主页 > 医药生命 >

【报导】A plain-text copy of this week in Science (August

This Week in SCIENCE
August 5 2005, 309 (5736)

THIS WEEK IN SCIENCE
--------------------

---------------------------------------------------------------------------
Squeezed Silica
---------------------------------------------------------------------------

The existence of a high-pressure form of silica with the pyrite structure
has long been speculated. Kuwayama et al. (p. 923) report experimental
evidence of a new high-pressure polymorph of SiO2 with a structure that
matches the theoretical predictions. Although it is unlikely that this
polymorph plays a role in the core of the Earth, this structure has
implications for the existence of SiO2 in the deep planetary interiors of
gas giants such as Uranus and Neptune.

CREDIT: KUWAYAMA ET AL

---------------------------------------------------------------------------
A Forensic Analysis
---------------------------------------------------------------------------

The legal system frequently faces situations in which scientifically valid
data would help determine the outcome of the case. Saks and Koehler (p.
892) review the state of forensic science and find it to be in transition.
Some areas, such as DNA fingerprinting, are increasingly well grounded in
scientific principles, whereas other areas are more subjective. The authors
discuss the various sources of error and offer some proposals for improving
the rigor of forensic science.

---------------------------------------------------------------------------
Flex and Rise
---------------------------------------------------------------------------

Earth models that have attempted to simulate the sea-level rise from ice
sheet melting after the Last Glacial Maximum have failed to reproduce the
changes recorded at the so-called "far-field" sites, such as Tahiti and the
Sunda Shelf. Bassett et al. (p. 925, published online 23 June 2005) have
used a model that combines a high-viscosity lower mantle and a significant
contribution from the Antarctic ice sheet to meltwater formation. The
reconstructed record and the data agree well, and these results also
provide another line of evidence that Antarctic ice was responsible for
more of the deglacial sea-level rise than was thought until recently.

---------------------------------------------------------------------------
Softer at the Edges
---------------------------------------------------------------------------

Metals become harder as grain sizes decrease, but at some point the grains
become so small that the deformation mechanisms change. Nanostructured
ceramics also show enhanced properties relative to their coarser-grained
counterparts, but do similar changes in deformation mechanisms occur in
these more brittle materials? Szlufarska et al. (p. 911) show that these
ceramics can be thought of as composites of hard nanoscale grains bounded
by softer, amorphous-like grain boundaries. A massive molecular dynamics
simulation shows that nanoindentation of a nanostructured silicon carbide
goes through four deformation regimes. The deformation changes from
cooperative grain sliding to a process dominated by amorphization of the
crystalline grains.

---------------------------------------------------------------------------
Patterns of Stress
---------------------------------------------------------------------------

During the fabrication of nanoparticles consisting of a silver core
surrounded by a silica shell, Li et al. (p. 909) found that controlling the
cooling rate could induce stresses in the silica that cause it to form a
dimpled pattern on the core sphere. The silica bumps take on either a
triangular or Fibonacci sequence pattern that minimizes the total strain
energy. These patterns are highly reminiscent of those seen in the
development of flowers and plants.

CREDIT: LI ET AL.

---------------------------------------------------------------------------
A Little Light Work
---------------------------------------------------------------------------

Light-driven structural changes in proteins that are required for function
are likely the result of photoexcitation processes redistributing charges.
However, measuring changes in charge distribution on the time scale of the
structural changes is challenging. Schenkl et al. (p. 917) have used Trp
residues that are close to the retinal-binding pocket in bacteriorhodopsin
to probe electric field changes. From the observed changes in Trp
absorbance, they calculate that the retinal dipole moment increases during
the first 200 femtoseconds after excitation. This change in charge
distribution precedes, and likely drives, isomerization.

---------------------------------------------------------------------------
Eukaryotic Potassium Channel Structure
---------------------------------------------------------------------------

阅读本文的人还阅读:

survivin、CD34、PCNA在大肠

【热评】Science痛批中国

【Fast Breaking Comments】G

【drug-news】新的家族性

【Science】2011年3月4日最

作者:admin@医学,生命科学    2011-03-14 17:14
医学,生命科学网