Rolling Out Single Photons
- Ian S. Osborne
Carbon nanotubes (CNTs) have been studied exhaustively in terms of electronic and mechanical properties. A large repertoire of potential applications has been demonstrated for these one-dimensional systems of rolled-up sheets of carbon. To date, most of the optical studies on CNTs have focused more on characterization. Högele et al. now show that CNTs exhibit quantum optical behavior. Photons, being bosons, like to bunch together. However, in quantum systems such as atoms and artificial atoms, the confinement of carriers can split the pack behavior, resulting in the photons being emitted one at a time. For applications such as ultrasecure communication, such antibunching behavior is desirable. These authors excite a single CNT with a laser pulse and observe that light is emitted as single photons upon relaxation. The results suggest that CNTs may extend their repertoire beyond logic circuits and other electronic devices, finding application in quantum optoelectronics. — ISO
Phys. Rev. Lett. 100, 217401 (2008).
Autoactivation Is the Key
- Nilah Monnier
Protein tyrosine kinases (PTKs) regulate cellular activities by transferring phosphate groups from ATP to other proteins. PTKs must first be activated by autophosphorylation of their own specific tyrosine residues. The structural basis for eukaryotic PTK activation involves displacement of an amino acid loop, which initially blocks access to the active site but shifts out of the way upon autophosphorylation. Much less is known about prokaryotic PTKs, which do not have significant sequence homology to eukaryotic PTKs.
Lee et al. have determined the crystal structure of a prokaryotic PTK domain from the Escherichia coli tyrosine kinase Etk, alone and with ADP bound. The side chain of tyrosine 574 (Y574), which must be autophosphorylated in order to activate Etk, is positioned facing ADP in the active site and blocks access by peptide substrates. Although Y574 is not part of a flexible loop, a phosphorylated Y574 side chain could rotate away from the active site into an alternate conformation that would be stabilized by a salt bridge to a nearby arginine and multiple hydrogen bonds to surrounding amino acids. The Etk structure therefore suggests a new mechanism of activation of PTKs, in which phosphorylation-triggered displacement of only a single amino acid side chain suffices to unlock the door to the active site. — NM*
EMBO J. 27, 10.1038/emboj.2008.97 (2008).
↵* Nilah Monnier is a summer intern in Science's editorial department.
Genomes of a Feather
- Laura M. Zahn
The zebra finch (Taeniopygia guttata) belongs to the most populous bird order (the perching birds or Passeriformes) and is having its genome sequenced. Stapley et al. have created a linkage map of the zebra finch and compared it to that of the distantly related chicken (Galliformes). There was extensive synteny, or conservation of genes on the same chromosomes, indicative of a low amount of interchromosomal rearrangement between bird orders. However, a number of intrachromosomal differences were found between the zebra finch, reed warbler (another Passerine), and chicken, especially on the five macrochromosomes, suggesting that gene order is not conserved even among close relatives. A comparison of recombination rates reveals that chickens have high rates relative to other birds whereas reed warblers experience less recombination. — LMZ
Genetics 179, 651 (2008).
- Julia Fahrenkamp-Uppenbrink
Fast detection of hazardous airborne materials in crowded settings such as airports has the potential to save many lives, but remains very challenging. The concentrations of hazardous material are likely to be extremely low, whereas other (harmless) aerosols are present in high concentrations. Furthermore, the response needs to be accurate to avoid costly false alarms that might require evacuation. Steele et al. have developed a single-particle aerosol mass spectrometry (SPAMS) system that addresses many of these issues. The system allows the detection, analysis, and identification of a wide range of hazardous aerosols, from chemical and biological to radioactive and explosive materials, within seconds. A 7-week field test at San Francisco International Airport showed a low false alarm rate. Key to the success of the method is not only the detailed characterization of individual particles without need for reagents, but also a complex software control system. However, challenges remain before the system can be deployed as a commercial detector; the current instrument is large and expensive, and further live-agent tests are required to test the system. — JFU
Anal. Chem. 80, 10.1021/ac8004428 (2008).
Multiple Choice Testing
- Peter Stern
When we make economic decisions, for example the purchase of a good or a service, our brain has to perform at least three computations. First, it has to assess the goal value of the good: in economic terms, our maximal willingness to pay. Second, it has to assess the decision value of the good: the goal value minus the unavoidable costs. Third, there is a prediction error, which indicates the deviation from one's expectations of reward; the prediction error is positive when something better than expected happens and negative when the opposite occurs. Unfortunately, these three related quantities are intermingled and are often highly correlated, making it challenging to isolate the neural regions performing these computations.
Hare et al. have attempted to measure goal value, decision value, and prediction error in a single neuroimaging task so that they could dissociate these parameters. They found that ventral striatum activation reflected prediction error and not goal or decision value. However, activity in the medial orbitofrontal cortex and the central orbitofrontal cortex correlated with goal value and decision value, respectively. — PRS
J. Neurosci. 28, 5623 (2008).
A Question of Geometry
- Gilbert Chin
The varied metal clusters that proteins use to handle diatomic gases are remarkable not only for the unsurpassed chemistry they support but also for the fine tuning of the geometric properties of the metal atoms by their ligands. By synthesizing a series of dinuclear FeNi complexes, Ohki et al. demonstrate how the Ni can be switched between octahedral, square pyramidal, and square planar geometries. In the precursor compound, the Fe atom carries three CO ligands, there are three bridging thiolates, and the Ni adopts a slightly distorted square pyramidal structure. Adding a bidentate thioether-thiolate ligand onto the Ni atom resulted in the release of one of the bridging thiolates and a square planar configuration. In contrast, adding a phenolate-thioether ligand yielded an octahedral Ni that retained the three Ni-μ-S bonds, with Ni-O and Ni-S links to the bidentate ligand and a Ni-O bond to a molecule of solvent (methanol). The reversible loss of methanol was accompanied by ejection of one of the bridging thiolates and conversion into the square planar configuration. Integrating these findings with earlier work on the [NiFe] hydrogenase and other dinuclear complexes (NiRu and GeRu), these authors propose that the hydrogenase NiFe cluster binds H2 by shifting the square pyramidal Ni into an octahedral configuration and that heterolysis of H2 leads to loss of the bridging OH ligand as H2O. — GJC
Proc. Natl. Acad. Sci. U.S.A. 105, 7652 (2008).
- Science Signaling
Signaling in Space
- L. Bryan Ray
The surfaces where T cells interact with antigen-presenting cells have a distinct spatial organization of membrane components known as the immunological synapse. Shen et al. have devised a way to control the spatial presentation of antibodies to the T cell receptor (anti-CD3) and antibodies to the costimulatory receptor CD28 (anti-CD28). They used microcontact printing to create a surface in which either anti-CD3 and anti-CD28 were presented together or anti-CD28 was segregated in dots around anti-CD3 regions. T cells exposed to the surface localized to the antibody-coated regions of the surface regardless of antibody segregation, but secretion of interleukin 2 was increased if anti-CD28 was segregated at the periphery of a central locus of anti-CD3 rather than uniformly distributed with it. Activation of the kinase Akt was also greater in cells exposed to the segregated signals. The dynamics of cell interaction with the receptors revealed that cells transiently contacted and released patches containing segregated anti-CD28 but remained associated with surfaces where the ligands were mixed. This system may foster insight into how the geometry of immunological synapses influences intracellular signaling. — LBR
Proc. Natl. Acad. Sci. U. S. A. 105, 7791 (2008).