Highly selective excitation in biomolecular NMR by frequency-switched single-transition cross-polarization

A new method for selective excitation in biomolecular NMR uses two-fold single-transition cross-polarization between protons and nitrogen-15 or carbon-13 nuclei. Switching the frequencies between the forward and backward transfer steps allows one to select a multiplet pattern that is associated with a single pair of spins in a medium-size protein. The efficiency of the transfer benefits from so-called TROSY line-narrowing effects which arise from interference between relaxation mechanisms.     

Density functional theory calculations of hydrogen-bond-mediated NMR J coupling in the solid state

A recently developed method for calculating NMR J coupling in solid-state systems is applied to calculate hydrogen-bond-mediated (2h) J NN couplings across intra- or intermolecular N-H...N hydrogen bonds in two 6-aminofulvene-1-aldimine derivatives and the ribbon structure formed by a deoxyguanosine derivative. Excellent quantitative agreement is observed between the calculated solid-state J couplings and those previously determined experimentally in two recent spin-echo magic-angle-spinning NMR studies ( Brown, S. P. ; et al. Chem. Commun. 2002, 1852-1853 and Pham, T. N. ; et al. Phys. Chem. Chem. Phys. 2007, 9, 3416-3423 ). For the 6-aminofulvene-1-aldimines, the differences in (2h) J NN couplings in pyrrole and triazole derivatives are reproduced, while for the guanosine ribbons, an increase in (2h) J NN is correlated with a decrease in the N-H...N hydrogen-bond distance. J couplings are additionally calculated for isolated molecules of the 6-aminofulevene-1-aldimines extracted from the crystal with and without further geometry optimization. Importantly, it is shown that experimentally observed differences between J couplings determined by solution- and solid-state NMR are not solely due to differences in geometry; long-range electrostatic effects of the crystal lattice are shown to be significant also. J couplings that are yet to be experimentally measured are calculated. Notably, (2h) J NO couplings across N-H...O hydrogen bonds are found to be of a similar magnitude to (2h) J NN couplings, suggesting that their utilization and quantitative determination should be experimentally feasible.     

Solid‐state NMR Spectroscopy of Quadrupolar Nuclei in Inorganic Chemistry

We here review the principles and applications of solid‐state NMR spectroscopy of quadrupolar nuclei, with a special emphasis on structural studies of inorganic solids. Most NMR‐observable nuclei have spin I > 1/2, and possess a quadrupole moment. The resulting quadrupolar interaction severely broadens the resonances, but also encapsulates valuable information about the symmetry of the electronic surroundings of the observed nucleus. The effect of the quadrupolar interaction, as well as that of the chemical shift and dipolar interaction, on solid‐state NMR spectra is examined in this article. To regain good resolution, specifically designed NMR techniques exist to remove the quadrupolar broadening, i.e. overtone and MQMAS spectroscopy, the principles of which are outlined here. In addition, the possibility of distance measurements via the dipolar interaction using the REDOR technique is discussed. The combined information derived from distance measurements, quadrupolar and chemical shift parameters can be helpful for determination of the crystal structure, or for detection of impurity phases, as illustrated by surveying a number of case studies covering spin I = 1, 3/2, 5/2 and 7/2.     

New solid state NMR techniques in coal analysis

Nuclear magnetic resonance (NMR) spectroscopy has become a valuable technique for the determination of the aromaticity of fossil fuels. Now, by exploiting ¹³C relaxation, intimate details of the structure of coal may be determined. This article briefly describes these new applications.     

Broadband rotational resonance in solid state NMR spectroscopy

A new technique for restoring nuclear magnetic dipole-dipole couplings under magic-angle spinning (MAS) in solid state nuclear magnetic resonance (NMR) spectroscopy is described and demonstrated. In this technique, called broadband rotational resonance (BroBaRR), the coupling between a pair of nuclear spins with NMR frequency difference close (but not necessarily equal) to the MAS frequency is restored by the application of a train of weak radio-frequency pulses at a carrier frequency close to the average of the two NMR frequencies. Phase or amplitude modulation of the pulse train at half the MAS frequency splits the carrier into sidebands close to the two NMR frequencies. The pulse train then removes offsets from the exact rotational resonance condition, leading to dipolar recoupling over a bandwidth controlled by the amplitude of the pulse train. (13)C NMR experiments on uniformly (15)N,(13)C-labeled L-valineHClH(2)O powder validate the theoretical analysis. BroBaRR will be useful in studies of molecular structures by solid state NMR, for example in the detection of long-range couplings between carbons in uniformly labeled organic and biological materials.     

Frequency-selective homonuclear dipolar recoupling in solid state NMR

We introduce a new approach to frequency-selective homonuclear dipolar recoupling in solid state nuclear magnetic resonance (NMR) with magic-angle spinning (MAS). This approach, to which we give the acronym SEASHORE, employs alternating periods of double-quantum recoupling and chemical shift evolution to produce phase modulations of the recoupled dipole-dipole interactions that average out undesired couplings, leaving only dipole-dipole couplings between nuclear spins with a selected pair of NMR frequencies. In principle, SEASHORE is applicable to systems with arbitrary coupling strengths and arbitrary sets of NMR frequencies. Arbitrary MAS frequencies are also possible, subject only to restrictions imposed by the pulse sequence chosen for double-quantum recoupling. We demonstrate the efficacy of SEASHORE in experimental (13)C NMR measurements of frequency-selective polarization transfer in uniformly (15)N, (13)C-labeled L-valine powder and frequency-selective intermolecular polarization transfer in amyloid fibrils formed by a synthetic decapeptide containing uniformly (15)N, (13)C-labeled residues.     

固体核磁共振研究分子筛的新进展

分子筛由于具有独特的孔道及可调控酸碱性等特征,被作为离子交换剂、吸附剂及催化剂而广泛应用于石油化工的各种催化过程中。固体核磁共振是研究分子筛结构、酸性及主客体相互作用的强有力谱学手段之一。简单概述了固体核磁共振研究分子筛的最近进展。     

Length scales in heterogeneous polymers from solid state NMR

New methods for probing structure and dynamics of heterogeneous polymers by multidimensional nuclear magnetic resonance (NMR) are described. On the molecular level high resolution multiple quantum spectroscopy of solids probes connectivities of different functional groups on length scales below 1 nm. On a mesoscopic scale NMR spin diffusion techniques probe phase separation and interfacial effects in polymer blends and block copolymers. On a macroscopic scale NMR imaging techniques allow to spatially resolve differences in order and mobility in the necking region or in shearbands of deformed polymers. The techniques are illustrated with amorphous polymers, elastomers, and core-shell systems.     

Symmetry-based constant-time homonuclear dipolar recoupling in solid state NMR

Constant-time dipolar recoupling pulse sequences are advantageous in structural studies by solid state nuclear magnetic resonance (NMR) with magic-angle spinning (MAS) because they yield experimental data that are relatively insensitive to radio-frequency pulse imperfections and nuclear spin relaxation processes. A new approach to the construction of constant-time homonuclear dipolar recoupling sequences is described, based on symmetry properties of the recoupled dipole-dipole interaction Hamiltonian under cyclic displacements in time with respect to the MAS sample rotation period. A specific symmetry-based pulse sequence called PITHIRDS-CT is introduced and demonstrated experimentally. (13)C NMR data for singly-(13)C-labeled amino acid powders and amyloid fibrils indicate the effectiveness of PITHIRDS-CT in measurements of intermolecular distances in solids. (15)N-detected and (13)C-detected measurements of intramolecular (15)N-(15)N distances in peptides with alpha-helical and beta-sheet structures indicate the utility of PITHIRDS-CT in studies of molecular conformations, especially measurements of backbone psi torsion angles in peptides containing uniformly (15)N- and (13)C-labeled amino acids.     

Chemically resolved solid state NMR images of polymers

Polymers are generally spatially heterogeneous, either in terms of morphology or as blends of various components. Solid state NMR imaging provides a means of characterizing both the chemical/morphological composition and its spatial variation. Here we discuss multiple-pulse line-narrowing approaches to acquiring high resolution NMR images and how these sequences can be modified so as to be sensitive to the chemical composition of a sample.     

Site discrimination in mixed-alkali glasses studied by cross-polarization NMR

Cation-cation interactions are thought to play a significant role in shaping the nonlinear compositional dependence of ionic conductivity, known as the mixed-alkali effect (MAE) in glassy solid electrolytes. For providing a structural rationale of this effect, the discrimination of various cation sites in mixed-alkali glasses is of interest. In the present study, cross-polarization (CP) experiments have been applied to glasses in the system [(Li(2)O)x(Na(2)O)(1-x)](0.3)[B(2)O(3)]0.7 to discriminate between alkali ions by virtue of different heteronuclear (7)Li-(23)Na dipole-dipole coupling strengths. Cross-polarization studies involving two types of quadrupolar nuclei (both (7)Li and (23)Na have a spin-quantum number I = 3/2) are complicated by spin state mixing under radio frequency irradiation and magic-angle spinning (MAS). Therefore careful validation and optimization protocols are reported for the model compound LiNaSO(4) prior to conducting the measurements on the glassy samples. (23)Na -->( 7)Li CP/MAS NMR spectra have been obtained on glasses containing the Na(+) ions as the dilute species. They reveal that those lithium species interacting particularly strongly with sodium ions have the same average (7)Li chemical shift as the entire lithium population; the symmetrical situation applies to the (23)Na nuclei at the sodium rich end of the composition range. On the other hand, a clear site discrimination is afforded by temperature-dependent static (23)Na -->( 7)Li CP experiments, indicating that the Li ions that are most strongly interacting with sodium ions are strongly immobilized. This finding provides the first direct experimental evidence for the proposed secondary mismatch concept invoked for explaining the strong MAE in the dilute foreign ion limit.     

Observation of solid-state 103Rh NMR by cross-polarization

Using 103Rh[1H] cross-polarization (CP) methods, we have obtained solid-state 103Rh NMR spectra for diamagnetic Rh(III) compounds. The isotropic chemical shift and chemical shift anisotropy (CSA) are reported for a crystalline form of the dihydroxy-bridged Rh(III) dimer and for a salt of the oxo-centered acetate-bridged Rh(III) trimer, from analysis of conventional CP/MAS spectra. Comparison of the CP kinetics of the dimer with new crystal structure data suggests a strategy for predicting 103Rh CP/MAS properties in solids.     

Structural characterization of bioactive glasses by solid state NMR

Nuclear magnetic resonance (NMR) spectroscopy offers an element-selective, inherently quantitative and experimentally very flexible approach for the structural elucidation of non-crystalline materials. The present review introduces the basic concepts of this technique, highlighting the use of advanced NMR methodology for characterizing short- and intermediate range order in bioactive glass systems. The current state of the literature in this field is summarized in a comprehensive manner. NMR can give clear-cut and quantitative answers about the extent of network polymerization, the spatial distribution of the network former and network modifier species, and the structural roles of Group III elements introduced into these networks. These results facilitate our understanding of the influence of bioglass compositions upon the dissolution kinetics and bioactivities of these glasses. A particular mission of this review is to highlight the utility of non-routine, more advanced experimentation, in the hope of their increased usage and circulation in future applications.

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The main six nuclear isotopes used in obtaining high-resolution magic-angle spinning NMR spectra for the structural characterization of bioactive glasses

     

重质稠环芳烃的固体核磁共振光谱研究

以固体核磁共振光谱为主要分析手段,比较研究了两种合成中间相沥青吡啶不溶组分的结构。结果表明：均四甲苯基中间相沥青吡啶不溶组分具有较低的芳香度和缩合度,富含甲基和环烷结构,分子呈渺位结合实际合构型;与此相反,四氢萘基中间相沥青则呈现很高的芳香度和缩合度,芳环上仅带有一些甲基取代基,平均分子为完全迫位缩合构型,造成这种结构差异的原因在于前驱齐聚物的结构及缩聚程度的不同。     

Solid state analysis in mechanochemistry by high resolution 27Al solid state NMR spectroscopy

Modern solid state NMR techniques [one-dimensional magic angle spinning (MAS) and two-dimensional quadrupole nutation experiments with MAS] are applied for the characterization of mechanochemically activated inorganic powders such as hydrargillite [γ-Al(OH)₃] and pseudo-boehmite [synthetic γ-AlO(OH)]. The materials were activated in a vibration mill.     

Dipolar and Scalar Couplings in Solid State NMR of Quadrupolar Nuclei

Summary.  Most NMR-active nuclei found in the periodic table have a quadrupole moment. In combination with a nonsymmetric electron distribution a strong NMR-active interaction results, which very often overshadows the dipolar and scalar couplings. This article aims at reviewing how these interactions manifest themselves in quadrupolar NMR and how they can be exploited for resonance assignment and structure elucidation, in spite of the presence of a strong quadrupolar interaction. E-mail: alexej.jerschow@nyu.edu Received April 16, 2002; accepted May 15, 2002     

Harnessing Water to Enhance Quadrupolar NMR Spectroscopy and Imaging

¹⁷O and ¹⁴N are attractive targets for in vivo NMR spectroscopy and imaging, but low gyromagnetic ratios γ and fast spin relaxation complicate observations. This work explores indirect ways of detecting some of these sites with the help of proton-detected double resonance techniques. As standard coherence transfer methods are of limited use for such indirect detection, alternative routes for probing the quadrupolar spectra on ¹H were tested. These centered on modulating the broadening effects imparted onto protons adjacent to the low-γ species through J couplings through either continuous wave or spin-echo double-resonance decoupling/recoupling sequences. As in all cases, the changes imparted by these double-resonance strategies were small due to the fast relaxation undergone by the quadrupoles, the sensitivity of these approaches was amplified by transferring their effects onto the abundant water ¹H signal. These amplifications were mediated by the spontaneous exchanges that the labile ¹Hs bound to ¹⁷O or ¹⁴N undergo with the water protons. In experiments designed on the basis of double-resonance spin echoes, these enhancements were imparted by looping the transverse encodings together with multiple longitudinal storage periods, leading to decoupling-recoupling with exchange (D-REX) sequences. In experiments designed on the basis of continuous on/off quadrupolar decoupling, these solvent exchanges were incorporated into chemical-exchange saturation transfer schemes, leading to decoupling-recoupling with saturation transfer (D-REST) sequences. Both of these variants harnessed sizable proportions of the easily detectable water signals, in order to characterize the NMR spectra and/or to image with atomic-site specificity the ¹⁷O and ¹⁴N species.