This Week in Science

Science  14 Apr 2000:
Vol. 288, Issue 5464, pp. 225
  1. Salty Interface

    Reactions between the atmosphere and sea salt aerosols are important for many tropospheric processes. Conventional gas and bulk aqueous phase chemical models have been unable to explain the observed chemistry of these particles, however. Knipping et al. (p. 301; see the Perspective by Seinfeld) have now combined experimental observation, computational kinetic modeling, and molecular dynamic and quantum mechanical calculations to show how the production of chlorine gas must occur from NaCl aerosols in the presence of OH. Their results demonstrate that the concentration of chloride ions is enhanced at the aerosol surface and that reactions at the air-water interface must dominate.

  2. Burning Questions

    Carbon monoxide is a pervasive atmospheric pollutant, and identifying its sources is important not only for scientific reasons but for questions related to environmental policy as well. Forest fires are one known source, and anthropogenic fossil fuel burning is another. Wotawa and Trainer (p. 324) have combined ground-based and aircraft measurements with a transport model to identify the origin of an episodic pollution event that affected the southeastern United States in the summer of 1995. By tracing the source of the carbon monoxide to emissions from forest fires in Canada, they show that boreal fires can be important contributors to the pollution budgets of mid-latitude regions.

  3. History in Titania

    The mineral phases within a rock can provide clues to its previous environments. Hwang et al. (p. 321) have uncovered a nanometer-sized inclusion of TiO2 (titania) with an α-PbO2-type structure sandwiched between two rutile crystals in the diamond-bearing ultrahigh pressure metamorphic rocks of the Erzgebirge in Germany. This phase of titania has not been observed previously in nature, and its recognition provides an important temperature and pressure indicator for the history of deformation in metamorphic rocks (for example, the diamonds in these rocks could have formed over a wider range of pressures and temperatures). This finding indicates that the massif experienced a rapid subduction and burial to depths of greater than 200 kilometers, one of the deepest recorded by any mineral phase, and then was rapidly exhumed.

  4. The Nanomechanics of Recognition

    Intermolecular forces arising from the adsorption of molecules on surfaces are known to induce surface stress. Fritz et al. (p. 316) exploit this effect to transduce molecular recognition into nanomechanical responses. Cantilevers were functionalized with biomolecules such as DNA. Molecular recognition of a ligand—such as a complementary DNA strand—was measured as a differential nanomechanical response between differently functionalized cantilevers. This method, which was also demonstrated for protein-protein recognition, does not require labeling, optical excitation, or external probes, is compatible with silicon technology, and may easily be run in parallel.

  5. Self-Contained Oxide Growth

    The layers of silicon dioxide that are used in the gates of MOSFETs (metal-oxide-semiconductor field-effect transistors) cease to be good insulators if they made only a few atomic layers in thickness. Further downsizing of these devices should be possible if other metal oxides with higher permittivity, such as alumina or zirconia, could be used as a dielectric, but if silicon is oxidized during their deposition, the lower capacitance of the silica layer will limit the effectiveness of this approach. Ritala et al. (p. 319) modified the method of atom layer deposition so that the reaction chemistry requires no separate oxygen sources. They can deposit thin layers of high-permittivity alumina and other oxides on silicon without forming an intermediate layer of silica.

  6. Stellar Remnants

    Titanium carbide grains, enclosed in amorphous graphite, have been described in meteorites and inferred to be evolved from asymptotic giant branch (AGB) stars. Von Helden et al. (p. 313) now provide a more direct link between these grains and the ejection of a carbon- and titanium-rich superwind from a post-AGB object after the death of the star. They measured the infrared wavelength spectra of TiC clusters in the laboratory and found a 20.1-micrometer peak with a similar width and intensity as the 20.1-micrometer peak that has only been detected in post-AGB objects by the Infrared Space Observatory. Combining meteoritic and spectral observations, they calculate the density and mass loss rate of the superwind coming from these post-AGB objects.

  7. Can Take the Heat

    The “heat” in hot peppers or from an acid burn turns out to be due to the triggering of one of the same receptors that contributes to the sensation of painful heat. Caterina et al. (p. 306; see the cover and the news story by Vogel) have constructed transgenic mice that are lacking the VR1 receptor, a cation channel selectively found in neural ganglia that mediate pain, which opens when activated by capsaicin or protons. These mice can eat chili peppers without painful consequences. The ability of these mice to sense noxious heat is impaired, but not completely eliminated, which suggests that other systems also contribute to heat detection. The VR1 receptor likely integrates stimuli released from tissue damage (such as protons) with the sensation of painful heat to send a rapid danger signal to the brain to remove the animal from the stimulus.

  8. Leaving Together

    The loss of evolutionary history and biodiversity incurred with the loss of entire genera of threatened animal species is very much worse than if extinction strikes at random, as is commonly assumed. Purvis et al. (p. 328) show that extinction risk is, in fact, unusually high for bird and mammal species that have few close relatives, and they also show that closely related species have unusually similar risks of extinction.

  9. Chaperone Selection

    Secretory and plasma membrane proteins begin their life with cotranslational translocation into the endoplasmic reticulum (ER). Inside the ER, the correct folding of newly synthesized proteins is assisted by a group of chaperone molecules including binding protein (BiP) and the lectins calnexin and calreticulin. Molinari and Helenius (p. 331) found that chaperone selection occurs cotranslationally and describe some of the criteria that select for a particular chaperone. The position of carbohydrate chains in the nascent chain of the protein diverted it from an interaction with BiP to an interaction with calnexin and calreticulin.

  10. Toxin and Phage Secretion

    Cholera toxin is secreted from Vibrio cholerae bacteria harboring the filamentous bacteriophage CTXϕ. Davis et al. (p. 333) examined the requirements for release of new phages from infected bacteria and discovered that a protein known to be involved in secretion of cholera toxin, EpsD, was also required for phage release. This finding is unexpected given the different size and nature of a protein toxin versus a protein-coated DNA molecule.

  11. Executing the Executioner

    Amyotrophic lateral sclerosis (ALS) is characterized by rapid death of motor neurons in the brain and spinal cord and results in paralysis and death, usually within 5 years of disease onset. The discovery of mutations in the SOD1gene, which encodes a copper zinc superoxide dismutase, in a familial form of the disease enabled the creation of a mouse model of ALS in which the mice overexpress a mutant version of human SOD1. Li et al. (p. 335; see the Perspective by Gurney et al.) now demonstrate that motor neurons in the ALS mice die by apoptosis. By blocking the activity of caspase enzymes, the cell's executioners during apoptosis, with the inhibitor zVAD-fmk, they dramatically prolonged survival of the ALS mice. These findings suggest that caspase inhibitors may be beneficial in the treatment of ALS, particularly if combined with other neuroprotective drugs.

  12. An Evolving View of HCV Infection

    Infection with hepatitis C virus (HCV) is a major public health problem that affects 170 million individuals worldwide. The vast majority of infected individuals develop chronic disease that can remain stable over many decades, whereas others successfully clear the virus soon after infection. To identify factors that might contribute to these diverse clinical outcomes, Farci et al. (p. 339) examined the evolution of sequences in the HCV envelope genes at early stages of infection. In patients who developed chronic disease, HCV evolved rapidly and showed a much greater genetic diversity than did HCV from patients who cleared the virus. These results suggest that analysis of viral sequence changes early in infection may be a useful tool for predicting clinical outcome.

  13. Dividing Up the Work Place

    During development, areas of the mammalian neocortex acquire specialization and thus different tasks, such as interpreting touch or directing reach, are parceled out to different areas. At least some of the specialization is directed by input from other regions of the brain. Bishop et al. (p. 344) now show that the specialization is not entirely dependent on neuronal input—rather, some component of the areal specialization is determined by genes expressed in the cortical cells themselves. Two genes, Emx2 and Pax6, act in opposition to determine the rostral-caudal pattern of neocortical areas, thus establishing the basics of cortical areal specification.

  14. Recognizing Language

    Although there might be general agreement that language, as commonly understood, is an ability limited to humans, it may nevertheless rely upon component skills that could be present to some extent in nonhuman primates. Ramus et al. (p. 349; see the Perspective by Werker and Vouloumanos) provide evidence for at least one of the skills using parallel training and testing paradigms to assess the comparative abilities of human newborns and tamarin monkeys. They find, replicating earlier work, that newborns can discriminate between rhythmically distinct languages when spoken forward but not backward. Surprisingly, the monkeys are equally adept, indicating that they do possess processing skills that help identify a sequence of sounds as language.