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Isomeric Composition (isomeric + composition)
Selected AbstractsComparison of the separation of aziridine isomers applying heptakis(2,3-di- O -methyl-6-sulfato),-CD and heptakis(2,3-di- O -acetyl-6-sulfato),-CD in aqueous and nonaqueous systemsELECTROPHORESIS, Issue 20 2005Yaser Bitar Abstract Aziridines are attracting interest as protease inhibitors, which might be used, e.g., for treatment of parasitic diseases. Within the framework of greater projects dealing with the search of new selective protease inhibitors, a huge number of aziridines with two stereogenic centers will be synthesized. Thus, a fast and reliable screening method for the evaluation of the isomeric composition is needed. Robust baseline separations were obtained using heptakis(2,3-di- O -acetyl-6-sulfato),-CD (HDAS) in acidic methanol and sulfated ,-CD in acidic phosphate buffer. With HDAS the resolutions were higher and migration times shorter. Thus, the method will be used as a screening method for further isomeric mixtures of aziridines. [source] A perspective on MALDI alternatives,total solvent-free analysis and electron transfer dissociation of highly charged ions by laserspray ionizationJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2010Sarah Trimpin Abstract Progress in research is hindered by analytical limitations, especially in biological areas in which sensitivity and dynamic range are critical to success. Inherent difficulties of characterization associated with complexity arising from heterogeneity of various materials including topologies (isomeric composition) and insolubility also limit progress. For this reason, we are developing methods for total solvent-free analysis by mass spectrometry consisting of solvent-free ionization followed by solvent-free gas-phase separation. We also recently constructed a novel matrix-assisted laser desorption ionization (MALDI) source that provides a simple, practical and sensitive way of producing highly charged ions by laserspray ionization (LSI) or singly charged ions commonly observed with MALDI by choice of matrix or matrix preparation. This is the first ionization source with such freedom,an extremely powerful analytical ,switch'. Multiply charged LSI ions allow molecules exceeding the mass-to-charge range of the instrument to be observed and permit for the first time electron transfer dissociation fragment ion analysis. Copyright © 2010 John Wiley & Sons, Ltd. [source] Isomer separation of hyperbranched polyesteramides with gas-phase H/D exchange and a novel MSn approach: DoDIPJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2002Sander Koster Abstract Two approaches are introduced that provide information about the isomeric composition of hyperbranched polyesteramides. The first approach is based on a novel tandem mass spectrometric (MSn) approach that allows the study of different types of isomeric structures by a separation based on their difference in appearance energy. The method is called DoDIP: dissociation of depleted ion populations. A first MS/MS step is used to fragment isomers with relatively low appearance energy. The isomers with higher appearance energy are fragmented in a second MS/MS step of higher energy. The second approach is based on gas-phase H/D exchange experiments that result in a bimodal isotopic distribution for oligomers XnDn+1 of which one distribution corresponds to a type of isomeric structure that exhibits H/D exchange behaviour and the other to an isomeric structure that does not exhibit H/D exchange behaviour. X is a difunctional anhydride of phthalic acid (P), 1,2-cyclohexanedicarboxylic acid (C), succinic acid (S) or glutaric acid (G). D in XnDn+1 is a trifunctional diisopropanolamine and n the degree of polymerization. The type of isomeric structure that does not exhibit H/D exchange behaviour has a non-alternating monomer sequence that contains an amine bond with a relatively high proton affinity. The other isomeric structure that does exhibit H/D exchange behaviour has an alternating monomer sequence containing only amide and ester bonds with relatively low proton affinity. Oligomer structures were confirmed with additional MS2 experiments after H/D exchange. H/D exchange experiments on the fragments obtained after MS2 of the parent ion show that next to previously postulated mechanisms for the cleavage of the ester and amide bond another reaction pathway must be operational. A new mechanism is introduced to explain the H/D exchange behaviour of the fragments that requires a cleavage of the amide bonds only. Two types of fragments are formed by this mechanism. One type is protonated due to the cleavage of the amide bond whereas the other type has an oxazolonium ion structure due to the loss of an additional H2O. Copyright © 2002 John Wiley & Sons, Ltd. [source] Quantitative 1H NMR spectroscopic determination of the E/Z isomer ratio of the antidepressant drug fluvoxamine for use in pharmaceutical analysisMAGNETIC RESONANCE IN CHEMISTRY, Issue 12 2002Ralph Deubner Abstract High-resolution NMR spectroscopy is a powerful tool in the elucidation of a structure with respect to constitution, configuration and conformation. In recent years, NMR has been increasingly used in quantitative analysis. In this study, we show the ability of 1H NMR to determine the isomeric composition of the antidepressant drug fluvoxamine. The activity of fluvoxamine resides on the E -isomer, and the current British Pharmacopoeia limits the content of the Z -isomer to 0.5%. The NMR method described here is able to determine the content of the Z -isomer down to the 0.2% level on a total amount of 15 mg of the substance. Copyright © 2002 John Wiley & Sons, Ltd. [source] Topologically Chiral Covalent Assemblies of Molecular Knots with Linear, Branched, and Cyclic ArchitecturesCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2004Oleg Lukin Dr. Abstract Selectively functionalized molecular knots (knotanes) of the amide-type have been used as building blocks in syntheses of higher covalent assemblies composed of up to four knotane units. Preparation of linear and branched tetraknotanes consisted of the consecutive selective removal of allyl groups followed by linking of the intermediate hydroxyknotanes with biphenyl-4,4,-disulfonyl chloride. Macrocyclic knotane oligomers involving two, three, and four knotane moieties were obtained by high-dilution cyclization of dihydroxyknotane and biphenyl-4,4,-disulfonyl chloride. Due to their relation with cyclophanes, the latter class of oligomeric knotanes was termed "knotanophanes". Chiral resolution analysis of new oligoknotanes has been attempted on chemically bonded Chiralpak AD stationary phases, however met severe difficulties due to their complex isomeric compositions, and in most cases a significant overlap of the isomer fractions was observed. In spite of the limits of presently available chiral stationary phases that allowed only partial resolution of the synthesized topologies, oligoknotanes have been shown to be of high fundamental interest due to their unprecedented chirality. The chirality descriptions of topologically chiral unsymmetrical dumbbell 4, and the linear tetraknotane 5 are analogous to the Fischer projections of erythrose/threose and hexaric acid, respectively, while the isomeric composition of the branched tetraknotane 8 is completely unique. Moreover, the linear and branched tetraknotanes are constitutional isomers. Chirality of knotanophanes represents, in turn, analogies to known cyclic forms of peptides or sugars with multiple stereogenic centers. [source] | ||||||||||