Editors' Choice

Science  04 Nov 2005:
Vol. 310, Issue 5749, pp. 747

    The Dynamics of Invasions

    The 1980 eruption of Mount St. Helens in the American northwest provided ecologists with 60 square km of primary successional habitat on which to study the dynamics of recolonization. Fagan et al. examine the role of interactions between species in determining the course of colonization and invasion on the fresh pumice slopes of the volcano. The spatial pattern of colonization by the prairie lupin, Lupinus lepidus, is governed by herbivore pressure. The plants are eaten by the leaf-tying larvae (caterpillars) of several lepidopteran species, and there is evidence for thresholds in the parameter ranges of plant spatial extent and timing of initial colonization that predict whether the herbivores can halt the invasion. As well as providing fresh insight into the dynamics of successional systems, these findings are relevant to the control of invasive plants because they suggest the possibility of developing protocols for the most effective timing and spatial deployment of herbivorous control agents. — AMS

    Am. Nat., in press.


    Boron Metathesis

    Metal carbene (M=C) complexes have proven highly useful because of their capacity to undergo metathesis reactions, which exchange the carbene group with another doubly bonded fragment. Recently, several different researchers have prepared boron analogs (M=B) of these compounds, with a double bond between a transition metal and a monovalent boron, or borylene, center.

    Kays et al. have shown that one such compound undergoes metathesis, much like its carbon cousin. The borylene fragment, bearing a diisopropyl amino group, is bonded to an organometallic iron center. Room-temperature exposure of this compound to a solution of triphenylphosphine sulfide (Ph3P=S) quickly cleaves the P=S linkage, pairing the aminoborylene with the sulfur and leaving the phosphine coordinated to the iron. Similarly, treatment with triphenylarsine oxide (Ph3As=O) produces a boron oxide compound and an arsine-coordinated metal complex. The analogous reaction with triphenylphosphine oxide (Ph3P=O) is slow enough to permit isolation of an intermediate, which the authors characterized by x-ray crystallography. This intermediate shows a P-O-B linkage and lengthened Fe-B bond, suggesting that these reactions proceed by initial attack of the boron center by the electronegative oxygen or sulfur. — JSY

    Angew. Chem. Int. Ed. 10.1002/anie.200502343 (2005).


    Rescuing Glycine

    The two ways to achieve structural stability of an integral membrane protein are to bundle α helices together, as in lactose permease, and to curl a β sheet (comprised of individual β strands) into a barrel, as exemplified in the family of porins. The β-barrel proteins are found in the outer membrane of bacteria and of organelles (mitochondria and chloroplasts) thought to have a bacterial heritage.

    Jackups and Liang have systematically analyzed the small, but growing, data set of three-dimensional structures of β-barrel membrane proteins in order to establish the propensities of interstrand amino acid neighbors. These values have current application to improving sequence-based alignments across proteins as well as the register between β strands within proteins. In addition, motifs and antimotifs of pairs of amino acids may find application in future studies of β-barrel membrane protein biogenesis (folding, translocation, and insertion). One such motif, originally identified in soluble β-barrel proteins, is termed aromatic rescue of glycine. The curvature of the inner (and often solvent-exposed) surface of a β barrel is facilitated by glycine residues, and the energetically unfavorable exposure of the peptide backbone can be mitigated by covering the glycine with an aromatic side chain, such as is found in tyrosine and phenylalanine, from a neighboring stave of the barrel. — GJC

    J. Mol. Biol. 10.1016/j.jmb.2005.09.094 (2005).


    Death in the Blink of an Iris

    In early development, the eye is covered with a membrane that includes blood vessels and that functions to nourish the developing lens and retina. This membrane, however, obscures the clear optical path needed for visual acuity.

    Studying rats, in which the eye matures postnatally, Morizane et al. uncover the link between maturation of the iris and the apoptosis of blood vessels supplying the immature eye. A key moment is when the iris begins to move, constricting and dilating in the way that will later control the light supply to the lens and improve focus. The constricting movements place pressure on the persistent blood vessels, causing the blood supply to stop intermittently. Pharmacologic intervention that paralyzed iris movements delayed regression of the vascular membrane. The authors propose that it is the increasing intermittency of the blood flow, rather than the mechanical shear stress induced by iris movement, that signals cells of this vascular membrane to initiate apoptosis. — PJH

    Am. J. Physiol. Regul. Integr. Comp. Physiol. 10.1152/ajpregu.00602.2005 (2005).


    Grabbing Hydrogen

    The activation of normally unreactive C-H bonds by metal complexes typically proceeds through metal-to-carbon hydrogen-atom transfers. This step need not be irreversible, but direct observation of hydrogen transfers in the other direction, from carbon to metal, has been lacking.

    Zhang et al. report time-resolved infrared spectra of this process as carried out by the osmium dimer [Cp(CO)2Os]2 (where Cp is cyclopentadienyl). The radical formed by homolytic cleavage of the Os-Os bond through photolysis is unlike other metal carbonyl radicals in that it does not redimerize, but instead attacks C-H bonds. The reaction of this radical with 1,4-cyclohexadiene could be followed on the microsecond time scale by the decay of the CO modes of the radical and the growth of new infrared bands assigned to Cp(CO)OsH. Electrochemical studies and thermochemical analysis revealed the driving force for this reaction: an exceptionally strong Os-H bond (82 kcal per mole) relative to M-H bonds in other metal carbonyls. — PDS

    J. Am. Chem. Soc. 10.1021/ja0555724 (2005).


    Thriving in a Sea of Methane

    Life on Earth relies on energy from chemical reactions or from sunlight. Chemical reactions between organic compounds and hydrogen have been proposed as a possibility for powering life forms on the surface of Titan, Saturn's largest moon.

    McKay and Smith calculated the amount of energy released by reactions of hydrocarbons present in Titan's dense smoggy atmosphere with hydrogen gas. They found that there would be more than enough fuel for specialized organisms such as the methanogenic bacteria found on Earth. Although methanogens could be sustained on Titan, it might be difficult for them to consume the chemicals, because organics may not dissolve readily in the methane lakes where life may reside. — JB

    Icarus 178, 274 (2005).


    Improving Child Health

    Measles tends to be overlooked in the context of developing countries that have to cope with assault from many other intractable infections, yet it still accounts for more than 350,000 deaths annually in Africa, despite the availability of an excellent vaccine. Currently, 22 measles virus genotypes have been recognized, but relatively little is known about the genotypes circulating in Africa.

    Muwonge et al., working in Uganda, have discovered a new genotype (d10) in a 2-year study that highlights the logistical difficulties of undertaking such surveillance in a developing country. The viruses they isolated showed uniformity within the country, but significant divergence from reference strains, and were highly distinct from other known African strains, too. It is possible that measles transmission dynamics in Uganda differs from that in developed countries. Genotype surveillance in Africa should be extended not only to monitor control programs but also to describe transmission patterns and hence whether approaches to control and vaccination need revising. — CA

    Emerg. Infect. Dis. 11, 1522 (2005).

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