Biomedical subjects
Publications and source records attributed to Edgar Weckert.
The optical rotatory power of achiral crystals of achiral pentaerythritol molecules was measured. The maximum rotations were found to be +/-6 degrees /mm. The quantum mechanically computed rotation of pentaerythritol molecules using linear response theory was 6 times larger although the experimental and theoretical tensors were similarly oriented to within 5 degrees .
For the second-generation asymmetric synthesis of the trans-tris(homoglutamic) acids via Strecker reaction of chiral ketimines, the cyanide addition as the key stereodifferentiating step produces mixtures of diastereomeric alpha-amino nitrile esters the composition of which is independent of the reaction temperature and the type of the solvent, respectively. The subsequent hydrolysis is exclusively achieved with concentrated H(2)SO(4) yielding diastereomeric mixtures of three secondary alpha-amino alpha-carbamoyl-gamma-esters and two diastereomeric cis-fused angular alpha-carbamoyl gamma-lactams as bicyclic glutamic acid derivatives, gained from in situ stereomer differentiating cyclisation of the secondary cis-alpha-amino alpha-carbamoyl-gamma-esters. Separation was achieved by CC. The pure secondary trans-alpha-amino alpha-carbamoyl-gamma-esters cyclise on heating and treatment with concentrated H(2)SO(4), respectively, to diastereomeric cis-fused angular secondary alpha-amino imides. Their hydrogenolysis led to the enantiomeric cis-fused angular primary alpha-amino imides. The configuration of all compounds was completely established by NMR methods, CD-spectra, and by X-ray analyses of the (alphaR,1R,5R)-1-carbamoyl-2-(1-phenylethyl)-2-azabicyclo[3.3.0]octan-3-one and of the trans-alphaS,1S,2R-2-ethoxycarbonylmethyl-1-(1-phenylethylamino)cyclopentanecarboxamide.
Structural analysis of the lectin SML-2 faced difficulties when applying standard crystallographic phasing methods. The connectivity-based ab initio phasing method allowed the computation of a 16 A resolution Fourier synthesis and the derivation of primary structural information. It was found that SML-2 crystals have three dimers in the asymmetric part of the unit cell linked by a noncrystallographic symmetry close to translation by (0, 0, 1/3). A clear identification of the noncrystallographic twofold axis explains the space-group transformation from the primitive P2(1)2(1)2(1) to the C-centred C222(1) observed during annealing procedures within an N(2) cryostream for cocrystals of SML-2 and galactose. Related packing considerations predict a possible arrangement of SML-2 molecules in a tetragonal unit cell. Multiple noncrystallographic symmetries and crystal forms provide a basis for further image improvements.
Perturbation of the two-beam diffracted power owing to the influence of a third lattice node has been examined for various three-beam cases in a small finite germanium crystal in the vicinity of the K-absorption edge. Although the crystal was slightly imperfect, the main parts of the experimental results are very well described within the framework of the fundamental theory of X-ray diffraction in conjunction with Cromer-Liberman calculations for the resonant scattering terms. Beam divergence and dynamical block size are treated as adjustable parameters in the analysis. Observed changes in the three-beam profile asymmetry are mainly attributed to size and not to resonance effects associated with the triplet phase sum of the involved reflections. Close to the absorption edge there is however some evidence indicating that f' values should be reduced in magnitude compared to the tabulated ones.
Based on the Takagi-Taupin equations, an analytical expression for the three-beam diffraction profile function in an elastically strained crystal plate is derived. The strain parameters appear in the shape functions governing both triplet-phase dependent and independent terms in the solution. Simulations show that three-beam profiles are significantly influenced. With increasing strain any phase information in the profiles is obscured and subsequently lost.
Differences in mosaicity between lysozyme crystals grown inside and outside a homogeneous magnetic field of 2.4 T and with and without agarose gel were investigated by X-ray diffraction rocking-curve measurements. High angular resolution was achieved using an Si(113) four-reflection Bartels monochromator. The results show that (i) all crystals were highly perfect, (ii) the mosaicities were clearly anisotropic and (iii) the mosaicities varied more strongly within each group of crystals (grown under identical conditions) than the average values across groups. In particular, the effect of the magnetic field on crystal mosaicity was found to be very small. Finally, the spatial distribution of mosaic blocks inside a protein crystal was visualized with a novel diffraction technique using a high spatial resolution two-dimensional CCD detector.
In addition to the two so far known types of metalloid Ga(22) clusters a new type is presented in two compounds containing the anions [Ga(22)Br[N(SiMe(3))(2)](10)Br(10)](3-) (1) and [Ga(22)Br(2)[N(SiMe(3))(2)](10)Br(10)](2-) (2). In both anions 10 Ga atoms of the icosahedral Ga(12) core are directly connected to further Ga atoms. The two remaining Ga atoms (top and bottom) of the Ga(12) icosahedron are bonded to one (1) and two Br atoms (2), respectively. The formation and structure of both compounds containing a slightly different average oxidation number of the Ga atoms is discussed and compared especially with regard to the Ga(84) cluster compound and similar metalloid Al(n) clusters. Finally, the consequences arising from the presence of two very similar but not identical Ga(22) cluster compounds are discussed and special consideration is given to the so far not understood physical properties (metallic conductivity and superconductivity) of the Ga(84) cluster compound.
The Ga(84)R(20)(4-) [R = N(SiMe(3))(2)] species, which represents the largest metalloid cluster entity structurally characterized so far, has been electronically and topologically modified: Via changing the redox potential of the reaction solution, crystals different from those containing the Ga(84)R(20)(4-) anion can be isolated, featuring similar Ga(84)R(20)(3-) entities. An accurate crystal structure analysis via synchrotron radiation is presented, which might be the first step toward an understanding of the metallic conductivity and superconductivity of the Ga(84)R(20)(4-) cluster compound, physical properties which are singular in the field of metalloid clusters so far.
Triplet phases recorded from insulin crystals were used to measure the improvement of phases during model refinement and to quantify the contribution made by each step in the refinement. Conventional amplitude data were recorded to 1.5 A resolution from rhombohedral pig insulin crystals using 1.54 A Cu Kalpha radiation. An initial atomic model and starting phases were obtained from a published structure and the atomic model was refined against the amplitude data using CNS. The refined phases were compared with 800 triplet phases that were measured from similar crystals using a three-beam interference technique and 1.1 A wavelength synchrotron radiation. The solvent region was improved further using a novel density-modification procedure. Calculated triplet phases were obtained from the model after each step in the refinement and were compared with the recorded triplet phases. The average difference between the recorded triplet phases and the calculated triplet phases was used as an unbiased measure of the correctness of the model at each stage in the refinement. The average individual phase error was estimated from discrepancies from triplet phases after each refinement step. Conventional atomic refinement of an approximate starting model reduced the average individual phase error from 21.6 to 14.7 degrees. Improvement of the solvent region, including the difference-map flattening procedure, reduced the individual phase error by a further 2.6 degrees. Modeling the discrete disorder of four amino acids accounted for an additional 0.5 degrees improvement and the final individual phase error was 11.6 degrees.
Changes in d-spacing have been measured for organic compounds as a function of temperature, wavelength and primary beam intensity using intense synchrotron radiation. The d-spacings of organic and protein crystals were found to increase irreversibly as a function of radiation dose. The activation energy of this thermally activated process was estimated. No evidence for a significant temperature increase of the sample due to exposure to intense X-ray beams could be found experimentally for flux densities up to 4 x 10(12) photons s-1 mm-2.