A simple approach for phase‐modulated single‐scan 2D NMR spectroscopyAndersen, Nikolas Salisbury; Köckenberger, Walter
doi: 10.1002/mrc.1624pmid: 16007727
Conventional NMR spectroscopy techniques require long acquisition times due to the recovery time between the repeated excitations necessary for each increment of the evolution times in the indirectly detected dimensions. Here we outline a pulse sequence element for gradient‐assisted ultrafast multidimensional NMR spectroscopy using frequency‐modulated ‘chirp’ pulses to generate phase‐modulated magnetization in an indirectly detected spectral dimension. The potential of this sequence element is demonstrated by acquiring a correlation spectroscopy (COSY) spectrum in 96 ms. This new pulse sequence element is an extension of ultrafast spectroscopy techniques based on the generation of amplitude modulation of the NMR signal in the indirectly detected spectral dimensions. The use of phase modulation instead of amplitude modulation helps broaden the applicability and may provide an increase of sensitivity in some experiments due to the ability to distinguish between positive and negative frequency offsets relative to the carrier frequency of the sequence element. Copyright © 2005 John Wiley & Sons, Ltd.
First example of conformational exchange in a natural taxane enolateShi, Qing Wen; Sauriol, Françoise; Park, Yong; Smith, V. H.; Lord, Gabriel; Zamir, Lolita O.
doi: 10.1002/mrc.1630pmid: 16028304
A detailed NMR study and full assignments of the 1H‐ and 13C‐NMR spectral data for a novel enolate taxane isolated from Taxus canadensis needles is described. The structures of two stable conformers were established using a combination of 1D and 2D NMR techniques including 1H, 1H‐COSY, gs‐HMQC, gs‐HMBC, NOESY and T‐ROESY. Ab initio quantum mechanical calculations were performed on the B3LYP/6‐31G* level of basis set to assist the NMR findings. Copyright © 2005 John Wiley & Sons, Ltd.
Exploiting the carboxylate chemical shift to resolve degenerate resonances in spectra of 13C‐labelled glycosaminoglycansColebrooke, Simon A.; Blundell, Charles D.; DeAngelis, Paul L.; Campbell, Iain D.; Almond, Andrew
doi: 10.1002/mrc.1620pmid: 15996005
Glycosaminoglycans (GAG) are important vertebrate extracellular matrix polysaccharides that comprise repeated units of an acidic and an N‐acetylated sugar. The constituent acidic sugars are central to their biological functions, but have been largely inaccessible to NMR because the 1H resonances overlap with those from other residues. Here, pulse sequences that address this failure are developed using 13C‐enriched oligosaccharides of the glycosaminoglycan, hyaluronan, as model systems. Two pulse sequences are presented that exploit the unique chemical shifts and scalar couplings present at the carboxylate moiety to filter out coherences from the N‐acetylated sugars and produce simple spectra containing only resonances from the acidic sugars. The first sequence uses one‐bond couplings to correlate the carboxylate carbon with the adjacent carbon and its directly attached proton, while the second sequence exploits a long‐range coupling to correlate the carboxylate carbon with the anomeric proton and carbon of the same residue. In addition, inclusion of an isotropic mixing block into these sequences allows resonances from the otherwise degenerate ring protons to be resolved. Spectra from the hyaluronan tetra‐ and hexasaccharides show that all glucuronic acid (GlcA) residues can be resolved from one another, allowing nuclei to be assigned in a sequence‐specific manner. However, in some spectra, resonances are observed at positions not predicted by spin‐operator analysis, and simulations reveal that these additional magnetisation transfers result from strong‐coupling. These experiments represent a foundation from which new structural and biochemical information can be obtained in a sequence‐specific manner for the acidic sugar residues in hyaluronan and other glycosaminoglycans. Copyright © 2005 John Wiley & Sons, Ltd.
Characterization of polyphenols from plant materials through their silylation and 29Si NMR spectroscopy—line assignment through 29Si, 13C spin–spin couplingsSchraml, Jan; Blechta, Vratislav; Sýkora, Jan; Soukupová, Ludmila; Cuřínová, Petra; Proněk, David; Lachman, Jaromír
doi: 10.1002/mrc.1638pmid: 16041773
The lines in 29Si NMR spectra of silylated polyphenols and some other compounds are difficult to assign owing to the absence of couplings with protons outside the silyl group. The assignment can be derived through small nJ(29Si, 13C) couplings (n > 1). Using a previously described method for measurements of these couplings, the assignment procedure is demonstrated here on three examples of trimethylsilylated phenols: 7‐hydroxyflavone, ferulic acid, and quercetin. In some cases the procedure can be used to identify carbon atoms to which the siloxy groups are attached. Copyright © 2005 John Wiley & Sons, Ltd.
The solvation of the mercury(II) ion—a 199Hg NMR studyMaliarik, Mikhail; Persson, Ingmar
doi: 10.1002/mrc.1625pmid: 16025553
The solvation of the mercury(II) ion in solvents with different solvation properties, water, dimethylsulfoxide, N,N‐dimethylthioformamide, and liquid ammonia, has been studied by means of 199Hg NMR. The 199Hg chemical shift shows a pronounced dependence on the coordination number of the mercury(II) ion in the solvates resulting in a difference of over 1200 ppm between basically tetrahedral and octahedral complexes. The chemical shifts can furthermore be associated with electron‐pair donor properties of the solvents. The spin‐lattice relaxation times of the 199Hg nucleus in the solvates have been measured at different applied magnetic fields, concentrations, temperatures, and isotope substitutions. Possible mechanisms for the 199Hg relaxation were proposed and the chemical shielding anisotropy in the solvates has been estimated. The 199Hg relaxation rates and the anisotropy are correlated with the structure of the solvate complexes in solution obtained from recent LAXS and EXAFS studies. Copyright © 2005 John Wiley & Sons, Ltd.
A two‐stage approach to automatic determination of 1H NMR coupling constantsCobas, J. C.; Constantino‐Castillo, V.; Martín‐Pastor, M.; del Río‐Portilla, F.
doi: 10.1002/mrc.1623pmid: 16025552
1H NMR scalar coupling constants are a rich source of information on molecular structure, but their extraction from spectra can be less than straightforward. Previous approaches to J extraction include methods proposed by Hoye, Golotvin, and the ‘modified J‐doubling’ method. Here we describe the ACCA method, currently implemented in the NMR package MestReC, which allows a high degree of automation in the extraction of coupling patterns even in the case of complex multiplets with sublinewidth splitting. The new approach is illustrated by application to strychnine, for which it has detected previously unreported couplings. Copyright © 2005 John Wiley & Sons, Ltd.
6/7Li NMR study of the Li1–zNi1+zO2 phasesChazel, Cédric; Ménétrier, Michel; Croguennec, Laurence; Delmas, Claude
doi: 10.1002/mrc.1639pmid: 16041774
A series of Li1−zNi1+zO2 materials have been synthesised by the coprecipitation route. An X‐ray diffraction study was carried out on these materials using the Rietveld method to determine the departure from the ideal stoichiometry z, which ranges from 0 to 0.138. The actual Li/Ni ratio was also checked by chemical analyses using inductively coupled plasma (ICP) for each sample. The stoichiometric sample (z ∼ 0) was obtained using a 15% Li excess. 6/7Li NMR results from LiNiO2 (z ∼ 0) show that the asymmetric shape of the NMR signal is due to anisotropy. Calculations give evidence that the paramagnetic dipolar interaction from the electron spins carried by Ni is anisotropic but does not completely explain the experimental anisotropy. 6Li MAS NMR (magic angle spinning NMR) experiments and temperature standardisation NMR measurements unambiguously assign the isotropic position at +726 ppm. The static‐echo NMR spectra of the non‐stoichiometric Li1−zNi1+zO2 phases also exhibit an asymmetric shape whose width increases with the departure from the ideal stoichiometry z. 6/7Li static and MAS NMR show that the 2zNi2+ ions thus formed modify the dipolar interaction within the materials and also affect the Fermi contact interaction, since a distribution of Li environments is observed using 6Li NMR for non‐stoichiometric samples. Copyright © 2005 John Wiley & Sons, Ltd.