77 resultados para 100 kyr tuning


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We examine the self-assembly of a peptide A6H comprising a hexa-alanine sequence A6 with a histidine (H) “head group”, which chelates Zn2+ cations. We study the self assembly of A6H and binding of Zn2+ ions in ZnCl2 solutions, under acidic and neutral conditions. A6H self-assembles into nanotapes held together by a β-sheet structure in acidic aqueous solutions. By dissolving A6H in acidic ZnCl2 solutions, the carbonyl oxygen atoms in A6H chelate the Zn2+ ions and allow for β-sheet formation at lower concentrations, consequently reducing the onset concentration for nanotape formation. A6H mixed with water or ZnCl2 solutions under neutral conditions produces short sheets or pseudocrystalline tapes, respectively. The imidazole ring of A6H chelates Zn2+ ions in neutral solutions. The internal structure of nanosheets and pseudocrystalline sheets in neutral solutions is similar to the internal structure of A6H nanotapes in acidic solutions. Our results show that it is possible to induce dramatic changes in the self-assembly and chelation sites of A6H by changing the pH of the solution. However, it is likely that the amphiphilic nature of A6H determines the internal structure of the self-assembled aggregates independent from changes in chelation.

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The Antarctic Peninsula region is currently undergoing rapid environmental change, resulting in the thinning, acceleration and recession of glaciers and the sequential collapse of ice shelves. It is important to view these changes in the context of long-term palaeoenvironmental complexity and to understand the key processes controlling ice sheet growth and recession. In addition, numerical ice sheet models require detailed geological data for tuning and testing. Therefore, this paper systematically and holistically reviews published geological evidence for Antarctic Peninsula Ice Sheet variability for each key locality throughout the Cenozoic, and brings together the prevailing consensus of the extent, character and behaviour of the glaciations of the Antarctic Peninsula region. Major contributions include a downloadable database of 186 terrestrial and marine calibrated dates; an original reconstruction of the LGM ice sheet; and a new series of isochrones detailing ice sheet retreat following the LGM. Glaciation of Antarctica was initiated around the Eocene/Oligocene transition in East Antarctica. Palaeogene records of Antarctic Peninsula glaciation are primarily restricted to King George Island, where glacigenic sediments provide a record of early East Antarctic glaciations, but with modification of far-travelled erratics by local South Shetland Island ice caps. Evidence for Neogene glaciation is derived primarily from King George Island and James Ross Island, where glaciovolcanic strata indicate that ice thicknesses reached 500–850 m during glacials. This suggests that the Antarctic Peninsula Ice Sheet draped, rather than drowned, the topography. Marine geophysical investigations indicate multiple ice sheet advances during this time. Seismic profiling of continental shelf-slope deposits indicates up to ten large advances of the Antarctic Peninsula Ice Sheet during the Early Pleistocene, when the ice sheet was dominated by 40 kyr cycles. Glacials became more pronounced, reaching the continental shelf edge, and of longer duration during the Middle Pleistocene. During the Late Pleistocene, repeated glacials reached the shelf edge, but ice shelves inhibited iceberg rafting. The Last Glacial Maximum (LGM) occurred at 18 ka BP, after which transitional glaciomarine sediments on the continental shelf indicate ice-sheet retreat. The continental shelf contains large bathymetric troughs, which were repeatedly occupied by large ice streams during Pleistocene glaciations. Retreat after the LGM was episodic in the Weddell Sea, with multiple readvances and changes in ice-flow direction, but rapid in the Bellingshausen Sea. The late Holocene Epoch was characterised by repeated fluctuations in palaeoenvironmental conditions, with associated glacial readvances. However, this has been subsumed by rapid warming and ice-shelf collapse during the twentieth century.

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The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditions in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia suggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, and in the northern and southern extratropics. The small differences among models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflect differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.

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The enzymatic cleavage of a peptide amphiphile (PA) is investigated. The self-assembly of the cleaved products is distinct from that of the PA substrate. The PA C16-KKFFVLK is cleaved by α-chymotrypsin at two sites leading to products C16-KKF with FVLK and C16-KKFF with VLK. The PA C16-KKFFVLK forms nanotubes and helical ribbons at room temperature. Both PAs C16-KKF and C16-KKFF corresponding to cleavage products instead self-assemble into 5-6 nm diameter spherical micelles, while peptides FVLK and VLK do not adopt well-defined aggregate structures. The secondary structures of the PAs and peptides are examined by FTIR and circular dichroism spectroscopy and X-ray diffraction. Only C16-KKFFVLK shows substantial β-sheet secondary structure, consistent with its self-assembly into extended aggregates, based on PA layers containing hydrogen-bonded peptide headgroups. This PA also exhibits a thermoreversible transition to twisted tapes on heating.

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The solar wind is an extended ionized gas of very high electrical conductivity, and therefore drags some magnetic flux out of the Sun to fill the heliosphere with a weak interplanetary magnetic field(1,2). Magnetic reconnection-the merging of oppositely directed magnetic fields-between the interplanetary field and the Earth's magnetic field allows energy from the solar wind to enter the near-Earth environment. The Sun's properties, such as its luminosity, are related to its magnetic field, although the connections are still not well understood(3,4). Moreover, changes in the heliospheric magnetic field have been linked with changes in total cloud cover over the Earth, which may influence global climate(5), Here we show that measurements of the near-Earth interplanetary magnetic field reveal that the total magnetic flux leaving the Sun has risen by a factor of 1.4 since 1964: surrogate measurements of the interplanetary magnetic field indicate that the increase since 1901 has been by a factor of 2,3, This increase may be related to chaotic changes in the dynamo that generates the solar magnetic field. We do not yet know quantitatively how such changes will influence the global environment.

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The state-resolved reactivity of CH4 in its totally symmetric C-H stretch vibration (�1) has been measured on a Ni(100) surface. Methane molecules were accelerated to kinetic energies of 49 and 63:5 kJ=mol in a molecular beam and vibrationally excited to �1 by stimulated Raman pumping before surface impact at normal incidence. The reactivity of the symmetric-stretch excited CH4 is about an order of magnitude higher than that of methane excited to the antisymmetric stretch (�3) reported by Juurlink et al. [Phys. Rev. Lett. 83, 868 (1999)] and is similar to that we have previously observed for the excitation of the first overtone (2�3). The difference between the state-resolved reactivity for �1 and �3 is consistent with predictions of a vibrationally adiabatic model of the methane reaction dynamics and indicates that statistical models cannot correctly describe the chemisorption of CH4 on nickel.

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We have investigated methane (CH4) dissociative chemisorption on the Ni{100} surface by first-principles molecular dynamics (MD) simulations. Our results show that this reaction is mode-specific, with the n1 state being the most strongly coupled to efficient energy flow into the reaction coordinate when the molecule reaches the transition state. By performing MD simulations for two different transition state (TS) structures we provide evidence of TS structure-specific energy redistribution in methane chemisorption. Our results are compared with recently reported state-resolved measurement of methane adsorption probability on nickel surfaces, and we find that a strong correlation exists between the highest vibrational efficacy measured on Ni{100} for the n1 state and the calculated highest fractional vibrational energy content in this mode.

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Although reviews of the association between polyphenol intake and cognition exist, research examining the cognitive effects of fruit, vegetable, and juice consumption across epidemiological and intervention studies has not been previously examined. Critical inclusion criteria were human participants, a measure of fruit, vegetable, or 100% juice consumption, an objective measure of cognitive function, and a clinical diagnosis of neuropsychological disease. Studies were excluded if consumption of fruit, vegetables, or juice was not assessed in isolation from other foods groups, or if there was no statistical control for education or IQ. Seventeen of 19 epidemiological studies and 3 of 6 intervention studies reported significant benefits of fruit, vegetable, or juice consumption for cognitive performance. The data suggest that chronic consumption of fruits, vegetables, and juices is beneficial for cognition in healthy older adults. The limited data from acute interventions indicate that consumption of fruit juices can have immediate benefits for memory function in adults with mild cognitive impairment; however, as of yet, acute benefits have not been observed in healthy adults. Conclusions regarding an optimum dietary intake for fruits, vegetables, and juices are difficult to quantify because of substantial heterogeneity in the categorization of consumption of these foods.

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Four alkyl substituted β-lactones were investigated as monomers in ring opening polymerisation to produce a family of poly(3-hydroxyalkanoate)s. Homopolymers were synthesised using a robust aluminium salen catalyst, resulting in polymers with low dispersity (Đ < 1.1) and predictable molecular weights. ABA triblock copolymers were prepared using poly(L-lactic acid) as the A block and the aforementioned poly(3-hydroxyalkanoate) as the B block via a sequential addition method. Characterisation of these copolymers determined they were well controlled with low dispersities and predictable molecular weight. DSC analysis determined copolymers prepared from β-butyrolactone or β-valerolactone yielded polymers with tunable and predictable thermal properties. Copolymers prepared from β-heptanolactone yielded a microphase separated material as indicated by SAXS, with two distinct Tgs. The polymers could be readily cast into flexible films and their improved tensile properties were explored.

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A new global synthesis and biomization of long (>40 kyr) pollen-data records is presented, and used with simulations from the HadCM3 and FAMOUS climate models to analyse the dynamics of the global terrestrial biosphere and carbon storage over the last glacial–interglacial cycle. Global modelled (BIOME4) biome distributions over time generally agree well with those inferred from pollen data. The two climate models show good agreement in global net primary productivity (NPP). NPP is strongly influenced by atmospheric carbon dioxide (CO2) concentrations through CO2 fertilization. The combined effects of modelled changes in vegetation and (via a simple model) soil carbon result in a global terrestrial carbon storage at the Last Glacial Maximum that is 210–470 Pg C less than in pre-industrial time. Without the contribution from exposed glacial continental shelves the reduction would be larger, 330–960 Pg C. Other intervals of low terrestrial carbon storage include stadial intervals at 108 and 85 ka BP, and between 60 and 65 ka BP during Marine Isotope Stage 4. Terrestrial carbon storage, determined by the balance of global NPP and decomposition, influences the stable carbon isotope composition (δ13C) of seawater because terrestrial organic carbon is depleted in 13C. Using a simple carbon-isotope mass balance equation we find agreement in trends between modelled ocean δ13C based on modelled land carbon storage, and palaeo-archives of ocean δ13C, confirming that terrestrial carbon storage variations may be important drivers of ocean δ13C changes.

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Approximate Bayesian computation (ABC) is a popular family of algorithms which perform approximate parameter inference when numerical evaluation of the likelihood function is not possible but data can be simulated from the model. They return a sample of parameter values which produce simulations close to the observed dataset. A standard approach is to reduce the simulated and observed datasets to vectors of summary statistics and accept when the difference between these is below a specified threshold. ABC can also be adapted to perform model choice. In this article, we present a new software package for R, abctools which provides methods for tuning ABC algorithms. This includes recent dimension reduction algorithms to tune the choice of summary statistics, and coverage methods to tune the choice of threshold. We provide several illustrations of these routines on applications taken from the ABC literature.

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A new global synthesis and biomization of long (> 40 kyr) pollen-data records is presented and used with sim- ulations from the HadCM3 and FAMOUS climate models and the BIOME4 vegetation model to analyse the dynamics of the global terrestrial biosphere and carbon storage over the last glacial–interglacial cycle. Simulated biome distribu- tions using BIOME4 driven by HadCM3 and FAMOUS at the global scale over time generally agree well with those in- ferred from pollen data. Global average areas of grassland and dry shrubland, desert, and tundra biomes show large- scale increases during the Last Glacial Maximum, between ca. 64 and 74 ka BP and cool substages of Marine Isotope Stage 5, at the expense of the tropical forest, warm-temperate forest, and temperate forest biomes. These changes are re- flected in BIOME4 simulations of global net primary pro- ductivity, showing good agreement between the two models. Such changes are likely to affect terrestrial carbon storage, which in turn influences the stable carbon isotopic composi- tion of seawater as terrestrial carbon is depleted in 13C.