This Week in Science

Science  14 Apr 2006:
Vol. 312, Issue 5771, pp. 155
  1. Catch, React, Release

    CREDIT: YOSHIZAWA ET AL.

    Enzymes derive much of their remarkable selectivity by orienting substrates in ways that facilitate specific reaction paths. Recently, chemists have sought to achieve similar effects using relatively simpler hollow structures, assembled from organic and metallic building blocks in solution. However, these synthetic hosts often bind products as effectively as they do reactants so that catalysis is inhibited. Yoshizawa et al. (p. 251) find that a palladium and triazine-based host that adopts a bowl shape can catalyze the Diels-Alder reaction of anthracenes and phthalimides in water because the product geometry is no longer compatible with the host. A related host structure, shaped like a cage, can be used stoichiometrically to orient the same reagents in a different way and yields an unusual Diels-Alder adduct at a terminal, rather than central, site on the anthracene framework.

  2. Sending Charges Their Separate Ways

    Devices implanted in the body require power, which is normally delivered by batteries, but a number of approaches have been proposed to tap into the power or fuel sources the body already provides. Wang and Song (p. 242) have converted mechanical energy into electrical energy by deflecting anchored ZnO nanowires with a conductive atomic force microscope tip. The strain field created by bending the nanowires with the tip caused charges to separate and build up on opposite sides of this polar material. The tip and nanowire form a rectifying Schottky barrier so that built-up charge is released as electrical current when the tip crosses from one face polarity to the other.

  3. Ductility Through Deformation

    In traditional metalworking, a metal is cold-deformed in order to introduce dislocations that make it stronger, and then annealed to restore its ductility. As the number of dislocations increases, their movement and ability to multiply are hindered, which is the source of the strengthening. X. Huang et al. (p. 249) now show that an opposite cycle of processes can be used to prepare ductile nanostructured aluminum. During heat treatment, dislocation sources are removed, making it harder for new dislocations to form, and a subsequent deformation step restores these sources, thus enhancing the ductility.

  4. Anode Alternatives

    In order for solid-oxide fuel cells to run directly on natural gas, improved anode materials will be needed to avoid problems such as carbon buildup and sulfur poisoning. Y.-H. Huang et al. (p. 254) report on the use of double-perovskite materials, Sr2Mg1-xMnxMoO6-δ, as anodes at temperatures between 650° and 1000°C. The active Mo(VI)-Mo(V) couple is paired with the Mg and Mn cations that are not reduced by the fuel. The Mg cation appears to be especially resistant to sulfur poisoning and allows for stable operation (200 hours) in the presence of 50 parts per million H2S.

  5. An Extra Thymus in Mice

    CREDIT: TERSZOWSKI ET AL.

    The thymus lies directly above the heart and acts as a cradle for developing T cells that will eventually protect the body from the many pathogens encountered during a lifetime. The thymus has been considered one of a kind, but Terszowski et al. (p. 284, published online 2 March; see the Perspective by von Boehmer) find that mice frequently possess a second, smaller thymus located in the neck. This “cervical” thymus displays all of the classical features that define the larger thoracic organ, including boundaries between distinct thymocyte compartments and markers for thymic epithelia and developing thymocytes. Moreover, T cells emerging from this smaller cousin also appear functionally competent and can populate athymic adult recipients after cervical thymus transplantation.

  6. Alkane Shuffle

    Olefin metathesis, which swaps molecular fragments on either side of a carbon-carbon double bond, has become an efficient and widely used chemical process. However, there are few effective catalysts for the analogous transformation of saturated hydrocarbons. Such rearrangements around C-C single bonds would be useful in generating fuel from lighter alkanes (methane to hexane) derived from sources other than petroleum. Goldman et al. (p. 257; see the news story by Service) achieve alkane metathesis by combining two catalysts. A molecular iridium catalyst first dehydrogenates alkanes to form olefins, which can be combined and rearranged with a well-established olefin metathesis catalyst. The iridium complex then rehydrogenates the rearranged products. In this way, two equivalents of hexane can be converted to decane and ethane, as well as a small distribution of other alkanes stemming from isomerization at the olefin stage.

  7. Bile Buildup and Liver Regeneration

    Numerous secreted factors, including growth factors and cytokines, have been implicated in regulating hepatocyte proliferation. W. Huang et al. (p. 233; see the news story by Vogel) report that bile acids are essential stimulatory factors for liver regeneration in mice. An increase in bile acids stimulates regeneration and requires the nuclear bile acid receptor FXR. The authors propose a homeostatic mechanism for determination of liver size, in which FXR and perhaps other nuclear receptors sense the levels of endogenous metabolites to determine the liver's functional capacity. When liver function is decreased as a result of injury, the resulting accumulation of bile acids activates FXR, which stimulates signaling pathways to protect the liver from bile acid toxicity and also promotes liver growth to handle the overload.

  8. Biochemical Quantum Tunneling

    Whether enzymes have evolved to use quantum tunneling to facilitate proton transfer is a topic of considerable debate. Masgrau et al. (p. 237; see the Perspective by Benkovic and Hammes-Schiffer) present an atomic-level description of the reaction pathway for tryptamine oxidation by aromatic amine dehydrogenase. Proton transfer occurs in a reaction dominated by tunneling over a distance of 0.6 angstrom. Tunneling does not require long-range motions coupled to the reaction coordinate but is promoted by short-range motion that reduces the proton-acceptor distance.

  9. Controlling a Plant's Water Balance

    Plants lose much of their water through stomata, tiny pores on their leaves. The opening and closing of these pores, and thus a considerable part of the plant's water management, is at least partially under control of the hormone abscisic acid (ABA). Mishra et al. (p. 264) have now elucidated the signaling pathway between ABA in the opening and closing of stomatal pores. This mechanistic understanding has the potential to be used to produce plants with enhanced water-usage efficiency and drought tolerance.

  10. Gamete Recognition and Reproductive Success

    To ensure the reproductive success of a species at fertilization, egg and sperm must be compatible. In field studies with the sea urchin Strongylocentrotus franciscanus, Levitan and Ferrell (p. 267) examine the selective factors that shape highly polymorphic bindin gamete recognition proteins involved in species specificity. The reproductive success of males was greatest with common genotypes relative to rare genotypes; however, females with common genotypes had half the reproductive success of females with rare genotypes. In addition, common bindin alleles are selected with low sperm densities, but rare bindin alleles are selected with high densities. The interaction between spawning density and genotype frequency provides insight into why different species evolve at different rates.

  11. Directing Nonribosomal Peptide Synthesis

    CREDIT: KOGLIN ET AL.

    Nonribosomal peptide synthetases (NRPSs) operate through an assembly line where peptidyl carrier proteins (PCPs) transfer peptide intermediates between acyl-adenylate-forming (A) domains and peptide-bond-forming (C) domains. Koglin et al. (p. 273) now show how PCP reorients a cofactor that holds the growing peptide chains. Both apo and holo-PCP exist in two stable conformations and have one conformation in common between them. This double two-state equilibrium facilitates directed movement of the cofactor and may modulate specific interactions with the A and C domains.

  12. Genetic Link to Obesity?

    Obesity is thought to be in part heritable, but the genes responsible are difficult to identify, probably because they are numerous and each exerts only a small effect. Herbert et al. (p. 279) scanned the genomes of a group of well-characterized patients with markers for over 86,000 genetic polymorphisms. One of the markers was associated with an index of obesity, the body mass index. In follow-up studies of five completely independent groups of people that included adults and children, the association was present in four of the groups. The polymorphism is located upstream of a gene involved in global fat metabolism and could potentially affect its function.

  13. Particulate Impact

    Aerosols influence climate mainly through cloud formation and their interactions with solar radiation. Anthropogenic aerosol emissions are fairly well documented, but much less is known about natural emissions, such as biogenic volatile organic carbon (BVOC) from boreal forests. Tunved et al. (p. 261) show that Scandanavian boreal forests provide significant natural fluxes of climate-relevant aerosols, and propose a straightforward relation between emissions of monoterpenes and gas-particle formation over regions where there are few to no local anthropogenic aerosol sources.

  14. Choosing the Path of Molecular Breakup

    There are a number of pathways by which a molecule can dissociate, and controlling the path and the direction in which the fragments move may provide a way to control the outcomes of elementary chemical reactions. Kling et al. (p. 246) use recent developments in the generation of few-cycle optical pulses that allow their amplitude, frequency, and phase to be controlled with great precision. They apply the pulses to look at the dissociation D2+ molecules and show that manipulation of the waveform results in a pronounced dependence of the direction of emission of the ions.

  15. Inter- and Intrachromosome Interactions in Gene Regulation

    Long-range interactions within chromosomes that occur through intrachromosomal loops can set up important gene regulatory interactions. For example, the imprinting control region (ICR) at the insulin-like growth factor Igf2/H19 locus on mouse chromosome 7 is involved in bringing distant enhancers into promoter regions to increase gene transcription. Ling et al. (p. 269; see the Perspective by Spilianakis and Flavell) now show that interchromosome as well as intrachromosome interactions are mediated through the Igf2/H19 ICR. The maternal Igf2/H19 ICR interacts with a specific region on paternal chromosome 11. This interaction requires an ICR binding protein and appears to play a role in the regulation of expression of the interacting genes on chromosome 11.

  16. Functional Conservation of Noncoding Sequences

    Sequences, and even expression patterns, are conserved across evolutionary history, but less is understood about the conservation of regulatory sequences. Fisher et al. (p. 276, published online 23 March) developed a rapid, transposon-based transgenic strategy in zebrafish to study the functional conservation of noncoding sequences. This assay avoided problems of mosaicism that have been observed previously. The sequences that control expression of RET in humans (a gene that is associated with neural crest disorders) also show appropriate tissue-specific expression in the zebrafish, despite the lack of overt sequence conservation between the regulatory elements.

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