American Association for the Advancement of Science (AAAS)
0036-8075
1095-9203
Cơ quản chủ quản: AMER ASSOC ADVANCEMENT SCIENCE , American Association for the Advancement of Science
Lĩnh vực:
History and Philosophy of ScienceMultidisciplinary
Các bài báo tiêu biểu
Pathways to a Protein Folding Intermediate Observed in a 1-Microsecond Simulation in Aqueous Solution An implementation of classical molecular dynamics on parallel computers of increased efficiency has enabled a simulation of protein folding with explicit representation of water for 1 microsecond, about two orders of magnitude longer than the longest simulation of a protein in water reported to date. Starting with an unfolded state of villin headpiece subdomain, hydrophobic collapse and helix formation occur in an initial phase, followed by conformational readjustments. A marginally stable state, which has a lifetime of about 150 nanoseconds, a favorable solvation free energy, and shows significant resemblance to the native structure, is observed; two pathways to this state have been found.
Tập 282 Số 5389 - Trang 740-744 - 1998
Remobilization in the Cratonic Lithosphere Recorded in Polycrystalline Diamond Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere.
Tập 289 Số 5482 - Trang 1182-1185 - 2000
Water in Earth's Mantle: The Role of Nominally Anhydrous Minerals
Most minerals of Earth's upper mantle contain small amounts of hydrogen, structurally bound as hydroxyl (OH). The OH concentration in each mineral species is variable, in some cases reflecting the geological environment of mineral formation. Of the major mantle minerals, pyroxenes are the most hydrous, typically containing ∼200 to 500 parts per million H
2
O by weight, and probably dominate the water budget and hydrogen geochemistry of mantle rocks that do not contain a hydrous phase. Garnets and olivines commonly contain ∼1 to 50 parts per million. Nominally anhydrous minerals constitute a significant reservoir for mantle hydrogen, possibly accommodating all water in the depleted mantle and providing a possible mechanism to recycle water from Earth's surface into the deep mantle.
Tập 255 Số 5050 - Trang 1391-1397 - 1992
Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO <sub>2</sub> Upset the Current Balance?
During the last glacial period, Earth's climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean's thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO
2
levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people.
Tập 278 Số 5343 - Trang 1582-1588 - 1997
Uranium-Thorium-Lead Isotope Relations in Lunar Materials
The lead isotopic compositions and uranium, thorium, and lead concentrations have been measured on six samples of material from the Sea of Tranquillity. The leads are moderately to very radiogenic; the initial lead concentrations are very low; the uranium and thorium levels are 0.26 to 0.88 and 0.87 to 3.35 parts per million, respectively. The Th/U ratios cluster about a 3.6 value. Apparent ages calculated for four rocks are 4.1 to 4.2 × 10
9
years. Dust and breccia yield apparent ages of 4.60 to 4.63 × 10
9
years. The uranium-lead ages are concordant, or nearly so, in all cases. The lunar surface is an ancient region with an extended record of events in the early history of the solar system. The discrepancy between the rock ages and dust ages poses a fundamental question about rock genesis on the moon.
Tập 167 Số 3918 - Trang 468-471 - 1970
Chemical Analysis of the Moon at the Surveyor VII Landing Site: Preliminary Results The alpha-scattering experiment aboard Surveyor VII has provided a chemical analysis of the moon in the area of the crater Tycho. The preliminary results indicate a chemical composition similar to that already found at two mare sites, but with a lower concentration of elements of the iron group (titanium through copper).
Tập 162 Số 3849 - Trang 117-118 - 1968
Ages, Irradiation History, and Chemical Composition of Lunar Rocks from the Sea of Tranquillity
The
87
Rb-
87
Sr internal isochrons for five rocks yield an age of 3.65 ± 0.05 × 10
9
years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 × 10
9
years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 × 10
9
years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable
131
Xe/
126
Xe ratio. No evidence for
129
I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.
Tập 167 Số 3918 - Trang 463-466 - 1970
Age of the Moon: An Isotopic Study of Uranium-Thorium-Lead Systematics of Lunar Samples
Concentrations of U, Th, and Pb in Apollo 11 samples studied are low (U. 0.16 to 0.87; Th, 0.53 to 3.4; Pb, 0.29 to 1.7, in ppm) but the extremely radiogenic lead in samples allows radiometric dating. The fine dust and the breccia have a concordant age of 4.66 billion years on the basis of
207
Pb/
206
Pb,
206
Pb/
238
U,
207
Pb/
235
U, and
208
Pb/
232
Th ratios. This age is comparable with the age of meteorites and with the age generally accepted for the earth. Six crystalline and vesicular samples are distinctly younger than the dust and breccia. The
238
U/
235
U ratio is the same as that in earth rocks, and
234
U is in radioactive equilibrium with parent
238
U.
Tập 167 Số 3918 - Trang 461-463 - 1970
Nuclear Activity of MLA Immune Receptors Links Isolate-Specific and Basal Disease-Resistance Responses Plant immune responses are triggered by pattern recognition receptors that detect conserved pathogen-associated molecular patterns (PAMPs) or by resistance (R) proteins recognizing isolate-specific pathogen effectors. We show that in barley, intracellular mildew A (MLA) R proteins function in the nucleus to confer resistance against the powdery mildew fungus. Recognition of the fungal avirulence A10 effector by MLA10 induces nuclear associations between receptor and WRKY transcription factors. The identified WRKY proteins act as repressors of PAMP-triggered basal defense. MLA appears to interfere with the WRKY repressor function, thereby de-repressing PAMP-triggered basal defense. Our findings reveal a mechanism by which these polymorphic immune receptors integrate distinct pathogen signals.
Tập 315 Số 5815 - Trang 1098-1103 - 2007