Enantio- and Diastereoselective Aldol-Reaction of 2,6-Dimethylphenyl Propionate Using Titanium-Carbohydrate Complexes

Chloro(cyclopentadienyl)bis(1,2:5,6-di-O-isopropylidene-α-D -glucofuranos-3-O-yl)titanium ( 1 ) is used for the transmetallation of Li-enolates obtained from propionyl derivatives. While such Ti-enolates of ketones and hydrazones appear to be unreactive, the (E)enolate 13 of 2,6-dimethylphenyl propionate ( 11 ) adds to the re-side of aldehydes, affording various syn-aldols 14 with high dia- and enantioselectivity (92–97% ds, 91–97% ee, cf. Scheme 2 and Table 1). Racemic syn-aldols (±)- 14 are obtained analogously from the achiral bis(2-propyloxy)-Ti-enolate 15 (Scheme 2 and Table 2). In contrast to the unstable Li-enolate 10 , the Ti-enolates 13 and 15 isomerize at ?30°, presumably to the thermodynamically more stable (Z)-enolates (Scheme 4), While the diastereoselectivity of the achiral enolate 15 is lost upon this equilibration, the chiral (Z)-enolate 27 quite unexpectedly affords anti-aldols 12 of high optical purity (94–98% ec) and, in most cases, with acceptable-to-good diastereoselectivity (82–90% ds). Notable exceptions are branched unsaturated and aromatic aldehydes which form a greater proportion of synepimers of moderate optical purity (Scheme 5 and Table 3). Consistent with these findings, re-facial-and ami -selective aldol-addition is also exhibited by the (Z)-configurated Ti-enolate 22 of N-propionyl-oxazolidi-none 19 (Scheme 3).     

Quantitative Auswertung von Dünnschicht-Chromatogrammen: On line-Kopplung mit programmierbaren elektronischen Tischrechnern

Reprocessing of spherical THTR fuel elements shall be tested in the Jülich pilot plant JUPITER. This fuel type differs significantly from other fuel elements with respect to shape, composition and fissile material content. It requests special provisions for reprocessing and the necessary material balancing and safeguarding. Two material balance areas (MBA) are defined: head end and chemical extraction process. Within the 1. MBA uranium and thorium are balanced mainly by using a combination of digital counting of the fuel spheres, gammaspectrometric burn-up determination of individual spheres and X-ray fluorescence determination of uranium and thorium in nitric acid solutions which have been obtained by dissolution in Thorex reagent of the heavy metal oxides after burning of the graphite matrix. The 2. MBA begins with the solution for the chemical extraction process, collected in the so called accountability tank. After extraction according to the Thorex flowsheet the process streams are monitored in line for process control, and off line for material balancing and safeguarding. This is performed mainly by X-ray fluorescence analysis, potentiometric titrations, alpha- and mass spectrometry.     

Mechanistic studies of the isomerization of peroxynitrite to nitrate catalyzed by distal histidine metmyoglobin mutants

Hemoproteins are known to react with the strong nitrating and oxidizing agent peroxynitrite according to different mechanisms. In this article, we show that the iron(iii) forms of the sperm whale myoglobin (sw Mb) mutants H64A, H64D, H64L, F43W/H64L, and H64Y/H93G catalyze the isomerization of peroxynitrite to nitrate. The two most efficient catalysts are H64A (k(cat) = (5.8 +/- 0.1) x 10(6) M(-1) s(-1), at pH 7.5 and 20 degrees C) and H64D metMb (k(cat) = (4.8 +/- 0.1) x 10(6) M(-1) s(-1), at pH 7.5 and 20 degrees C). The pH dependence of the values of k(cat) shows that HOONO is the species which reacts with the heme. In the presence of physiologically relevant concentrations of CO(2) (1.2 mM), the decay of peroxynitrite is accelerated by these metMb mutants via the concurring reaction of HOONO with their iron(iii) centers. Studies in the presence of free added tyrosine show that the metMb mutants prevent peroxynitrite-mediated nitration. The efficiency of the different sw metMb mutants correlates with the value of k(cat). Finally, we show that sw WT-metMb is nitrated to a larger extent than horse heart metMb, a result that suggests that the additional Tyr151 is a site of preferential nitration. Again, the extent of nitration of the tyrosine residues of the metMb mutants correlates with the values of k(cat).     

Determination of cobalt in urine by gas chromatography-mass spectrometry employing nickel as an internal standard.

In addition to the electric field and pH gradient used in isoelectric focusing, a recently introduced technique, isoelectric focusing (or electrical hyperlayer) field-flow fractionation, employs the flow of the liquid carrier through a thin separation channel as a third factor affecting separation. Focusing of cytochrome c (CYTC) in a trapezoidal cross-section channel of 0.875 ml volume and 25 cm length was investigated as a function of the injection procedure, relaxation time, flow-rate of the carrier ampholyte solution and applied electric power. The influence of different initial conditions was also investigated by computer simulation. Both computed and experimental data showed an important contribution of the injection procedure and relaxation time on the retention and shape of the CYTC zone. It follows from these data that the sample should be injected as a narrow zone into the centre of the stream rather than homogeneously together with the carrier solution. For the described experimental set-up it could be demonstrated that the time necessary for zone formation should be at least 15 min and that relaxation times in excess to 20 min do not influence the final shape of the CYTC zone. It could further be shown experimentally that the sample must be injected under an applied electric field, that the relaxation time should be about 10 min, that the elution flow-rate should not be larger than 100 μl/min, that focusing becomes more efficient with increasing electric fields and that, for a given assembly and specified flow conditions, there is an electric power window only within which proper operation is possible.     

Unfolding of truncated and wild type aspartate aminotransferase studied by size-exclusion chromatography

The reversible unfolding of globular proteins with increasing concentration of guanidinium chloride (GuCl) can be analysed by size-exclusion chromatography, because the hydrodynamic volume of the proteins increases during unfolding. The dimeric enzyme aspartate aminotransferase (AAT) shows an uncoupled dissociation of the identical subunits followed by the unfolding of the monomers. During the monomer unfolding formation of an intermediate is observed. A monomeric mutant of AAT unfolds with a similar shape of the unfolding transition phase, but is less stable, as shown by a shift of the transition mid-point from 1.7 M GuCl for the wild type to 1.3 M GuCl for the mutant.     

H-bonding dependent structures of (NH4+)3H+(SO4 2-)2. Mechanisms of phase transitions

The role of different H-bonds in phases II, III, IV, and V of triammonium hydrogen disulfate, (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), has been studied by X-ray diffraction and (1)H solid-state MAS NMR. The proper space group for phase II is C2/c, for phases III and IV is P2/n, and for phase V is P onemacr;. The structures of phases III and IV seem to be the same. The hydrogen atom participating in the O(-)-H(+).O(-) H-bond in phase II of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) at room temperature is split at two positions around the center of the crucial O(-)-H(+).O(-) H-bonding, joining two SO(4)(2)(-) tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The NH(4)(+) ions participate mainly in bifurcated H-bonds with two oxygen atoms from sulfate anions. On cooling, the major contribution of the bifurcated H-bond becomes stronger, whereas the minor one becomes weaker. This is coupled with rotation of sulfate ions. In all the phases of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), some additional, weak but significant, reflections are observed. They are located between the layers of the reciprocal lattice, suggesting possible modulation of the host (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) structure(s). According to (1)H MAS NMR obtained for phases II and III, the nature of the acidic proton disorder is dynamic, and localization of the proton takes place in a broader range of temperatures, as can be expected from the X-ray diffraction data.