Characterization of proton‐conducting organic–inorganic polymeric materials by ASAXS

The distribution of ZrO₂ and phosphotungstic acid (PTA) in a matrix of sulfonated polyether ketone was investigated by anomalous small‐angle X‐ray scattering (ASAXS). Scattering curves were obtained using X‐ray energies near the Zr and W absorption edges, allowing the independent analysis of the distribution of ZrO₂ and PTA in the sample. The interaction between both inorganic components improved their dispersion considerably when compared with films containing just one of the additives. The synergism was correlated to previous investigations concerning proton conductivity and permeability of the membranes developed for direct methanol fuel cell. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2981–2992, 2005     

Thickening of Crosslinked Ethyl Methacrylate-Acrylic Acid Latexes

Latexes based on ethyl methacrylate (EMA), acrylic acid (AA), crosslinked with 1,6-hexanediol propoxylate diacrylate (HPDA), were synthesized via emulsion polymerization, yielding dispersions with different acidic group contents and crosslinking densities. The latexes were thickened using various NaOH/AA molar ratios and the resultant dispersions were characterized by pH-metry, zeta potentiometry, and turbidimetry. The process of thickening by carboxyl neutralization was shown to be dependent on the neutralization effect on particle–particle interactions and particle solubilization. Turbidimetry, pH-metry, and zeta potential measurements showed that neutralization resulted in partial latex solubilization and viscometry indicated that, for a given latex, an optimum thickening was achieved for and specific NaOH/AA molar ratio, showing the existence of a relationship between thickening, particle–particle interactions, latex solubilization, and polyelectrolyte effect.     

X‐ray powder diffraction at the XRD1 beamline at LNLS

Various upgrades have been completed at the XRD1 beamline at the Brazilian synchrotron light source (LNLS). The upgrades are comprehensive, with changes to both hardware and software, now allowing users of the beamline to conduct X‐ray powder diffraction experiments with faster data acquisition times and improved quality. The main beamline parameters and the results obtained for different standards are presented, showing the beamline ability of performing high‐quality experiments in transmission geometry. XRD1 operates in the 5.5–14 keV range and has a photon flux of 7.8 × 10⁹ photons s^?1 (with 100 mA) at 12 keV, which is one of the typical working energies. At 8 keV (the other typical working energy) the photon flux at the sample position is 3.4 × 10¹⁰ photons s^?1 and the energy resolution ΔE/E = 3 × 10^?4.     

Electrochemical Glyphosate Sensor Based on Hybrid Material SiO2/Sm2O3/Carbon Modified with Meldola Blue

In this work, we have proposed an electrochemical sensor for the detection of pesticides by using a ceramic composite with a SiO₂ surface modified with Sm₂O₃ nanoparticles and C-graphite (SSMG), obtained by the sol-gel process and immobilized by adsorption in Meldola Blue cationic dye (MB). The composite was called (SSMG/MB), which was characterized by spectroscopic, electrochemical techniques, Fourier-transform infrared spectroscopy, and Cyclic Voltammetry. The proposed sensor was applied for the glyphosate electrochemical detection, using Differential Pulse Voltammetry, and, under optimized parameters has presented the linear response for the pesticide in the concentration range from 0.99 to 7.94 (μmol L⁻¹; R²=0.9963; n=8). The calculated values for the detection limit and the quantification limit were 0.15 and 0.49 μmol L⁻¹, respectively. Therefore, the new electrochemical sensor based on SiO₂, NPsSm₂O_3, C-graphite, and MB hybrid material was developed for the first time for glyphosate determination, which has demonstrated high potential for the development of new hybrid devices for environmental control.     

Differential pulse voltammetric determination of benznidazole loaded in nanostructured lipid carriers and urine**

Benznidazole (BZN) is the first-choice drug for treating Chagas disease (CD). However, it is not ideal for this purpose as it is highly toxic and has irregular pharmacokinetics due to factors such as its low aqueous solubility. These factors necessitate the development of reliable and effective alternative methods for the analytical determination of BZN in biological and pharmaceutical samples. In this context, we present a new electroanalytical method for quantifying BZN using differential pulse voltammetry (DPV) and a glassy carbon electrode (GCE). This method was applied to urine and a pharmaceutical formulation of BZN incorporated into nanostructured lipid carriers (NLC-BZN). The proposed method provided a linear analytical range of 1.00–10.6 μmol L⁻¹ (R=0.999), with a detection limit of 0.044 μmol L⁻¹ and a quantification limit of 0.13 μmol L⁻¹. The relative standard deviation of the intra-day and inter-day precision was below 2.50 %. Through interference studies, the methodology proved to be selective for BZN, because there was no significant potential interference in any of the samples. The recovery tests showed that the accuracy was within the limits recommended in the literature. Therefore, the developed DPV/GCE method can be successfully applied as an alternative method for detecting BZN in NLC-BZN pharmaceutical formulations and human urine.     

Covalent attachment of Candida rugosa lipase on chemically modified hybrid matrix of polysiloxane-polyvinyl alcohol with different activating compounds

Candida rugosa lipase was immobilized by covalent binding on hybrid matrix of polysiloxane–polyvinyl alcohol chemically modified with different activating agents as glutaraldehyde, sodium metaperiodate and carbonyldiimidazole. The experimental results suggested that functional activating agents render different interactions between enzyme and support, producing consequently alterations in the optimal reaction conditions. Properties of the immobilized systems were assessed and their performance on hydrolytic and synthetic reactions were evaluated and compared with the free enzyme. In hydrolytic reactions using p-nitrophenyl palmitate as substrate all immobilized systems showed higher thermal stability and optima pH and temperature values in relation to the free lipase. Among the activating compounds, carbonyldiimidazole resulted in a total recovery of activity on the support and the highest thermal stability. For the butyl butyrate synthesis, the best performance (molar conversion of 95% and volumetric productivity of 2.33 g L⁻¹ h⁻¹) was attained with the lipase immobilized on POS–PVA activated with sodium metaperiodate. The properties of the support and immobilized derivatives were also evaluated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopies and chemical composition (FTIR).     

Vibrational Study on the Local Structure of Post‐Synthesis and Hybrid Mesoporous Materials: Are There Fundamental Distinctions?

Organic-inorganic mesoporous materials of the MCM-41 type are important materials that can be prepared by either post-synthesis or one-pot synthesis procedures. A complete control of the characteristics at a local level is of the utmost importance in view of the applications of such materials. However, there are not many studies relating such features with synthetic approaches. In this work, we prepared samples by post-synthesis derivatization of materials from Si-based MCM-41, with bidentate nitrogen ligands bearing one or two silylated arms, and by one-pot synthesis of organic-inorganic hybrid materials. The bulk properties of the two kinds of materials were comparable. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and Raman spectroscopy were used to investigate the local environment, namely, the number of OH groups and distribution of SiO(4) units (large and small ring units). Hydrophilicity correlates with both the type of organic moiety used (mono- or disilylated), as well as with the synthetic procedure. The same vibrational studies showed how the structure in the channels changes as a function of pressure, reflecting the low mechanical stability of the mesoporous materials.     

Energetic differences between the five- and six-membered ring hydrocarbons: strain energies in the parent and radical molecules

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.     

The <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis> Rv2540c DNA sequence encodes a bifunctional chorismate synthase

Background

The emergence of multi- and extensively-drug resistant Mycobacterium tuberculosis strains has created an urgent need for new agents to treat tuberculosis (TB). The enzymes of shikimate pathway are attractive targets to the development of antitubercular agents because it is essential for M. tuberculosis and is absent from humans. Chorismate synthase (CS) is the seventh enzyme of this route and catalyzes the NADH- and FMN-dependent synthesis of chorismate, a precursor of aromatic amino acids, naphthoquinones, menaquinones, and mycobactins. Although the M. tuberculosis Rv2540c (aroF) sequence has been annotated to encode a chorismate synthase, there has been no report on its correct assignment and functional characterization of its protein product.

Results

In the present work, we describe DNA amplification of aroF-encoded CS from M. tuberculosis (Mt CS), molecular cloning, protein expression, and purification to homogeneity. N-terminal amino acid sequencing, mass spectrometry and gel filtration chromatography were employed to determine identity, subunit molecular weight and oligomeric state in solution of homogeneous recombinant Mt CS. The bifunctionality of Mt CS was determined by measurements of both chorismate synthase and NADH:FMN oxidoreductase activities. The flavin reductase activity was characterized, showing the existence of a complex between FMN_ox and Mt CS. FMN_ox and NADH equilibrium binding was measured. Primary deuterium, solvent and multiple kinetic isotope effects are described and suggest distinct steps for hydride and proton transfers, with the former being more rate-limiting.

Conclusion

This is the first report showing that a bacterial CS is bifunctional. Primary deuterium kinetic isotope effects show that C₄-proS hydrogen is being transferred during the reduction of FMN_ox by NADH and that hydride transfer contributes significantly to the rate-limiting step of FMN reduction reaction. Solvent kinetic isotope effects and proton inventory results indicate that proton transfer from solvent partially limits the rate of FMN reduction and that a single proton transfer gives rise to the observed solvent isotope effect. Multiple isotope effects suggest a stepwise mechanism for the reduction of FMN_ox. The results on enzyme kinetics described here provide evidence for the mode of action of Mt CS and should thus pave the way for the rational design of antitubercular agents.

     

Surface dangling-bond States and band lineups in hydrogen-terminated Si, Ge, and Ge/si nanowires

We report an ab initio study of the electronic properties of surface dangling-bond (SDB) states in hydrogen-terminated Si and Ge nanowires with diameters between 1 and 2 nm, Ge/Si nanowire heterostructures, and Si and Ge (111) surfaces. We find that the charge transition levels epsilon(+/-) of SDB states behave as a common energy reference among Si and Ge wires and Si/Ge heterostructures, at 4.3+/-0.1 eV below the vacuum level. Calculations of epsilon(+/-) for isolated atoms indicate that this nearly constant value is a periodic-table atomic property.