Mining Uranium from Seawater: A Coordination Chemistry Approach

Poly(acrylamidoxime) fibers are the current state-of-the-art adsorbent for mining uranium from seawater. However, the amidoxime group is not perfectly selective towards the uranyl cation, in particular, competition with transition metal cations remains a major challenge. In order for subsequent generations of chelating polymer adsorbents to be improved, the coordination chemistry of amidoxime-uranyl and -transition metal cation complexes needs to be better understood. While the coordination mode of amidoxime-uranyl complexes has been established in the literature, a number of amidoxime-transition metal cation complex binding motifs can be observed on the Cambridge Structrural Database. Likewise, the formation constants, or log K values, of a number of essential amidoxime-uranyl and -transition metal cation complexes remain largely unresolved due to the wide range of conflicting acid dissociation constants, or pKa [pKa] values, that have been reported for representative acyclic amidoxime ligands in the literature. Therefore, in Chapter 2 we use spectroscopic titrations to resolve the pKa values of acetamidoxime and benzamidoxime. Subsequently, we use those pKa values to develop computational protocols for predicting the pKa values of aqueous oxoacid ligands. In Chapters 3 and 4, we computationally investigate the binding motif of formamidoximate-dioxovanadium(V) and –oxovanadium(IV) complexes, major competing ions in seawater by utilizing density functional theory and wave-function methods in conjunction with continuum solvation calculations. Our investigations of these formamidoximate complexes universally identified the most stable binding motif to be a tautomerically rearranged imino hydroxylamine chelate formed via coordination of the imino nitrogen and hydroxylamine oxygen. In Chapter 5, we build on the design principles acquired in Chapters 2-4 to design a ligand, salicylaldoxime that is more selective towards utanyl than competing transition metal cations. Finally, in Chapter 6 we potentiometrically determine the proton affinity distribution of the classical poly(acrylamidoxime) fiber between pH 2 and pH 10 via the Stable Numerical Solution of the Adsorption Integral Equation Using Splines (SAIUS) algorithm. This work lays the foundation for resolving the metal cation affinity distribution of the poly(acrylamidoxime) fibers, which can aid in improving the uranium selectivity of subsequent generations of chelating polymers

Determinants of Survival and Success among Nascent Entrepreneurs: A Cohort Analysis

This paper explores determining factors of startups’ survival rate and subsequent factors that play a role in the growth of those start-ups into established firms. We use data from the Panel Study of Entrepreneurial Dynamics (PSED) of five cohorts of 3,910 nascent ventures across four countries; the United States, Australia, Sweden, and China. Our results indicate that (1) survival within the first year seems mainly determined by select procedural activities such as employee recruitment and initiation of financial projections, as well as the age of the team leader and cohort of the firm; (2) gender, education, and growth preference of the firms’ leader appear as the biggest predictors of performance after five years; and (3), a positive innovation-performance relationship through R&D prioritization, patenting, and product novelty may exist

Magnetic-Field Tuning of Light-Induced Superconductivity in Striped La2−x_{2-x}Bax_xCuO4_4

Optical excitation of stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ has been shown to transiently enhance superconducting tunneling between the CuO$_2$ planes. This effect was revealed by a blue-shift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling in La$_{2-x}$Ba$_x$CuO$_4$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.Comment: 35 pages, 13 figure

Computational approaches for identifying inhibitors of protein interactions

Inter-molecular interaction is at the heart of biological function. Proteins can interact with ligands, peptides, small molecules, and other proteins to serve their structural or functional purpose. With advances in combinatorial chemistry and the development of high throughput binding assays, the available inter-molecular interaction data is increasing exponentially. As the space of testable compounds increases, the complexity and cost of finding a suitable inhibitor for a protein interaction increases. Computational drug discovery plays an important role in minimizing the time and cost needed to study the space of testable compounds. This work focuses on the usage of various computational methods in identifying protein interaction inhibitors and demonstrates the ability of computational drug discovery to contribute to the ever growing field of molecular interaction. A program to predict the location of binding surfaces on proteins, STP (Mehio et al., Bioinformatics, 2010, in press), has been created based on calculating the propensity of triplet-patterns of surface protein atoms that occur in binding sites. The use of STP in predicting ligand binding sites, allosteric binding sites, enzyme classification numbers, and binding details in multi-unit complexes is demonstrated. STP has been integrated into the in-house high throughput drug discovery pipeline, allowing the identification of inhibitors for proteins whose binding sites are unknown. Another computational paradigm is introduced, creating a virtual library of -turn peptidomimetics, designed to mimic the interaction of the Baff-Receptor (Baff-R) with the B-Lymphocyte Stimulator (Blys). LIDAEUS (Taylor, et al., Br J Pharmacol, 2008; 153, p. S55-S67) is used to identify chemical groups with favorable binding to Blys. Natural and non-natural sidechains are then used to create a library of synthesizable cyclic hexapeptides that would mimic the Blys:Baff-R interaction. Finally, this work demonstrates the usage and synergy of various in-house computational resources in drug discovery. The ProPep database is a repository used to study trends, motifs, residue pairing frequencies, and aminoacid enrichment propensities in protein-peptide interaction. The LHRLL protein-peptide interaction motif is identified and used with UFSRAT (S. Shave, PhD Thesis, University of Edinburgh, 2010) to conduct ligand-based virtual screening and generate a list of possible antagonists from the EDULISS (K. Hsin, PhD Thesis, University of Edinburgh, 2010) compound repository. A high throughput version of AutoDock (Morris, et al., J Comput Chem, 1998; 19, p. 1639-62) was adapted and used for precision virtual screening of these molecules, resulting in a list of compounds that are likely to inhibit the binding of this motif to several Nuclear Receptors

Ultrafast Dynamics of Photo-Doped Mott Antiferromagnets

Strong coupling between spin and charge degrees of freedom in two-dimensional spin-1/2 Mott antiferromagnets (AFMs) creates a rich platform to study quantum many-body physics. For decades, the consequences of these interactions have been intensely studied in thermal equilibrium, where the introduction of charge carriers through chemical doping has been shown to generate a vibrant phase diagram rich with unconventional types of charge, spin, and orbital ordering. In recent years, however, attention has grown to include the study of these materials as they are driven far from equilibrium using intense pulses of light produced by femtosecond laser sources. In addition to fundamental interest in the resultant dynamics, recent experimental and theoretical studies have suggested that driven Mott insulators can host states of matter that cannot be accessed in thermal equilibrium. While many driving protocols have been developed---spanning from the selective excitation of bosonic modes to photon-dressing via coherent time-periodic driving---the simplest conceptual approach to engineering Mott insulators with light is known as photo-doping. In this procedure, the material is impulsively driven resonantly with a transition from a filled band to an empty band, transiently producing charge carriers. Given the impact of chemical doping in thermal equilibrium, photo-doping has garnered interest as an important tool in the study of driven Mott insulators. Early successes in the study of photo-doped Mott AFMs include the observation of ultrafast demagnetization and the prediction of non-thermal magnetic states, charge density waves, and superconductivity. Photo-doping thus holds promise to generate an out-of-equilibrium phase diagram that is equally rich to that found in equilibrium. Yet, many open questions about the basic properties of photo-doped Mott insulators remain unresolved. Whether charge instabilities exist as a result of interactions between the photo-dopants has yet to be examined. Moreover, while theoretical studies have suggested that antiferromagnetic correlations can enhance attractive interactions between photo-dopants, evidence of the resultant bound states remain elusive. Even the light-matter interactions that generate the photo-dopants are in need of investigation, as the fate of a Mott insulator driven by strong electric fields remains a fundamental open theoretical and experimental problem. In this thesis, I present a series of experiments designed to answer each of these questions. After describing the properties of Mott insulators in Chapter 1, I present the experimental details of the tools that enable these studies in Chapter 2. Taking a multi-messenger approach to ultrafast spectroscopy, a suite of ultrafast probes simultaneously track the spin and charge degrees of freedom to paint a holistic picture of the out-of-equilibrium state. In Chapter 3, I use ultrafast THz conductivity to establish the existence of an insulating photo-excited fluid of Hubbard excitons (HEs), which are bound states that are thought to form as a result of attractive spin-mediated interactions. This magnetic binding mechanism is studied in more detail in Chapter 4 by examining the properties of these HEs in the magnetic critical region of several materials that lie in different magnetic universality classes. In Chapter 5, I study the effects of HE formation on the ultrafast demagnetization that is known to occur following photo-doping. Finally, I turn my attention towards the photo-dopant generation mechanism in Chapter 6, exploring the effects of strong electric field driving in Mott insulators. I find signatures of the so-called Keldysh crossover from a multiphoton-absorption- to a quantum-tunneling-dominated pair production regime. Altogether, this work establishes photo-doped Mott insulators as a rich playground to engineer non-equilibrium phases of matter and study quantum many-body dynamics.</p

Determination of software quality through a generic model

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal

Self-rated health disparities among disadvantaged older adults in ethnically diverse urban neighborhoods in a Middle Eastern country.

OBJECTIVES: This paper examines differentials in self-rated health (SRH) among older adults (aged 60+ years) across three impoverished and ethnically diverse neighborhoods in post-conflict Lebanon and assesses whether variations are explained by social and economic factors. DESIGN: Data were drawn from the Older Adult Component (n = 740) of the Urban Health Survey, a population-based cross-sectional study conducted in 2003 in a formal community (Nabaa), an informal settlement (Hey El-Sellom), and a refugee camp for Palestinians (Burj El-Barajneh) in Beirut, Lebanon. The role of the social capital and economic security constructs in offsetting poor SRH was assessed using multivariate ordinal logistic regression analyses. RESULTS: Older adults in Nabaa fared better in SRH compared to those in Hey El-Sellom and Burj El-Barajneh, with a prevalence of good, average, and poor SRH being respectively, 41.5%, 37.0%, and 21.5% in Nabaa, 33.3%, 23.9%, and 42.7% in Hey El-Sellom, and 25.2%, 31.3%, and 43.5% in Burj El-Barajneh. The economic security construct attenuated the odds of poorer SRH in Burj El-Barajneh as compared to Nabaa from 2.57 (95% confidence interval, CI: 1.89-3.79) to 1.42 (95% CI: 0.96-2.08), but had no impact on this association in Hey El-Sellom (odds ratio, OR: 2.12, 95% CI: 1.39-3.24). The incorporation of the social capital construct in the fully adjusted model rendered this association insignificant in Hey El-Sellom (OR: 1.49, 95% CI: 0.96-2.32), and led to further reductions in the magnitude of the association in Burj El-Barajneh camp (OR: 1.18, 95% CI: 0.80-1.76). CONCLUSIONS: The social context in which older adults live and their financial security are key in explaining disparities in SRH in marginalized communities. Social capital and economic security, often overlooked in policy and public health interventions, need to be integrated in dimensions of well-being of older adults, especially in post-conflict settings

Signatures of Ultrafast Reversal of Excitonic Order in Ta₂NiSe₅

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta₂NiSe₅ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta₂NiSe₅ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials