Amphibians as indicators of early tertiary "out-of-India" dispersal of vertebrates     

Bossuyt F  Milinkovitch MC 《Science (New York, N.Y.)》2001,292(5514):93-95

Sixty-five million years ago, massive volcanism produced on the India-Seychelles landmass the largest continental lava deposit (Deccan Traps) of the past 200 million years. Using a molecular clock-independent approach for inferring dating information from molecular phylogenies, we show that multiple lineages of frogs survived Deccan Traps volcanism after millions of years of isolation on drifting India. The collision between the Indian and Eurasian plates was followed by wide dispersal of several of these lineages. This "out-of-India" scenario reveals a zoogeographical pattern that might reconcile paleontological and molecular data in other vertebrate groups.  相似文献   

Structural biology. Robots enter the race to analyze proteins     

Service RF 《Science (New York, N.Y.)》2001,292(5515):187-188

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Dynamics of the vocal imitation process: how a zebra finch learns its song     

Tchernichovski O  Mitra PP  Lints T  Nottebohm F 《Science (New York, N.Y.)》2001,291(5513):2564-2569

Song imitation in birds provides good material for studying the basic biology of vocal learning. Techniques were developed for inducing the rapid onset of song imitation in young zebra finches and for tracking trajectories of vocal change over a 7-week period until a match to a model song was achieved. Exposure to a model song induced the prompt generation of repeated structured sounds (prototypes) followed by a slow transition from repetitive to serial delivery of syllables. Tracking this transition revealed two phenomena: (i) Imitations of dissimilar sounds can emerge from successive renditions of the same prototype, and (ii) developmental trajectories for some sounds followed paths of increasing acoustic mismatch until an abrupt correction occurred by period doubling. These dynamics are likely to reflect underlying neural and articulatory constraints on the production and imitation of sounds.  相似文献   

Superconductivity. Physicists scramble to recapture the magic     

Service RF 《Science (New York, N.Y.)》2001,291(5512):2295

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In situ measurement of grain rotation during deformation of polycrystals     

Margulies L  Winther G  Poulsen HF 《Science (New York, N.Y.)》2001,291(5512):2392-2394

Texture evolution governs many of the physical, chemical, and mechanical properties of polycrystalline materials, but texture models have only been tested on the macroscopic level, which makes it hard to distinguish between approaches that are conceptually very different. Here, we present a universal method for providing data on the underlying structural dynamics at the grain and subgrain level. The method is based on diffraction with focused hard x-rays. First results relate to the tensile deformation of pure aluminum. Experimental grain rotations are inconsistent with the classical Taylor and Sachs models.  相似文献   

Organic chemistry. Sugars join the automation rush     

Service RF 《Science (New York, N.Y.)》2001,291(5505):805-806

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Materials science. The simplicity of complexity--rational design of giant pores     

Férey G 《Science (New York, N.Y.)》2001,291(5506):994-995

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Uniform binding of aminoacyl-tRNAs to elongation factor Tu by thermodynamic compensation     

LaRiviere FJ  Wolfson AD  Uhlenbeck OC 《Science (New York, N.Y.)》2001,294(5540):165-168

Elongation factor Tu (EF-Tu) binds all elongator aminoacyl-transfer RNAs (aa-tRNAs) for delivery to the ribosome during protein synthesis. Here, we show that EF-Tu binds misacylated tRNAs over a much wider range of affinities than it binds the corresponding correctly acylated tRNAs, suggesting that the protein exhibits considerable specificity for both the amino acid side chain and the tRNA body. The thermodynamic contributions of the amino acid and the tRNA body to the overall binding affinity are independent of each other and compensate for one another when the tRNAs are correctly acylated. Because certain misacylated tRNAs bind EF-Tu significantly more strongly or weakly than cognate aa-tRNAs, EF-Tu may contribute to translational accuracy.  相似文献   

Electron paramagnetic resonance investigations of free radicals in extra virgin olive oils     

Ottaviani MF  Spallaci M  Cangiotti M  Bacchiocca M  Ninfali P 《Journal of agricultural and food chemistry》2001,49(8):3691-3696

Free radicals in olive oils were identified and quantified by EPR, by means of the spin-trapping technique making use of alpha-phenylbutylnitrone (PBN) as spin trap. The radical species were identified as PBN-trapped hydroxyl radicals (PBN-*OH) in the water microdroplets inside the fat medium. The largest radical concentration was 12.5 microM identical with 100%. The following were the relative concentrations of the radicals under different conditions: (1) Two oils, produced by continuous centrifugation, aged for 1 year, showed a 25-30% increase in the radicals compared to nonaged oils; 1-year-old oil, produced by pressure, did not differ from the nonaged oil. (2) Radical production was markedly reduced by N(2) bubbling; it was increased by heating, whereas it showed a biphasic pattern by air bubbling over time. (3) Radical concentration as a function of the UV irradiation time increased up to a maximum, after which it decreased and finally remained constant. The phenolic and oxygen contents were related to the radical content. This study demonstrates that the EPR technique is suitably applied to the detection of free radicals in olive oil and that storage, handling, and stress conditions of the oils significantly influence the radical concentration.  相似文献   

Photochemical degradation of acifluorfen in aqueous solution     

Vulliet E  Emmelin C  Scrano L  Bufo SA  Chovelon JM  Meallier P  Grenier-Loustalot MF 《Journal of agricultural and food chemistry》2001,49(10):4795-4800

To elucidate the photochemical behavior of diphenyl ether herbicides in superficial waters, the photodegradation of acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzo?c acid (CAS Registry No. 50594-66-6), was studied in water and acetonitrile. All experiments were carried out under laboratory conditions using a solar simulator (xenon arc) or jacket Pyrex reaction cell equipped with a 125 W high-pressure mercury lamp. The calculated polychromatic quantum efficiencies (Phi(solvent)) of acifluorfen in different solvents are as follows (units are degraded molecules photon(-1)): Phi(water) = 10(-4), Phi(acetonitrile) = 10(-4), Phi(methanol) = 10(-4), and Phi(hexane) = 10(-2). The results obtained in this work are in good agreement with the literature value of monochromatic quantum yield. HPLC-MS analysis (APCI and ESI in positive and negative modes) was used to identify acifluorfen photoproducts. These results suggest that the photodegradation of acifluorfen proceeds via a number of reaction pathways: (1) decarboxylation, (2) dehalogenation, (3) substitution of chlorine group by hydroxyl or hydrogen groups, and (4) cleavage of ether linkage, giving phenols. Photorearrangement products were studied by other investigators. No such products were observed. In addition, it was found that the trifluoro functional group on acifluorfen was not affected by any transformation, and no products of a nitro group reduction were found.  相似文献