Exploiting C3-symmetry in the dynamic coordination of a chiral trisoxazoline to copper(II): improved enantioselectivity, and catalyst stability in asymmetric lewis acid catalysis

Chiral C3-symmetric trisoxazolines are highly efficient stereodirecting ligands in enantioselective Cu(II) Lewis acid catalysis which is based on the concept of a stereoelectronic hemilability of the divalent copper; in direct comparison with the analogous bisoxazoline systems they are more efficient in the enantioselective alpha-amination as well as the enantioselective Mannich reaction of prochiral beta-ketoesters.     

Highly selective entry to the azadirachtin skeleton via a Claisen rearrangement/radical cyclization sequence

[formula: see text] A highly diastereoselective, microwave-induced Claisen rearrangement of an appropriately substituted propargylic enol ether allows the formation of the sterically congested C8-C14 bond of azadirachtin. When combined with a radical-mediated cyclization of the corresponding allene, this sequence offers rapid entry to the framework of azadirachtin.     

Fluorescence-quenched solid phase combinatorial libraries in the characterization of cysteine protease substrate specificity

To map the substrate specificity of cysteine proteases, two combinatorial peptide libraries were synthesized and screened using the archetypal protease, papain. The use of PEGA resin as the solid support for library synthesis facilitated the application of an on-resin fluorescence-quenched assay. Results from the screening of library 2 indicated a preference for Pro or Val in the S3 subsite and hydrophobic residues in S2; the most prevalent residue not being Phe but Val. The S1 subsite exhibited a dual specificity for both small, nonpolar residues, Ala or Gly, as well as larger, Gln, and charged residues, Arg. Small residues predominated in the S1'-S4' subsites. Active peptides from the libraries and variations thereof were resynthesized and their kinetics of hydrolysis by papain assessed in solution phase assays. Generally, there was a good correlation between the extent of substrate cleavage on solid phase and the kcat/KM's obtained in solution phase assays. Several good substrates for papain were obtained, the best substrates being Y(NO2)PMPPLCTSMK(Abz) (kcat/KM = 2109 (mM s)-1), Y(NO2)PYAVQSPQK(Abz) (kcat/KM = 1524 (mM s)-1), and Y(NO2)PVLRQQRSK(Abz) (kcat/KM = 1450 (mM s)-1). These results were interpreted in structural terms by the use of molecular dynamics (MD). These MD calculations indicated two different modes for the binding of substrates in the narrow enzyme cleft.     

Isolierung von Magnesium aus biologischem Material für die massenspektrometrische Isotopenanalyse

Magnesium is precipitated as hydroxide after elimination of interfering elements by means of ion-exchange procedures (phosphate, iron, trace elements) and masking with EDTA (calcium). The sample is applied in form of MgCl₂ to the tungsten filament of an 1-filament-ion-source for surface ionization.     

Radiochemische Untersuchungen zur Verteilung von129J und3H in Bestrahltem Kernbrennstoff aus Leistungsreaktoren

The working techniques used for the investigation of¹²⁹I and³H sectional distribution in highly irradiated UO₂ pellets are described. The analytical samples are taken by a micro drilling technique. For the determination of¹²⁹I concentration in the UO₂ samples neutron activation analysis following iodine separation from uranium and bulk fission products is used.³H is measured by liquid scintillation counting after distillation. Reliability of the analytical results is discussed as well as possible interferences.     

Output power enhancement of the 632.8 nm monomode He-Ne/I2-laser

For the He-Ne/I₂-laser, a stable monomode operation has been realized when the tube pressure exceeds a certain level. An output power of more than 1 mW has been obtained at the 632.8 nm laser transition. Within the monomode tuning range of the laser, the d, e, f, g, and h, i, j iodine hyperfine components of the R(127) line of the 11–5 band of the B ³ II ⁺ _0u–X ¹⁺ _0g electronic transition have been observed. The mode selection method used makes it possible to increase the contrast of the iodine hyperfine components.     

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Ohne Zusammenfassung     

Acidic properties of TiO2?SiO2 catalysts

The acidic properties of a series of TiO₂–SiO₂ catalysts with different Ti/Si mole ratios have been studied. In order to determine the amount of acid centers and the acid strength distribution, the Benesi method of discontinuous titration has been used. Thermogravimetric measurements were also carried out.

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TiO₂–SiO₂ Ti/Si. , - . .

     

Sign of nuclear electric quadrupole coupling constants in solids fromγ-ray anisotropy measurements after capture of polarized neutrons

An experimental method is presented which allows the determination of the sign of nuclear electric quadrupole interactions in solids. Activated target nuclei with a purely dipolar spin polarization are produced by capture of polarized thermal neutrons. The quadrupole coupling of the nuclei to the electric field gradient tensor in the target crystal converts this (dipolar) polarization partly into a (quadrupolar) alignment, which can be measured by the anisotropy of the-ray emission in a succeeding nuclear transition. The sign of the alignment created in this reorientation process depends on the sign of the electric quadrupole interaction. The reorientation effect can be enhanced by selective induction of nuclear magnetic resonance transitions. The method has been applied to measure the sign of the crystal electric field gradient (efg) in tetragonal MgF₂. Further, the sign of an efg in cubic CaF₂ originating from a¹⁹F interstitial adjacent to the activated²⁰F probe nucleus has been determined. The method is in principle applicable to a considerable number of nuclides.