High-resolution NMR Spectra Research Articles

High-resolution NMR with resistive and hybrid magnets: Deconvolution using a field-fluctuation signal

A method for compensating effect of field fluctuation is examined to attain high-resolution NMR spectra with resistive and hybrid magnets. In this method, time dependence of electromotive force induced for a pickup coil attached near a sample is measured synchronously with acquisition of NMR. Observed voltage across the pickup coil is converted to field fluctuation data, which is used to deconvolute NMR signals. The feasibility of the method is studied by 79Br MAS NMR of KBr under a 30 T magnetic field of a hybrid magnet. Twenty single-scan NMR signals were accumulated after the manipulation, resulting in a high-resolution NMR spectrum.

27Al double rotation two-dimensional spin diffusion NMR: Complete unambiguous assignment of aluminium sites in 9Al 2O 3 · 2B 2O 3

High-resolution two-dimensional 27Al spin diffusion NMR spectra of 9Al 2O 3 · 2B 2O 3 recorded under double rotation for different mixing times are presented. The two crystallographically distinct pentahedral Al sites are distinguished by the presence or absence of correlation peaks with neighboring tetrahedral sites.

An algorithm for the automated quantitation of metabolites in in vitro NMR signals

The quantitation of metabolite concentrations from in vitro NMR spectra is hampered by the sensitivity of peak positions to experimental conditions. The quantitation methods currently available are generally labor intensive and cannot readily be automated. Here, an algorithm is presented for the automatic time domain analysis of high-resolution NMR spectra. The TARQUIN algorithm uses a set of basis functions obtained by quantum mechanical simulation using predetermined parameters. Each basis function is optimized by subdividing it into a set of signals from magnetically equivalent spins and varying the simulated chemical shifts of each of these groups to match the signal undergoing analysis. A novel approach to the standard multidimensional minimization problem is introduced based on evaluating the fit resulting from different permutations of possible chemical shifts, obtained from one-dimensional searches. Results are presented from the analysis of (1)H proton magic angle spinning spectra of cell lines illustrating the robustness of the method in a typical application. Simulation was used to investigate the biggest peak shifts that can be tolerated.

Synthesis and high-resolution NMR spectra of A-nor-derivatives of 11-deoxyglycyrrhetic acid

Several skeletal and oxidative transformations of 11-deoxyglycyrrhetic acid were carried out to produce A-nor-derivatives. Signals for protons and C atoms in the PMR and 13C NMR spectra of the A-nor-derivatives were assigned using high-resolution 1H (400 MHz) and 13C (125 MHz) NMR spectroscopy.

Orientation of molecules by magnetic field as a new source of information on their structures

A complex procedure for quantitative allowance for small but significant effects of molecular orientation by strong static magnetic fields was elaborated. A series of high-resolution 1H NMR spectra of 1,2,3-trichloronaphthalene recorded at magnetic field strength varied over a wide range was analyzed in the framework of a unified approach with high accuracy. The spin-spin coupling constants and the dipole-dipole coupling constants for all pairs of 1H nuclei and the anisotropy and rhombicity parameters of the magnetic susceptibility tensor of the molecule were determined. Ab initio CSGT/RHF quantum chemical calculations of this property using a wide range of conventional diffuse and polarization basis set functions were carried out. Augmentation of the basis set with polarization functions affects the values of the calculated parameters to a lesser extent compared to augmentation with diffuse functions. The results of calculations using the 6-311G(df) and 6-311++G(df) basis sets are in good agreement with the experimental values of the magnetic susceptibility anisotropy for 1,2,3-trichloronaphthalene. The advantages of the method proposed and specific features of the effects of orientation by magnetic field as a new source of information on the structure of molecules in solution are discussed.

Detection of Peptide−Phospholipid Interaction Sites in Bilayer Membranes by 13C NMR Spectroscopy: Observation of 2H/31P-Selective 1H-Depolarization under Magic-Angle Spinning

We have developed a solid-state NMR method for observing the signals due to 13C spins of a peptide in the close vicinity of 31P and 2H spins in deuterated phospholipid bilayers. The signal intensities in 13C high-resolution NMR spectra directly indicate the depolarization of 1H by 1H-31P and 1H-2H dipolar couplings under multiple-contact cross-polarization. This method was applied to a fully 13C-, 15N-labeled 14-residue peptide, mastoparan-X (MP-X), bound to phospholipid bilayers whose fatty acyl chains are deuterated. The 13C NMR spectra for the depolarization were simulated from the chemical shifts and structure of membrane-bound MP-X previously determined and the distribution of 2H and 31P spins in lipid bilayers. The minimization of RMSD between the simulated and the experimental spectra showed that the amphiphilic alpha-helix of MP-X was located in the interface between the water layer and the hydrophobic domain of the bilayer, with nonpolar residues facing the phosphorus atoms and alkyl chains of the lipids.

Characterisation of the 1H and 13C NMR spectra of methylcitric acid

Methylcitric acid (MCA) was synthesised in Reformatsky reaction (2 RS, 3 RS stereoisomers) and in the nucleophilic addition (2 RS, 3 SR stereoisomers). The stereoselectivity of these reactions was analysed. 1H and 13C NMR spectra of diastereoisomers of methylcitric acid were recorded and interpreted. The values of 1H chemical shifts and 1H– 1H coupling constants were analysed. Proton-decoupled high-resolution 13C NMR spectra of MCA diastereoisomers were measured in a series of dilute water solutions of various acidities. These data may provide a basis for unequivocal determination of the presence of MCA in the urine samples of patients’ suffering from propionic acidemia, methylmalonic aciduria, or holocarboxylase synthetase deficiency. NMR spectroscopy enables determination of MCA diastereoisomers in body fluids and can be a complementary and useful diagnostic tool.

Removal of a time barrier for high-resolution multidimensional NMR spectroscopy

We introduce the recursive multidimensional decomposition (R-MDD) method to speed recording of high-resolution NMR spectra. The measurement time is logarithmically dependent on the sizes of indirect spectral dimensions. R-MDD has the sensitivity and resolution advantages of optimized nonuniform acquisition schemes and is applicable to all types of biomolecular spectra. We demonstrated it for triple resonance experiments on three globular proteins (ubiquitin, azurin and the barstar-barnase complex) of 8-22 kDa.

Microcoil High-Resolution Magic Angle Spinning NMR Spectroscopy

We report the construction of a dual-channel microcoil nuclear magnetic resonance probehead allowing magic-angle spinning for mass-limited samples. With coils down to 235 mum inner diameter, this allows high-resolution solid-state NMR spectra to be obtained for amounts of materials of a few nanoliters. This is demonstrated by the carbon-13 spectrum of a tripeptide and a single silk rod, prepared from the silk gland of the Bombyx mori silkworm. Furthermore, the microcoil allows for radio frequency field strengths well beyond current probe technology, aiding in getting the highest possible resolution by efficiently decoupling the observed nuclei from the abundantly present proton nuclei.

Structure of polypiperylene chain segments according to high-resolution 13C NMR spectra

The microstructure of stereoregular 1,4-trans-and 1,4-cis-polypiperylenes, as well as polymers prepared from the trans-and cis-piperylene isomers via cationic polymerization in the presence of TiCl4, was studied by high-resolution 13C NMR spectroscopy. Polypiperylene synthesized through the cationic polymerization of the cis isomer had a more diversified morphology of the macromolecular chain, i.e., had higher relative amounts of 1,2-cis-units and combinations of irregular-addition 1,4-trans-units. It was shown that 13C NMR spectra give the most comprehensive and independent information on the details of structure of the piperylene macromolecular chain.

Crystallization of Zeolites from Organo-Silicic Colloids

As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200 degrees C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85 degrees C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (approximately = 1 day) at 80 degrees C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si-O-Si angle in the silica cluster seems to be of paramount importance.

Deterministic and statistical methods for reconstructing multidimensional NMR spectra

Reconstruction of an image from a set of projections is a well-established science, successfully exploited in X-ray tomography and magnetic resonance imaging. This principle has been adapted to generate multidimensional NMR spectra, with the key difference that, instead of continuous density functions, high-resolution NMR spectra comprise discrete features, relatively sparsely distributed in space. For this reason, a reliable reconstruction can be made from a small number of projections. This speeds the measurements by orders of magnitude compared to the traditional methodology, which explores all evolution space on a Cartesian grid, one step at a time. Speed is of crucial importance for structural investigations of biomolecules such as proteins and for the investigation of time-dependent phenomena. Whereas the recording of a suitable set of projections is a straightforward process, the reconstruction stage can be more problematic. Several practical reconstruction schemes are explored. The deterministic methods-additive back-projection and the lowest-value algorithm-derive the multidimensional spectrum directly from the experimental projections. The statistical search methods include iterative least-squares fitting, maximum entropy, and model-fitting schemes based on Bayesian analysis, particularly the reversible-jump Markov chain Monte Carlo procedure. These competing reconstruction schemes are tested on a set of six projections derived from the three-dimensional 700-MHz HNCO spectrum of a 187-residue protein (HasA) and compared in terms of reliability, absence of artifacts, sensitivity to noise, and speed of computation.

High-level bacterial expression and purification of human SirT2 protein for NMR studies

Silent information regulator 2 (Sir2) proteins are a class of protein deacetylase enzymes that play key roles in transcriptional gene silencing, DNA repair, and aging. Here, we describe the high-level bacterial expression and purification of a human SirT2 construct that yields high resolution NMR spectra. By removing the N-terminal helix α0 and using Thioredoxin as a fusion partner, greater than 10 mg/L of purified protein can be obtained from minimal media. The protein is fully functional and enables NMR-based screening and structural studies of this important protein.

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.

Microstructure evolution of hydrated cement pastes

We propose an original method based on both proton nuclear magnetic relaxation dispersion and high-resolution NMR spectra to investigate the microstructure of synthesized Ca3SiO5-hydrated cement paste. This method allows a clear assessment of the local proton chemical sites as well as the determination of dynamical information of moving proton species in pores. We show also how the microstructure evolves during and after completion of hydration in a range of length scales between 2 and 500 nm. In particular, we show how the pore size distribution of the cement paste reaches progressively a power-law characteristic of a surface-fractal distribution with a dimension Df = 2.6, which takes into account the hierarchical order in the material. Last, we study how this pore size distribution is modified during setting by varying either the water-to-cement ratio or addition of ultrafine particles. This shows that our method could be relevant to relate the mechanical properties to the microstructure of the material. This proposed NMR method is general enough for the characterization of microstructure of any porous media with reactive surface involving water confinement.

Feasibility of the overtone NMR for half-integer quadrupolar nuclei

We present a theoretical description and computer simulations for the overtone NMR of half-integer quadrupolar nuclei in solids. It is shown that the overtone NMR has the ability to produce high-resolution solid NMR spectra for the quadrupolar nuclei under magic angle spinning conditions. We demonstrate that when polycrystalline 23Na 2MoO 4 is placed in the static magnetic field of 7.05 T, the direct detection of the overtone NMR is possible. It becomes, however, apparent that indirect detection schemes are required to obtain the usual NMR spectra that have separated resonance lines corresponding to chemically non-equivalent sites.

Investigating Sorption on Iron−Oxyhydroxide Soil Minerals by Solid-State NMR Spectroscopy: A 6Li MAS NMR Study of Adsorption and Absorption on Goethite

High-resolution 2H MAS NMR spectra can be obtained for nanocrystalline particles of goethite (alpha-FeOOH, particle size approximately 4-10 nm) at room temperature, facilitating NMR studies of sorption under environmentally relevant conditions. Li sorption was investigated as a function of pH, the system representing an ideal model system for NMR studies. 6Li resonances with large hyperfine shifts (approximately 145 ppm) were observed above the goethite point of zero charge, providing clear evidence for the presence of Li-O-Fe connectivities, and thus the formation of an inner sphere Li+ complex on the goethite surface. Even larger Li hyperfine shifts (289 ppm) were observed for Li+-exchanged goethite, which contains lithium ions in the tunnels of the goethite structure, confirming the Li assignment of the 145 ppm Li resonance to the surface sites.

High‐resolution nuclear magnetic resonance spectroscopy of biological tissues using projected magic angle spinning

High-resolution NMR spectra of materials subject to anisotropic broadening are usually obtained by rotating the sample about the magic angle, which is 54.7 degrees to the static magnetic field. In projected magic angle spinning (p-MAS), the sample is spun about two angles, neither of which is the magic angle. This provides a method of obtaining isotropic spectra while spinning at shallow angles. The p-MAS experiment may be used in situations where spinning the sample at the magic angle is not possible due to geometric or other constraints, allowing the choice of spinning angle to be determined by factors such as the shape of the sample, rather than by the spin physics. The application of this technique to bovine tissue samples is demonstrated as a proof of principle for future biological or medical applications.

Solid-state high-resolution NMR studies on spin fluctuation associated with valence tautomerism of metal–benzoquinone system: Cobalt complex in the stable and meta-stable phases

Abstract Equilibrium between low-spin [CoIII(SQ)(Cat)(N–N)] and high-spin [CoII(SQ)2(N–N)] redox isomers, where SQ is semiquinonate (charge: −1, spin: 1/2), Cat is catecholate (charge: −2, spin: 0) and N–N is chelating nitrogen donor ligand, respectively, is a representative valence tautomeric phenomenon. To elucidate independently the spin state of the cobalt ion and that of benzoquinone-derived ligands in the solid state, we measured 13C MAS NMR spectrum of 3,5-di-t-butyl-1,2-benzoquinone and 2H MAS NMR spectrum of deuterated 2,2′-bipyridine for [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2′-bipyridine)] · x(C6H5CH3) and its deuterated analogue in a temperature range of 200–350 K. Irreversible change of an effective magnetic moment μeff of a virgin sample was observed around 370 K due to a partial loss of crystal solvent and a change of crystal structure, whereas the sample annealed at 390 K showed a crystal structure different from the reported one and a reversible change of μeff, which is ascribed to equilibrium between Co(III)-form (S = 1/2) and Co(II)-form (S = 3/2). Based on the shifts and the number of NMR peaks for the annealed sample, we concluded that (1) interconversion between redox isomers occurs faster than NMR time scale (>104 s−1) in the solid state, (2) intraconversion between SQ and Cat in Co(III)-form also occurs much faster than 5 × 104 s−1 even at 198 K and (3) electron spins on SQ ligands in Co(II)-form are quenched probably due to a strong antiferromagnetic coupling between the two SQ ligands. The enthalpy and the entropy of the interconversion were estimated to be 17 kJ/mol and 54 J/(K mol), respectively. For the virgin metastable phase, SQ and Cat were clearly distinguished by 13C MAS NMR spectrum. The solid-state high-resolution NMR spectrum is useful to detect independently the change of spin states of benzoquinone-derived radical and metal ion.

NMR spectra of paramagnetic complexes of 1-vinylimidazole with iron group metals

The high-resolution 1H and 13C NMR spectra of 1-vinylimidazole complexes with iron group metals were recorded. The contact coupling in these systems was established in the 1H and 13C NMR spectra. The applicability of the NMR spectra transformed by long-range hyperfine coupling for elucidating the molecular structure of the ligand was shown.