In situ SAXS investigation of dibromomethane adsorption in ordered mesoporous silica

A SAXS/WAXS apparatus with the aid of a specially designed sample cell capable for performing both SAXS and WAXS experiments was used for adsorption studies in nanoporous materials. The applicability of the instrument for structural investigations and its ability for adsorption experiments because of the advanced sample environment were demonstrated by carrying out in situ SAXS measurements during gas physisorption. SAXS profiles of ordered mesoporous silica were measured at selected equilibrium points alongside a dibromomethane (CH₂Br₂) adsorption isotherm at 293 K. SBA-15 was the adsorbent of choice because it consists of a regular 2D hexagonal array of cylindrical mesopores that gives rise to Bragg reflections in the small-angle regime. CH₂Br₂ was selected as a contrast-matching fluid because it has almost the same electron density as silica. We obtained high-quality data comparable to those resulting from experiments performed in synchrotron light sources which produce intense beams of x-rays and support advanced instrumentation for high-resolution diffraction and SAXS studies. The Bragg peaks of the pore lattice are clearly visible for the evacuated sample and at the early stages of the adsorption process. The intensity decrease and the elimination of the Bragg peaks for the saturated sample suggest that an almost perfect contrast matching was achieved. A model has been used for monitoring the fluid condensation and evaporation regime in SBA-15 by taking into account both the Bragg scattering and the diffuse scattering for spatially random pore filling. The results show the absence of spatial correlations between filled pores suggesting random pore filling.     

In situ small angle X-ray scattering and benzene adsorption on polymer-based carbon hollow fiber membranes

The structural changes and the mechanism of benzene adsorption on microporous carbon hollow fiber membranes with different surface and pore network properties have been investigated by in situ small-angle X-ray scattering (SAXS) and benzene adsorption. Benzene adsorption measurements have been carried out in situ with SAXS alongside an adsorption/desorption isotherm cycle at 293 K with the aid of a specially constructed adsorption sample cell. In addition low-pressure C₆H₆ and high-pressure CO₂, CH₄ and N₂ adsorption isotherms have been performed. Two carbon hollow fiber membranes, both prepared by controlled pyrolysis procedures of polyimide membrane precursor, were under study. During benzene adsorption the intensity of the SAXS curves changes in a way that depends on how the pores are filled and the contrast fluctuations occur. The SAXS data have been modeled by evaluating the form factor of lamellar micropores upon filling with C₆H₆. The existence of ultra micropores within the surrounding matrix was also taken into account. The results suggest that the arrangement of the ultra micropores on the non-activated membrane is in such a way that the access of benzene to the micropores is restricted, resulting in an incomplete filling. On the other hand, the activation process generates a more accessible pore network where the micropores are completely filled.     

Generation of the [NiFe] Hydrogenase Active Site (Sulfur's the One!)

Abstract

Chemical and biochemical methods were used to unravel the unprecedented pathway by which the CN ligands of iron in [NiFe] hydrogenase are introduced. Carbamoyl phosphate is the one carbon precursor of these ligands, and reactions involving a protein cysteinyl sulfur are key for processing this precursor into CN ligands.     

Modelling Affinity Chromatography

Abstract

Affinity chromatography plays a significant role in the separation and purification of biologically active macromolecules in laboratory and large-scale applications. There is a need for models which could be used to predict accurately the dynamic behavior of affinity chromatography separations, in order to permit the design, optimization, control, and process scale-up of affinity chromatography systems. Furthermore, the construction and use of such models will contribute to a better fundamental understanding of the physicochemical and biospecific mechanisms involved in affinity chromatography processes. The parameters of the models should be obtainable by using information from a small number of experiments.

This work reviews the modeling of affinity chromatography, and presents general models that could be used to describe the dynamic behavior of the adsorption, wash, and elution stages of affinity chromatography systems. Certain model structures, modeling approaches and operational strategies for systems having porous or nonporous adsorbent particles are also suggested, and experiments are proposed whose data are necessary for parameter estimation and model discrimination studies in affinity chromatography.

Particular emphasis is given to :he modeling of the intrinsic mechanisms of intraparticle diffusion, adsorption, and desorption, because the intrinsic mechanisms are normally independent of the mode of operation (i.e., batch, fixed bed, fluidized bed, continuous countercurrent, or others).     

Ultraviolet-protected inflation

In natural inflation, the inflaton is a pseudo-Nambu-Goldstone boson which acquires a mass by explicit breaking of a global shift symmetry at scale f. In this case, for small field values, the potential is flat and stable under radiative corrections. Nevertheless, slow roll conditions enforce f ? M(p), making the validity of the whole scenario questionable. In this Letter, we show that a coupling of the inflaton kinetic term to the Einstein tensor allows f ? M(p) by enhancing the gravitational friction acting on the inflaton during inflation. This new unique interaction (a) keeps the theory perturbative in the whole inflationary trajectory, (b) preserves the tree-level shift invariance of the pseudo-Nambu-Goldstone boson, and (c) avoids the introduction of any new degrees of freedom with respect to standard natural inflation.     

Surfactant-enhanced liquid-liquid microextraction coupled to micro-solid phase extraction onto highly hydrophobic magnetic nanoparticles

We are presenting a simplified alternative method for dispersive liquid-liquid microextraction (DLLME) by resorting to the use of surfactants as emulsifiers and micro solid-phase extraction (μ-SPE). In this combined procedure, DLLME of hydrophobic components is initially accomplished in a mixed micellar/microemulsion extractant phase that is prepared by rapidly mixing a non-ionic surfactant and 1-octanol in aqueous medium. Then, and in contrast to classic DLLME, the extractant phase is collected by highly hydrophobic polysiloxane-coated core-shell Fe₂O₃@C magnetic nanoparticles. Hence, the sample components are the target analyte in the DLLME which, in turn, becomes the target analyte of the μ-SPE step. This 2-step approach represents a new and simple DLLME procedure that lacks tedious steps such as centrifugation, thawing, or delicate collection of the extractant phase. As a result, the analytical process is accelerated and the volume of the collected phase does not depend on the volume of the extraction solvent. The method was applied to extract cadmium in the form of its pyrrolidine dithiocarbamate chelate from spiked water samples prior to its determination by FAAS. Detection limits were brought down to the low μg L^?1 levels by preconcentrating 10 mL samples with satisfactory recoveries (96.0–108.0 %).

Synthetic,structural, spectroscopic and solution speciation studies of the binary Al(III)–quinic acid system. Relevance of soluble Al(III)–hydroxycarboxylate species to molecular toxicity

Efforts to delineate the interactions of Al(III), a known metallotoxin, with low molecular mass physiological substrates involved in cellular processes led to the investigation of the structural speciation of the binary Al(III)–quinic acid system. Reaction of Al(NO₃)₃ · 9H₂O with d-(−)-quinic acid at a specific pH (4.0) afforded a colorless crystalline material K[Al(C₇H₁₁O₆)₃] · (OH) · 4H₂O (1). Complex 1 was characterized by elemental analysis, FT-IR, DSC–TGA, ¹³C-MAS NMR, solution ¹H and ¹³C NMR, and X-ray crystallography. The structure of 1 reveals a mononuclear octahedral complex of Al(III) with three singly ionized quinate ligands bound to it. The three ligand alcoholic side chains do not participate in metal binding and dangle away from the complex. The concurrent study of the aqueous speciation of the binary Al(III)–quinic acid system projects a number of species complementing the synthetic studies on the binary system Al(III)–quinic acid. The structural and spectroscopic data of 1 in the solid state and in solution emphasize its physicochemical properties emanating from the projections of the aqueous structural speciation scheme of the Al(III)–quinic acid system. The employed pH-specific synthetic work (a) exemplifies essential structural and chemical attributes of soluble aqueous species, arising from biologically relevant interactions of Al(III) with natural α-hydroxycarboxylate substrates, and (b) provides a potential linkage to the chemical reactivity of Al(III) toward O-containing molecular targets influencing physiological processes and/or toxicity events.     

Design, synthesis, and evaluation of a lanthanide chelating protein probe: CLaNP-5 yields predictable paramagnetic effects independent of environment

Immobilized lanthanide ions offer the opportunity to refine structures of proteins and the complexes they form by using restraints obtained from paramagnetic NMR experiments. We report the design, synthesis, and spectroscopic evaluation of the lanthanide chelator, Caged Lanthanide NMR Probe 5 (CLaNP-5) readily attachable to a protein surface via two cysteine residues. The probe causes tunable pseudocontact shifts, alignment, paramagnetic relaxation enhancement, and luminescence, by chelating it to the appropriate lanthanide ion. The observation of single shifts and the finding that the magnetic susceptibility tensors obtained from shifts and alignment analyses are highly similar strongly indicate that the probe is rigid with respect to the protein backbone. By placing the probe at various positions on a model protein it is demonstrated that the size and orientation of the magnetic susceptibility tensor of the probe are independent of the local protein environment. Consequently, the effects of the probe are readily predictable using a protein structure only. These findings designate CLaNP-5 as a protein probe to deliver unambiguous high quality structural restraints in studies on protein-protein and protein-ligand interactions.     

Synthesis, characterization, and study of octanuclear iron-oxo clusters containing a redox-active Fe4O4-cubane core

A one-pot synthetic procedure yields the octanuclear Fe(III) complexes Fe(8)(micro(4-)O)(4)(micro-pz(*))(12)X(40, where X = Cl and pz(*) = pyrazolate anion (pz = C(3)H(3)N(2)-) (1), 4-Cl-pz (2), and 4-Me-pz (3) or X = Br and pz(*) = pz (4). The crystal structures of complexes 1-4, determined by X-ray diffraction, show an Fe(4)O(4)-cubane core encapsulated in a shell composed of four interwoven Fe(micro-pz(*))(3)X units. Complexes 1-4 have been characterized by 1H NMR, infrared, and Raman spectroscopies. M?ssbauer spectroscopic analysis distinguishes the cubane and outer Fe(III) centers by their different isomer shift and quadrupole splitting values. Electrochemical analyses by cyclic voltammetry show four consecutive, closely spaced, reversible reduction processes for each of the four complexes. Magnetic susceptibility studies, corroborated by density functional theory calculations, reveal weak antiferromagnetic coupling among the four cubane Fe centers and strong antiferromagnetic coupling between cubane and outer Fe atoms of 1. The structural similarity between the antiferromagnetic Fe(8)(micro(4-)O)(4) core of 1-4 and the antiferromagnetic units contained in the minerals ferrihydrite and maghemite is demonstrated by X-ray and M?ssbauer data.