Thermodynamics and mechanism of protonated cysteine decomposition: a guided ion beam and computational study

pre-printA quantitative molecular description of the decomposition of protonated cysteine, H+Cys, is provided by studying the kinetic energy dependence of threshold collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Primary dissociation channels are deamidation (yielding both NH3 loss and NH4 + formation) and (H2O + CO) loss reactions, followed by an additional six subsequent decompositions. Analysis of the kinetic energy-dependent CID cross sections provides the 0 K barriers for six different reactions after accounting for unimolecular decay rates, internal energy of reactant ions, multiple ion-molecule collisions, and competition among the decay channels. To identify the mechanisms associated with these reactions, quantum chemical calculations performed at the B3LYP/6-311+G(d,p) level were used to locate the transition states (TSs) and intermediates for these processes. Single point energies of the reactants, products, and key optimized TSs and intermediates are calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311+G(2d,2p) basis set. The computational characterization of the elementary steps of these reactions including the structures of the final products is validated by quantitative agreement with the experimental energetics. In agreement with previous work, deamidation is facilitated by anchimeric assistance of the thio group, which also leads to an interesting rearrangement of the intact amino acid identified computationally

The formation of stoichiometric uranium brannerite (UTi2O6) glass-ceramic composites from the component oxides in a one-pot synthesis

Brannerite glass-ceramic composites have been suggested as suitable wasteform materials for high-actinide content wastes, but the formation of glass-ceramic composites containing stoichiometric uranium brannerite (UTi2O6) has not been well-studied. Uranium brannerite glass-ceramic composites were synthesised at by a one-pot cold-press and sinter route from the component oxides. As a comparison, two further samples were produced using an alkoxide-nitrate route. A range of compositions with varying molar ratios of uranium and titanium oxides (from 1:2 to 1:3.20) were synthesised, with a range of different heat treatments (1200 °C for 12–48 h, and 1250 °C for 12 h). All compositions were analysed by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray near-edge spectroscopy, and found to contain UTi2O6 as the majority crystalline phase forming within a glass matrix of nominal stoichiometry Na2AlBSi6O16. In compositions with UO2:TiO2 ratios of 1:2 and 1:2.28, particles of UO2 were observed in the glass matrix, likely due to dissolution of TiO2 in the glass phase; this was prevented by the addition of excess TiO2. This work demonstrates the suitability of this system to produce highly durable wasteforms with excellent actinide waste loading, even with a simple one-pot process. Some grains of brannerite consist of a UO2 particle encapsulated in a shell of UTi2O6, suggesting that brannerite crystallises around particles of UO2 until either the UO2 is fully depleted, or the kinetic barrier becomes too large for further diffusion to occur. We propose that the formation of brannerite within glass-ceramic composites at lower temperatures than that for pure ceramic brannerite is caused by an increase in the rate of diffusion of the reactants within the glass

Solution composition and particle size effects on the dissolution and solubility of a ThO2 microstructural analogue for UO2 matrix of nuclear fuel

The objective of this study was to investigate the dissolution rate of ThO2 which was synthesised to approximate, as closely as possible, the microstructure of UO2 in a nuclear fuel matrix. The optimal sintering temperature for ThO2 pellets was found to be 1750 ℃, which produced pellets with a microstructure similar to UO2 nuclear fuel pellets, with randomly oriented grains ranging in size from 10 to 30 μm. Dissolution was conducted using ThO2 particles of different size fractions (80 to 160 μm and 2 to 4 mm) in the presence and absence of carbonate, in solutions with pH from 2 to 8 and at 80 ℃. Dissolution rates were calculated from Th released from the solid phase to solution. Particles of ThO2 were also leached with 1 M HNO3 at 80 ℃ in order to investigate the morphological changes at the particle surfaces. The concentration of Th was found to be ≥ 10–9 mol/L at pH ≤ 4, lower than the theoretical solubility of crystalline ThO2. At higher pH values, from 4 to 8, the measured concentrations (10−10 to 10–12 mol/L) were between the theoretical solubility of ThO2 and Th(OH)4. Grain boundaries were shown to exert an influence on the dissolution of ThO2 particles. Using high resolution aqueous solution analysis, these data presented here extend the current understanding of Th solubility in solutio

The extremophilic pharmacy:drug discovery at the limits of life

[No abstract

The extremophilic pharmacy:drug discovery at the limits of life

[No abstract

Insights into the fabrication and structure of plutonium pyrochlores

Rare earth zirconates, such as Nd2Zr2O7, crystallise with the pyrochlore structure and are a group of materials which have been suggested as potential nuclear waste forms for actinide immobilisation. In this work, a new hydroxide co-precipitation route is presented to investigate the incorporation of Pu into Nd2Zr2O7. The plutonium content was varied between 5 and 10 mol% and the structural uptake and Pu oxidation state were probed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS). The experimental findings were complemented by DFT ab initio calculations. For all the incorporation mechanisms studied PuO2 was used as the reference reactant state to allow for a direct comparison between the possible Pu uptake scenarios. Analysis of the experimental data suggests that Pu(IV) cations substitute for Nd(III) cations leading to structural distortion of the pyrochlore A-sites. The computed solution energies and bond-distances corroborate the experimental findings and indicate that the excess charge is balanced via the introduction of oxygen at formerly vacant sites

Bottleable neutral analogues of [B2H5]- as versatile and strongly binding eta2 donor ligands

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X = Cl, OTf, C6F5) lies opposite the bridging hydrogen of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B-B bond account for their formation. Astoundingly, these copper σB-B-complexes are inert to ligand exchange with pyridine under either heating or photoirradiation

Sexual risk behavior among HIV-positive persons in Jamaica.

Background: HIV/AIDS remains a global public health challenge, especially in sub-Saharan Africa and the Caribbean. Sexual risk behaviors among HIV-positive persons place their partners at risk for HIV transmission and other sexually transmitted infections. Stopping transmission acts among HIV-positive people is crucial in reversing HIV incidence. Objective: This study aimed to assess the prevalence and predictors of sexual risk behaviors among HIV-positive individuals in clinical care in Northwestern Jamaica. Methods: A cross-sectional survey of 118 (33 males and 85 females) HIV-positive individuals was used to assess demographic and health characteristics, HIV/AIDS knowledge, attitudes, and beliefs and sexual risk behaviors. Results: About 12% of the study population stated that they had unprotected anal or vaginal sex without disclosing their HIV status. Participants who agreed that condoms reduce the risk of HIV transmission were 13.1 times more likely to use condoms during their last sexual encounters(95% CI: 2.1-79.0) than those who disagreed. About 75% of participants reported using a condom every time they had sexual intercourse in the past year, while 25% used condoms irregularly. Participants who had unprotected anal or vaginal sex without disclosing their status were less likely to have used condoms during the last sexual encounter (OR=0.1; 95% CI: 0.02-0.5). Conclusion: The prevalence of unsafe sex remains high among sexually active people living with HIV/AIDS in Jamaica. Study participants who engaged in unprotected sex without disclosing their HIV-positive status potentially place their partners at risk for HIV transmission and other sexually transmitted infections. The study findings highlight the need to promote safe sexual behaviors and a positive social environment for people living with HIV/AIDS in Jamaica

Krypton irradiation damage in Nd-doped zirconolite and perovskite as potential ceramics for inert matrix fuel and plutonium disposition

ABSTRACT Understanding the effect of radiation damage and noble gas accommodation in potential ceramic hosts for plutonium disposition is necessary to evaluate the long-term behaviour during geological disposal. Polycrystalline samples of Nd-doped zirconolite and Nd-doped perovskite were irradiated ex-situ with 2 MeV Kr + at a dose of 5x10 15 ions.cm -2 to simulate plutonium nuclei recoil during alpha decay. The feasibility of thin section preparation of both pristine and irradiated samples by Focussed Ion Beam sectioning was demonstrated. After irradiation, the Nd-doped zirconolite revealed a well defined amorphous region separated from the pristine material by a thin (40-60 nm) damaged interface. The Nd-doped perovskite contained a defined irradiated region composed of an amorphous region surrounded by damaged regions. In both samples, as revealed by electron diffraction, the damaged regions and interface have a structure in which the fluorite sublattice is present while the pristine lattice is absent. In addition in Nddoped perovskite, the amorphisation dose depended on crystallographic orientation and possibly sample configuration (thin section and bulk). In Nd-doped perovskite, Electron Energy Loss Spectroscopy study revealed a change in Ti coordination associated with the crystal to amorphous transition

Isolierung neutraler, mono- und dikationischer B2P2-Ringe durch Addition eines Diphosphans an eine Bor-Bor-Dreifachbindung

Das NHC-stabilisierte Diborin B2(SIDep)2 (SIDep=1,3-Bis(2,6-diethylphenyl)imidazolin-2-yliden) unterzieht sich bei Raumtemperatur einer P-P-Bindungsaktivierung mit Tetraethyldiphosphan, wobei mittels 1,2-Diphosphinierung über ein Diphosphoryldiboren in hohen Ausbeuten B2P2-Heterocyclen gebildet werden. In Abhängigkeit vom verwendeten Oxidationsmittel und Gegenion kann dieser Heterocyclus zu einem Radikalkation beziehungsweise Dikation oxidiert werden. Beginnend mit dem planaren, neutralen 1,3-Bis(alkyliden)-1,3-diborata-2,4-diphosphoniocyclobutan führt jeder Oxidationsschritt zu einer verminderten B-B-Bindungslänge und dem Verlust der Planarität durch die Kationisierung. Röntgenstrukturanalysen in Kombination mit DFT- und CASSCF/NEVPT2-Rechnungen offenbaren für die NHC-stabilisierten dikationischen B2P2-Ringe geschlossenschalige, schmetterlingsartige Strukturen, wovon die diradikaloiden Isomere mit planarem Ring in energetischer Nähe liegen