Heteronuclear Chemical Shift Correlation Research Articles

Anti-AIDS agents, 4. Tripterifordin, a novel anti-HIV principle from Tripterygium wilfordii: isolation and structural elucidation.

A new kaurane-type diterpene lactone, tripterifordin [1], has been isolated from the roots of Tripterygium wilfordii. The structure of 1 was elucidated by spectroscopic methods, including the concerted application of a number of 2D nmr techniques that involved the 1H-1H COSY, heteronucleus-detected variants of the heteronuclear chemical shift correlation (HETCOR), phase-sensitive NOESY, and long-range HETCOR. Compound 1 shows anti-HIV replication activity in H9 lymphocyte cells with an EC50 of 1 microgram/ml.

1H and 13C NMR spectra, 13C NMR relaxation times and molecular modelling studies of the narcotic agonist–antagonist α‐( − )‐N‐propynylnormetazocine

AbstractThe analysis of high‐frequency 1H and 13C NMR spectra and both 1H1H homonuclear and 1H13C heteronuclear chemical shift correlation experiments indicate that N‐propynlnormetazocine (NPMH+) is configurationally heterogeneous in solution, with the N‐equatorial isomer being more populated than the N‐axial isomer. The experimental proton–proton coupling constants measured for both diastereoisomers were in agreement with the calculated values for a chair conformation of the piperidine ring. AM1 and MM2 theoretical calculations both indicated the chair conformation as that preferred by the piperidine ring, and the N‐equatorial isomer as being energetically favoured with respect to the N‐axial form. The best fit of the 13C T1 relaxation time data and the related motional analysis of NPMH+ was carried out using the anisotropic model of reorientation with superimposed internal motion. The results showed an increase of the rotational anisotropy of the molecule on passing from the gas phase to the solution state.

Antigenic and structural relationships within group 19 Streptococcuspneumoniae: chemical characterization of the specific capsular polysaccharides of types 19B and 19C

The specific capsular polysaccharides of Streptococcuspneumoniae types 19B and 19C (American types 58 and 59) were investigated by a combination of 1H, 13C, and 31P nuclear magnetic resonance spectroscopy and analytical methods based on mass spectrometry. The two polysaccharides were found to be high molecular weight polymers composed of L-rhamnose, 2-amino-2-deoxy-D-mannose, D-ribose, D-glucose, and phosphate. Homo- and heteronuclear chemical shift correlation techniques and nuclear Overhauser enhancement experiments led to the unambiguous assignment of the 1H and 13C resonances from the glycose residues and established their sequence within repeating oligosaccharide units. The oligosaccharide units are polymerized through phosphate diester linkages.[Formula: see text]Both polysaccharides share a common hexasaccharide structural unit and they differ only in the degree of substitution at the branched 2-acetamido-2-deoxy-β-D-mannopyranosyl residue: the 19C polysaccharide is O-6 linked by β-D-glucopyranosyl end groups to form a heptasaccharide repeating unit, while the 19B polysaccharide is unsubstituted at that position. The serologic cross-reactivity between S. pneumoniae serotypes 19B and 19C can now be related to the structural similarity of the antigenic capsular polysaccharides. Keywords: Streptococcuspneumoniae, capsular polysaccharide, immuno chemistry.

Structure elucidation of the oligosaccharide calonyctin A by the concerted use of 1D and 2D NMR techniques

AbstractThe chemical structure of a plant growth substance, calonyctin A, was determined by NMR spectroscopy. The 13C and 1H spectral assignments were obtained using a combination of DEPT, heteronuclear chemical shift correlation, 1H–1H COSY, 1H–1H NOESY and inverse long‐range 1H–13C correlation. Calonyctin A was found to be a mixture of two homologous glycosides, each containing four rhamnoses and two aglycones. The configuration of the rhamnoses and the structural linkage between the six molecular fragments are reported.

13C NMR investigation of tautomeric and acid‐base equilibria of pyrazolone T and three analogs

AbstractPyrazolone T and three derivatives have been characterized by 13C and, in part, 15N nmr at several pH values. The 13C chemical shifts have been assigned at, or near, the equivalence points and pKa values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3‐bond C‐H couplings. The 13C chemical shift data indicate the existence of both tautomeric and acid‐base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values.

Spectral reassignment and structure elucidation of scopolamine free base through two‐dimensional NMR techniques

AbstractThe combination of 1D and 2D NMR techniques, especially the long‐range heteronuclear chemical shift correlation experiment, has permitted the reassignment of some carbons and protons of scopolamine free base in CDCl3 solution and the differentiation of each atom of the piperidine ring. In contrast to literature data, and to the protonated state conformation, these studies showed that the methyl group is equatorially disposed relative to the piperidine ring. NMR and IR spectroscopic results demonstrated that the OH group is connected by a hydrogen bond to the phenyl ring at a low concentration, but an intermolecular hydrogen bond is formed at high concentration.

An explanation of anomalous artifact intensity in fully 1H–1H decoupled two‐dimensional chemical shift correlation spectroscopy

AbstractTwo‐dimensional heteronuclear chemical shift correlation experiments which attempt decoupling in f1 have been shown to give artifacts in strongly coupled systems. In particular, COLOC and HETRES experiments on methylene groups show an artifact midway between the two proton lines. This paper provides an explanation of the artifact intensity behaviour with respect to the strength of coupling between the methylene protons. Experimental results on 1,2‐dibromoethylbenzene are presented, and theoretical calculations were carried out on the COLOC experiment and a mathematical model was developed which predicted the artifact appearance and intensity. Simulations were also performed using the simulation program SIMPLTN to verify the experimental and theoretical results.

Complete13C and1H NMR Chemical Shift Assignments of 1-Nitropyrene

Abstract Ambiguities in the 13C NMR chemical shift assignments of 1-nitropyrene were resolved using direct and long-range two-dimensional heteronuclear chemical shift correlation experiments. The latter was modified with a bilinear rotational decoupling (BIRD) pulse sequence, which enabled detection of all three-bond long-range correlations.

Long-range heteronuclear correlation 2D NMR by MLEV-16 isotropic mixing

Coherence transfer by MLEV-16 isotropic mixing is shown to be an efficient way of obtaining phase-sensitive heteronuclear chemical shift correlation 2D spectra. Both long- and short-range correlations are possible by appropriate choice of mixing time. At long mixing times, the contribution of coherence transfer pathways through multiple couplings extends the range of correlation. Long mixing time experiments may therefore be useful for structure determination in total unknowns. Results are presented for a 13C-detected isotropic mixing experiment on menthol.

Improvements in proton-detected NMR spectroscopy using spin-flip decoupling. An application to heteronuclear chemical-shift correlation

Improvements in proton-detected NMR spectroscopy using spin-flip decoupling. An application to heteronuclear chemical-shift correlation

Structural analysis of the specific capsular polysaccharide of Rhodococcus equi serotype 2

The specific capsular polysaccharide produced by Rhodococcus equi serotype 2 is a high-molecular-weight acidic polymer composed of d-glucose, d-mannose, d-glucuronic acid and 3- O-[( S)-1-carboxyethyl]- l-rhamnose in equimolar proportions. Structural analysis, employing a combination of chemical and n.m.r. techniques, established that the polysaccharide is composed of linear repeating tetrasaccharide units, ▪ in which the β- d-mannose residues carry O-acetyl groups at O-2 and O-3 to the extent of 1.7 mol equivalents. Unequivocal determination of the absolute chirality of the 3- O-[( S)-1-carboxyethyl]-α- l-rhamnose residues was achieved by chemical correlation with an authentic synthetic sample. The 1H and 13C-n.m.r. resonances of the native and O-deacetylated serotype 2 polysaccharides were fully assigned by homo- and heteronuclear chemical-shift correlation methods.

Unusual Hindered Rotation of an Unsubstituted Phenyl Group. Variable Temperature1H NMR Studies and Preliminary13C Assignments in Ketazolam

Abstract An unusual hindered rotation of an unsubstituted phenyl group in the drug, ketazolam, has been found at ambient temperature. Variable temperature lH NMR has been employed to examine the two- fold rotation about the C(sp2)-C(sp3) bond. Chemical shift as- signments of the IH NMR spectra and partial assignments of the 13C spectra are supported by off-resonance decoupling, gated decou- pling, and low temperature 2D l3C-1H heteronuclear chemical shift correlation experiments at 200.1 MHz for IH and 50.3 MHz for 13C.

Determination of the structure and absolute configuration of the glycerolphosphate-containing capsular polysaccharide of Pasteurellahaemolytica serotype T3 by high-resolution nuclear magnetic resonance spectroscopy

The specific capsular polysaccharide antigen produced by Pasteurellahaemolytica serotype T3 was found to be a high molecular weight teichoic acid composed of equal molar amounts of D-galactose, glycerol, and phosphate and a nonstoichiometric amount of O-acetate. Structural analysis of the polymer employed a combination of one- and two-dimensional 1H and 13C nuclear magnetic resonance methods at high field. Homo- and heteronuclear chemical shift correlation techniques, together with J-resolved and nuclear Overhauser effect (nOe) difference experiments led to the unambiguous assignments of the 1H and 13C resonances corresponding to the D-galactose and glycerol structural units and established that the repeating unit of the polysaccharide antigen was composed of → 4)-α-D-Galp-(1 → 1)-L-glycerol-(3 → units joined by phosphate diester groups. The chirality of the glycerol moiety was established by conformational analysis of a 1-O-α-D-galactopyranosyl-S-glycerol fragment (2) obtained from the teichoic acid by depolymerization with HF, from which the stereochemistry of the glycerol methine carbon centre was related to that of the D-glycose from 1H–1H nOe and 3J values. The chirality was confirmed by gas–liquid chromatographic and mass spectral analysis of the (−)-camphanate ester of 2,3-di-O-methylglycerol derived from permethylated 2. The D-galactosyl residues are partially mono-O-acetylated at C-2 and C-3. Keywords: Pasteurellahaemolytica, capsular polysaccharide, NMR analysis.

Configuration and Total Assignment of the 1H- and 13C-nmr Spectra of the Alkaloid Holstiine

The alkaloid holstiine was reisolated from Strychnos benningsii and subjected to a total structural assignment through the concerted application of a number of two-dimensional nmr techniques that included COSY, HC-COSY, HOHAHA, NOESY, and proton-detected long-range heteronuclear chemical shift correlation (HMBC)

Spectral artifacts in heteronuclear chemical shift correlation experiments with homonuclear decoupling via a constant evolution time

AbstractExperimental and theoretical studies carried out to investigate the influence of strong coupling effects in the spectra generated through the commonly used heteronuclear chemical shift correlation (HETCOR) sequences involving a constant evolution time are reported. It is seen that the spectra generated via such sequences are prone to artifacts of appreciable intensity. Failure to recognize these can result in the misinterpretation of spectral data.

Structural analysis of the specific capsular polysaccharide of Rhodococcus equi serotype

The specific capsular polysaccharide produced by Rhodococcus equi serotype 1 was found to be a high molecular weight acidic polymer composed of D-glucose, D-mannose, and D-glucuronic acid. Structural analysis of the polysaccharide employed a combination of chemical and nuclear magnetic resonance techniques, from which it was determined that the polysaccharide possessed a linear repeating tetrasaccharide unit containing a single O-acetyl substituent and and acetal-linked pyruvic acid moiety: [formula: see text] The 1H and 13C nuclear magnetic resonances of O-deacetylated and pyruvic-free serotype 1 polysaccharides were fully assigned by homo- and hetero-nuclear chemical shift correlation methods.

Deoxyribose conformation in [d(GTATATAC)]2: evaluation of sugar pucker by simulation of double-quantum-filtered COSY cross-peaks

Exchangeable and nonexchangeable proton and phosphorus resonances (11.75 T) of [d(GTATATAC)]2 in aqueous solution were assigned by using proton two-dimensional nuclear Overhauser effect (2D NOE) spectra, homonuclear proton double-quantum-filtered COSY (2QF-COSY) spectra, proton spin-lattice relaxation time measurements, and 31P1H heteronuclear shift correlation spectra. Due to the large line widths, it was not possible to directly extract vicinal proton coupling constant values from any spectrum including ECOSY or 2QF-COSY. However, comparison of quantitative 2QF-COSY spectral simulations with experimental spectra enabled elucidation of coupling constants. The scope and limitations of this approach were explored by computation and by use of experimental data. It was found that proton line widths exhibit some variability from one residue to the next as well as from one proton to the next within a residue and the exact line width is critical to accurate evaluation of coupling constants. Experimental 2QF-COSY spectra were not consistent with a rigid deoxyribose conformation for any of the nucleotide residues. A classical two-state model, with rapid jumps between C2'-endo (pseudorotation angle P = 162 degrees) and C3'-endo (P = 9 degrees) conformations, was able to account for the spectral characteristics of terminal residue sugars: 60% C2'-endo and 40% C3'-endo. However, the 2QF-COSY cross-peaks from the -TATATA- core could be simulated only if the classical two-state model was altered such that the dominant conformer had a pseudorotation angle at 144 degrees instead of 162 degrees. In this case, the major conformer amounted to 80-85%. Alternatively, the spectral data were consistent with a three-state model in which C2'-endo and C3'-endo conformations had the largest and smallest populations, respectively, but a third conformer corresponding to C1'-exo (P = 126 degrees) was present, consistent with recent molecular dynamics calculations. This alternative yielded populations of 50% (P = 162 degrees), 35% (P = 126 degrees), and 15% (P = 9 degrees) for the -TATATA- sugars. The spectral results indicate little variation of sugar pucker between T and A. Small differences in cross-peak component intensities and characteristic spectral distortions, however, do suggest some unquantified variation. 31P1H heteronuclear chemical shift correlation spectra manifested alternating chemical shifts and coupling constants suggestive of phosphodiester backbone conformational differences between TA and AT junctions.

A proton-detected heteronuclear chemical-shift correlation experiment with improved resolution and sensitivity

A proton-detected heteronuclear chemical-shift correlation experiment is proposed that partially eliminates the induced heteronucleas-remote-proton coupling present in the routinely used heteronuclear multiple-quantum correlation (HMQC) experiment. As is demonstrated with natural-abundance 133C1H chemical-shift correlations of lysozyme, the partial remote-proton decoupling obtained in the new experiment results in significant improvements in resolution and sensitivity as compared to the HMQC experiment.

Palominol, a new diterpene based on the dolabellane skeleton from the caribbean gorgonian octocorals eunicea calyculata and eunicea laciniata

A new bicyclic diterpenoid, palominol ( 2), of the dolabellane ring system has been isolated from the Caribbean gorgonians Eunicea calyculata and Eunicea laciniata . The structure of this new compound was assigned on the basis of chemical and spectral studies, particularly heteronuclear chemical shift correlation methods, homonuclear spin decoupling experiments and nuclear Overhauser enhancement difference spectroscopy (NOEDS).

Didemnin B Conformation and dynamics of an antitumour and antiviral depsipeptide studied in solution by 1H and 13C. n.m.r. spectroscopy

The solution conformation of didemnin B, the most potent member of a family of depsipeptides that shows antitumour, antiviral, and immunosuppressive activity, has been studied in chloroform solution using n.m.r. spectroscopy. 1H and 13C spectra have been assigned from analysis of a number of two-dimensional homonuclear and heteronuclear chemical shift correlation experiments which confirm the recently corrected primary structure of the molecule. The conformation of the peptide has been deduced from measurements of the temperature dependence of the NH chemical shifts, analysis of coupling constant data and primarily through NOE effects observed in the rotating frame. Interproton distance bounds determined from a quantitative analysis of the ROE data provide 41 constraints from which a family of closely related structures were calculated using distance geometry algorithms. A type II beta-turn involving residues Thr6, Leu7, and Pro8 is well represented in the computed conformers as is a hydrogen bonding interaction between the NH of Leu3 and the carbonyl oxygen of Thr6. This latter interaction causes the linear portion of the structure to fold back over the depsipeptide ring, imparting to it a degree of structural stability as well as giving the molecule a somewhat globular character. Only one transannular hydrogen bond, between Ist1 NH and Leu3 carbonyl, stabilizes the conformation of the depsipeptide, which has an irregular non-planar configuration. The small temperature coefficients (less than 2.0 x 10(-3) ppm/degrees C) for the NHs of Ist1 and Leu3 are consistent with their involvement in these hydrogen bonding interactions. We find that many of the structural features observed in the crystalline form of didemnin B are conserved in solution. Analysis of the 13C spin-lattice relaxation rates of the protonated carbons reveals small variations in effective correlation times at specific sites in the molecule. The data suggests that the peptide segment encompassing residues Leu3 through to Thr6 is in a more motionally restricted part of the structure.