This Week in Science

Science  04 May 2001:
Vol. 292, Issue 5518, pp. 805
  1. Oil and Water Try to Mix

    The interfaces between different liquids are notoriously difficult to probe experimentally because the signature of the molecules at the interface must be separated from that of the surrounding bulk liquid. Vibrational sum frequency spectroscopy is one of the few techniques that can overcome this problem. Scatena et al. (p. 908) used this method to probe the hydrogen bonding and orientation of water molecules at organic-water interfaces. Contrary to the conventional picture of strong hydrogen bonding between water molecules near fluid hydrophobic surfaces, they find that the water molecules are weakly bonded and are partially oriented by interactions with the organic phase.

  2. Monitoring Electron Paths in Atoms

    When an intense laser field interacts with an atom, the excited electrons driven by the laser field can be pulled from the nucleus, perform some complex orbits, and then be driven back to the nucleus, where they can scatter or recombine. Although it is often possible to calculate quantum-mechanical descriptions of such processes, they often can be difficult to appreciate. Feynman's approach to quantum mechanics, which involves summing over all possible paths, or quantum trajectories, provides a somewhat more intuitive description of the processes involved, but many experiments have been difficult to describe in this fashion because of the shear number of paths involved. Salières et al. (p. 902; see the news story by Seife) used a polarized laser field to limit the number of possible paths and show that the quantum orbit approach can describe the processes.

  3. A Tight Squeeze

    What happens to fluids when they are confined to very small spaces? The surface force apparatus, in which molecules are confined between cylindrical mica surfaces whose separation is carefully controlled, has been used to determine forces as a function of separation distance. Heuberger et al. (p. 905; see the Perspective by Israelachvili and Gourdon) have built an extended surface force apparatus in which fast spectral correlation improves the instrument resolution by a factor of 10 to 30. They probed the behavior of liquids for separation distances much smaller than molecular dimensions and could measure both the pressure and specific volume of the samples. They present data both for the case where the sample coexists as a liquid and a gas and for the case where only one fluid phase is present and continuum behavior no longer applies.

  4. Rotating with Light

    Microscale objects can be readily translated with light beams (“optical tweezers”), but the methods that have been developed to add rotational control have been limited by the properties of the materials to be manipulated. Paterson et al. (p. 912) introduce a generalized technique that allows rotational control over arbitrarily shaped objects. The interference of a laser beam with a helical component in the electric field and a plane wave results in the formation of a light beam with a corkscrew-shaped intensity profile. By varying the path-difference between the two beams, any object trapped within the optical field is also rotated as the corkscrew is turned.

  5. Ironing Out Vibrations

    Phonons are elementary vibrations of solids that influence many of their fundamental properties, but describing their variations with pressure and temperature, especially at extreme conditions, can be difficult to do in practice. For example, for iron, which primarily makes up Earth's core, many of these properties have yet to match seismic measurements or heat-flow data accurately. Mao et al. (p. 914) now combine both an experimental approach using a modified diamond-anvil press and nuclear-resonant inelastic x-ray scattering measurements with ab initio calculations to infer the phonon density of states at pressures up to those of Earth's core. These results indicate that the mean atomic number for the core is higher than that for pure iron and suggest the presence of some nickel in the core.

  6. Organics in the Oceans

    Much of the organic carbon in the ocean is actually not in organisms themselves but is present as dissolved organic matter (DOM) in seawater. It has been difficult to characterize DOM, yet understanding its origin, dynamics, and its further availability to organisms is critical to quantifying the role of the oceans in the global carbon budget. In a laboratory study, Ogawa et al. (p. 917) examined and traced organic matter as it was processed by naturally occurring marine bacteria. Ocean bacteria play a critical role in processing most of the oceans' DOM as well as producing the large amount of DOM that is refractory or that has low bioavailability.

  7. Helping with Her Coat

    Phospholipase C (PLC) hydrolyzes a membrane phospholipid to produce two second messengers that can in turn function in cellular signaling pathways. Fukami et al. (p. 920) examined the PLCδ4 isoform, which is localized to the acrosomal region of sperm. Upon disrupting the PLCδ4 gene in mice, the resultant males were either sterile or produced very few progeny. The sperm were unable to interact appropriately with the egg coat, the zona pellucida. Because PLCδ4 is essential for an early step in mammalian fertilization, greater insight into the function of this protein may aid in understanding human fertility and have application to contraception.

  8. Putting the Pieces Together

    Atomic-resolution snapshots of the large and small subunits of the ribosome have now been used to determine the structure of whole ribosome and to tease out some of its inner workings (see the Perspective by Dahlberg). Yusupov et al. (p. 883; see the cover and 30 March news story by Pennisi) have used the subunit structures to resolve a complex of the whole ribosome (large and small subunits) at 5.5 angstroms (Å) with bound messenger RNA (mRNA) and transfer RNAs (tRNAs) that enter at the A or acceptor site, move into the P or peptidyl transferase site, and exit via the E site. What is now visible are the precise physical relations between the three tRNA binding sites and the interfacial bridges that underlie relative movement of the subunits, which is the postulated means of coordinating the in-register translocation of tRNAs and mRNA. Ogle et al. (p. 897) show how the ribosome discriminates against near-cognate tRNA from their structures of the small ribosomal subunit in complex with A-site tRNA and mRNA, with and without the antibiotic paramomycin, at resolutions between 3.1 and 3.3 Å. Structural changes in the small subunit allow specific interactions with the codon-anticodon double helix that require Watson-Crick base pairing of the first two base pairs. The antibiotic paromomycin partially induces these structural changes and so disrupts decoding by facilitating binding of near-cognate tRNAs.

  9. Glial Cells and the Synapse

    Glial cells participate in the regulation of synaptic transmission by controlling neurotransmitter diffusion and concentration in the synaptic cleft. Two reports explore the role of glutamate and its receptors in this control (see the Perspective by Gallo and Chittajallu). To investigate the role of glial glutamate uptake, Oliet et al. (p. 923) took advantage of a substantial anatomical rearrangement between astrocytes and neurons in the hypothalamus during changes in reproductive state. During lactation, astrocyte ensheathment of synapses and hence glutamate removal are reduced, which leads to changes in the amplitude of evoked excitatory postsynaptic currents caused by activation of presynaptic metabotropic glutamate receptors. Glial cells express AMPA glutamate receptor subunits, the physiological role of which has been poorly understood. Iino et al. (p. 926) altered the function of Bergmann glial cells in the cerebellum by adding an AMPA receptor subunit that renders the channels calcium impermeable. They observed major changes in the morphology of the glial cell specializations around the synapses: The glial envelopment around Purkinje cell dendritic spines became retracted. Removal of released glutamate was impaired and led to an unusual multiple climbing fiber innervation of Purkinje cells.

  10. Managing Metabolic Models

    Understanding gene function in complete genomes will require the integration of different kinds of information in a way that will facilitate analysis and the generation of testable hypotheses. Ideker et al. (p. 929) have developed a systems approach to DNA microarrays, proteomics, and physical interactions for probing the yeast galactose utilization pathway. The pathway was perturbed by mutation and growth in selective media, and the changes were assayed in approximately 6200 yeast genes. They generated and tested hypotheses regarding the regulatory capabilities of Gal-1-P and galactose transporter.

  11. A Natural Take on Preservation

    Natural killer (NK) cells provide a vital link between innate and adaptive immunity and play a specific role in protection against tumors and viruses. Several lines of study have shown that NK cells operate through signals delivered by inhibitory and activation receptors, yet direct evidence for activation receptors in protection against pathogens has been lacking. Brown et al. (p. 934) show that the NK activation receptor—LY-49H—plays a critical role in the resistance of mice to infection with cytomegalovirus. LY-49 contains immunoreceptor tyrosine activation motif signaling motifs normally found in lymphocyte membrane receptors, which suggests that NK cells may operate through similar pathways of cell signaling as the cells of the adaptive immune response.