Editors' Choice

Science  29 Oct 2004:
Vol. 306, Issue 5697, pp. 781
  1. MICROFLUIDICS

    Breaking Up Is Hard To Do

    1. Marc S. Lavine

    Emulsions consist of droplets of one liquid encased within a second immiscible liquid and are important, for example, in the manufacture of cosmetics and foods. Double emulsions consist of droplets that encase finer droplets that are suspended in a continuous phase, such as water in oil in water. Double emulsions can be made using vigorous mixing techniques or by forcing single emulsions through membranes or nozzles, but in all cases a broad size distribution is obtained. Okushima et al. have turned to microfluidic devices, which have already shown a lot of promise in making droplets of controlled sizes using T junctions and variations in flow conditions. A series of T junctions where the properties of the channel changed at the junction were used to make double emulsions. To make water droplets within an oil phase, the junctions were designed so that water flowed into a hydrophobic channel, causing it to pinch off. This stream then flowed into a second hydrophilic channel, thus creating the double emulsion, with a very narrow distribution in the size of the droplets. With a slight adjustment of the flow conditions, it was also possible to encapsulate two water droplets from different sources within a single oil droplet, and by reversing the order of the channels they were also able to make oil-water-oil emulsions. —MSL

    Langmuir 10.1021/la0480336 (2004)

  2. CHEMISTRY

    Ambiguous Acidity

    1. Jake S. Yeston

    Acidic strength is often broadly defined as the ease with which an acid loses H+ to a base. Stoyanov et al. show that in nonpolar media, where acid and base stay closely associated after reacting, the relative strength of two acids varies with the base. They use infrared spectroscopy to compare two similar acids, both common synthetic reagents: TfOH and Tf2NH (where Tf is a CF3SO2 group). When water acts as the base in benzene or dichloroethane solution, TfOH is the better H+ donor, forming a (H3O+)(TfO) ion pair; Tf2NH forms only a weak hydrogen bond to water. In contrast, trioctylamine base accepts H+ more readily from Tf2NH than from TfOH.

    The data suggest that the discrepancy results from extra hydrogen bonding between H3O+ and the SO2 group in TfO, which geometry precludes in the Tf2NH complex. Thus, the acidity of HX depends not only on the stability of X but on direct molecular interactions between X and the base. — JSY

    J. Phys. Chem. A 108, 9310 (2004).

  3. BIOCHEMISTRY

    Sensing an Opening

    1. Phillip D. Szuromi

    One way to monitor enzyme activity is to use synthetic membrane pores. Vesicles containing blocked pores can be loaded with fluorescent dye at high enough concentrations for self-quenching; enzyme activity then removes the blockers, and the efflux of the dye allows fluorescence. One drawback of these approaches is that the enzyme must be able to enter the pores. Gorteau et al. describe an approach in which products of enzyme activity bind to pore precursors to create nascent open pores that permeabilize the pores. A rigid-rod octiphenyl core bearing peptide side chains with a central arginine group forms a β-barrel pore upon binding of acidic ligands with hydrophobic tails. The authors followed the activity of pig liver esterase using pyrenebutyrate methylester as a substrate, which generated pyrene carboxylate as a product to create open pores. The pores are lined internally with His-His groups and can be blocked by the addition of larger molecules such as poly-L-glutamate, probably through the formation of an α-helix. — PDS

    J. Am. Chem. Soc. 126, 13592 (2004).

  4. PALEOCLIMATE

    Long-Term Links

    1. H. Jesse Smith

    The Gulf Stream transports large amounts of heat from low to high latitudes in the North Atlantic region. The Florida Current is a branch of the Gulf Stream that flows through the Straits of Florida, transporting large volumes of warm water in a northerly direction, and plays a key role in regulating the climate of the circum-North Atlantic region. Although good observational records of its strength extend roughly 100 years into the past, little is known about its variability on time scales greater than that. Lund and Curry present oxygen isotopic analyses of planktonic forams from three marine sediment cores taken from near the Florida Keys, which show multiple sea surface temperature oscillations at centennial to millennial scales over the past 5200 years. The Florida Current was either cooler or more saline during the Little Ice Age and was relatively warm or fresh during the Medieval Warm Period. The Florida Current oxygen isotope signal varied coherently with atmospheric radiocarbon over periods of 1000 years or more, reinforcing suggestions that ocean circulation may help to modulate atmospheric radiocarbon on millennial time scales. — HJS

    Paleoceanography 10.1029/2004PA001008 (2004).

  5. ECOLOGY/EVOLUTION

    Vulnerable Vultures

    1. Andrew M. Sugden

    Over the past decade, the populations of Gyps vulture species across the Indian subcontinent have crashed, in many areas by more than 95%. The dramatic decline and potential extinction of vultures have serious implications for a human-dominated ecosystem in which scavengers (rather than predators) play such an important role, with heightened risk of disease from decaying unconsumed carcasses and from proliferating four- footed scavengers — dogs, cats, and rats. At first mysterious, the likely cause of the vulture decline in Pakistan was recently pinpointed as the widely-used veterinary anti-inflammatory drug diclofenac administered to cattle: Vultures fed on carcasses of diclofenac-treated cattle develop fatal kidney failure.

    Green et al. now show that diclofenac is the probable cause of Gyps decline across the entire subcontinent. A simulation model of vulture demography provides a quantitative range of estimates of the proportion of cattle that would need to be treated with diclofenac in order to produce the observed levels of vulture decline. Fewer than 1% of cattle would be sufficient to produce the catastrophic declines observed. To stave off the possible imminent extinction of Gyps species, an urgent search for alternatives to diclofenac is required. Captive breeding programs may also be necessary to maintain stocks of vultures for eventual reintroduction — AMS

    J. Appl. Ecol. 41, 793 (2004).

  6. MEDICINE

    Help Rather Than Hinder

    1. Stephen J. Simpson

    The immune system has been thought to present a barrier to the normal development of the fetus through maternal immunity to “foreign” paternal proteins. However, Hibey et al. now find that efficient activation of innate natural killer (NK) cells may actually help fetal growth.

    The multiple receptors carried by NK cells exist in two basic forms, which lead to either cellular activation or inhibition. It is the balance between these signals that ultimately dictates the NK cell's response. Pregnant women carrying particular combinations of receptor genes and their polymorphic ligands that favor NK cell inhibition were more likely to develop the potentially lethal condition preeclampsia, in which placental blood flow becomes impaired. Thus, uterine NK cells and their cytokines may promote the trophoblast remodeling of the placental spiral arteries that generates the essential increase in blood flow required during placenta formation. Thus, by inhibiting, rather than activating NK cells, it is possible that this remodeling becomes less efficient and sets the stage for preeclampsia. — SJS

    J. Exp. Med. 200, 957 (2004).

  7. STKE

    Waving to Far-Off Neurons?

    1. Elizabeth M. Adler

    Glial cells participate in bidirectional interactions with neurons and help sculpt brain function. For instance, astrocytes clear extracellular glutamate released during synaptic activity and can themselves release glutamate in response to increases in intracellular calcium ([Ca2+]i). Astrocytes propagate long-distance waves of increased [Ca2+]i through mechanisms that involve ATP release, purinergic receptor stimulation, and mobilization of inositol 1,4,5-trisphosphate (IP3) from intracellular stores. However, the function of these waves is not clear. Bernardinelli et al. visualized changes in [Na+]i and [Ca2+]i in primary cultures of mouse cortical astrocytes and observed propagation of Na+ and Ca2+ waves after electrical or mechanical stimulation of a single cell. The kinetics of Na+ and Ca2+ waves were distinct: whereas propagation of both was blocked by a purinergic receptor antagonist or by a cell-permeant Ca2+ chelator, only the propagation of Na+ waves was substantially reduced by gap junction blockade. Glutamate is released during Ca2+ waves, and inhibition of the glutamate transporter inhibited Na+ wave propagation. Increased [Na+]i stimulates astrocyte metabolic activity; thus; glutamate released during the Ca2+ wave may elicit a secondary Na+-mediated metabolic wave, potentially providing products of glycolysis to distant neurons. — EMA

    Proc. Natl. Acad. Sci. U.S.A. 101, 14937 (2004).