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Argentophilicity-Dependent Colossal Thermal Expansion in Extended Prussian Blue Analogues
No full textArgentophilicity-Dependent Colossal Thermal Expansion in Extended Prussian Blue Analogues ... Representations of the crystal structure of the A3[M(CN)6] extended Prussian blue framework (a) and the contraction along the trigonal axis toward the ideal cubic framework (b) effected by expansion of the Kagome Ag/D lattices (c). ... (12) The contrasting behavior of 3 relative to that of Ag-containing analogues 1 and 2 is a strong indication that some property of the Ag atoms is responsible for the colossal thermal expansion effects. ..
Cranial modularity and sequence heterochrony in mammals
No full textHeterochrony, the temporal shifting of develop- cranial bone ossification sequences of 12 therian mammals. A
dataset of 12--18 developmental events was used to assess if
mental events relative to each other, requires a degree of
modularity in developmental sequences corresponds to six
autonomy among those processes or structures. Modularity, the
phenotypic modules, derived from a recent morphometric
division of larger structures or processes into autonomous sets
analysis of cranial modularity in mammals. Kendall’s t was
of internally integrated units, is often discussed in relation to the
used to measure rank correlations, with randomization tests
concept of heterochrony. However, the relationship between the
for significance. If modularity in developmental sequences
developmental modules derived from studies of heterochrony
corresponds to observed phenotypic modules, bones within
and evolutionary modules, which should be of adaptive
a single phenotypic module should show integration of
importance and relate to the genotype–phenotype map, has
developmental timing, maintaining the same timing of
not been explicitly studied. I analyzed a series of sectioned and
ossification relative to each other, despite differences in
whole cleared-and-stained embryological and neonatal
overall ossification sequences across taxa. Analyses did not
specimens, supplemented with published ontogenetic data, to
find any significant conservation of developmental timing
test the hypothesis that bones within the same phenotypic
within the six phenotypic modules, meaning that bones that are
modules, as determined by morphometric analysis, are
highly integrated in adult morphology are not significantly
developmentally integrated and will display coordinated
integrated in developmental timing.
heterochronic shifts across taxa. Modularity was analyzed i
Origins and assessment of snowball Earth hypotheses
No full textRecords of Precambrian glaciation onwards from the late nineteenth century led to the concept of one or more major ice ages. This concept was becoming well advanced by the mid 1930s, particularly through the compilation of Kulling in 1934. Even so tillite stratigraphy shows that glaciation was exceptional rather than typical of Earth history. Some Proterozoic tillites, sandwiched between warm marine facies, indicate low, even equatorial palaeolatitudes as determined magnetically, and more recently led to ideas of a snow- and ice-covered ‘snowball Earth’. However, interbedded non-glacial facies as well as thick tillite successions requiring abundant snowfall both militate against the hypothesis of extreme prolonged freezing temperatures referred to here as an ‘iceball Earth’ in which all oceans and seas were sealed in continuous ice cover. On the other hand tropical environments were interrupted by glaciation several times in the Proterozoic, something that did not recur in the Phanerozoic. The term ‘snowball Earth’ is consistent with the established view of extremely widespread Proterozoic glaciation, but the ‘iceball Earth’ version of this is not compatible with the geological record
Neutron diffraction of magnetic materials
No full textNeutron diffraction is a powerful tool for studying magnetic materials. Neutrons have a magnetic moment, and are scattered by the magnetic moments of atoms in a sample. The cross section for magnetic scattering is sensitive to the relative orientation of the neutron magnetic moment, the atomic magnetic moment, and the scattering vector. This allows magnetic structures to be determined from the intensities of magnetic diffraction peaks, in much the same way that crystal structures are determined from the intensities of nuclear diffraction peaks (Rodríguez-Carvajal 1993). Small-angle neutron scattering and polarized-neutron reflectometry can yield magnetic information over a range length scales, and can be applied to the study of magnetic nanoparticles, spin glasses, and magnetic multilayers (Arai et al. 1985a,b; Arai and Ishikawa 1985; Mangin et al. 1993; Ott et al. 2004). Inelastic magnetic scattering can be used to probe a range of magnetic excitations, providing quantitative information about the magnetic exchange forces between neighboring spins (Brockhouse 1957; Alperin et al. 1967; Samuelsen 1969; Samuelsen and Shirane 1970; Hansen et al. 1997, 2000; Lefmann et al. 1999, 2001; Klausen et al. 2003, 2004)
Novel Structural Features of CDK Inhibition Revealed by an ab Initio Computational Method Combined with Dynamic Simulations
No full textThe rational development of specific inhibitors for the 500 protein kinases encoded in the human genome
is impeded by a poor understanding of the structural basis for the activity and selectivity of small molecules
that compete for ATP binding. Combining classical dynamic simulations with a novel ab initio computational
approach linear-scalable to molecular interactions involving thousands of atoms, we have investigated the
binding of five distinct inhibitors to the cyclin-dependent kinase CDK2. We report here that polarization
and dynamic hydrogen bonding effects, so far undetected by crystallography, affect both their activity and
selectivity. The effects arise from the specific solvation patterns of water molecules in the ATP binding
pocket or the intermittent formation of hydrogen bonds during the dynamics of CDK/inhibitor interactions
and explain the unexpectedly high potency of certain inhibitors such as 3-(3H-imidazol-4-ylmethylene)-5-
methoxy-1,3-dihydro-indol-2-one (SU9516). The Lys89 residue in the ATP-binding pocket of CDK2 is
observed to form temporary hydrogen bonds with the three most potent inhibitors. This residue is replaced
in CDK4 by Thr89, whose shorter side-chain cannot form similar bonds, explaining the relative selectivity
of the inhibitors for CDK2. Our results provide a generally applicable computational method for the analysis
of biomolecular structures and reveal hitherto unrecognized features of the interaction between protein kinases
and their inhibitors
Into the field again: re-examining Charles Darwin's 1835 geological work on Isla Santiago (James Island) in the Galapagos archipelago
No full textIn 1835 Charles Darwin’s geological observations on Isla Santiago (James Island) in the Galápagos Islands led him to important insights as to the process by which different varieties of igneous rock might be produced from the same volcanic vent. His work figured in a tradition of
interpretation that began with the work of George Poulett Scrope and would end in the twentieth century with the theory of magmatic differentiation of igneous rocks through the process of crystal fractionation. This article reports on the findings of an expedition to Isla Santiago in July
2007 during which we were able to locate samples of igneous rocks similar to those collected by Darwin. We have used these, together with Darwin’s original specimens and transcriptions of his field notes, to examine how his
understanding of the separation of the trachytic and basaltic series of magmas developed from his initial field observations through to publication of Volcanic Islands in 1844
An 85-ka record of climate change in lowland Central America
No full textDrill cores obtained from Lake Petén Itzá, Petén, Guatemala, contain a not, vert, similar85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. Sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions, respectively. From not, vert, similar85 to 48 ka, sediments were dominated by carbonate clay indicating moist conditions during Marine Isotope Stages (MIS) 5a, 4, and early 3. The first gypsum layer was deposited at not, vert, similar48 ka, signifying a shift toward drier hydrologic conditions and the onset of wet–dry oscillations. During the latter part of MIS 3, Petén climate varied between wetter conditions during interstadials and drier states during stadials. The pattern of clay–gypsum (wet–dry) oscillations during the latter part of MIS 3 (not, vert, similar48–23 ka) closely resembles the temperature records from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. The most arid periods coincided with Heinrich Events when cold sea surface temperatures prevailed in the North Atlantic, meridional overturning circulation was reduced, and the Intertropical Convergence Zone (ITCZ) was displaced southward. A thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake, and pollen accumulated during the same period indicates vegetation consisted of a temperate pine-oak forest. This finding contradicts previous inferences that climate was arid during the Last Glacial Maximum (LGM) chronozone (21±2 ka). At not, vert, similar18 ka, Petén climate switched from moist to arid conditions and remained dry from 18 to 14.7 ka during the early deglaciation. Moister conditions prevailed during the warmer Bolling–Allerod (14.7–12.8 ka) with the exception of a brief return to dry conditions at not, vert, similar13.8 ka that coincides with the Older Dryas and meltwater pulse 1A. The onset of the Younger Dryas at 12.8 ka marked the return of gypsum and hence dry conditions. The lake continued to precipitate gypsum until not, vert, similar10.3 ka when rainfall increased markedly in the early Holocene
On the thermal buffering of naturally ventilated buildings through internal thermal mass
No full textIn this paper we examine the role of thermal mass in buffering the interior temperature of a naturally ventilated building from the diurnal fluctuations in the environment. First, we show that the effective thermal mass which is in good thermal contact with the air is limited by the diffusion distance into the thermal mass over one diurnal temperature cycle. We also show that this effective thermal mass may be modelled as an isothermal mass. Temperature fluctuations in the effective thermal mass are attenuated and phase-shifted from those of the interior air, and therefore heat is exchanged with the interior air. The evolution of the interior air temperature is then controlled by the relative magnitudes of (i) the time for the heat exchange between the effective thermal mass and the air; (ii) the time for the natural ventilation to replace the air in the space with air from the environment; and (iii) the period of the diurnal oscillations of the environment. Through analysis and numerical solution of the governing equations, we characterize a number of different limiting cases. If the ventilation rate is very small, then the thermal mass buffers the interior air temperature from fluctuations in the environment, creating a near-isothermal interior. If the ventilation rate increases, so that there are many air changes over the course of a day, but if there is little heat exchange between the thermal mass and interior air, then the interior air temperature locks on to the environment temperature. If there is rapid thermal equilibration of the thermal mass and interior air, and a high ventilation rate, then both the thermal mass and the interior air temperatures lock on to the environment temperature. However, in many buildings, the more usual case is that in which the time for thermal equilibration is comparable to the period of diurnal fluctuations, and in which ventilation rates are moderate. In this case, the fluctuations of the temperature of the thermal mass lag those of the interior air, which in turn lag those of the environment. We consider the implications of these results for the use of thermal mass in naturally ventilated buildings
Melt-solid dihedral angles of common minerals in natural rocks.
No full textThe melt–solid dihedral angle has been measured in a range of
igneous rock types, ranging in composition from picrite, through
basalt, phonolite, andesite and rhyolite, for the minerals quartz,
leucite, plagioclase, olivine, amphibole and clinopyroxene. Populations
of up to 104 true 3-D angles were measured in each sample
using a universal stage mounted on an optical microscope. The
median and standard deviation of the angle populations for each
mineral are distinct (plagioclase 25, with standard deviation
(SD) 11; clinopyroxene 38, with SD 14; olivine 29, with
SD 13; quartz 18, with SD 9; leucite 20, with SD 11),
with no control by either melt composition or degree of approach of
the grains to their equilibrium shapes
Probing the ferroelectric phase transition through Raman spectroscopy in Pb(Fe~2~/~3W~1~/~3)~1~/~2Ti~1~/~2O~3 thin films (3 pages)
No full textThe present work investigates the evolution of micro-Raman spectra of (1−x)Pb(Fe2/3W1/3)O3−xPbTiO3 (PFWT) (x=0.50) thin films in the temperature range from 80 to 600 K. Raman and dielectric data indicate that the crystal structure changes from tetragonal to cubic, i.e., a ferroelectric phase transition at 575 K. The dielectric properties of PFWT thin films were studied in the temperature range of 80–600 K over a wide range of frequencies. The slope of the reciprocal of the dielectric constant is 2:1, matched well with the simplest Landau free energy model, and it indicates a continuous second order displacive ferroelectric phase transition. ©2007 American Institute of Physic