1,721,119 research outputs found
Accurate measurement of heteronuclear long-range coupling constants from 1D-subspectra in crowded spectral regions
Get PDFImproved Sensitivity in Indirect Monitoring of Chemical Shifts of Proton-Heteronuclear spin pairs (1H-13C and 1H-15N) in 3D and 4D NMR Spectroscopy
Get PDFIn three-dimensional and four-dimensional experiments on doubly labelled proteins not only heteronuclear (13C or 15N) but also proton (1H) frequencies are often indirectly monitored, rather than being directly observed. In this communication we show how in these experiments by overlaying 1H and heteronuclear evolutions one can obtain decreased apparent relaxation rates of 1H signals, yielding improved sensitivity. The new method applies to spin pairs like 1H-15N, as in amide groups, or 1H-13C, as in methine groups of alpha or aromatic systems
Additivity constants for the prediction of 1H-1H coupling constants in thio-pyranose derivatives
No full textPseudorotational analysis of 4-thiohexofuranose derivatives from 1H-1H coupling constants
No full textOn the basis of Altona's empirical generalization of the Karplus equation, an expression of 3JH,H as function of pseudorotation parameters for five-membered rings, was deduced. The resulting equation was parametrized by means of molecular mechanics calculations for tetrahydrofuran and tetrahydrothiophene. Estimation of 3JH,H for the full pseudorotational itinerary was made. From the best fit between calculated and measured 3JH,H values the preferred conformation of those rings may be established. The procedure was applied for the conformational analysis of 4-thiohexofuranose derivatives, having α and β, -galacto and D- manno, and α--talo configurations
Conformational analysis of methyl tetrofuranosides and pentofuranosides from 1H, 1H coupling constants
No full textA new three-dimensional pulse sequence for correlating intraresidue NH, N, and CO chemical shifts in 13C, 15N labelled proteins
Get PDFGoing Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A test of the single conformation hypothesis in the analysis of NMR data for small molecules
Get PDFA tricyclic ketone with seven stereogenic centers produced in >95:5 diastereomeric excess by an asymmetric Diels−Alder reaction has been subjected to a careful 2-D NMR(CDCl3)/MD analysis by Reggelin and co-workers and interpreted in terms of a single conformation in CDCl3 solution. The present work examines the validity of this interpretation. Specifically, the conformational profile of the endo isomer of the tricyclic ketone was examined by Monte Carlo searches with both the MM2* and MMFF force fields in MacroModel using the GB/SA CHCl3 solvent continuum model. The two sets of conformations were then combined and optimized with the MM3(96) force field. The structures of the resulting conformations and the NMR-derived geometric variables were together subjected to a NAMFIS analysis (NMR analysis of molecular flexibility in solution). Ideally, the procedure deconvolutes the thermally averaged NMR data into a small family of conformations that optimally represents the J-derived torsions and the NOE-derived distances. It is concluded that the tricyclic ketone under study is not well-characterized by a single conformation in CDCl3 solution, but by a set of rapidly equilibrating conformers that produces a deceptively averaged NMR spectrum. It would appear that the previously suggested structure is a virtual conformation obligated to compress multiconformer features into a single 3-D construct. In additon, we find that current force fields do not uniformly represent the conformational profile of polar molecules such as 1. The cause is traced primarily to the variable treatment of electrostatic effects
Nucleophilic displacement-reactions of the 4-sulfonyloxy group in derivatives having the D-manno configuration
No full textMethyl 2,3-di-O-benzoyl-6-deoxy-4-O-(p-tolylsulfonyl) and 4-O-[(p-nitrophenyl)sulfonyl]-α-d-mannopyranosides (9 and 10) were prepared by three different routes from methyl α-d-mannopyranoside (1). The analogous 4-sulfonyloxy derivatives having HO-2 and HO-3 free (14 and 15) were also synthesized from 1. Nucleophilic substitution of the sulfonyloxy group of 9, 10, 14, and 15 by potassium thiocyanate in N,N-dimethylformamide was attempted. Compounds 9 and 10 gave a mixture of solvolysis products: methyl 2,3-di-O-benzoyl-6-deoxy-α-d-talopyranoside (17), methyl 3,4-di-O-benzoyl-6-deoxy-α-d-talopyranoside (18), and methyl 2,4-di-O-benzoyl-6-deoxy-α-d-talopyranoside (19), which are evidently formed by intramolecular displacement of the 4-sulfonate by backside attack of the C-2 benzoyloxy substituent, followed by benzoyl migration. The structure of compounds 17–19 was established by spectroscopic analysis, and then chemically confirmed. Although compound 14 decomposed during the substitution reaction, the 4-p-nitrophenylsulfonyl derivative 15 gave a 2:1 mixture of the 4-thiocyano derivatives with inversion [methyl 4,6-dideoxy-4-thiocyano-α-d-talopyranoside (22)] and retention [methyl 4,6-dideoxy-4-thiocyano-α-d-mannopyranoside (23)] of the C-4 configuration
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