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41.
M. J. Gaylard G. C. MacLeod D. P. Smits M. E. West D. J. Walt 《Journal of chemical crystallography》1993,23(8):691-691
Future contributions to Journal of Crystallographic and Spectroscopic Research 相似文献
42.
J. Cable J. K. MacLeod M. R. Vegar R. J. Wells 《Journal of mass spectrometry : JMS》1973,7(10):1137-1152
By the use of site-specific and stereospecific deuterium labelling, it has been possible to show that loss of the elements of water from all possible bicyclo[3.3.1]nonanols (I to V) in the mass spectrometer, takes place in a substantially stereospecific manner. The position of abstraction of the hydrogen atom by the hydroxyl group can be related to the proximity effects that are present in the ground state conformers, while the [M]+ / [M – H2O]+ ratio appears to depend on the minimum H–-OH intramolecular distance in the more stable conformations. 相似文献
43.
BACKGROUND: Peptide-ligating technologies facilitate a range of manipulations for the study of protein structure and function that are not possible using conventional genetic or mutagenic methods. To different extents, the currently available enzymatic and nonenzymatic methodologies are synthetically demanding, sequence-dependent and/or sensitive to denaturants. No single coupling method is universally applicable. Accordingly, new strategies for peptide ligation are sought. RESULTS: Site-specific variants (Ser195-->Gly, S195G, and Ser195-->Ala, S195A) of Streptomyces griseus protease B (SGPB) were generated that efficiently catalyze peptide ligation (i.e., aminolysis of ester-, thioester- and para-nitroanilide-activated peptides). The variants also showed reduced hydrolytic activity relative to the wild-type enzyme. The ratio of aminolysis to hydrolysis was greater for the S195A variant, which was also capable of catalyzing ligation in concentrations of urea as high as 2 M. CONCLUSIONS: Mutagenic substitution of the active-site serine residue of SGPB by either glycine or alanine has created a unique class of peptide-ligating catalysts that are useful for coupling relatively stable ester- and para-nitroanilide-activated substrates. Ligation proceeds through an acyl-enzyme intermediate involving His57. Serine to alanine mutations may provide a general strategy for converting proteases with chymotrypsin-like protein folds into peptide-coupling enzymes. 相似文献
44.
Yan X MacLeod AM Gillespie WA Knippels GM Oepts D van Der Meer AF Seidel W 《Physical review letters》2000,85(16):3404-3407
Time-resolved measurements of the transverse electric field associated with relativistic electron bunches are presented. Using an ultrafast electro-optic sensor close to the electron beam, the longitudinal profile of the electric field was measured with subpicosecond time resolution and without time-reversal ambiguity. Results are shown for two cases: inside the vacuum beam line in the presence of wake fields, and in air behind a beryllium window, effectively probing the near-field transition radiation. Especially in the latter case, reconstruction of the longitudinal electron bunch shape is straightforward. 相似文献
45.
A high yielding synthesis of 2-phenethylglucosinolate is described. The method should also be directly applicable to most (if not all) other glucosinolates. 相似文献
46.
47.
The nucleide 73Kr has been identified by on-line mass separation as a precursor of β-delayed proton emission. The proton branch is (6.8 ±1.2) × 10−3 proton/decay. The protons populate the ground state and also the first excited 2+ state at 866 keV in 72Se with a relative intensity of (35±9) %. The value of QEC−Bp, where Bp is the proton separation energy for the nucleus 73Br, is found to be 4.85 ±0.30 MeV based on the fraction of proton events preceded by positron decay. 相似文献
48.
MacLeod JK Flanigan IL Williams JF Collins JG 《Journal of mass spectrometry : JMS》2001,36(5):500-508
The (EIMS) electron ionization mass spectrometric fragmentation patterns of the methoxime- and ethoxime-trimethylsilyl (TMS) derivatives of C(4) to C(7) sugars involved as phosphates in the Calvin pathway of photosynthesis in plants were analysed by gas chromatography/EIMS using specifically labelled (13)C analogs. In general, most but not all of the major ions in the mass spectra arise from single carbon-carbon bond cleavages of the straight-chain derivatives. The results confirm that GC/MS of the alkoxime-TMS derivatives is a viable method for measuring (13)C incorporations at individual carbon atoms in each of the sugar phosphates during photosynthetic experiments with (13)CO(2). 相似文献
49.
Although the loss of a C2H4O molecule from the molecular ions of the isomeric bicyclo[3.3.1]nonan-2- and 3-ones gives rise to the base peak at m/e 94 in both spectra, deuterium labelling results show that the processes leading to this ion are markedly different in the two cases. While the 2-one follows essentially (>80%) the same [M ? C2H4O]+ fragmentation pathway described for other 2-alkylcyclohexanones, the 3-one exhibits a more complex decomposition pathway in which one α-hydrogen is transferred away from the departing C2 unit and three hydrogen atoms are subsequently rearranged to the eliminated C2H4O molecule. Similar competing fragmentation schemes have been invoked to explain deuterium labelling results in cycloheptanone, 2-methylcycloheptanone and cyclooctanone. 相似文献
50.
Paula J. MacLeod Richard Barber Peter G. Odell Barkev Keoshkerian Michael K. Georges 《Macromolecular Symposia》2000,155(1):31-38
Stable emulsions of polystyrene have been prepared by nitroxide-mediated living-radical miniemulsion polymerization. Optimization of the ratios and amounts of surfactant, cosurfactant, and nitroxide provided both good latex properties and good living-radical chemistry. Results were superior to those obtained in comparable bulk polymerizations. Conversions of 87% were obtained in 6 hours at 135°C without the use of a rate accelerant and the molecular weight distributions were typically in the 1.1 range. 相似文献