This Week in Science

Science  28 Mar 2008:
Vol. 319, Issue 5871, pp. 1729
  1. Picosecond Pirouette

    CREDIT: JAMES CAHOON

    The time resolution available to track chemical reactions and rearrangements has steadily increased into the femtosecond range of atomic vibrational periods, but most techniques required photoinitiation to achieve the necessary precision. Two-dimensional infrared spectroscopy has very recently been shown to overcome this limitation by tracking vibrational energy migration during rapid thermally driven processes. Cahoon et al. (p. 1820) apply this method to quantify the transition-state dynamics of the thermally driven ligand rearrangement, or fluxionality, in Fe(CO)5. By modeling the data at several different temperatures and comparing the results with theory, they obtain direct evidence for the long-postulated Berry pseudorotation mechanism in which axial and equatorial CO ligands switch places through a fleeting square pyramidal geometry on a picosecond time scale.

  2. Semiconductor Doping Writ Small

    Adding dopants or impurities is a known method to change the electronic properties of semiconductor materials. In theory, it should also be a useful trick for altering the properties of semiconductor nanocrystals or quantum dots. However, as the particle size gets smaller, it becomes increasingly difficult to dope the particles uniformly or to avoid unwanted chemical reactions of dopants that introduce an extra electron or hole into the particle. Norris et al. (p. 1776) review a number of techniques that have been developed to overcome the challenges of doping nanocrystals.

  3. Careful Hold of a Sharp Tool

    The squid beak is an organic hard tissue embedded in soft muscle tissue. Miserez et al. (p. 1816; see the Perspective by Messersmith) question how the squid can use the sharp and rigid beak without causing damage to its own muscle tissue that holds and powers it. They find that there is a gradient in the properties along the beak such that only the cutting end is stiff and hard; the end that is held by the soft tissues is soft and compliant. Gradient materials have been found in nature before, but in this case the authors map and correlate the mechanical properties with the local chemistry. In particular, they find that the stiffness gradient can be tied to mixtures of chitin and a histidine-rich protein family that contains 3, 4-dihydroxyphenyl-L-alanine.

  4. Ionically Driven Membrane Assembly

    Interfaces can help drive the self-assembly of molecules into larger well-ordered structures. For example, Langmuir-Blodgett films form from amphiphilic molecules at air-water interfaces. Capito et al. (p. 1812) show that when aqueous solutions of a high molecular weight polysaccharide and an oppositely charged low molecular weight peptide amphiphile are mixed, polymer sacs on the millimeter scale form instantly. The membranes possess a hierarchical structure in which the high molecular weight polymer extends across the membrane. The membranes are self-healing and are robust enough to withstand suturing. Thus, it may be possible to use them to encapsulate cells or other objects.

  5. Evolving Neutron Stars

    After a star has spent its nuclear fuel, it may collapse to form a compact neutron star. If this very dense object is rotating quickly, it may then become a pulsar and emit a rotating beam of radio waves that can be detected on Earth as a regular series of radio pulses. Neutron stars can also evolve into other exotic objects called magnetars, which emit x-ray bursts powered by the star's extremely strong magnetic fields. Gavriil et al. (p. 1802, published online 21 February) have now found a long-sought missing link between these two kinds of neutron stars by examining data from a pulsar in the Aquila constellation. A series of magnetar-like x-ray bursts was observed, and the spectrum of the bursts suggests that the neutron star is changing from a pulsar into a magnetar.

  6. Finely Spaced Clock Ticks

    CREDIT: ROSENBAND ET AL.

    Atomic clocks based on Cs atoms are the standard timekeepers of today, but clocks based on optical transitions of trapped atoms and ions offer the potential of better precision because they operate at much higher frequency (see the Perspective by Kleppner). One requirement for any new timekeeping standard is the ability to compare the operation of one remote clock with another. Ludlow et al. (p. 1805, published online 14 February) make a comparison over an optic fiber line of two different optical clocks (one Sr and the other Ca) 4 kilometers apart, and demonstrate a fractional uncertainty in the ticking of both clocks of 1 × 10−16 that surpasses even the best Cs-based standards. Rosenband et al. (p. 1808, published online 6 March) take two different optical clocks based on singly trapped Al and Hg ions and show that the ratio of the frequency of the clocks can be measured with a fractional error of 5.2 × 10−17. Taking measurements spanning the length of 1 year, they then show that if there are changes in the fundamental constants, measured in terms of the fine structure constant, then such changes are constrained to less than (−1.6 ± 2.4) × 10−17 per year.

  7. They Are What They Eat

    CREDIT: FERMILAB

    Whether a honeybee larva will become a worker or a queen is largely determined by what it eats—only future queens are fed royal jelly. Kucharski et al. (p. 1827, published online 13 March) hypothesized that this developmental decision was epigenetically controlled by DNA methylation. Silencing DNA methyl transferase 3 (Dnmt3) mimicked the effects of royal jelly on early developmental processes. When Dnmt3 expression was reduced, even in the absence of royal jelly, larvae developed into queens rather than workers.

  8. Viral Activation Strategy

    Flaviviruses, such as dengue, West Nile, or yellow fever viruses, are first assembled as immature, intracellular particles that must be processed and secreted through the secretory pathway. Intracellular cleavage of the precursor membrane protein (prM) in immature particles is essential for infectivity and correlates with virus pathogenicity. Yu et al. (p. 1834) and L. Li et al. (p. 1830) provide insight into the structural transitions that occur during virus maturation and how these relate to function. On immature virus, prM undergoes a large conformational change at low pH during transit through the Golgi complex, which exposes its furin cleavage site. The prM protein is cleaved, but remains associated with the virion envelope glycoprotein (E), which masks its membrane fusion loop and prevents premature membrane fusion. On exposure to the neutral pH of the extracellular milieu, prM dissociates from E to give mature virus that can undergo membrane fusion during the infection of a new host cell.

  9. Smell and Repel

    Blood-feeding insects are responsible for spreading some of the deadliest infectious diseases. Topically applied insect repellents play a crucial role in protecting humans from these insects. The most widely used of these compounds, DEET, has been around for more than 50 years, but its function is still poorly understood. Ditzen et al. (p. 1838, published online 13 March; see the 14 March news story by Leslie) found in both fruit flies and in the malaria mosquito that DEET acts on the insect olfactory system by inhibiting olfactory neurons that mediate responses to attractive substances. It seems that DEET functions by masking the host odor through blocking odorant receptors that require the olfactory co-receptor OR83b.

  10. The Smell of Fear

    Research on classical conditioning has largely concentrated on understanding how an organism learns to associate sensory stimuli with biologically salient events. Can aversive learning directly modify sensory perception of the conditioned stimulus itself? W. Li et al. (p. 1842) show that classical conditioning can indeed have a direct effect on perceptual discrimination abilities by using odors that exist in enantiomeric forms that cannot normally be distinguished. One form was associated with a negative experience (an electric shock). After repeated exposure, human subjects clearly showed improved discrimination. Conditioning caused representation of the target odors to be reorganized in piriform cortex.

  11. Of Rats and Rules

    The learning of rules and the ability to generalize between learned events and novel instances is a fundamental attribute of human cognition. Murphy et al. (p. 1849) investigated rule learning in nonprimates by asking whether rats can also learn “rules” that are analogous to the rules of grammar—for example, triads like subject-verb-object. Specifically, in a Pavlovian conditioning procedure, rats were trained to discriminate certain triplet sequences from other sequences. The animals' ability to discriminate correct from incorrect sequences developed gradually over a number of training sessions, which suggests that in rats transfer to novel stimuli—learning a rule—is possible.

  12. Clearing the Cleft

    Ionic currents through postsynaptic receptors can produce an electric field inside the synaptic cleft, which raises the possibility that they can affect the dwell time of electrically charged neurotransmitters. Sylantyev et al. (p. 1845) analyzed excitatory postsynaptic currents in acute rat brain slice cultures and in cultured neurons. Electric fields present in the extracellular space of the synaptic cleft generated by open ion channels during synaptic transmission accelerated diffusion of charged glutamate molecules out of the cleft via electrodiffusion, which altered signal integration in hippocampal pyramidal cells.

  13. Setting Many Small Traps

    Trapping of molecules onto surfaces through weak interactions can be useful not only for fundamental studies, as weak binding minimizes alteration of the molecule's bonding, but can be useful in applications where molecules are presented as targets for binding. Dil et al. (p. 1824) have examined the nanopore network formed by boron nitride grown on the (001) surface of ruthenium or rhodium. The 2-nanometer holes of the network can trap individual large organic molecules such as phthalocyanines that would otherwise form clusters. Through a combination of photoemission studies of adsorbed xenon and density functional calculations, the authors show that the holes are regions of low work function and that the trapping potential is created by a ring of dipoles at the rims of the holes.

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