Editors' Choice

Science  05 Aug 2005:
Vol. 309, Issue 5736, pp. 852

    Turn On, Tune In, Drop Out

    1. Marc S. Lavine

    In a liquid crystal display, the brightness or color of a pixel is controlled by the orientation of the molecules and can be changed by turning on an electric field. In order to ensure uniform alignment within a domain, the glass surface is treated to make it grooved or otherwise anisotropic, which lowers the overall surface energy between the liquid crystal molecules and the glass.

    Having shown previously that an oligosiloxane compound deposited onto glass covered with indium tin oxide (ITO) spontaneously forms rigid oligomers that amplify the grooves in the ITO coating, Hoogboom et al. use a pyridine-functionalized siloxane that forms an alignment layer capable of binding to the dye zinc phthalocyanine (ZnPc). The ZnPc molecules form epitaxial stacks whose height can be controlled by varying the immersion time, thus providing an opportunity to tune the sizes of domains. After liquid crystal molecules are deposited, it is generally difficult to alter the strength of the surface interactions or to correct defects. However, in this system, adding nitrogen-containing compounds partially dissolves the ZnPc stacks, which drop out and hence provide a second chance to tune the device. — MSL

    J. Am. Chem. Soc. 10.1021/ja051865l (2005).


    Luck of the Draw

    1. Lisa D. Chong

    Genetically identical organisms that have been raised in identical environments age at different rates, suggesting that in addition to genes and environment, chance physiological phenomena can influence life span. Rea et al. report that the stress response system of Caenorhabditis elegans is subject to an underlying physiological randomness that affects how it copes with environmental insults. They placed the gene encoding green fluorescent protein (GFP) under the control of the regulatory region from the gene encoding a heat shock protein, creating an easily scored biomarker. Upon exposure to heat, isogenic worms exhibited considerable variation in fluorescence, and those expressing the highest amount of GFP tolerated heat the best and lived the longest. The physiological state indexed by GFP expression level was not heritable, and the authors suggest that stochastic variation in molecular and biochemical reactions could account for the variation in individual robustness and longevity. — LDC

    Nat. Genet. 10.1038/ng1608 (2005).


    Black Carbon

    1. H. Jesse Smith

    Global climate models are often used for detection and attribution experiments that assign cause to observed variations in climate. These studies have shown that most of the global warming that has occurred over the past 100 years has been caused by increasing concentrations of atmospheric greenhouse gases, and that warming has been moderated by the cooling effect of sulfate aerosols (which reflect sunlight back into space). However, black carbon aerosols have not been included explicitly in these simulations, despite suspicions that they could have a significant effect on the global radiative energy balance, perhaps even outweighing that of sulfate aerosols, because black carbon, unlike sulfate, absorbs solar radiation and causes atmospheric heating.

    Jones et al. report results from a detection and attribution analysis that includes black carbon aerosols, as well as sulfate aerosols and greenhouse gases. They find that black carbon is not as important as sulfate and that its inclusion does not change the conclusion that 20th-century warming is due mostly to the positive forcing of greenhouse gas variations. Nevertheless, the magnitude of the effect of black carbon aerosols cannot be evaluated precisely, and black carbon can influence the radiative properties of Earth in other ways, such as by decreasing the albedo of snow. — HJS

    Geophys. Res. Lett. 32, 10.1029/2005GL023370 (2005).


    Who's the Most Proficient of All?

    1. Gilbert J. Chin

    It might seem a straight-forward matter to determine how much the enzyme urease accelerates the rate of decomposition of urea, which, after all, contains only four nonhydrogen atoms. The problem, however, is that in the nonenzymatic pathway, the elimination of ammonia precedes the addition of water, whereas the enzyme promotes the nucleophilic attack of water, yielding a tetrahedral intermediate and a different reaction pathway to the same products.

    Previously, Estiu and Merz carried out a computational analysis of the uncatalyzed reaction, relying both on earlier studies with small dinickel molecules and on structural analysis of the dinickel cluster at the urease active site. They found that the catalytic proficiency of the enzyme, calculated by dividing the biochemical quantity kcat/Km by the rate constant of the noncatalyzed reaction (knon), is many orders of magnitude greater than that of the reigning champion, ornithine 5'-monophosphate decarboxylase. In contrast, Callahan et al. have measured the hydrolysis of substituted ureas, which cannot undergo elimination, and extrapolated from these data to arrive at a much faster non-catalyzed hydrolysis rate—one that confirms urease as a proficient, but not record-setting, catalyst. — GJC

    J. Am. Chem. Soc. 126, 6932; 10.1021/ja0525399 (2005).


    Surf's Up

    1. Stella M. Hurtley

    In order to infect a target cell, enveloped animal viruses must gain access to the cell's interior. The early stage of virus infection involves attachment to the cell surface and is frequently followed by endocytosis. Often, viruses are seen to bind to cellular extensions such as microvilli or filopodia. Lehmann et al. asked whether such binding is a productive interaction for the virus, which needs to access the cell body (which can be far away) for successful infection. In vivo imaging studies revealed that after viruses bind to filopodia, they travel in a surfing type of movement along the cell surface toward the cell body, where they then can enter the cell. Filopodia are filled with actin microfilaments, and it is these filaments, in conjunction with cellular myosin II, that promote virus surfing. Disruption of surfing can reduce the efficiency of viral infection. — SMH

    J. Cell Biol. 170, 317 (2005).


    A Matter of Choice

    1. Stephen J. Simpson

    Thymocytes develop into two principal lineages: CD4+ or CD8+ T cells. In arriving at either fate, these cells first pass through a double-positive stage in which both CD4 and CD8 co-receptors are expressed, with one or the other later becoming permanently turned off.

    To explain how this is regulated, Sarafova et al. extend their kinetic signaling model in which cell fate is determined by the context of T cell receptor (TCR) signals during the initial CD8 down-regulation that takes place in all double-positive thymocytes. The model predicts that continued signaling in these cells (facilitated through TCR and CD4) would maintain CD4 transcription. However, if signaling were not sustained (as would be the case for TCR signals that depend on CD8 receptors), then CD4 expression would stop and CD8 transcription would resume. To test this, thymocytes from CD4-deficient mice were engineered to express a CD4 transgene under the control of immature CD8 transcriptional elements. In response to CD4-dependent TCR activation, these cells down-regulated the CD4 transgene (as they also did for endogenous CD8), but subsequently re-started CD8 gene transcription to become functional CD8+ T cells. This supports the idea that regardless of TCR and co-receptor specificity, the fate of thymocytes is dictated by the presence or absence of a sustained T cell signal that mediates transcriptional cross-regulation of co-receptor expression. — SJS

    Immunity 23, 75 (2005).


    Esters with Ease

    1. Jake S. Yeston

    Organic esters are widely used as fragrances and in the synthesis of pharmaceutical compounds. Among the many synthetic routes to esters, the oxidative dimerization of alcohols is direct and involves the endothermic liberation of a dihydrogen equivalent from each alcohol; efficient reactivity therefore requires another component, a stoichiometric H2 acceptor.

    Zhang et al. have developed a homogeneous ruthenium catalyst to couple primary alcohols into esters in the absence of any extra reagents. The reaction occurs in toluene (115°C) at 0.1 mole % catalyst loading; continuous purging of H2 from the system drives the equilibrium to >90% yield of the ester for butanol, hexanol, and benzyl alcohol. The key component of the catalyst is a tridentate ligand, consisting of diethyl-amino and di-tert-butylphosphino coordinating groups appended to a pyridine ring, and the low kinetic barrier may be due to lability of the diethylamino arm at the Ru center. Preliminary studies of the mechanism support the initial oxidation of one alcohol to the aldehyde, followed by addition of the second alcohol to form a hemiacetal, which in turn loses H2 to give the ester. — JSY

    J. Am. Chem. Soc. 10.1021/ja052862b (2005).