Carbon dioxide activation by a uranium(III) complex derived from a chelating bis(aryloxide) ligand

The new dianionic ligand, C6H4{p-C(CH3)2C6H2Me2O−}2 (= p-Me2bp), featuring two aryloxide donors and a central arene ring, has been synthesized, and used to prepare the mixed-ligand U(III) compound, [U(Cp*)(p-Me2bp)] which exhibits an η6-interaction with the uranium center. Reductive activation of CO2 was investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave a dinuclear uranium carbonate complex,{U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO2 showed the formation of a rare U(IV) oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the latter two products is temperature dependent: at low temperatures (-78 ˚C) oxalate formation is favored, whilst at room temperature the carbonate is the dominant product. The U(IV) iodide, [U(Cp*)(p-Me2bp)I], was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIII redox couple of −2.18 V vs FeCp2+/0 as well as an possible electrochemically accessible UIII/UII reduction process at −2.56 V vs FeCp2+/0

State based model of long-term potentiation and synaptic tagging and capture

Recent data indicate that plasticity protocols have not only synapse-specific but also more widespread effects. In particular, in synaptic tagging and capture (STC), tagged synapses can capture plasticity-related proteins, synthesized in response to strong stimulation of other synapses. This leads to long-lasting modification of only weakly stimulated synapses. Here we present a biophysical model of synaptic plasticity in the hippocampus that incorporates several key results from experiments on STC. The model specifies a set of physical states in which a synapse can exist, together with transition rates that are affected by high- and low-frequency stimulation protocols. In contrast to most standard plasticity models, the model exhibits both early- and late-phase LTP/D, de-potentiation, and STC. As such, it provides a useful starting point for further theoretical work on the role of STC in learning and memory

Interstitial Fractionalization and Spherical Crystallography

Finding the ground states of identical particles packed on spheres has relevance for stabilizing emulsions and a venerable history in the literature of theoretical physics and mathematics. Theory and experiment have confirmed that defects such as disclinations and dislocations are an intrinsic part of the ground state. Here we discuss the remarkable behavior of vacancies and interstitials in spherical crystals. The strain fields of isolated disclinations forced in by the spherical topology literally rip interstitials and vacancies apart, typically into dislocation fragments that combine with the disclinations to create small grain boundary scars. The fractionation is often into three charge-neutral dislocations, although dislocation pairs can be created as well. We use a powerful, freely available computer program to explore interstitial fractionalization in some detail, for a variety of power law pair potentials. We investigate the dependence on initial conditions and the final state energies, and compare the position dependence of interstitial energies with the predictions of continuum elastic theory on the sphere. The theory predicts that, before fragmentation, interstitials are repelled from 5-fold disclinations and vacancies are attracted. We also use vacancies and interstitials to study low energy states in the vicinity of "magic numbers" that accommodate regular icosadeltahedral tessellations.Comment: 21 pages, 9 figure

Analysis of Electrodeposited Nickel-Iron Alloy Film Composition Using Particle-Induced X-Ray Emission

The elemental composition of electrodeposited NiFe thin films was analyzed with particle-induced X-ray emission (PIXE). The thin films were electrodeposited on polycrystalline Au substrates from a 100mM NiSO4, 10 mM FeSO4, 0.5M H3BO3, and 1M Na2SO4 solution. PIXE spectra of these films were analyzed to obtain relative amounts of Ni and Fe as a function of deposition potential and deposition time. The results show that PIXE can measure the total deposited metal in a sample over at least four orders of magnitude with similar fractional uncertainties. The technique is also sensitive enough to observe the variations in alloy composition due to sample nonuniformity or variations in deposition parameters

Talking Through the Problems: A Study of Discourse in Peer-Led Small Groups

Increasingly, studies are investigating the factors that influence student discourse in science courses, and specifically the mechanisms and discourse processes within small groups, to better understand the learning that takes place as students work together. This paper contributes to a growing body of research by analyzing how students engage in conversation and work together to solve problems in a peer-led small-group setting. This qualitative study evaluates video of Peer-Led Team Learning (PLTL) sessions in general chemistry, with attention to both the activity structures and the function of discourse as students undertook different types of problems across one semester. Our findings suggest that students talk their way through the problems; practicing a combination of regulative and instructional language to manage the group dynamics of their community of peer learners while developing and using specific disciplinary vocabulary. Additionally, student discourse patterns revealed a focus on the process of complex problem-solving, where students engage in joint decision-making by taking turns, questioning and explaining, and building on one another’s ideas. While students in our study engaged in less of the deeper, meaning-making discourse than expected, these observations about the function of language in small-group learning deepens an understanding of how PLTL and other types of small-group learning based on the tenets of social constructivism may lead to improvements in science education, with implications for the structure of small-group learning environments, problem design, and training of peer group leaders to encourage students to engage in more of the most effective discourse in these learning contexts

Statistical Mechanics of Semiflexible Bundles of Wormlike Polymer Chains

We demonstrate that a semiflexible bundle of wormlike chains exhibits a state-dependent bending stiffness that alters fundamentally its scaling behavior with respect to the standard wormlike chain. We explore the equilibrium conformational and mechanical behavior of wormlike bundles in isolation, in crosslinked networks, and in solution.Comment: 4 pages, 3 figures, final version as publishe

VennVax, a DNA-prime, peptide-boost multi-T-cell epitope poxvirus vaccine, induces protective immunity against vaccinia infection by T cell response alone

The potential for smallpox to be disseminated in a bioterror attack has prompted development of new, safer smallpox vaccination strategies. We designed and evaluated immunogenicity and efficacy of a T-cell epitope vaccine based on conserved and antigenic vaccinia/variola sequences, identified using bioinformatics and immunological methods. Vaccination in HLA transgenic mice using a DNA-prime/peptide-boost strategy elicited significant T cell responses to multiple epitopes. No antibody response pre-challenge was observed, neither against whole vaccinia antigens nor vaccine epitope peptides. Remarkably, 100% of vaccinated mice survived lethal vaccinia challenge, demonstrating that protective immunity to vaccinia does not require B cell priming

Near-Earth initiation of a terrestrial substorm

Despite the characterization of the auroral substorm more than 40 years ago, controversy still surrounds the processes triggering substorm onset initiation. That stretching of the Earth's magnetotail following the addition of new nightside magnetic flux from dayside reconnection powers the substorm is well understood; the trigger for explosive energy release at substorm expansion phase onset is not. Using ground-based data sets with unprecedented combined spatial and temporal coverage, we report the discovery of new localized and contemporaneous magnetic wave and small azimuthal scale auroral signature of substorm onset. These local auroral arc undulations and magnetic field signatures rapidly evolve on second time scales for several minutes in advance of the release of the auroral surge. We also present evidence from a conjugate geosynchronous satellite of the concurrent magnetic onset in space as the onset of magnetic pulsations in the ionosphere, to within technique error. Throughout this time period, the more poleward arcs that correspond to the auroral oval which maps to the central plasma sheet remain undisturbed. There is good evidence that flows from the midtail crossing the plasma sheet can generate north-south auroral structures, yet no such auroral forms are seen in this event. Our observations present a severe challenge to the standard hypothesis that magnetic reconnection in stretched magnetotail fields triggers onset, indicating substorm expansion phase initiation occurs on field lines that are close to the Earth, as bounded by observations at geosynchronous orbit and in the conjugate ionosphere

Trends in Causes and Distribution, and Effects of Whitebark Pine Decline on Grizzly Bear Mortality in the Greater Yellowstone Ecosystem

Documented grizzly bear (Ursus arctos) mortalities have been increasing in recent years in the Greater Yellowstone Ecosystem (GYE), due, in part, to increases in bear numbers and range expansion. Previous research has documented that variable seed production of whitebark pine (WBP; Pinus albicaulis), an important fall food, is inversely related to grizzly bear fall mortality.  However, WBP has experienced widespread mortality during the last decade because of mountain pine beetle (Dendroctonus ponderosae) infestations. We investigated trends in causes and distribution of human-caused mortalities for independent-aged (? 2 yrs old) grizzly bears in the GYE during 1975–2012, and the effect of WBP cone production on numbers of fall (> 1 August) mortalities (n = 172) during the period of WBP decline (2000-2012) using Poisson regression. During 1975–1982, 91 percent of mortalities occurred within the Grizzly Bear Recovery Zone and primary causes were poaching/malicious killings and losses related to conflicts with livestock. During the two most recent decades most mortalities were associated with ungulate hunting, usually involving self-defense kills, or anthropogenic sites, and an increasing percentage of mortalities occurred outside the recovery zone. Using predictor variables of cone production, sex, location in or out of the Recovery Zone, and year suggests: 1) annual cone production was still predictive of human-caused fall mortalities, 2) no evidence of a difference in annual numbers of fall mortalities between males and females, and 3) an increase in annual mortalities over the study period, with most of this increase outside the Recovery Zone