoidable reaction, which is also catalyzed by rubisco (see Tcherkez et al. for more on this abominably perplexing phenomenon), uses O2 as a substrate and generates one molecule each of glyoxylate and glycerate (instead of two equivalents of glycerate). Glyoxylate is then converted—via subsequent reactions in the peroxisome and mitochondrion—into glycerate, but in doing so one-quarter of the already fixed carbon atoms are lost as CO2 with the concomitant debiting of already fixed nitrogen atoms in the form of ammonia. Increasing the local concentration of CO2 relative to O2 is an evolutionary achievement found in C4 plants (such as corn), and efforts to introduce a CO2-concentrating module into C3 plants (such as rice) have been pursued.

    Kebeish et al. describe a means of reducing the material cost of carbon-atom recovery from glyoxylate. They have engineered the targeting of three bacterial enzymes to the chloroplast in Arabidopsis. The result is that when two molecules of glyoxylate are converted into one of glycerate, the CO2 that is liberated is not lost, but is recaptured by rubisco; the consequences are a decrease in photorespiration, an increase in photosynthesis, and more biomass (leaves and roots) produced. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 103, 7246 (2006); Nat. Biotechnol. 25, 10.1038/nbt1299 (2007).


    Are We Close Yet?

    1. Laura M. Zahn

    Large-scale genome-based surveys that look for correlations of phenotype with genotype typically examine large numbers of individuals; the results often depend on assumptions, which may not always withstand close scrutiny, about the underlying structure of the populations from which these individuals are drawn. Building on analysis of variance tests that assess whether the observed variation between populations is significant and on cluster analytic methods, Nievergelt et al. introduce the generalized analysis of molecular variance (GAMOVA). This approach extends a previous technique known as the analysis of molecular variance by creating a genetic background distance matrix and applying it to a multivariate regression analysis to test hypotheses about population structure. Several large human data sets (Centre d'Etude du Polymorphisme-Human Genome Diversity Project; Howell's craniometric characters; and HapMap) were reanalyzed with GAMOVA in order to demonstrate its potential for detecting population-level structure even among individuals in regions of low population differentiation. — LMZ

    PLoS Genet. 3, e51 (2007).


    Heptacoordinate Mercury

    1. Jake S. Yeston

    Although diffraction techniques have offered detailed pictures of atomic arrangements in solids, determining the corresponding structures in solution, where most reactions occur, is hindered by rapid fluctuations in the coordination environment. The solvation shell structure of aqueous mercuric ions is of interest on account of the metal's toxicity, but has proven to be an especially elusive target because of the absence of strong characteristic features in the visible absorption spectrum. Inferences from the solid state have favored a distorted octahedral, or hexacoordinate, arrangement of water molecules around the central Hg(II) ion. Chillemi et al. present experimental and theoretical evidence implicating the presence of an extra water molecule in the shell, giving rise to an unusual seven-coordinate arrangement. Primary support for this claim emerges from x-ray absorption near-edge spectra, which are not consistent with an octahedral shell. Quantum chemical calculations and accompanying molecular dynamics simulations paint a picture of a flexible seven-membered shell that persists for several nanoseconds, while occasionally accepting or expelling water to create much shorter-lived six-and eight-coordinate environments. — JSY

    J. Am. Chem. Soc. 129, 10.1021/ja066943z (2007).


    A Ribbon-Cutting Ceremony

    1. Stella M. Hurtley

    The Golgi complex is a collection of stacked and interconnected membranes found in a juxta-nuclear position in most nucleated animal cells. During cell division, the Golgi complex fragments, presumably to allow for the partitioning of Golgi membranes to both daughter cells, and a protein referred to as BARS (brefeldin A-ADP ribosylated substrate, also known as CtBP1-S) is important in this process. The BARS protein acts to disconnect Golgi stacks from one another, and this fissional step has been shown to be required for successful mitosis. How then can some cells divide without BARS? Colanzi et al. addressed this issue by examining Golgi characteristics in a variety of cell types. They found that fibroblasts from mice genetically deficient in BARS did not possess an interconnected Golgi ribbon, and that BARS activity was not required for the completion of mitosis. On the other hand, in normal fibroblasts, where Golgi stacks were robustly linked, BARS-mediated scission was essential. — SMH

    EMBO J. 26, 10.1038/sj.emboj.7601686 (2007).


    Alleviating Allergies

    1. Stephen J. Simpson

    The aberrant activation of T helper 2 CD4+ lymphocytes can result in damaging allergic responses, and hence a great deal of effort has been directed toward understanding the mechanisms that normally regulate these cells. Grohmann et al. show that a soluble form of the glucocorticoid-inducible tumor necrosis factor receptor (GITR) cross-regulates allergic responses in mice by signaling through its own ligand. This causes plasmacytoid dendritic cells (pDCs) to produce indoleamine 2,3-dioxygenase (IDO), which mediates strong immunomodulatory effects though the catabolism of tryptophan. Administration of the synthetic glucocorticoid dexamethasone reduced symptoms of allergic responses in mice, including airway inflammation, and this effect depended on GITR-induced IDO, suggesting that this pathway may promote some actions of corticosteroids. In another study, Xanthou et al. observed that the regulatory cytokine osteopontin is expressed in the lungs of asthma patients and can directly affect allergic airway inflammation in mice, again via the activities of pDCs. In this system, however, allergic responses were promoted by osteopontin during the primary phase of antigen challenge, whereas it exerted an anti-inflammatory influence during secondary challenge. The two mediators identified in these studies—GITR-induced IDO and osteopontin—may offer targets for the treatment of asthma. — SJS

    Nat. Med. 13, 10.1038/nm1563; 10.1038/nm1580 (2007).


    An Electrical Spin on Magnetism

    1. Ian S. Osborne

    Magnetic ferroelectrics (multiferroics) are materials that can respond to electric and magnetic fields. It is common in devices such as hard drives to take advantage of the large response (to an applied magnetic field) of the electronic state of a material with giant magnetoresistance. In contrast, the inverse effect, control of the magnetic structure by an applied electric field, is comparatively rare. Because of the prospect of faster switching times in smaller memory devices, there is much interest in developing such electrically controlled magnetic materials.

    Recent theoretical work suggested that a key property to look for in such a material is spin chirality: It was proposed that materials in which the magnetic moments of the individual atoms form a spiral structure should also exhibit an electrical response. Yamasaki et al. go some way toward putting that theory into practice by showing that the spin helicity in single crystalline TbMnO3 can be electrically switched from rotating clockwise to counterclockwise by application of an electric poling field as the material is cooled through the helical spin transition temperature. Probing by neutron scattering revealed that the handedness of the chiral spin structure is controlled by the polarity of the poling field. — ISO

    Phys. Rev. Lett. 98, 147204 (2007).