Acidic and reductive dissolution of magnetite in aqueous sulfuric acid: Site-binding model and experimental results

The open circuit dissolution of ionic metal oxides in mineral acids is modelled assuming that the rate is controlled by the transfer of metal ions in hydrolytic equilibrium with bulk metal ions, from the metal oxide surface to the Stern plane. The site-binding model of the double layer metal oxide/electrolyte solution is used to obtain the pH dependence of surface and Stern potentials. The nature of the active sites is discussed and their surface concentration is assumed to be proportional to suface charge σ₀. Again, the site-binding model is used to detemine the pH dependence of σ₀. It is thus shown that the rate order in c_H⁺ is essentially defined by the potential dependence of the charge transfer process, for oxides with points of zero charge near neutrality that dissolve in mildly or strongly acidic solutions. The role of surface complexation is also discussed in terms of the site-binding model and the difficulties in interpreting dissolution experiments under constant external applied potential are discussed in terms of the complexity of the semiconductor oxide/electrolyte solution interfacial region in magnetite.An experimental study of the open circuit dissolution of magnetite in sulfuric acid is presented and interpreted according to the proposed model.The reductive dissolution of magnetite is modelled by extension of the Valverde-Wagner model of oxide dissolution. Experimental results are presented to demonstrate that the reductive dissolution rate of magnetite in ferrous containing solutions is controlled by the rate of electron transfer from adsorbed Fe(II) to Fe(III) surface states of magnetite.     

Effect of conformational changes on a one-electron reduction process: evidence of a one-electron P-P bond formation in a bis(phosphinine)

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P-P bond (3.479 A), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P-P distance in neutral 1 is large (5.8 A). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P-P single bond (P-P 2.389 A). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C-Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P-P distance. The ability of the phosphinine radical anion to reversibly form weak P-P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P-P bond.     

Synthesis and pharmacological evaluation of new 16-methyl pregnane derivatives

The pharmacological activity of several new pregnane derivatives 15-19 were determined on gonadectomized male hamster flank organs, seminal vesicles and in vitro conversion of testosterone (T) to dihydrotestosterone (DHT) as 5alpha-reductase inhibitors. Steroids 15-19 decreased the diameter of the pigmented spot in the flank organs as compared to the T treated animals; in this model, steroids 16 and 19 showed a higher activity than the commercially available finasteride 3. Injection of T increased the weight of the seminal vesicles. Compounds 15-19 when injected together with T decreased the weight of the seminal vesicles thus showing an antiandrogenic effect. The trienone 19 exhibited a considerably higher activity than finasteride. Steroids 15-19 inhibited the in vitro metabolism of [3H]T to [3H]DHT in seminal vesicles homogenates of gonadectomized male hamsters. Compounds 18 and 19 showed a much higher antiandrogenic effect than finasteride. This enhancement of the biological activity could probably be attributed to the coplanarity of the steroidal skeleton as previously observed by our group. The high antiandrogenic activity of the epoxy compound 16 is probably the result of the ring opening of the oxiran ring with the nucleophilic part of the enzyme 5alpha-reductase thus leading to a stable adduct with concomitant deactivation of this enzyme.     

Characterization of lactosylated proteins of infant formula powders using two-dimensional gel electrophoresis and nanoelectrospray mass spectrometry

Infant formula powders were analyzed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) to assess the whey proteins quality, which may be altered by the heat treatment used during the processing conditions. Lactosylation was found to be the major chemical modification occurring in whey proteins. In parallel, a two-dimensional (2-D) gel electrophoresis was performed on the milk sample and the entire protein patterns were analyzed by nano-ESI-MS after cutting the different gel spots and in-gel trypsin digestion. A highly selective and specific tandem MS technique has been developed to characterize and localize up to ten lactosylation sites in beta-lactoglobulin (beta-Lg) and alpha(S2)-casein. alpha-Lactalbumin (alpha-La), with five lactosylated peptides, was found to be an interesting protein marker in the milk powder sample to detect chemical modification induced by the processing/storage conditions.     

15N,(1)H Heteronuclear correlation NMR of gramicidin A in DMPC-d(67)

We have studied gramicidin A, an environmentally sensitive polymorphic pentadecapeptide, fully 15N-labelled and dispersed in a highly deuterated phospholipid bilayer system. By submitting the sample to fast magic angle spinning, we were able to reduce the polypeptide amide hydrogen linewidths to 160 Hz, and hence to partially resolve them. By correlating these resonances with the 40 Hz wide dipolar coupled 15N in a 2D-CROPSY (cross-polarization spectroscopy) experiment, it was possible to observe the 20 partially overlapping 1H-15N signal pairs from the polypeptide backbone and sidechains. Both chemical shift distributions closely match those of the same peptide in SDS micelles, but only poorly match those of conformationally different gramicidin A in trifluoroethanol, dimethylsulfoxide, or methanol/chloroform mixture. Our results are indicative of the N-to-N right-handed beta6.3-helix conformation of gramicidin A and offer sufficient resolution to encourage development of experiments to measure orientational or distance restraints using through-space dipolar couplings.     

Direct conversion of macrocyclic furans into macrocyclic isothiazoles

The first calixhetarenes with more than one heteroatom in the constituent rings are prepared in one step by treatment of calix[4]furan 1a and calix[6]furan 1b with ethyl carbamate, thionyl chloride and pyridine to give 2, 3, 4 and 5, 6, 7 respectively; these products have been characterised by X-ray crystallography which reveals that in 2 all eight heteroatoms lie on one face of the macrocyle.     

Trends in Preconcentration Procedures for Metal Determination Using Atomic Spectrometry Techniques

 Methods for metal preconcentration are often described in the literature. However, purposes are often different, depending on whether the methods are applied in environmental, clinical or technological fields. The respective method needs to be efficient, give high sensitivity, and ideally also is selective which is useful when used in combination with atomic spectroscopy. This review presents the actual tendencies in metal preconcentration using techniques such as Flame Atomic Absorption Spectrometry (FAAS), Electrothermal Atomic Absorption Spectrometry (ETAAS), Hydride Generation Atomic Absorption Spectrometry (HGAAS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Procedures based on related to electrochemical, coprecipitation/precipitation, liquid–liquid and solid–liquid extraction and atom trapping mechanisms are presented. Correspondence: Department of Analytical Chemistry, Universidade Estadual de Campinas – UNICAMP, P.O. Box 6154, 13083-970 Campinas, Brazil. e-mail: zezzi@iqm.unicamp.br Received December 20, 2001; accepted October 11, 2002     

Regioselectivity of Birch reductive alkylation of biaryls

[reaction: see text] The regioselectivity of the Birch reductive alkylation of polysubstituted biaryls has been investigated. Results indicate that regioselectivity is affected by the electronic nature of substituents on both aromatic rings. The electron-rich 3,5-dimethoxyphenyl moiety is selectively reduced and then alkylated, while phenols and aniline are not dearomatized under these conditions. Biaryls possessing a phenol moiety are alkylated on the second ring, providing that the acidic proton has been removed prior to the Li/NH3 reduction.     

Selective liquid adsorption and structural properties of montmorillonite and its hexadecylpyridinium derivatives

Summary An X-ray diffraction method was used to study the structures of montmorillonites, the surfaces of which were covered to a systematically increasing extent with hexadecylpyridinium (HDP) cations. It was found that, depending on the degree of cover of the surface, the HDP cations form a mono- or bimolecular layer in the interlayer space. A detailed investigation was made of the selective sorption of binary mixtures on montmorillonite and on HDP-montmorillonites with different surface coverages. The adsorption excess isotherms were determined, as were the adsorption capacities from analysis of the isotherms, and also the equivalent specific surface areas.It was established that from benzene-n-heptane mixtures the benzene is adsorbed only on the external surface of montmorillonite, while in the case of organocomplexes it causes moderate disaggregation.From benzene, alcohols with different numbers of carbon atoms are also preferentially adsorbed on the internal surfaces of montmorillonite; with the rise in the number of carbon atoms, the extent of interlamellar adsorption decreases. In the case of HDP-montmorillonites in methanol-benzene mixtures, the adsorption capacity exhibits a monotonous increase with the degree of organopbility in the range under consideration. In the event of optimum organophility the disaggregation of the organocomplex is complete, and the equivalent specific surface area is 764-778 m² · g^–1. The swelling of the organocomplex, which can be characterized by X-ray diffraction measurement, is in close correlation with the interlamellar liquid adsorption. The results show modification of the surface of montmorillonite with HDP cations fundamentally determines the nature and extent of the selective liquid adsorption.

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Zusammenfassung Die Struktur von mit Hexadecyl-pyridinium(HDP)-Kationen im systematisch gesteigerten Maße bedeckten Montmorilloniten wurde mit der Röntgendiffraktions-Methode untersucht. Es wurde festgestellt, daß die HDP-Kationen, von der Bedeckung der Oberfläche abhängend, im Schichtzwischenraum eine mono- bzw. bimolekulare Schicht bilden. Die selektive Sorption binärer Gemische wurde an Montmorilloniten und an HDP-Montmorilloniten verschiedener Bedeckung eingehend untersucht. Es wurden die Adsorptionsisothermen von Flüssigkeitsgemischen und aus deren Analyse die Adsorptionskapazitäten und die äquivalenten spezifischen Oberflächen bestimmt.Es wurde festgestellt, daß aus Benzol-n-Heptan-Gemischen das Benzol nur an der äußeren Montmorillonite-Oberfläche adsorbiert wird, während es bei Organokomplexen eine mäßige Desaggregation verursacht. Die Adsorption von Alkoholen verschiedener Kohlenstoffzahl aus Benzol erscheint auch an inneren Montmorillonit-Oberflächen bevorzugt; mit zunehmender Kohlenstoffzahl nimmt das Ausmaß der interlamellaren Adsorption ab. Bei HDP-Montmorilloniten zeigt die Adsorptionskapazität aus Methanol-Benzol-Gemischen mit zunehmender Organophilität im untersuchten Gebiet eine monotone Steigerung. Im Falle optimaler Organophilität ist die Desaggregation vollständig, die äquivalente spezifische Oberfläche beträgt 764-778 m²/g. Die durch Röntgenn-diffraktionsmessung charakterisierbare Quellung des Organokomplexes steht mit der interlamellaren Flüssig-keitsadsorption in engern Zusammenhang. Unsere Ergebnisse zeigen, daß Charakter und Ausmaß der selektiven Flüssigkeitsadsorption durch die Modifizierung der Montmorillonit-Oberfläche mit HDP-Kationen grundlegend verändert wird.

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With 14 figures and 4 tables     

Direct conversion of N-methoxy-N-methylamides (Weinreb amides) to ketones via a nonclassical Wittig reaction

[reaction: see text] N-Methoxy-N-methylamides (Weinreb amides) are converted efficiently into ketones by reaction with alkylidenetriphenylphosphoranes and in situ hydrolysis of the product.