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131.
Two general methods for the preparation of β-silyl-substituted divinyl ketones have been developed starting from either α, β-unsaturated aldehydes or simple ketones. Anhydrous FeCl3 induces the cyclization to cyclopentenones under mild conditions and in good yields with predictable and complete control over the position of the double bond in the five-membered ring. The observed effects of substituents on rate can be explained by a rate-determining cationic electrocyclization. Silyl substitution has been shown to retard the reaction. 相似文献
132.
Two methods of determining trace levels of platinum and gold in aqueous solutions with high concents of total dissolved solids were investigated. The first involves preconcentration and separation of the precious metals from the interfering matrix by solvent extraction, followed by graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman effect background correction. The direct determination of Pt and Au in solutions of high ionic strength by GFAAS is not desirable because of interference between elements in the matrix and the analyte, increased imprecision of analysis, greatly increased background absorbance leading to increased detection limits and rapid deterioration of the graphite tube. All the extraction methods for gold examined in this study resulted in decreased imprecision, increased sensitivity and lower background absorbance compared with direct measurements on the aqueous solution. All techniques also exhibited good recoveries (> 8%) and reproducibilities (relative standard deviation < 10%). The highest sensitivities for gold extraction from distilled water were obtained for dibutyl sulfide (DBS)—toluene and the lowest for cyanide—dibutyl ketone. The degree of extraction of Au was, however, dependent on the composition of the solution, indicating that standard and sample matrices should be closely matched even when employing solvent extraction. Solvent extraction was generally less successful for Pt. In order to obtain an acceptable imprecision in the Pt extractions, it was found that the use of SnCl2 as a labilizing agent is essential for most of the techniques investigated.The second method was direct measurement by inductively coupled plasma mass spectrometry (ICP—MS). ICP—MS offers the advantages of a very low detection limit (100 ng l?1 or better) without preconcentration and a large dynamic range. However, severe matrix effects can occur in concentrated solutions. Whereas high concentrations in solution of both sodium perchlorate and sodium chloride decrease the sensitivity, the presence of sulfide and natural organic (fulvic) acid increase the sensitivity for Pt and Au by a factor of up to 4. Sulfate, on the other hand, decreases the sensitivity of ICP-MS for Pt. The method of standard additions or isotope dilution is recommended for routine use to circumvent this problem, especially when the nature of the matrix is unknown or cannot be easily matched in the standards. 相似文献
133.
Russell SC Czerwieniec G Lebrilla C Tobias H Fergenson DP Steele P Pitesky M Horn J Srivastava A Frank M Gard EE 《Journal of the American Society for Mass Spectrometry》2004,15(6):900-909
The appearance of informative signals in the mass spectra of laser-ablated bio-aerosol particles depends on the effective ionization probabilities (EIP) of individual components during the laser ionization process. This study investigates how bio-aerosol chemical composition governs the EIP values of specific components and the overall features of the spectra from the bio-aerosol mass spectrometry (BAMS). EIP values were determined for a series of amino acid, dipicolinic acid, and peptide aerosol particles to determine what chemical features aid in ionization. The spectra of individual amino acids and dipicolinic acid, as well as mixtures, were examined for extent of fragmentation and the presence of molecular ion dimers, which are indicative of ionization conditions. Standard mixtures yielded information with respect to the significance of secondary ion plume reactions on observed spectra. A greater understanding of how these parameters affect EIP and spectra characteristics of bio-aerosols will aid in the intelligent selection of viable future biomarkers for the identification of bio-terrorism agents. 相似文献
134.
We have investigated the dielectrophoretic assembly of colloidal gold, carbon black, and carbon nanotubes into electrical wires. The resulting microwires have diameters less than 1 microm, with lengths ranging from 5 microm to 3 mm. Current-voltage curves for these wires indicate an ohmic response, where the resistance is determined by the type of colloid and by the frequency of the alternating field used to grow the wires. The predicted frequency dependence of dielectrophoresis is confirmed by experiment. Measurements of the threshold voltage for initial wire growth are also presented. These experiments demonstrate that a variety of nanoparticles can be assembled into microwires for sensor applications. 相似文献
135.
A variety of 1-(-alkylphenyl)- and 1-(-'-dialkylphenyl)- pyrrolidines and -piperidines were prepared by the gas phase alumina mediated condensation of tetrahydrofuran (THF), tetrahydropyran (THP) or the corresponding diols with primary aromatic amines in fair to high yield. This methodology can also be used for the synthesis of 1-phenylhexahydroazepine from aniline. A mechanistic interpretation of the catalytic action of alumina is presented. 相似文献
136.
Springfield SA Marcantonio K Ceglia S Albaneze-Walker J Dormer PG Nelson TD Murry JA 《The Journal of organic chemistry》2003,68(11):4598-4599
In this paper, we disclose an efficient one-pot procedure for the preparation of substituted 1,8-naphthyridin-4-one analogues. Previous efforts to effect this type of transformation were complicated by the formation of benzene tricarboxylate. Via the use of excess base, the impurity formation was completely inhibited. This allowed for the clean preparation of the desired intermediate and subsequent formation of naphthyridone analogues in a single flask, which could then be crystallized directly from the reaction mixture in good yield and high purity. 相似文献
137.
Conboy JC McReynolds KD Gervay-Hague J Saavedra SS 《Journal of the American Chemical Society》2002,124(6):968-977
The interaction of recombinant HIV-1 surface glycoprotein gp120 (rgp120) with natural isolates of lactosylceramide (LacCer), glucosylceramide (GlcCer), and galactosylceramide (GalCer) has been quantitatively measured under equilibrium conditions using total internal reflection fluorescence (TIRF) spectroscopy. The binding affinity (K(a)) of rgp120 to these glycosphingolipids (GSLs), reconstituted at 5 mol % in supported planar lipid bilayers composed of 95 mol % POPC, is ca. 10(6) M(-1) for dissolved rgp120 concentrations greater than 25 nM. In contrast, at concentrations of rgp120 between 0.2 and 15 nM, rgp120 does not bind significantly to LacCer and GlcCer, but has a high affinity for GalCer with a measured K(a) value of 1.6 x 10(9) M(-1). However, protein surface coverage measurements show that this strong binding process accounts for very little of the total protein adsorbed over the entire concentration range studied. At a protein concentration of ca. 20 nM, the surface coverage is only 3% of that achieved at apparent saturation (i.e., when the protein concentration is ca. 220 nM). Thus the "high affinity" binding sites comprise only a small fraction of the total number of binding sites. Several other variables were investigated. Rgp120 binding behavior at membranes doped with alpha-hydroxygalactosylceramide (alpha-GalCer) was very similar to that observed with GalCer, showing that the presence/absence of an alpha-hydroxy moiety does not significantly affect galactosylceramide recognition. Phase segregation of GalCer, which occurs when the mole fraction of this GSL in a POPC bilayer exceeds ca. 0.1, was also investigated and showed no effect on binding affinity at low rgp120 concentrations. To investigate the influence of fatty acid chain length, GSLs with monodisperse C(18) and C(24) chain lengths, both with and without an alpha-hydroxy moiety, were synthesized, and their binding affinity to rgp120 was examined. Relative to the natural isolates (which contain a mixture of chain lengths), minimal differences were observed; thus among the compounds tested, fatty acid chain length does not affect GSL recognition. The results of this work should aid efforts to design anti-HIV-1 agents based on membrane-tethered, carbohydrate-based receptors for rgp120. 相似文献
138.
A silanediol inhibitor of the metalloprotease thermolysin was prepared for comparison to a known phosphinic acid inhibitor, providing the first comparison of these second-row element based transition-state analogues. Inhibition of thermolysin by the silanediol (K(i) = 41 nM) was comparable to that of the phosphinic acid (K(i) = 10 nM) even though the silanediol is uncharged and thereby lacks the intrinsic Coulombic attraction of the phosphinate anion to the active-site zinc cation. This silanediol protease inhibitor is the least sterically encumbered example prepared to date and, therefore, the most prone toward polymerization. Hydrolysis of a difluorosilane intermediate to the silanediol leads cleanly to a monomeric product. 相似文献
139.
Wong KL Bitter M Hammett GW Heidbrink W Hendel H Kaita R Scott S Strachan JD Tait G Bell MG Budny R Bush C Chan A Coonrod J Efthimion PC England AC Eubank HP Fredrickson E Furth HP Goldston RJ Grek B Grisham L Hawryluk RJ Hill KW Johnson D Kamperschroer J Kugel H Ma C Mansfield D Manos D McCune DC McGuire K Medley SS Mueller D Nieschmidt E Owens DK Paré VK Park H Ramsey A Rasmussen D Roquemore AL Schivell J Sesnic S Taylor G Williams MD Zarnstorff MC 《Physical review letters》1985,55(23):2587-2590
140.