Editors' Choice

Science  20 Apr 2007:
Vol. 316, Issue 5823, pp. 341

    Sorting Out Lead Levels

    The toxicity of lead has prompted significant concern over the presence of the heavy metal in various sources of drinking water. However, measuring the relatively low concentrations at issue can be an analytical challenge, particularly because of the need to avoid contamination of the vessels and apparatus. As a result, there have been conflicting reports of the lead concentrations measured in bottled waters.

    Shotyk and Krachler have applied clean-room procedures and high-sensitivity detection technology—developed to gauge the nanogram-per-liter concentrations of lead present in polar ice—to the measurement of lead levels in a sample of 125 commercially available bottled waters from across the world. For comparison, they also assayed artesian flow sources in southern Ontario, Canada. In all cases, the measured concentrations were several orders of magnitude below the 10μg/liter level considered hazardous. However, glass bottles appeared to leach lead over time, with concentrations roughly doubling (from ~100 to ~200 ng/liter) in water stored in them over a 6-month period. Concentrations measured in plastic bottles were generally much lower, and ranged from <1 to 761 ng/liter with a median of 8.5 ng/liter. The artesian sources exhibited a median lead concentration of 5.1 ng/liter in a much narrower range. — JSY

    Environ. Sci. Technol. 41, 10.1021/es062964h (2007).


    Bridging System and Individual

    Equal opportunity is not, of course, quite the same thing as equal outcome, as an examination of even a small number of individuals will confirm. Inequalities abound—whether of wealth, educational achievement, or employment history—and can be the source of emotional distress, especially among the advantaged, when they are perceived as having gained these unfairly. Wakslak et al. show how adoption of a justification for inequality (that success is the product of effort and ability) alleviates three facets of emotional distress and then assess which of these mediates enervated support for redistributive social policies. They find that existential guilt and depressed mood (both of which are inwardly directed) were not primary motivations behind attitudes on redistribution but that an other-regarding moral outrage was. — GJC

    Psychol. Sci. 18, 267 (2007).


    Subversions and Transformations

    Epithelial cells are polarized, with their apical surface oriented toward the lumen of an organ and their basolateral side facing the blood. Layers of epithelial cells play an important and essential role in keeping the outside out and the inside in. Helping them to achieve this are the so-called tight junctions, which link neighboring cells and resist any intercellular infiltration of macromolecules. Kierbel et al. have studied how the pathogenic bacterium Pseudomonas aeruginosa (a major contributor to nosocomial infections) manages to invade epithelia, despite having a preference for binding to the basolateral surfaces of epithelial cells. Most of the invading bacteria will, in fact, be confronted by an apical surface when they attempt to colonize a new host. The authors find that the bacterium actually induces the remodeling of a small portion of the apical membrane into a basolateral-like protrusion. P. aeruginosa binds to epithelial monolayers and recruits phosphatidylinositol 3-kinase (PI3K). The PI3K generates phosphatidylinositol 3,4,5-trisphosphate (PIP3), which attracts actin filaments and the delivery of basolateral membrane proteins. The invading bacteria then become engulfed by the transplanted patch of basolateral membrane and thus succeed in breaching the barrier defenses of the epithelium. — SMH

    J. Cell Biol. 177, 21 (2007).


    Pampered by PAAMPSA

    The promise of polymer electronics depends in part on the development of stable conductive materials that can be inexpensively processed. One material that has been extensively studied is polyaniline (PANI), which when doped with acids becomes conductive but has limited solubility in common solvents. An alternative is to polymerize the aniline onto a polymer acid template that provides the acid doping, may enhance the PANI crystallinity, and may also provide excess pendant groups to enhance solubility. Yoo et al. explored the strongly acidic poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) as a template material. For the highest-molecular-weight PAAMPSA studied, the conductivity was high even though the acid doping was found to be incomplete. The best results (1.1 S/cm) were obtained for the lowest-molecular-weight template. 15N solid-state nuclear magnetic resonance spectroscopy suggested that the enhanced conductivity in these systems stemmed from a nitrogen environment in which the protonated amines can delocalize charge along the PANI-PAAMPSA backbone, an effect not seen in other polymer acid-doped systems. — MSL

    J. Mater. Chem. 17, 1268 (2007).


    Backdrop for the Future

    One of the most frequently invoked potential effects of global warming is an abrupt change in precipitation patterns. In order to assess the onset of such a change, it is necessary to have baseline knowledge of the variability of precipitation in the past. Narisma et al. have analyzed the record of global rainfall for the 20th century in order to establish the spatial and temporal distributions of abrupt decreases in rainfall over that period. They find that about 30 regional instances of large, sudden decreases in precipitation have occurred over the past 100 years, all of which deviated from the climatological norm by at least 10% and lasted for 10 years or more. The authors also observe that these sudden decreases in rainfall occurred mostly in arid and semi-arid regions, which is consistent with the results of climate modeling studies, and they suggest that this finding could be a consequence of a strong positive feedback between vegetation and climate. — HJS

    Geophys. Res. Lett. 34, L06710 (2007).


    Simply Hexagonal Stacks

    When nanoparticles pack into colloidal crystals, they usually adopt a hexagonally close-packed (hcp) structure in which two layers stack in an ABAB sequence, even though hard-sphere models favor a face-centered cubic (fcc) structure in which three layers stack ABCABC. Talapin et al. found that when they grew superlattices on carbon substrates of more polar, nearly spherical, semiconducting PbS, PbSe, or γ-Fe2O3 nanoparticles that were also somewhat larger (>7 nm in diameter), the layers did not stack in threefold hollow sites but rather directly on top of the underlying nanoparticle in a simple hexagonal (sh) lattice. Transmission electron microscopy images were analyzed to show that the stacking was not simply a projection plane of an fcc lattice. The authors calculated total electrostatic and dispersive energies for different lattices and attribute the stability of the sh superlattices to interactions of nonlocal dipoles between individual nanoparticles and with the underlying substrate, favoring the more open sh packing arrangement. — PDS

    Nano Lett. 7, 10.1021/nl070058c (2007).

  7. STKE

    Sigmoid Overrules Hyperbolic

    Many cell-surface proteins, such as growth factor receptors, are glycosylated on asparagines residues as they transit the biosynthetic pathway. Proteins with more richly branched N-glycans will display more poly-Nacetyllactosamine and thus bind more tightly to galectins, which are lattice- forming cell-surface proteins. Interaction with galectin appears to hinder receptor endocytosis; the longer residence time supports a greater capacity for signal transduction via these receptors. The synthesis of glycan branches requires an intermediate produced by the enzyme N-acetylglucosaminyltransferase V (Mgat5), whose substrate is UDP-N-acetylglucosamine (GlcNAc), which is a product of the hexosamine pathway.

    Lau et al. found that adding GlcNAc to Mgat5−/− cells increased the interaction of epidermal growth factor receptors (EGFRs) and transforming growth factor-β receptors (TGFβRs) with galectin. Surprisingly, EGFR (with eight N-glycosylation sites) showed a hyberbolic increase in response (phosphorylation of its downstream effector ERK), whereas TGFβR (with only one or two glycosylation sites) showed a sigmoid or switchlike increase in response (nuclear translocation of its downstream effector Smad) when the Mgat5−/− cells were supplemented with increasing concentrations of GlcNAc. Based on these observations, they built a mathematical model that described the dependence of cell-surface growth-regulating receptor abundance on glycosylation state and on metabolic flux through the hexosamine pathway. Receptors that promote proliferation generally have higher numbers of glycosylation sites than those that promote growth arrest and differentiation, which keeps the surface number of the latter relatively low until high concentrations of GlcNAc become available. — NRG

    Cell 129, 123 (2007).