2009年04月08日

Twist

The spins of a layer of manganese atoms on a tungsten surface from appairal pattern with a unique turning sense. such 'chiral magnetic order' might exist in other, similar contects.

Chirality is a fasinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physis (in the chage-parity violation of electroweak in teractions). (Siegel,J.S. Single-handed cooperation. Nature 409,777-778/2001)(Ellis,J. Antimatter matters. Nature 424,631-634/2003)

In condensed matter, magnetic materials canalso display single-handed, or homochiral, spin tructures. This may be coused by the Dzyaloshinskkii-Moeiya interction, wichi arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field.(Dzialoshinskii,I.E. Thermodynamic theory of "Weak" ferromagnetism in antiferromagnetic substances. Sov Phys. JETP 5,1259-1262/1957)(Moriya,T. Anistotropic superexchange interaction and weak ferromagetism. phys.Rev. 120,91-98/1960)

This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion summetry, so that homochiral spin structures may occur.(Fert,A. Magnetic and transport properties of metallic multilayers. Mater. Aci froum 59&60, 439-443/1990)

### data ###
Nature 447,115-230 / 2007
Let's twist again / Cristian Pfleiderer and Ulich K. RoBler
Chiral magnetic order at surfaces driven by inversion asymmetry / M Bode et al (University of Hamburg)
posted by 0≠素子 at 09:16| Comment(0) | condensed-matter physics | このブログの読者になる | 更新情報をチェックする

2009年03月25日

telomerase RNA

Telomeres cap the ends of chromosomes and provide a means to complete replication. The DNA portion of telomeres is synthesized by the enzyme telomerase using part of an RNA subunit as a tenplate for reverse transcription. How the mature 3' end of telomerase RNA is generated has so far remained elusive.

Here we show that in Schizosaccharomyces pombe telomerase RNA transcriots must be rocessed to generate functional telomerase. Charecterization of the maturation pathway uncovered un unexpected role for the spliceosome, which normally catalyses splicing of pre-messenger RNA.

The first spliceosomal cleavage reaction generates the mature 3' end of telomerase RNA (TER1, the functional RNA encoded by the ter1^+ gene), releasing the active from of the RNA without exon ligation.

Blocking the first step or permitting completion of spllicing generates inactive forms of TER1 and causes progressive telomere shortening.

We establish that 3'end processing of TER1 is critical for telomerase function and describe a previously unknown mechanism for RNA maturation that uses the ability of the spliceosome to mediate site-specific cleavage.


### DataBace ###
nature Vol.456 837-1008 no.7224
Spliceosome meets tekimerase / Sophie Bonnal & Juan Valacarcel
Spliceosomal clavage generates the 3' end of telomerase RNA / J A Box et al. (Stowers Institure for Medical Research)

posted by 0≠素子 at 09:46| Comment(0) | biogeochemistry | このブログの読者になる | 更新情報をチェックする

2009年03月21日

Reassessing the first appearance of eukaryotes and cyanobacteria

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to sharp rise in atomo-spheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages.(Beekker,A. at al. Dating the rise of atmospheric oxygen. nature427,117-120/2004)(Canfield,D.E The early history of atmospheric oxygen annu. Rev. Earth Planet. Sci. 33,1-36/2005)(Kopp,R.E., Kirschvink, J.L., Hilburn,I.A. &Nash,C.Z. The Paleoproterozoic snowball Earth : a climate disaster triggered by the evolution of oxygenic photosynthesis.Proc. Natl Acad. Sci. USA 102,11131-11136/2005)

The oldest widely accepted evidace for oxygenic photosynthesis has come from hydorocarbons extracted from 〜2.7-Gyr-old shales in the Pilbara Craton, Australia,whicth contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria.(brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Archean molecular fossils and the early rise of eukaryotes. Science 285,1033-1036/1999)(Brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)(Brocks,J.J., Buick,R., Summons, R. E. & G. A. A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia. Geochim. Cosmochim. Acta 67,4321-4335/2003)(Simmons,R.E., Jahnke,L.L., Hope,J.M. & Logan,G.A. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400,554-557/1999)

The soluble hydrocarbons were interpreted to be indigenous and syngentic despite metamorphic alteration and extreme enrichment (10-20‰) of ^13~C relative to bulk sedimentary organic matter.(Brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)(Heyes, J.M., Kaplan,I.R> & Wedeking,K.W. in Earth's Earliest Biosphere : Its Oligin and Evolution / Ed. Schopf,J.W / 93-134 / Princeton Iniv. Press,1983)

Here we present micrometre-scale, in situ ^13~C/^12~C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that priginally yielded biomarkers.

Our results show that both kerogen and pyrobitumen are stotongly depleted in ^13~C, indicating that indigenous petroleum is 10-20‰ lighter than the extracted hydrocarbons.(brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)

These ersults are inconsistent with an indigenous origin for the biomarkers.

Whatever their origin, the biomarkers must have entered the rock after peak metamorphism 〜2.2Gyr ago and thusdo not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon.(Rasmussen,B., Fletcher,I.R. & Sheppard,S. Isotopic dating of the migration of a low-grade metamorphic front during orogensis. Geology 33,773-776/2005)

The old est fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68Gyr ago and 〜2.15Gyr ago, respectively(Knoll,A.H., Javaux, E.J., Hewitt,D & Cohen, P. Eukarryyotic organisms in Proterrozoic oceans. Phil. Trans R. Soc. B 361,1023-1038/2006)(Hofmann,H.J. Precambrian microflora, Belcher Lslands, Canada:Significance and systematics. J.Paleontol, 50,1040-1073/1976)

Our results eliminate the evidence for oxygenic photosynthesis 〜2.7Gyr ago and exclude previous biomarker evidence for a long delay (〜300 million years) between the appearance of oxygen-producing cyanobacteria and tha rise in atmospheric axygen 2.45-2.32Gyr ago.(Beekker,A. at al. Dating the rise of atmospheric oxygen. nature427,117-120/2004)


### DataBace ###
nature Vol.445 1007-1148 Issue no.7216 23 October 2008
Letter p.1101 / Reassessing the first appearance of eukaryotes and cyanobacteria / B Rasmussen et al. (Cartin Univercity of technology)


posted by 0≠素子 at 12:26| Comment(0) | evolution | このブログの読者になる | 更新情報をチェックする

2009年03月19日

Life before the rise of oxgen

The discovery og molecular fossils in 2.7-billion-year-old rocks prompted are-evaluation og microbial evolution, and of the advent of photosynthesis and rize of atmospheric oxgen.

That discovery now comes into question.


Go Back to Archaean time, the interval of Earth's History between about 4 billion and 2.5 bilion years ago, and we're in largely unknown biological territory.

Attenpts to identifly a fossil recorf of life have prodused meager ersults. and controversy presists obout whether certain microfossil-like structuress are of biological orgin.(Schopf,J.W.Phill Trans. R. Soc B.361-869-885/2006)(Brasier,M.D.et.al. Nature416,76-81/2002)

Almost a decade ago, however, Archaean palaeontology received a big boost with the discovery by Brocks et al. of a diverse assenblage og lipid 'biomarkers' in 2.7-billion year-old geological samples from Western Australia.

Biomarkers, or molecular fossils, are natural products (often hydrocarbons) whose synthesis can be linked to a specific biological origin - and, by physiological proxy, to enbiron mental conditions.

posted by 0≠素子 at 10:29| Comment(0) | biogeochemistry | このブログの読者になる | 更新情報をチェックする

2009年03月15日

Oxidation in transtion

Many important biological and chemical processes, including photocatalytic water oxydation to molecular oxgen (of interrest as a route to artifical photosynthesis) and the activation of dioxygen on metal surfaces, are thought to involve transition metal terminal oxo complexes.

Poverenv et al. now report the synthersis of a platinum-based oxdizing reagent with potentially uzeful characteristics.

It's d^6 Pt(IV) terminal oxo complex that is not stabilized by anelectron accepting ligand framework, and so exhivits reactivity as both an inter-and intra-molecular oxygen donor and as an electrophile.

It also undergoes water activation to produce a terminal dihydroxo complex, which may be of relevance to the mechanism of water oxidation and other catalytic reactions.

posted by 0≠素子 at 00:30| Comment(0) | chemistry | このブログの読者になる | 更新情報をチェックする

2009年03月14日

Plant invaders

The current episode of climate warming is resulting in range shifts of some plants and animals from lower to hugher latitudes and altitudes.

A study of the growth of fifteen plant species - six that had shoen rapid range expansion and nine related natives - now shows that exotic plant species expanding therir range into North-Western Europe will have less exposure than natives to both below-ground enemies and above-ground generalist herbivores.

One implication of this work is that some of the plants shifting their range towards higher katitudes and altitudes will be invasive with potential deleterious effects on biodiversity in temperate and notrthern latitudes.


### DataBace ###
nature Vol.456 837-1008 Issue no.7224


posted by 0≠素子 at 15:00| Comment(0) | ecology | このブログの読者になる | 更新情報をチェックする

2009年03月12日

Patteerning of sodium ions and the control of electrons in sodium bobaltate

Sodium cobaltate (NaxCoO2) has emierged as material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physis of the superconducting copper oxides.(Terasaki,I., Sasago,Y., & Uchinokuma,K. Large thermoelectic power in NaxCoO2 single crystals. Phys. Rev. B56, R12685-12687/1997)(Lee,M.et al. Large enhancement of the thermopower in NaxCoO2 at high Na doing Nture Mater. 5,537-540/2006)(Takada,K. et al. Superconductivity in tow-dimensional NaxCoO2 layers. Nature 422,53-55/2003)

The densty x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers.(Dalmas,C. et al Electrochemical interclation of sodium in NaxCoO2 bronzes. Soild State lonics 3/4,165-169/1981)

Recent electron diffraction measurements revel a kaleidoscope of Na^+ ion patterns as a function of concentration.(Zandbergen,H.W. et al. Sodium ion ordering in NaxCoO2 : Electron diffraction study. Phys. Rev. B70, 024101/2004)

Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-rage three-dimensional superstructures of these ions.

We are goveerned by pure electrostaties, and that the origanizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings.

Our results provide a good startingpoint to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostaric potential from the Na superstructures.(Hubbard,J, Electron correlations in narrow energy bands. Proc. R. Soc Land. A276,238-257/1963)

The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes perferentially occupy the lowest potential regions.

Thus we conclude that the Na^+ ion patterning has a decisive role in the transpot and magnatic properties.


### DataBace ###
nature Vol.445 567-682 Issue no.7128 8 Febuary 2007
Letter p.631 / Patteerning of sodium ions and the control of electrons in sodium bobaltate / M Roger et al. (CEA Saclay)

posted by 0≠素子 at 04:16| Comment(0) | matter physics | このブログの読者になる | 更新情報をチェックする

STRUCTURL BIOLOGY : Molecular mecanichnery in action

If biology were a car, structural biologists would be looking under the bonnet to find out how the engine works.

Put more prosaically, structural biology aims to understand how biology works at the molecular level.

Much information is gleaned by studying at atomic resolution the three-dimensional structurs of molecules that make up living organisms, and the interactions of these molecules with one another.


### DataBace ###
nature Vol.445 567-682 Issue no.7128 8 Febuary 2007
News and Views p.609 / STRUCTURL BIOLOGY : Molecular mecanichnery in action / Ad Bax and Dennis A. Torchia
Articles p.618 / Quantitative dynamics and binding studies of the 20S proteasome by NMR / R Sprangers & L E Kay (The Universuty of Tronto)


続きを読む
posted by 0≠素子 at 00:10| Comment(0) | sturacuture - biology | このブログの読者になる | 更新情報をチェックする

2009年03月05日

Earthquaks : the endgame

Once on Earthquake has been initiated the question arises, where will it end ? Hou big an earthque will it be? Analysis of the mapped surface rupture trances of 22 historical strike-slip earthquaks, the first at San Andreas, California in 1857 and the moust reccent at Denali, Alaska in 2002, Shows that rupture endpoints frequenty coincide with fault steps or the termini of active fault traces, and that a fault step of more than 3 or 4 km effectively stops an earthquake rupture in tracks.

These findings are relevant to seismic hazard analysis by suggesting limits on the probable extent of future earthquaks on known active faults.


### DataBace ###
nature Vol.444 243-400 Issue no.7117 16 November 2007
News and Views p.276 / SEISMOLOGY : Greatness Thrust upon then / James F. Doldan
Letter p.358 / Predicting the endpoints of earthquake rupturs / Steven G. Wesnousky (University of Nevada)

続きを読む
posted by 0≠素子 at 10:07| Comment(0) | quantum information | このブログの読者になる | 更新情報をチェックする

Pandemic potential

The fact that the H5N1 bird flu virus circulating in Asia, Europe and Africa is unable to attach to fuman-type cell receptors has heleped to prevent it from causing world-wide epidemic of a human viriant of the disease.

Now a study of H5N1 isolates from some of the new humans that have been identified two mutations in a virlhaemagglutinin that allow it toboth human and avain receptors.

These mutations might be of use as molecular markers for assesing the pandemic potential of H5N1.


### DataBace ###
nature Vol.444 243-400 Issue no.7117 16 November 2007
Letter p.378 / Haemagglutinin mutations responsible for the bunding of H5N1 influenza A Viruses to human-type receptors / S Yamada (University of Tokyo, Japan Science and Technology Agency)/ www.nature.com/podcast


posted by 0≠素子 at 09:39| Comment(1) | diary | このブログの読者になる | 更新情報をチェックする
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