Magnetic and Mössbauer Studies of Magnetite-Loaded Polyvinyl Alcohol Hydrogels

Materials producing strain in a magnetic field are known as magnetoelastic or magnetostrictive materials. A new type of material that is able to produce giant strain in a nonhomogeneous magnetic field has been developed. In these magnetic-field-sensitive gels (ferrogels) fine colloidal particles having superparamagnetic behavior are incorporated into a highly swollen elastic polymer network. Magnetic properties of ferrogels have been investigated using electron microscopy, static magnetization measurements, and M?ssbauer spectroscopy. Analysis of the data yielded information on the superparamagnetic behavior of ferrogels and made it possible to estimate the size distribution of the magnetic cores of magnetite particles made by chemical precipitation and built into a chemically cross-linked polyvinyl alcohol matrix. The results are interpreted on the basis of a core-shell model. Copyright 2000 Academic Press.     

First-principles calculations of K-edge X-ray absorption near edge spectra of some transition metal dimers

Theoretical X-ray absorption near-edge spectra have been evaluated for different bond lengths of Mn₂, Co₂ and Ni₂ using a modified Xα scattered-wave procedure.     

Solid-state NMR study of intercalated species in poly(epsilon-caprolactone)/clay nanocomposites

The structure and dynamics of surfactant and polymer chains in intercalated poly(epsilon-caprolactone)/clay nanocomposites are characterized by (31)P magic-angle spinning (MAS) and (13)C cross-polarization MAS NMR techniques. To obtain hybrid materials with the low polymer content required for this study, in situ intercalative polymerization was performed by adapting a published procedure. After nanocomposite formation, the chain motion of the surfactant is enhanced in the saponite-based materials but reduced in the Laponite ones. Compared to the starting clay, the trans conformer population of the surfactant hydrocarbon chain in the nanocomposite decreases for the saponite systems. Mobility of the polymer chain is higher in the nanocomposites than in the bulk phase. The charge of the modified saponite does not significantly influence chain mobility in the nanocomposites.     

Direct high-performance liquid chromatographic separation of unusual secondary amino acids and a comparison of the performances of Chirobiotic T and TAG columns

Two macrocyclic glycopeptide antibiotic-type chiral stationary phases (CSPs) based on native teicoplanin and teicoplanin aglycone, Chirobiotic T and TAG, respectively, were evaluated with regard to the high-performance liquid chromatographic separation of the enantiomers of 10 secondary alpha-amino acids (imino acids). The chromatographic results are given as the retention, separation and resolution factors, together with the enantioselective free energy difference corresponding to the separation of the enantiomers. By application of these two CSPs, excellent resolutions were achieved for the investigated compounds by using reversed-phase mobile mode systems. The separation conditions were optimized by variation of the mobile phase composition. The difference in enantioselective free energy between the aglycone CSP and the teicoplanin CSP for these particular amino acids ranged between 0.70 and -1.83 kJ mol(-1). It was established that better enantioseparations of the secondary alpha-amino acids were attained in most cases on the aglycone CSP.     

Voltammetric determination of L-dopa using an electrode modified with trinuclear ruthenium ammine complex (Ru-red) supported on Y-type zeolite

The L-dopa is the immediate precursor of the neurotransmitter dopamine. Unlike dopamine, L-dopa easily enters the central nervous system and is used in the treatment of Parkinson’s disease. A sensitive and selective method is presented for the voltammetric determination of L-dopa in pharmaceutical formulations using a carbon paste electrode modified with trinuclear ruthenium ammine complex [(NH₃)₅Ru^IIIORu^IV(NH₃)₄ORu^III(NH₃)₅]⁶⁺ (Ru-red) incorporated in NaY zeolite. The parameters which influence on the electrode response (paste composition, potential scan rate, pH and interference) were also investigated. The optimum conditions were found to an electrode composition (m/m) of 25% zeolite containing 6.7% Ru, 50% graphite and 25% mineral oil in acetate buffer at pH 4.8. Voltammetric peak currents showed a linear response for L-dopa concentration in the range between 1.2×10⁻⁴ and 1.0×10⁻² mol l⁻¹ (r=0.9988) with a detection limit of 8.5×10⁻⁵ mol l⁻¹. The variation coefficient for a 1.0×10⁻³ mol l⁻¹ L-dopa (n=10) was 5.5%. The results obtained for L-dopa in pharmaceutical formulations (tablet) was in agreement with compared official method. In conclusion, this study has illustrated that the proposed electrode modified with Ru-red incorporated zeolite is suitable valuable for selective measurements of L-dopa.     

Mercury removal from aqueous and organo-aqueous solutions by natural Mexican erionite

The sorption of Hg(II) from aqueous and organo aqueous solutions was investigated by Mexican natural erionite. The mercury chemical species (anionic, cationic or neutral) were determined by high voltage electrophoresis, and the mercury chemical species present in the aqueous media were simulated by a program MEDUSA. The mercury sorption process was monitored during 48 hours. The mercury content was determined by neutron activation analysis. Mixtures of benzene/water[Hg(II)], toluene/water[Hg(II)] and ethanol/water[Hg(II)] were chosen as organo-aqueous media. It was found that both the mercury chemical species and the dielectric constant of solvents play an important role in the mercury sorption by erionite.     

CoI- and Co0-bipyridine complexes obtained by reduction of CoBr2bpy: electrochemical behaviour and investigation of their reactions with aromatic halides and vinylic acetates

The electrochemical behaviour of CoBr(2)bpy (bpy=2,2'-bipyridine) catalyst precursor in acetonitrile has been studied, revealing its possible reduction into the corresponding Co(I) and Co(0) complexes. These low-valent cobalt species appear to be stable on the time scale of cyclic voltammetry. In the presence of aromatic halides, both complexes undergo oxidative addition, the latter Co(0) species allowing the activation of poorly reactive aromatic chlorides. The arylcobalt(III) and arylcobalt(II) obtained are reduced at the same potential as the original Co(II) and Co(I) complexes, respectively, resulting in the observation of overall ECE mechanisms in both cases. The electrochemical study shows that vinylic acetates competitively react with electrogenerated Co(0) species, leading to a labile complex. Preparative scale electrolyses carried out from solutions containing aromatic halides (ArX), vinyl acetate (vinylOAc) and a catalytic amount of CoBr(2)bpy lead to a mixture of biaryl (Ar-Ar) and arene (ArH) as long as the potential is set on the plateau of the Co(II) right arrow over left arrow Co(I) reduction wave. The coupling product (Ar-vinyl) is formed only if the electrolysis is performed on the plateau of the Co(I)/Co(0) reduction wave. A mechanism is proposed for the overall cobalt-catalyzed coupling reaction between aromatic halides and allylic acetates.     

On the [2,3] sigmatropic rearrangement of allylic nitro compounds

Allylic nitro compounds undergo relatively clean [2,3] sigmatropic rearrangement upon heating in refluxing 1,2-dichlorobenzene in the presence of DABCO to give the corresponding allylic alcohols via the intermediate allylic nitrite. [reaction: see text]     

Toward direct determination of conformations of protein building units from multidimensional NMR experiments. V. NMR chemical shielding analysis of N-formyl-serinamide,a model for polar side-chain containing peptides

Knowledge of chemical shift-structure relationships could greatly facilitate the NMR chemical shift assignment and structure refinement processes that occur during peptide/protein structure determination via NMR spectroscopy. To determine whether such correlations exist for polar side chain containing amino acid residues the serine dipeptide model, For-L-Ser-NH(2), was studied. Using the GIAO-RHF/6-31+G(d) and GIAO-RHF/TZ2P levels of theory the NMR chemical shifts of all hydrogen ((1)H(N), (1)H(alpha), (1)H(beta1), (1)H(beta2)), carbon ((13)C(alpha), (13)C(beta), (13)C') and nitrogen ((15)N) atoms have been computed for all 44 stable conformers of For-L-Ser-NH(2). An attempt was made to establish correlation between chemical shift of each nucleus and the major conformational variables (omega(0), phi, psi, omega(1), chi,(1) and chi(2)). At both levels of theory a linear correlation can be observed between (1)H(alpha)/phi, (13)C(alpha)/phi, and (13)C(alpha)/psi. These results indicate that the backbone and side-chain structures of For-L-Ser-NH(2) have a strong influence on its chemical shifts.