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81.
Functionalized keto-enamines 6 were obtained by nucleophilic addition of enol ethers to the imine moiety of 2-azadienes derived from dehydroaspartic esters 4. Reactions of 2-azadiene 4c containing three electron-withdrawing substituents (CO(2)R) with enol ethers 5 in the presence of lithium perchlorate led to the formation of tetrahydropyridine derivatives 7 in a regio- and stereoselective fashion. 2H-[1,3]-oxazines 10 and pyridine derivatives 12 and 13 were obtained by heterocycloaddition reactions of electron-poor azadienes 4d-g containing two electron-withdrawing substituents (4-O(2)N-C(6)H(4), CO(2)R) in positions 1 and 4 with carbonyl derivatives (ethyl glyoxalate 9a and diethyl ketomalonate 9b) and the electron-deficient olefin tetracyanoethylene 11. 相似文献
82.
This work serves as a follow-up to Part I of experiments designed to determine the underlying principles in the formation of pseudomolecular, or adduct, ions during electrospray ionization. Aromatic acids were studied by flow injection analysis in the negative ionization mode of electrospray ionization mass spectrometry. Part I dealt with common acidic anti-inflammatory pharmaceuticals. such as ibuprofen and related analogues. Part II deals with functionally less complex molecules, namely benzoic acid (BA) and substituted benzoic acids. Halide-substituted molecules are investigated to deduce the effect of electron-withdrawing substituents (bromo-, chloro-, and fluoro-) and ring position (ortho-, meta- and para-) on the response of a traditional deprotonated molecular ion ([M-H]-) and a sodium-bridged dimer ion ([2M-2 H+Na]-). Amino-substituted benzoic acids are also analyzed in order to study the effect of an additional ionizable group on the molecule, and para-tert.-butyl-BA is analyzed to study the effect of increased hydrophobicity, as they relate to the formation of pseudomolecular ions. This study shows that solution character [octanol-water partition coefficient (or log P) and pKa] of the model compounds controls the relative efficiency of formation of [M-H]- and [2M-2H+Na]- ions. However the relative gas phase character (gas phase basicity and proton affinity) also has a significant effect on the formation of the sodium-bridged dimer ion. For the halide-substituted species, placement of the electron-withdrawing atom at the meta-position gives the greatest enhancement in sensitivity. Observations also show that as the structural complexity of the model compound increases, predictions relating analyte acidity to sodium-bridged dimer ion formation give way to a stronger dependence between log P values and ionization efficiency. Supporting this hypothesis is the nearly ten-fold enhancement in signal for tert.-butyl BA relative to BA. due to the greater hydrophobicity, and consequently, increased surface activity in an electrosprayed droplet of the analyte molecule. 相似文献
83.
Breakstone A Crawley HB Dallavalle GM Doroba K Drijard D Fabbri F Firestone A Fischer HG Frehse F Geist W Giacomelli G Gokieli R Gorbics M Hanke P Heiden M Herr W Kluge EE Lamsa JW Lohse T Meyer WT Mornacchi G Nakada T Panter M Putzer A Rauschnabel K Rimondi F Siroli GP Sosnowski R Szczekowski M Ullaland O Wegener D 《Physical review letters》1985,55(20):2180-2183
84.
Estimation of diuretic drugs in biological fluids by HPLC 总被引:1,自引:0,他引:1
Summary This critical review of different methods proposed for the determination and screening of diuretics is directed mainly, because of its potential application, towards highperformance liquid chromatography. 相似文献
85.
Molecular mechanics and dynamics calculations have been used in conjunction with experimental data to study the effects of amine ligand bulk on the formation of both guanine and methionine complexes with platinum diamine compounds. The AMBER force field has been supplemented with previous modifications [Yao; et al. Inorg. Chem. 1994, 33, 6061-6077. Cerasino; et al. Inorg. Chem. 1997, 36, 6070-6079] and has been further modified to include parameters for platinum bound to the sulfur atom of methionine. Molecular mechanics calculations with this modified AMBER force field have suggested that a platinum complex with two sulfur-bound methionine ligands and a bulky diamine ligand (N,N,N',N'-tetramethylethylenediamine, Me(4)en) would have severe interligand clashes; such interligand clashes are less pronounced in bis(9-ethylguanine) complexes. Consistent with these observations, NMR studies with [Pt(Me(4)en)(D(2)O)(2)](2+) have indicated that guanine 5'-monophosphate reacts in a 2:1 guanine:platinum ratio while both methionine and N-acetylmethionine react with only a 1:1 stoichiometry. Methionine forms a chelate via the sulfur and nitrogen atoms whereas N-acetylmethionine forms a chelate via the sulfur and oxygen atoms. The oxygen of the latter chelate can be displaced by the addition of guanosine 5'-monophosphate, although complete displacement of the N-acetylmethionine was not observed. 相似文献
86.
Wong JW Maleknia SD Downard KM 《Journal of the American Society for Mass Spectrometry》2005,16(2):225-233
The calcium-dependent interaction of calmodulin and melittin is studied through the application of a radical probe approach in which solutions of the protein and peptide and protein alone are subjected to high fluxes of hydroxyl and other oxygen radicals on millisecond timescales. These radicals are generated by an electrical discharge within an electrospray ion source of a mass spectrometer. Condensation of the electrosprayed droplets followed by proteolytic digestion of both calmodulin and melittin has identified residues in both which participate in the interaction and/or are shielded from solvent within the protein complex. Consistent with other theoretical models and available experimental data, the tryptophan residue of melittin at position 19 is shown to be critical to the formation of the complex with the C-terminal domain of peptide enveloped by and protected from oxidation upon binding to the protein. Furthermore, the N-terminal domain (to residue 36) and tyrosine at position 99 in calmodulin are significantly protected from limited oxidation upon the binding of melittin while exposing the phenylalanine residue at position 92 of the flexible loop domain. The N-terminus (through residue 36) of calmodulin is shown to lie in closer proximity to the melittin helix than its C-terminal counterpart (residues 127-148) based upon the protection levels measured at reactive residues within these segments of the protein. 相似文献
87.
We propose and test a pair potential that is accurate at all relevant distances and simple enough for use in large-scale computer simulations. A combination of the Rydberg potential from spectroscopy and the London inverse-sixth-power energy, the proposed form fits spectroscopically determined potentials better than the Morse, Varnshi, and Hulburt-Hirschfelder potentials and much better than the Lennard-Jones and harmonic potentials. At long distances, it goes smoothly to the London force appropriate for gases and preserves van der Waals's "continuity of the gas and liquid states," which is routinely violated by coefficients assigned to the Lennard-Jones 6-12 form. 相似文献
88.
Kevin F. Morris Eugene J. Billiot Fereshteh H. Billiot Jordan A. Ingle Kevin B. Krause Corbin R. Lewis 《Journal of Dispersion Science and Technology》2019,40(5):716-727
Molecular dynamics (MD) simulations were used to investigate the binding of six chiral compounds to the amino acid-based molecular micelle (MM) poly-(sodium undecyl-(L)-leucine-leucine) or poly(SULL). The MM investigated is used as a chiral selector in capillary electrophoresis. The project goal was to characterize the chiral recognition mechanism in these separations and to move toward predictive models to identify the best amino acid-based MM for a given separation. Poly(SULL) was found to contain six binding sites into which chiral compounds could insert. Four sites had similar sizes, shapes, and electrostatic properties. Enantiomers of alprenolol, propranolol, 1,1′-bi-2-naphthyl-2,2′-diyl hydrogen phosphate, 1,1′-bi-2-naphthol, chlorthalidone, or lorazepam were separately docked into each binding pocket and MD simulations with the resulting intermolecular complexes were performed. Solvent-accessible surface area calculations showed the compounds preferentially associated with binding sites where they penetrated into the MM core and shielded their non-polar atoms from solvent. Furthermore, with five of the six compounds the enantiomer with the most favorable free energy of MM association also experienced the most favorable intermolecular hydrogen bonding interactions with the MM. This result suggests that stereoselective intermolecular hydrogen bonds play an important role in chiral discrimination in separations using amino acid-based MMs.GRAPHICAL ABSTRACT 相似文献
89.
Moroney C Lehane M Braña-Magdalena A Furey A James KJ 《Journal of chromatography. A》2002,963(1-2):353-361
Azaspiracids have been identified as the cause of a new toxic syndrome called azaspiracid poisoning (AZP) that has led to incidents of human intoxications throughout Europe following the consumption of mussels. Although five AZP toxins have been structurally elucidated to-date, azaspiracid (AZA1), 8-methylazaspiracid (AZA2) and 22-demethylazaspiracid (AZA3) are the predominant toxins. Separation of the three main AZP toxins was achieved using reversed-phase liquid chromatography (LC) and coupled to an electrospray ionisation source of an ion-trap mass spectrometer. Five reversed-phase (C18) and three diol solid-phase extraction (SPE) cartridges were compared for their efficacy in the cleanup of shellfish matrix. The comparison was based on the optimum recoveries of AZA1, AZA2 and AZA3 from extracts of mussel tissues. LC-electrospray MS3 analysis was used to quantify the AZP toxins in wash and eluate fractions in the SPE studies. Good recovery and reproducibility data were obtained for one diol SPE cartridge and two C18 SPE cartridge types. 相似文献
90.
W. Herr und R. Wölfle 《Fresenius' Journal of Analytical Chemistry》1965,209(1):213-226
Zusammenfassung Aufgrund ihrer komplexen und charakteristischen Zerfallsschematalassen sich eine Reihe von beispielsweise durch Neutroneneinfang entstandenen Radionukliden selektiv durch ,-Koinzidenz-Messungen nachweisen und quantitativ bestimmen. Die Methode wird hier besonders auf die Elemente Selen (75Se) und Iridium (192Ir) angewendet. Sie bietet den Vorzug extrem hoher Nachweisempfindlichkeit (etwa 10–10 g) und gestattet es darüber hinaus, in geeigneten Grundmaterialien wie Eisen und Platin zerstörungsfrei zu arbeiten. Eine Beeinflussung durch die Gegenwart anderer -Strahler kann durch diskrete Wahl und Einstellung der Koinzidenzbereiche weitgehend verhindert werden. Die besonderen Vorteile der Methode liegen in einer beachtlichen Vereinfachung und Verkürzung der chemischen Aufbereitung.
Es ist uns eine angenehme Pflicht, der Reaktorbetriebsgruppe des FRJ 1 in Jülich für die freundlicherweise durchgeführten Bestrahlungen zu danken; Herrn Heinz Ollig möchten wir für seine Hilfe bei der Durchführung der Messungen Dank sagen. 相似文献
Summary On the basis of their complex and characteristic decay schemes, a number of radionuclides formed for example by neutron capture can be selectively detected and quantitatively determined by means of ,-coincidence measurements. The method is specifically applied to the elements selenium (75Se) and iridium (192Ir). It offers the advantage of extremely high sensitivity of detection (ca. 10–10 gram) and permits in addition the performance of non-destructive analyses in suitable matrix materials, such as iron and platinum. Disturbances due to the presence of other -emitters can largely be prevented by discrete choice and adjustment of the coincidence ranges. The influence of the sample composition and dimensions are discussed. The particular advantage of the method consists in a considerable simplification and reduction of the time of chemical treatment.
Es ist uns eine angenehme Pflicht, der Reaktorbetriebsgruppe des FRJ 1 in Jülich für die freundlicherweise durchgeführten Bestrahlungen zu danken; Herrn Heinz Ollig möchten wir für seine Hilfe bei der Durchführung der Messungen Dank sagen. 相似文献