The transplutonium elements (atomic numbers 95-103) are a group of metals that lie at the edge of the periodic table. As a result, the patterns and trends used to predict and control the physics and ...chemistry for transition metals, main-group elements and lanthanides are less applicable to transplutonium elements. Furthermore, understanding the properties of these heavy elements has been restricted by their scarcity and radioactivity. This is especially true for einsteinium (Es), the heaviest element on the periodic table that can currently be generated in quantities sufficient to enable classical macroscale studies
. Here we characterize a coordination complex of einsteinium, using less than 200 nanograms of
Es (with half-life of 275.7(5) days), with an organic hydroxypyridinone-based chelating ligand. X-ray absorption spectroscopic and structural studies are used to determine the energy of the L
-edge and a bond distance of einsteinium. Photophysical measurements show antenna sensitization of Es
luminescence; they also reveal a hypsochromic shift on metal complexation, which had not previously been observed in lower-atomic-number actinide elements. These findings are indicative of an intermediate spin-orbit coupling scheme in which j-j coupling (whereby single-electron orbital angular momentum and spin are first coupled to form a total angular momentum, j) prevails over Russell-Saunders coupling. Together with previous actinide complexation studies
, our results highlight the need to continue studying the unusual behaviour of the actinide elements, especially those that are scarce and short-lived.
The solution‐state interactions of plutonium and berkelium with the octadentate chelator 3,4,3‐LI(1,2‐HOPO) (343‐HOPO) were investigated and characterized by X‐ray absorption spectroscopy, which ...revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X‐ray absorption near‐edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white‐line position energies for Pu‐343‐HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk‐343‐HOPO results were found to mirror the XANES behavior of Bk(III)‐DTPA. Extended X‐ray absorption fine structure results revealed An—OHOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343‐HOPO via density functional theory calculations. Pu(III)‐ and Bk(III)‐343‐HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)‐, Cm(III)‐, Cf(III)‐, and Es(III)‐343‐HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series.
X‐ray absorption spectroscopy was used to probe the interactions between an octadentate hydroxypyridinone chelator and two transuranic elements in microgram quantities – plutonium and berkelium – within buffered solutions. Despite the precedence for chelation‐driven stabilization of the tetravalent oxidation state of actinides with this ligand, in situ reductive decomposition yielded plutonium(III) and berkelium(III) coordination complexes.
The separation of trivalent lanthanides and actinides is challenging because of their similar sizes and charge densities. S-donating extractants have shown significant selectivity for trivalent ...actinides over lanthanides, with single-stage americium/lanthanide separation efficiencies for some thiol-based extractants reported at >99.999%. While such separations could transform the nuclear waste management landscape, these systems are often limited by the hydrolytic and radiolytic stability of the extractant. Progress away from thiol-based systems is limited by the poorly understood and complex interactions of these extractants in organic phases, where molecular aggregation and micelle formation obfuscates assessment of the metal–extractant coordination environment. Because S-donating thioethers are generally more resistant to hydrolysis and oxidation and the aqueous phase coordination chemistry is anticipated to lack complications brought on by micelle formation, we have considered three thioethers, 2,2′-thiodiacetic acid (TDA), (2R,5S)-tetrahydrothiophene-2,5-dicarboxylic acid, and 2,5-thiophenedicarboxylic acid (TPA), as possible trivalent actinide selective reagents. Formation constants, extended X-ray absorption fine structure spectroscopy, and computational studies were completed for thioether complexes with a variety of trivalent lanthanides and actinides including Nd, Eu, Tb, Am, Cm, Bk, and Cf. TPA was found to have moderately higher selectivity for the actinides because of its ability to bind actinides in a different manner than lanthanides, but the utility of TPA is limited by poor water solubility and high rigidity. While significant competition with water for the metal center limits the efficacy of aqueous-based thioethers for separations, the characterization of these solution-phase, S-containing lanthanide and actinide complexes is the most comprehensively available in the literature to date. This is due to the breadth of lanthanides and actinides considered as well as the techniques deployed and serves as a platform for the further development of S-containing reagents for actinide separations. Additionally, this paper reports on the first bond lengths for Cf and Bk with a neutral S donor.
EXAFS provides the capability to interrogate nanoparticle (NP) structure in atomistic detail without relying on long‐range crystallinity. There is a limitation in that EXAFS provides averaged ...structural information, making it difficult to separate a small amount of heterogeneous structure from bulk. In this work, models were developed to extract surface‐specific information from conventional EXAFS measurements collected on UO2 NPs of varying size. Specifically, the surface terminating species of UO2 NPs was determined from comparison of coordination numbers with geometric models while the origin of static disorder was interrogated from user‐defined simulations. Results show that the degree of oxygenation on the NP surface does not significantly deviate from bulk surface and that static disorder is highly enhanced in NP surface layers but cannot be attributed to surface relaxation effects alone. The approach described herein has the potential to be adapted to a range of inorganic NP systems to interrogate surface structure.
Various sizes of UO2 NPs were modeled with user‐defined parameters considering surface termination effect, relaxation, and disorder. The surface coordination of the NPs was backtracked by comparing the EXAFS simulated from the model with actual EXAFS experimental data.
Octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) siderophore analogues are known to be efficacious chelators of the actinide cations, and these ligands are also capable of facilitating ...both activation and reduction of actinyl species. Utilizing X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) spectroscopies, as well as cyclic voltammetry measurements, herein, we elucidate chelation‐based mechanisms for driving reactivity and initiating redox processes in a family of neptunyl–HOPO and CAM complexes. Based on the selected chelator, the ability to control the oxidation state of neptunium and the speed of reduction and concurrent oxo group activation was demonstrated. Most notably, reduction kinetics for the NpVO2+//NpIV redox couple upon chelation by the ligands 3,4,3‐LI(1,2‐HOPO) and 3,4,3‐LI(CAM)2(1,2‐HOPO)2 was observed to be faster than ever reported, and in fact quicker than we could measure using either X‐ray absorption spectroscopy or electrochemical techniques.
Spontaneous chelation‐driven reduction of the NpO2+ moiety was observed in the condensed phase. X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) spectroscopy on Np‐343 ligand complexes confirmed metal‐ion reduction, providing some of the first structural data on a mixed valent NpV/NpIV chelate (see scheme).
Effects of HIV-1 Tat on enteric neuropathogenesis Ngwainmbi, Joy; De, Dipanjana D; Smith, Tricia H ...
The Journal of neuroscience,
2014-Oct-22, 2014-10-22, 20141022, Letnik:
34, Številka:
43
Journal Article
Recenzirano
Odprti dostop
The gastrointestinal (GI) tract presents a major site of immune modulation by HIV, resulting in significant morbidity. Most GI processes affected during HIV infection are regulated by the enteric ...nervous system. HIV has been identified in GI histologic specimens in up to 40% of patients, and the presence of viral proteins, including the trans-activator of transcription (Tat), has been reported in the gut indicating that HIV itself may be an indirect gut pathogen. Little is known of how Tat affects the enteric nervous system. Here we investigated the effects of the Tat protein on enteric neuronal excitability, proinflammatory cytokine release, and its overall effect on GI motility. Direct application of Tat (100 nm) increased the number of action potentials and reduced the threshold for action potential initiation in isolated myenteric neurons. This effect persisted in neurons pretreated with Tat for 3 d (19 of 20) and in neurons isolated from Tat(+) (Tat-expressing) transgenic mice. Tat increased sodium channel isoforms Nav1.7 and Nav1.8 levels. This increase was accompanied by an increase in sodium current density and a leftward shift in the sodium channel activation voltage. RANTES, IL-6, and IL-1β, but not TNF-α, were enhanced by Tat. Intestinal transit and cecal water content were also significantly higher in Tat(+) transgenic mice than Tat(-) littermates (controls). Together, these findings show that Tat has a direct and persistent effect on enteric neuronal excitability, and together with its effect on proinflammatory cytokines, regulates gut motility, thereby contributing to GI dysmotilities reported in HIV patients.
Summary
Purpose: Clinically, perturbations in the semaphorin signaling system have been associated with autism and epilepsy. The semaphorins have been implicated in guidance, migration, ...differentiation, and synaptic plasticity of neurons. The semaphorin 3F (Sema3F) ligand and its receptor, neuropilin 2 (NPN2) are highly expressed within limbic areas. NPN2 signaling may intimately direct the apposition of presynaptic and postsynaptic locations, facilitating the development and maturity of hippocampal synaptic function. To further understand the role of NPN2 signaling in central nevous system (CNS) plasticity, structural and functional alterations were assessed in NPN2 deficient mice.
Methods: In NPN2 deficient mice, we measured seizure susceptibility after kainic acid or pentylenetetrazol, neuronal excitability and synaptic throughput in slice preparations, principal and interneuron cell counts with immunocytochemical protocols, synaptosomal protein levels with immunoblots, and dendritic morphology with Golgi‐staining.
Results: NPN2 deficient mice had shorter seizure latencies, increased vulnerability to seizure‐related death, were more likely to develop spontaneous recurrent seizure activity after chemical challenge, and had an increased slope on input/output curves. Principal cell counts were unchanged, but GABA, parvalbumin, and neuropeptide Y interneuron cell counts were significantly reduced. Synaptosomal NPN2 protein levels and total number of GABAergic synapses were decreased in a gene dose‐dependent fashion. CA1 pyramidal cells showed reduced dendritic length and complexity, as well as an increased number of dendritic spines.
Discussion: These data suggest the novel hypothesis that the Sema 3F signaling system's role in appropriate placement of subsets of hippocampal interneurons has critical downstream consequences for hippocampal function, resulting in a more seizure susceptible phenotype.
Neptunium (237Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its ...tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe5O8H·4H2O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O2+) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np LIII-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (Oax and Oeq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O2+ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O2 solid. An analogous experiment was also performed with U(VI)O22+, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O2). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O2+) species.
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