High-resolution Proton NMR Spectra Research Articles

Proton-proton homonuclear dipolar decoupling in solid-state NMR using rotor-synchronized z-rotation pulse sequences

We present a theoretical analysis of rotor-synchronized homonuclear dipolar decoupling schemes that cause a z-rotation of the spins. These pulse sequences applicable at high spinning rates (nu(r) > or = 30 kHz) yield high-resolution proton NMR spectra that are free of artifacts, such as zero lines and image peaks. We show that the scaled isotropic chemical-shift positions of proton lines can be calculated from the zero-order average Hamiltonian and that the scaling factor does not depend on offset. The effects of different adjustable parameters (rf field, spinning rate, pulse shape, offset) on the decoupling performance are analyzed by numerical simulations of proton spectra and by (1)H solid-state NMR experiments on NaH(2)PO(4) and glycine.

The unambiguous assignment of NMR spectra of per-O-methylated 6-mono and 6,6-diamino-β-cyclodextrins

The step-by-step analysis of high-resolution proton and 13C NMR spectra of per-O-methylated-6-monoamino and 6,6-diamino-A,D-β-cyclodextrins is described. Selected 2-D experiments (COSY, NOESY, HSQC, DEPT, and HMBC) allowed us to challenge the current interpretation. The full assignment of the proton and carbon-13 NMR spectra of these two compounds was performed enabling us to complete the assignment of NMR spectra acquired at 500 and 800 MHz for these important synthetic intermediates.Key words: COSY, NOESY, HSQC, DEPT, HMBC, NMR, 6-amino-β-cyclodextrins, 6,6-diamino-β-cyclodextrins.

Assigning powders to crystal structures by high-resolution1H–1H double quantum and1H–13C J-INEPT solid-state NMR spectroscopy and first principles computation. A case study of penicillin G

We show how powder samples at natural isotopic abundance can be assigned to crystal structures by using high-resolution proton and carbon-13 solid-state NMR spectra in combination with first principles calculations. Homonuclear proton double-quantum spectra in combination with through-bond proton-carbon HSQC spectra are used to assign the NMR spectra. We then show that the proton chemical shifts can be included in the process of assigning the spectra to a crystal structure using first principles calculations. The method is demonstrated on the K salt of penicillin G.

High-resolution proton solid-state NMR spectroscopy by phase-modulated Lee–Goldburg experiment

In this Letter, we present a pulse scheme for coherent averaging of spin–spin interactions called phase-modulated Lee–Goldburg (PMLG) which is aimed at achieving high-resolution proton NMR spectra in the solid state. Our objective is mainly to discuss the design and execution of this experiment and to show some preliminary experimental results as evidence of the far-reaching potentials of this technique.

The NMR spectra of lead stabilizers

AbstractHigh resolution proton and 13C NMR spectra of lead‐based heat stabilizers and their reaction products, obtained using the magic angle spinning technique, support structural assignments inferred from IR spectra. These demonstrate that lead stabilizers are unique compounds, rather than the double salts of lead oxide found in textbooks.

Fast Magic-Angle Proton NMR Studies of Polymer Interfaces in Poly(ethyl acrylate)/Vycor Composites

Fast magic-angle spinning solid-state proton NMR has been used to study the structure and dynamics of poly(ethyl acrylate) in bulk and polymerized in a Vycor glass with 40 A pores. Bulk poly(ethyl acrylate) is a low-T g material, and high-resolution solid-state proton NMR spectra are observed at ambient temperature because the dipolar interactions are averaged by the combination of chain motion and fast magic-angle sample spinning. The spectrum of the poly(ethyl acrylate)/Vycor composite shows signals in addition to those of the polymer that are assigned to water and surface hydroxyl groups in the porous glass. Integration of the composite and bulk poly(ethyl acrylate) signals reveals that the volume fraction of polymer in the pores is 0.56 and that the polymer occupies the central 30 A in a 40 A pore. Spin diffusion studies demonstrate that filling of the poly(ethyl acrylate)/Vycor is incomplete, and a fraction of the water in the pores is isolated from the polymer. Multiple-quantum NMR studies reveal a large difference in chain dynamics between the bulk poly(ethyl acrylate) and the porous glass composite, and 2D wide line separation NMR shows that the polymer is restricted at the surface of the pores but has bulklike mobility in the center of the pore.

Measurement and Analysis of the Molecular Ordering Tensors of Two Enantiomers Oriented in a Polypeptide Liquid Crystalline System

High-resolution proton and natural abundance carbon-13 NMR spectra of a rigid chiral molecule, (&)-p(trichloromethy1)-p-propiolactone, dissolved in the liquid-crystalline phase of poly(y-benzyl-L-glutamate) (PBLG), in methylene chloride, were analyzed. From the NMR data and the X-ray structure, the full order matrix was determined and analyzed in detail for each optical isomer of the molecule. It is shown that the spectral discrimination of the enantiomers in this lyotropic medium arises mainly from the difference in their average molecular orientation and the principal axis systems of orientation are compared.

Spectral Simplification and “Pseudo Decoupling” in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients

High-resolution proton NMR spectra of complex molecules are generally crowded due to the homonuclear spin–spin couplings. Knowledge of chemical-shift frequencies from such crowded spectra are of utmost importance in identifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decoupling. Since the pioneering work of Aue et al. (1) , two- dimensional J-resolved spectroscopy seems to have offered the best route to ‘‘broadband-decoupled’’ proton spectra. Subsequently, various attempts have been made to improve upon the technique (2–9) , of which the most promising solution seems to be ‘‘purged J spectroscopy’’ ( 8, 9) . Recently, a new proton ‘‘decoupling’’ method has been proposed (10) which uses the classic spin-echo sequence without any purging pulse. A new era has started in the field of NMR with the use of pulsed-field gradients, which in turn, is a result of technological progress in the design of high- doublequality shielded gradients. We have reported the use of these gradients for editing one-dimensional proton NMR spectra (11) , where the gradients are used for suppressing artifacts arising from $180^o$ pulse imperfections and for z filtering. It was shown that the experiment can be used to identify unambiguously singlets and triplets overlapping with other multiplets. This was done by suppressing the signals arising from doublets and quartets by matching the interval between the two $90^o_x$ pulses with \tau= 1/2J and with the use of gradient pulses (Fig. 1a) . This process leaves the coherences of the singlets unaffected. For triplets, the central line is unaffected and the outer lines are inverted (Fig. 2a) . We report here the extensions of this pulse sequence to filter singlets and triplets without any distortions (Fig. 1b) and to achieve ‘‘pseudo-decoupling’’ of the triplets in the proton NMR spectra (Figs. 1c and 1d).

Aggregation properties of semisynthetic GD1a ganglioside (IV 3Neu5ACII 3Neu5AcGgOse 4Cer) containing an acetyl group as acyl moiety

GD1a ganglioside containing an acetyl group as aryl moiety, GD1a(acetyl), was synthesized from natural GD1a. The aggregative properties in aqueous solution of GD1a(acetyl) have been studied by static and dynamic laser light-scattering measurements. GD1a(acetyl) spontaneously aggregates as small micelles showing a hydrodynamic radius and molecular mass of 33 Å and 96 kDa, respectively. Vibrio cholerae sialidase showed a very high activity on the micelles of GD1a(acetyl), compared to GDla. This has been explained as a consequence of the high surface curvature of the the small micelles. High resolution proton NMR spectra were recorded from micelles of GD1a(acetyl) in deuterated water. The low overall correlation time of the GDla(acetyl) micelles was calculated to be about 2 × 10 −8s, a value one order of magnitude lower than that determined for natural GD1a.

Elimination of Spin-Spin Splittings from High-Resolution Proton NMR Spectra

The scalar couplings that result in the splitting of the signals in the NMR spectrum arise due to the interaction of the nuclear spins, whereby the spin polarization is transmitted through chemical bonds. The interaction strengths depend inter alia on the number of consecutive chemical bonds intervening between the two interacting spins and on the molecular conformation. The pairwise interaction of many spins in a molecule resulting in a complex spectrum poses a severe challenge to analyse the spectrum and hence the determination of magnitudes and signs of homo- and heteronuclear couplings. The problem is more severe in the analysis of 1H spectra than the spectra of most of the other nuclei due to the often very small chemical shift dispersion. As a consequence, the straightforward analysis and the accurate extraction of the coupling constants from the 1H spectrum of a complex spin system continues to remain a challenge, and often may be a formidable task. Over the years, the several pure shift-based one‐dimensional and two‐dimensional methodologies have been developed by workers in the field, which provide broadband homonuclear decoupling of proton spectra, removing the complexity but at the cost of the very informative scalar couplings. To circumvent this problem, several one‐dimensional and two‐dimensional NMR experiments have been developed for the determination of homonuclear and heteronuclear couplings (nJHX, where n = 1,2,3) while retaining the high resolution obtained by implementing pure shift strategies. This review attempts to summarize the extensive work reported by a large number of researchers over the years for the accurate determination of homo- and heteronuclear scalar couplings.

High-Resolution Proton NMR Spectra in Solids by Single-Pulse Excitation

High-Resolution Proton NMR Spectra in Solids by Single-Pulse Excitation

"Decoupled" Proton NMR Spectra

High-resolution proton NMR spectra are recorded in a new form where all resonances are singlets at the chemical-shift frequencies, with no spin-spin splittings. These "decoupled" proton spectra are derived from two-dimensional J spectra after real Fourier transformation (without frequency discrimination in F1) so that each spin multiplet lies along both the 45° and the 135° diagonal, forming a pattern similar to St. Andrew′s cross, with C4 symmetry. The chemical shifts are located by searching for these centers of symmetry with a postacquisition data-processing algorithm. This is designed to facilitate the separation of overlapping and interpenetrating spin multiplets. The method is illustrated with applications to the 400 MHz high-resolution proton spectra of dehydrotestosterone and 4-androsten-3,17-dione. It is also possible to separate the spectra of components in a mixture and this is illustrated by breaking down the spectrum of an aqueous solution of D-glucose into subspectra from the α and β anomers, in order to follow the time evolution of the mutarotation.

Polymer conformation effects in solid-state high-resolution proton NMR spectra obtained by combined rotation and multiple pulse spectroscopy (CRAMPS)

It is shown that CRAMPS can be used to characterize conformation in crystalline polymers and conformational changes and/or specific interactions in miscible polymer blends

Correlation between nmr spectral parameters of nucleosides and its implication to the conformation about the glycosyl bond

Analyses of high resolution proton and carbon NMR spectra of a series of guanine nucleosides in DMSO have revealed a near linear correlation between the chemical shift of the H2, atom of the sugar moiety and the vicinal coupling constant 3JC4-H1'. This unexpected result provides evidence that the variations in the glycosyl torsion angle between nucleosides in solution are less that those which have previously been reported in crystals and it is an experimental basis for analyzing the syn and anti populations from chemical shift and coupling constant data.

Localized proton NMR spectroscopy in different regions of the human brain in vivo. Relaxation times and concentrations of cerebral metabolites.

High-resolution proton NMR spectra of normal human brain in vivo have been obtained from selected 27- and 64-ml volumes-of-interest (VOI) localized in the insular area, the occipital area, the thalamus, and the cerebellum of normal volunteers. Localization was achieved by stimulated echo (STEAM) sequences using a conventional 1.5-T whole-body MRI system (Siemens Magnetom). The proton NMR spectra show resonances from lipids, lactate, acetate, N-acetylaspartate (NAA), gamma-aminobutyrate, glutamine, glutamate, aspartate, creatine and phosphocreatine, choline-containing compounds, taurine, and inositols. While T1 relaxation times of most of these metabolites were about 1100-1700 ms without significant regional differences, their T2 relaxation times varied between 100 and 500 ms. The longest T2 values of about (500 +/- 50) ms were observed for the methyl protons of NAA in the white matter of the occipital lobe compared to (320 +/- 30) ms in the other parts of the brain. No significant regional T2 differences were found for choline and creatine methyl resonances. The relative concentrations of NAA in gray and white matter were found to be 35% higher than those in the thalamus and cerebellum. Assuming a concentration of 10 mM for total creatine the resulting NAA concentrations of 13-18 mM are by a factor of 2-3 higher than previously reported using analytical techniques. Cerebral lactate reached a maximum concentration of about 1.0 mM.

Localized high‐resolution proton NMR spectroscopy using stimulated echoes: Initial applications to human brain in vivo

Water-suppressed localized proton NMR spectroscopy using stimulated echoes has been successfully applied to detect metabolites in the human brain in vivo. The STEAM spectroscopy sequence allows single-step localization by exciting three intersecting slices. Water suppression is achieved by preceding chemical-shift-selective (CHESS) rf pulses. High-resolution (0.05 ppm) proton NMR spectra of healthy volunteers have been High-resolution (0.05 ppm) proton NMR spectra of healthy volunteers have been obtained on a conventional 1.5-T whole-body MRI system (Siemens Magnetom). Volumes-of-interest (VOI) of 64 ml (4 x 4 x 4 cm3) were localized in the occipital area of the brain and spectra were recorded within measuring times ranging from 1 s (single scan) to about 10 min. The experimental procedure is described in detail. Resonance assignments include acetate, N-acetyl aspartate, gamma-amino butyrate, glutamine, glutamate, aspartate, creatine and phosphocreatine, choline-containing compounds, taurine, and inositols. Cerebral lactate was found to be at a maximum concentration of 0.5 mM when assuming N-acetyl aspartate in white matter to be 6 mM.

1H and13C NMR study of 2-alkyl thio-5-phenyl-1,3,4-thiadiazole; The S-oxidation and dioxidation effects

Abstract High resolution proton and carbon-13 NMR spectra of 2-alkyl thio-; 2-alkyl sulfinyl, and 2-alkyl sulfonyl-5-phenyl-1, 3,4-thiadiazoles were analysed. The chirality of the sulfoxide group induced proehiralty at the oα-methylene or β-methylene carbon atom. Further more the S-oxidation and dioxidation effects on the carbon-13 chemical shifts were summarised.

High-resolution proton NMR spectra of human arterial plaque

Well-resolved proton ( 1H) NMR spectra of solid human arterial plaque can be acquired. Studies have been carried out of human fatty plaque obtained postmortem ( ex vivo ), the total lipids extracted from human atheroma, and a model mixture of cholesteryl esters whose lipid composition resembles that of human atheroma. In each case, well-resolved 1H NMR spectra were obtained at body temperature (37°C), with little or no underlying broad signal. Such sharp 1H NMR spectra are typical of isotropic fluids, whereas solid and liquid-crystalline materials give rise to much broader spectral lines. The results suggest the sharp 1H NMR spectra of human atheromatous lesions at body temperature are due largely to the presence of intracellular and extracellular droplets of cholesteryl esters in the isotropic liquid phase. These findings provide a necessary basis for use of 1H NMR techniques to image quantitatively the lipid constituents of human atheroma in vivo , and to study their chemical and physical properties.

Assignment of resonances for ‘acute-phase’ glycoproteins in high resolution proton NMR spectra of human blood plasma

Broad resonances at 2.04 and 2.08 ppm in 500 MHz Hahn spin-echo 1H NMR spectra of human blood plasma are assigned to the N-acetyl groups of mobile carbohydrate side-chains (largely N-acetylglucosamine and N-acetylneuraminic acid) of glycoproteins such as α 1,-acid glycoprotein. Their intensities in spin-echo spectra correlate with clinical conditions in which an elevation of the level of ‘acute-phase’ glycoproteins is expected, and so may be of value in the study of certain diseases.

Analysis of human aqueous humour by high resolution 1H NMR spectroscopy

High-resolution proton NMR spectra from nine samples of human aqueous humour extracted from the anterior chamber of the eye are reported. They were obtained within the order of 45 min from small volumes (ca 0·3 ml) of sample with minimal pretreatment and the method was non-destructive. Metabolites detected included acetate, acetoacetate, alanine, ascorbate, citrate, creatine, formate, glucose, glutamine-glutamate, β-hydroxybutyrate, lactate, threonine and valine. Concentrations of lactate, valine, alanine and acetate were determined. These novel metabolite profiles may be of value in the study of clinical disorders.