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41.
Yoel Tsur Yitzchak L. Freilich Moshe Levy 《Journal of polymer science. Part A, Polymer chemistry》1974,12(7):1531-1539
The degradation of a series of polybenzimidazoles containing metylene groups in the chain was studied by TGA and MS. On heating the polymer derived from m-phenylenediacetic acid and 3,3′-diaminobenzidine to about 560°C in argon, partial fast decomposition into a solid sublimate was observed. The sublimate consisted of a mixture of fractions derived from the breaking of the chain at the methylene bonds. These methylene groups can be preoxidized at 250°C to carbonyl groups, eliminating thereby this mode of decomposition. At temperature higher than 560°C, gaseous products similar to those of the corresponding all-aromatic PBI are obtained. 相似文献
42.
Y. Kashman 《Tetrahedron》1979,35(2):263-266
The isolation and structure determination of two oxygenated calamenene sesquiterpenes from a gorgonian is reported. The NMR (1H and 13C) and CD spectra of these new compounds are interpreted and the stereochemistry of the system discussed. 相似文献
43.
[structure: see text] A novel cyclic peptide, callynormine A, was isolated from the Kenyan marine sponge Callyspongia abnormis and its structure elucidated by interpretation of its NMR data and X-ray diffraction analysis. Callynormine A represents a new class of heterodetic cyclic peptides (designated endiamino peptides) possessing an alpha-amido-beta-aminoacrylamide cyclization functionality. 相似文献
44.
Yoel Tikochinsky 《International Journal of Theoretical Physics》1988,27(5):543-549
Starting with very simple assumptions, Feynman rules for the quantum mechanical amplitudes and the associated probabilities are derived. These rules emerge as the only consistent rules for manipulating complex amplitudes assigned to processes. The probability of a process to which an amplitudex has been assigned is determined asp(x)=|x|
, 0<2. If virtual processes are allowed,=2. 相似文献
45.
46.
Hagit Sorek Sophie Gueta Maria Jesus Martin Emile Gaydou Yoel Kashman 《Tetrahedron》2006,62(37):8838-8843
In our continuing program to identify bioactive compounds from marine invertebrates, the MeOH/EtOAc (1:1) extract of three collections of the Madagascar sponge, Biemna laboutei, was found to be cytotoxic to a series of human tumor cells. From the two sponges, seven new guanidine alkaloids, designated netamines A-G (1-7), have been isolated and their structures elucidated. Compounds 3 and 4 were found to be cytotoxic against three tumor cells with GI50 values in the micromolar range. 相似文献
47.
Novaxenicins A-D and xeniolides I-K, seven new diterpenes from the soft coral Xenia novaebrittanniae
Seven new diterpenes, novaxenicins A-D (1-4) and xeniolides I-K (5-7), have been isolated from the Kenyan soft coral Xenia novaebrittanniae. The structures and relative stereochemistry of the compounds were elucidated by interpretation of MS, COSY, HSQC, HMBC, and NOESY experiments. The structure of novaxenicin A (1) was secured by X-ray diffraction analysis. Compound 5 possesses anti-bacterial activity at a concentration of 1.25 μg/ml and compound 2 induces apoptosis in transformed mammalian cells at a concentration of 1.25 μg/ml. 相似文献
48.
Two new compounds, designated as itampolins A (1) and B (2) were isolated from the sponge Iotrochota purpurea collected at Itampolo, southwest of Tuléar, Madagascar. The structures of the compounds were elucidated mainly on the basis of 1D and 2D NMR and mass spectroscopic data and, in the case of 1, also by chemical degradation. 相似文献
49.
Thiols are oxidized to the corresponding disulfides with a mild base, anhydrous potassium phosphate, under ambient conditions in the presence of air. 相似文献
50.
Ronald R. Coifman Yoel Shkolnisky Fred J. Sigworth Amit Singer 《Applied and Computational Harmonic Analysis》2010,28(3):296-312
Recovering the three-dimensional structure of molecules is important for understanding their functionality. We describe a spectral graph algorithm for reconstructing the three-dimensional structure of molecules from their cryo-electron microscopy images taken at random unknown orientations.We first identify a one-to-one correspondence between radial lines in three-dimensional Fourier space of the molecule and points on the unit sphere. The problem is then reduced to determining the coordinates of points on the sphere given a subset of their pairwise geodesic distances. To recover those coordinates, we exploit the special geometry of the problem, as rendered by the Fourier projection–slice theorem, to construct a weighted graph whose vertices are the radial Fourier lines and whose edges are linked using the common line property. The graph organizes the radial lines on the sphere in a global manner that reveals the acquisition direction of each image. This organization is derived from a global computation of a few eigenvectors of the graph's sparse adjacency matrix. Once the directions are obtained, the molecule can be reconstructed using classical tomography methods.The presented algorithm is direct (as opposed to iterative refinement schemes), does not require any prior model for the reconstructed object, and is shown to have favorable computational and numerical properties. Moreover, the algorithm does not impose any assumption on the distribution of the projection orientations. Physically, this means that the algorithm is applicable to molecules that have unknown spatial preference. 相似文献