NMR spectra of a microcrystalline protein at 30 kHz MAS

Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS.     

Classification of Poincaré duality algebras over the rationals

The Poincaré duality algebras over Q play a key role in the rational homotopy classification of closed manifolds [3]. In this paper we give a way of classifying general Poincaré duality algebras and then specialize to the case of algebras which are generated by some homogeneous component and show how the classification reduces to the linear classification of certain homogeneous polynomials and exterior forms.     

Incoherent tunnelling of positive muons and trapping by vacancies in electron-irradiated aluminium

We have measured the transverse spin relaxation of positive muons ⁺ in Al single crystals after irradiation at 150 K with 3 MeV electrons. The relaxation functions agree with those expected for diffusion-limited trapping of the ⁺ in monovacancies. Between 215 K and 60 K the ⁺ diffusivity is well described by the Flynn-Stoneham law (multi-phonon incoherent tunnelling between ground states) with an activation enthalpyH _a = (30±2) meV. At lower temperatures, few-phonon (in particular one-phonon) processes become important. The decrease of the vacancy concentration by a factor of 100 during annealing between 227 K and 267 K has been studied.     

Mass-selected resonance-enhanced multiphoton ionisation spectra of laser-desorbed molecules for environmental analysis: 16 representative polycyclic aromatic compounds

Mass-selected gas-phase UV spectra of laser-desorbed molecules at room temperature have been measured via resonance-enhanced multiphoton ionisation (REMPI) and time-of-flight mass selection. The wavelength range of 260 to 320 nm is optimal for detection of polycyclic aromatic hydrocarbons (PAHs) using REMPI-mass spectrometry. A new laser desorption/laser ionisation source has been used which features a compact size and thermal equilibrium of the desorbed molecules. 16 PAHs have been investigated which have been selected by the US Environmental Protection Agency (EPA). These 16 EPA-PAHs are commonly used world-wide to characterise the PAH-load of environmental samples.     

A 13C endor study on Copper(II)/Zinc(II) bis-(diethyl-dithiocarbamate) Cu/Zn(et2dtc)2

¹³C ENDOR spectra of Cu(et₂dtc)₂ substituted into a single crystals of Zn(et₂dtc)₂ are reported. The symmetry of the incorporated guest molecule appears to be considerably influenced by the Zn(et₂dtc)₂ host lattice. The unexpectedly large isotropic ¹³C hfs coupling can be understood assuming a “transannular overlap” mechanism.     

Trennung und Identifizierung von Bactericiden auf der Basis von halogenierten Aromaten durch Dünnschicht-Chromatographie

Plates coated with silica gel with addition of fluorescence indicator are used for thin-layer chromatography with a solvent mixture of benzene and acetone (8∶2). The substances are made visible with UV-light and by spraying with solutions of 2,6-dibromoquinonechloroimide and sodium carbonate. The R_f-values, the fluorescence, and the colour of the spots allow to detect and to identify the bactericides rapidly and without further chemical techniques.     

Structure and annealing of (n, γ)-Induced point defects in CaF2

Neutron activated²⁰F nuclei (T _1/2=11 s) in CaF₂ serve for two purposes: first they produce well defined point defects by their (n, )-production mechanism, second, they act as probes to detect the properties of these defects in a- and-ray detected nuclear magnetic resonance experiment. Below about 70 K an interstitial defect is observed due to a quadrupolar splitting of the²⁰F-NMR line. The defect induced electric field gradient is oriented along the 111-axis of the crystal and causes a quadrupole coupling constante ² qQ/h=–2.0(1) MHz at the²⁰F site. The defect anneals above 70 K with an Arrhenius like temperature dependence corresponding to an activation energyE _M =0.17(4) eV.     

Einsatz der echten Integration in der Atomabsorptions-Spektroskopie

An improvement of the signal-to-noise ratio in atomic absorption spectroscopy may be obtained by each of the following three methods of signal averaging:

increase of the time constant

digital signal averaging

true integration of the signal

The most adequate of these methods appears to be true integration. Two examples are presented to demonstrate the improved signal-to-noise ratio due to the application of this method:

trace analysis of mercury in water by means of flameless AAS. The detection limit appears to be 0.2 ng which corresponds to a concentration of 2 · 10^?13 g/ml.

trace analysis of copper in human urine by means of flame AAS: 1,3 · 10^?8 g/ml copper could be detected without any solvent extraction process.

     

Effect of quadrupolar interactions on the magnetic transitions of the terbium-monopnictides

The thermodynamic properties below the Neél temperatures of the single-groundstate magnets TbP, TbAs, TbSb and TbBi are analyzed within the mean-field approximation. If, in addition to the magnetic interaction, also quadrupole-quadrupole couplings between the Tb³⁺ are taken into account, the self-consistent calculations describe experimental data on the spontaneous dipolar and quadrupolar moments and the specific heat quite satisfactorily up toT _N . In a general phase diagram, the validity of the MFA is traced to the quadrupolar coupling, pushing the transitions to first or almost first order. Going from TbP to TbBi, a systematic strengthening of the exchange coupling is observed which is probably due to the conduction electrons. The ferrodistortive quadrupolar interaction, decreasing from TbP to TbBi, is also dominated by exchange interactions.Project of the SFB 65 Festkörperspektroskopie