Structural characterization of Acetobacter diazotropicus levansucrase by matrix-assisted laser desorption/ionization mass spectrometry: identification of an N-terminal blocking group and a free-thiol cysteine residue

High-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to characterize the primary structure of the levansucrase (EC 2.4.1.10) secreted by Acetobacter diazotropicus SRT4. The technique permitted not only the reading frame of this enzyme, the amino acid sequence of which was deduced from DNA, but also the elucidation of an N-terminal blocking group and the position of a disulfide bridge between Cys309 and Cys365 among the three Cys residues. A free cysteine (Cys127) was identified by modifying an intact molecule with a sulfhydryl reagent, 5-(octyldithio)-2-nitrobenzoic acid, under non-reducing conditions. In addition, the enzyme obtained by site-directed mutagenesis at Asp279 to Asn279 was also identified by the above methods. Post-source decay analysis of the tryptic peptide containing the mutation site unequivocally revealed an Asn residue at position 279.     

Characterization of magnetic materials by low-field microwave absorption techniques

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe H_DC+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool.     

Monte carlo calculation of transport for three-dimensional magnetohydrodynamic equilibria

A Monte Carlo algorithm has been implemented which couples the results of a three-dimensional magnetohydrodynamic equilibrium code with particle transport calculations similar to those of Boozer and Kuo-Petravic. The equilibrium results are in a form which lends itself to easy and accurate computation of the coefficients required to follow particle orbits in flux variables at finite pressure. In particular, the parallel current and the Clebsch potential for the current are obtained from Fourier series solutions of first-order partial differential equations with constant coefficients. Confinement times are estimated from the exponential decay of expected values of appropriately chosen functionals of the particle distribution. The observation that these functionals satisfy boundary conditions helps us to compute confinement times over a wide range of collision frequencies, including cases where losses due to particle trapping are very high. Initial conditions are chosen to optimize the Monte Carlo calculation by using known information about the expected particle distribution. Results for actual and proposed stellarator experiment are given. Electron and ion confinement times are compared, since this is relevant to the issue of the effect of ambipolar electric fields on stellarator confinement. A spectral method is given to determine the electric field from charge neutrality, and numerical evidence is presented to suggest that anomalous electron transport may be due to small resonant terms in the electric potential. Also, comparisons are made with a simplified theory of particle transport already incorporated in the equilibrium code.     

Pt-Cu / ZSM-5 for removal of nitrates from drinking water

Removal of nitrates from drinking water by catalytic hydrogenation over ZSM-5 supported Pt-Cu catalysts was studied. Bimetallics Pt-Cu were prepared by ion exchange of copper on a parent monometallic platinum catalyst. Monometallic platinum catalysts are inactive for nitrate reduction, while Pt-Cu bimetallic catalysts are active for nitrate removal. In the bimetallic catalyst, the role of copper is probably to reduce nitrate according to a redox reaction. The addition of copper to Pt catalysts decreases the production of ammonium ions     

Effects of torsion stress on the domain wall unpinning AC field in amorphous FeCoBSi wires

The unpinning or propagation field, h_p, where domain walls are unpinned and start their displacement under the AC magnetic field of as-cast amorphous wires of nominal composition (Co₉₄Fe₆)_72.5B₁₅Si_12.5, was determined for each torsion angle between –90° (counterclockwise) and +150° (clockwise) at a constant frequency of 1 kHz. A plot of h_p as a function of torsion angle showed a sharp minimum at a torsion angle of +120. The results are interpreted in terms of the counterbalance effect of the torsion against the intrinsic helical anisotropy induced during the wire fabrication.     

Coercivity mechanism in nanophase (Nd–Pr)–Fe–B melt spun alloys

The nucleation mechanism to predict coercivity values in melt-spun exchange-coupled (Nd_1−xPr_x)_yFe_94−yB₆ alloys for various Nd:Pr ratios x, and Fe:RE ratios y, was tested using the dependence of the anisotropy constant K₁ on Pr content x for the minimum nucleation field H_N^min in the modified Brown's equation. Very good agreement was found between experimental data and theoretical values, confirming the predominance of the nucleation of reverse domains over the wall pinning process in the coercivity mechanism of melt spun REFeB alloys.     

Hexadecameric self-assembled dendrimers built from 2'-deoxyguanosine derivatives

Herein we describe the construction of hexadecameric self-assembled dendrimers (SADs) using a series of dendronized 8-(m-acetylphenyl)-2'-deoxyguanosine (mAG) subunits. The azido-substituted mAG subunits were covalently linked to alkynyl polyester dendrons using a copper-catalyzed 1,3-dipolar cycloaddition reaction. Discrete SADs are formed with high fidelity and thermal stability even with the increased steric hindrance offered by the dendrons.