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121.
Correction for pile-up losses in the amplifier is possible by the dead-time fraction indicator of the ADC in case of long-lived radionuclides. If the dead-time meter has been calibrated, an accuracy of 1.5% is feasible up to a dead-time fraction of 25%. The precision decreases from 1.5% at 10% dead-time fraction to 3% at a deadtime fraction of 30%. 相似文献
122.
Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier. 相似文献
123.
Marchand C Nguyen CH Ward B Sun JS Bisagni E Garestier T Hélène C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(9):1559-1563
Sequence-specific DNA recognition can be achieved by oligonucleotides that bind to the major groove of oligopyrimidine x oligopurine sequences. These intermolecular structures could be used to modulate gene expression and to create new tools for molecular biology. Here we report the synthesis and biochemical characterization of triple helix-specific DNA cleaving reagents. It is based on the previously reported triplex-specific ligands, benzo[e]pyridoindole (BePI) and benzo[g]pyridoindole (BgPI), covalently attached to ethylenediaminotetraacetic acid (EDTA). In the presence of iron, a reducing agent and molecular oxygen, BgPI-EDTA x FeII but not BePI-EDTA x FeII induced a double-stranded cut in a plasmid DNA at the single site where a triplex-forming oligonucleotide binds. At single nucleotide resolution, it was found that upon triplex formation BePI-EDTA x FeII led to cleavage of the pyrimidine strand and protection of the purine strand. BgPI-EDTA x FeII cleaved both strands with similar efficiency. The difference in cleavage efficiency between the two conjugates was rationalized by the location of the EDTA x FeII moiety with respect to the grooves of DNA (major groove: BePI-EDTA x FeII, minor groove: BgPI-EDTA x FeII). This work paves the way to the development of a new class of triple helix directed DNA cleaving reagents. Such molecules will be of interest for sequence-specific DNA cleavage and for investigating triple-helical structures, such as H-DNA, which could play an important role in the control of gene expression in vivo. 相似文献
124.
Institute of Chemistry, Moldavian SSR Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 759–760, September–October, 1988. 相似文献
125.
Nguyen Van Suc Nguyen Mong Sinh 《Journal of Radioanalytical and Nuclear Chemistry》1993,176(5):383-389
Radiochemical neutron activation analysis (RNAA) has been applied for determination of rare earth elements (REE) in Vietnamese monazite. The chemical separation procedure used is based on the chromatographic elution of rare earth groups, after the separation of233Pa(Th) in irradiated monazite samples by coprecipitation with MnO2, the rare earth elements were retained by Biorad AG1×8 resin column in 10% 15.4M HNO3-90% methanol solution. The elution of heavy rare earth (HREE) and middle rare earth (MREE) groups was carried out with 10% 1M HNO3-90% methanol and 10% 0.05M HNO3-90% methanol solution, respectively; while the light rare earths (LREE) were eluted from the column by 0.1M HNO3 solution. The accuracy of the method was checked by the analysis of granodiorite GSP-I and the rare earth values were in good agreement. 相似文献
126.
Anusuya Choudhury Michael E. Pierce Dieu Nguyen Louis Storace Pat N. Confalone 《Tetrahedron letters》2005,46(47):8099-8102
D-D4FC (1) is an anti-HIV agent currently under phase II clinical trial (Pharmaset Inc). Its molecular architecture is suitable for a Ferrier rearrangement kind of operation on a furanoid glycal to fix the position of the double bond and the relative stereochemistry. Despite the fact that classical Ferrier rearrangement does not work on furanoid glycals, a palladium mediated modified protocol has been developed for the glycosidation of an aromatization prone xylo-furanoid glycal (5) for the synthesis of D-D4FC. 相似文献
127.
128.
O. P. Charkin N. M. Klimenko T. P. Nguyen D. O. Charkin Y. -S. Wang H. -C. Chang S. H. Lin 《Russian Journal of Inorganic Chemistry》2006,51(10):1613-1622
The electronic and geometric structures, energy stability, normal mode frequencies, and spin density distribution have been calculated by the density functional theory B3LYP method with the Gen = 6-31+G*(Fe) + 6-31G(C,H,N,O), 6-31G*, and 6-311++G** basis sets for the deep fragmentation products of the free hemin ion with successive removal of methyl and vinyl groups in the electronic states with different multiplicities. The computation results are compared with the available experimental data and previous computation results for the fragmentation products of the isolated heme molecule and hemin ion with removal of carboxymethyl groups. The trends in the behavior of these properties are analyzed as a function of multiplicity, external charge, and the number of peripheral substituents at the porphyrin core. 相似文献
129.
130.