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991.
W. Görner 《Journal of Radioanalytical and Nuclear Chemistry》1975,28(1-2):249-255
Pulser and live timer are alternate tools. Dead time effects can be expressed in terms of a pulse rate dependent factor of the counting yield. The task of their correction should be shifted from the live timer of the ADC to a central timing unit. A new method is proposed, combining the advantages of the pulser and the live timer, where by each selected and accepted event is adjoined to a clock time interval and each selected but not accepted event to a dead time interval. The length of each interval is determined by the arrival of the next selected event. 相似文献
992.
Roswitha Stejskal Ernst Urban Horst Völlenkle 《Monatshefte für Chemie / Chemical Monthly》1991,122(3):145-156
Summary Oxidation of isobenzofuranone1 yielded in a diastereoselective reaction epoxide2. Acidolysis of2 resulted in a mixture oftrans-glycols6 a (88%) and7 a (4%), which were separated by crystallization. The relative configuration of6 a and7 a at the chiral centers 3 a, 5, 6, and 7 a was determined by1H-NMR-spectroscopy and X-ray analysis of O-acetylated and 7 a-methylated derivatives. 相似文献
993.
The hyperfine structure of the X2∑+, υ = 0 state of 88Sr35Cl has been investigated using microwave—optical polarization spectroscopy. Resolution of the hyperfine structure in transitions between low-lying rotational states has allowed the first determination of magnetic dipole and electric quadrupole coupling constants. The results are: b = 18.663(89) MHz, c = 7.72(30)MHz, and eqQ = 3.96(84) MHz. 相似文献
994.
SiO reacts with F2, in an argon matrix after photolysis with a high-pressure mercury lamp to form OSiF2. Isotopic splitting (16O/18O and 28Si/29Si) and force constant calculations show that the 6 fundamentals observed can be assigned to the planar molecule OSiF2. The value of the force constant of the SiO double bond, which was calculated as approximately 9 × 102 N m?1 in earlier investigations, is confirmed by this work. 相似文献
995.
Serber Z Straub W Corsini L Nomura AM Shimba N Craik CS Ortiz de Montellano P Dötsch V 《Journal of the American Chemical Society》2004,126(22):7119-7125
Studying protein components of large intracellular complexes by in-cell NMR has so far been impossible because the backbone resonances are unobservable due to their slow tumbling rates. We describe a methodology that overcomes this difficulty through selective labeling of methyl groups, which possess more favorable relaxation behavior. Comparison of different in-cell labeling schemes with three different proteins, calmodulin, NmerA, and FKBP, shows that selective labeling with [(13)C]methyl groups on methionine and alanine provides excellent sensitivity with low background levels at very low costs. 相似文献
996.
Molt O Rübeling D Schäfer G Schrader T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(17):4225-4232
Host molecule 1 displays a high affinity in water towards catecholamines and especially related structures such as beta-blockers with extended aromatic pi-faces (up to 7x10(3) M(-1) for each single complexation step or 5x10(7) M(-2) for both steps). The amphiphilic structural design leads to an extensive self-association of host molecules through their aromatic flanks. Above a cmc (critical micelle concentration) of 3x10(-4) M, host 1 forms micelles that produce a favorable microenvironment for hydrophobic interactions with the included guest molecules. Electrostatic attraction of the ammonium alcohol by the phosphonate anions is thus combined with hydrophobic contributions between the aromatic moieties. Ionic hydrogen bonds with polar OH or NH groups of the guest enforce the non-covalent interactions, and finally lead to increased specificity. Both its affinity and its selectivity towards adrenergic receptor substrates are greatly enhanced if the receptor molecule 1 is transferred from water into a lipid monolayer. Catecholamines and beta-blockers lead to drastically different effects at concentrations approaching the micromolar regime. Especially beta-blockers with minute structural changes can be easily distinguished from each other. In both cases, extensive hydrophobic interactions with a self-associated and/or self-organized microenvironment are largely responsible for the observed high efficiency and specificity. 相似文献
997.
Hofmann C Boll R Heitmann B Hauser G Dürr C Frerich A Weitnauer G Glaser SJ Bechthold A 《Chemistry & biology》2005,12(10):1137-1143
The oligosaccharide antibiotic avilamycin A is composed of a polyketide-derived dichloroisoeverninic acid moiety attached to a heptasaccharide chain consisting of six hexoses and one unusual pentose moiety. We describe the generation of mutant strains of the avilamycin producer defective in different sugar biosynthetic genes. Inactivation of two genes (aviD and aviE2) resulted in the breakdown of the avilamycin biosynthesis. In contrast, avilamycin production was not influenced in an aviP mutant. Inactivation of aviGT4 resulted in a mutant that accumulated a novel avilamycin derivative lacking the terminal eurekanate residue. Finally, AviE2 was expressed in Escherichia coli and the gene product was characterized biochemically. AviE2 was shown to convert UDP-D-glucuronic acid to UDP-D-xylose, indicating that the pentose residue of avilamycin A is derived from D-glucose and not from D-ribose. Here we report a UDP-D-glucuronic acid decarboxylase in actinomycetes. 相似文献
998.
The protonated form of 1,8-bis(dimethylamino)naphthalene (DMANH(+)) consists of a rigid, aromatic framework, substituted by two amino groups that are connected by a strong, symmetric (on the NMR time-scale) hydrogen bond bridge. The reorientational motion of the molecule in dimethylformamide-d7 solution was characterized by T(1) and NOE measurements for aromatic (13)C nuclei. Treating the reorientation of DMANH(+) as anisotropic rotational diffusion of a rigid body, the diffusion tensor was determined with good accuracy. Measurements and interpretation of (15)N T(1) and NOE indicate that the proton transfer between potential minima in the hydrogen bond bridge is faster than the molecular reorientation. 相似文献
999.
Chibisov AK Slavnova TD Görner H 《Journal of photochemistry and photobiology. B, Biology》2003,72(1-3):11-16
Chlorophyll a (Chla) in aqueous solution (2-6% acetone) is present as mono- and dihydrated aggregated forms which are characterized by specific ground state absorption spectra. The amount of dihydrated form is larger in the presence of macromolecules, such as bovine serum albumin (BSA), lysozime and polyvinyl alcohol (PVA), increasing from BSA to lysozime and PVA. Chla in aqueous acetone with and without macromolecules is characterized by low fluorescence and the absence of triplet-triplet (T-T) absorption. The ratio of dihydrated to monohydrated forms is significantly influenced by triton X-100. For lower triton X-100 concentrations, i.e. smaller than the critical micelle concentration of 0.26 mM (cmc), dihydrated forms are converted into monohydrated in both aqueous acetone and the presence of BSA or lysozime. In the presence of PVA dihydrated forms appeared to be resistant to triton X-100 action. Moreover, for triton X-100 concentrations of 2-3 times higher than cmc the amount of these forms is increased with time. T-T absorption of both mono- and dihydrated Chla aggregates was not detected in the presence of [triton X-100] < cmc. The lack of T-T absorption in aqueous acetone solution as well as in the presence of macromolecules implies that the triplet lifetime of the chlorophyll aggregates is short (tauT<10 ns) and/or the quantum yield of intersystem crossing is small (<5 x 10(-3)). The Chla monomers start to be formed as solubilized in the micelle for [triton X-100] larger than cmc, showing substantial fluorescence and T-T absorption. 相似文献
1000.
Thermogravimetric studies, supported by IR and X-ray diffraction measurements, were carried out for La2(SeO4)3 · 5H2O in order to establish its decomposition scheme under various experimental conditions. Several analytical methods were tested for the simultaneous determination of (Se(IV) and Se(VI) in the quenched samples. TG weight losses and analyses of the solid intermediates indicated that after dehydration La2(SeO4)3 decomposes in nitrogen mainly to oxyselenite(IV) compoundsvia the normal selenite(IV) but in air the oxyselenate La2O2SeO4 was detected as one of the intermediates. The final product of the thermal degradation is La2O3. TG curves of La2(SeO4)3 · 5H2O were also compared with those obtained for the isostructural neodymium compound. 相似文献