Recoupling of chemical-shift anisotropy powder patterns in MAS NMR

Scaled recoupling of chemical shift anisotropies at high magnetic fields under MAS with interspersed C-elements

The power of chemical shift anisotropy (CSA) measurements for probing structure and dynamics of molecules has been long recognized. NMR pulse sequences that allow measurement of CSA values in an indirect dimension of a protein correlation spectrum have been employed for aliphatic groups, but for practical reasons, carbonyl functional groups have been little studied, despite the fact that carbonyls are expected to give particularly varied and informative CSA values. Specifically, the wide spectral widths of carbonyl tensors make their measurements difficult with typically attainable spectrometer settings. We present here an extended family of experiments that enable the recovery of static CSA lineshapes in an indirect dimension of magic angle spinning (MAS) solid-state NMR experiments, except for various real valued scaling factors. The experiment is suitable for uniformly labeled material, at moderate MAS rates (10 kHz-30 kHz) and at higher magnetic fields (ν_0H > 600 MHz). Specifically, the experiments are based on pulse sequence elements from a previous commonly used pulse sequence for CSA measurement, recoupling of chemical shift anisotropy (ROCSA), while modification of scaling factors is achieved by interspersing different blocks of C-elements of the same Cn_n ¹ cycle. Using experimental conditions similar to the parent ROCSA sequence, a CSA scaling factor between 0 and 0.272 can be obtained, thus allowing a useful practical range of possibilities in experimental conditions for measurement of larger CSA values. Using these blocks, it is also possible to make a constant-time CSA recoupling sequence. The effectiveness of this approach, fROCSA, is shown on model compounds 1-¹³C-Gly, U-¹³C,¹⁵N-l-His, and microcrystalline U-¹³C,¹⁵N-Ubiquitin.

Proton-detected 3D 1H anisotropic/14N/1H isotropic chemical shifts correlation NMR under fast magic angle spinning on solid samples without isotopic enrichment

The chemical shift anisotropy (CSA) interaction of a nucleus is an important indicator of the local electronic environment particularly for the contributions arising from hydrogen (H)-bonding, electrostatic and π-π interactions. CSAs of protons bonded to nitrogen atoms are of significant interest due to their common role as H-bonding partners in many chemical, pharmaceutical and biological systems. Although very fast (∼100 kHz) magic angle sample spinning (MAS) experiments have enabled the measurement of proton CSAs directly from solids, due to a narrow chemical shift (CS) distribution, overlapping NH proton resonances are common and necessitate the introduction of an additional frequency dimension to the regular 2D ¹H CSA/¹H CS correlation method to achieve sufficient resolution. While this can be accomplished by using the isotropic shift frequency of ¹⁴N or ¹⁵N nuclei, the use of the naturally-abundant ¹⁴N nucleus avoids ¹⁵N isotopic labeling and therefore would be useful for a variety of solids. To this end, we propose a proton-detected 3D ¹H CSA/¹⁴N/¹H CS correlation method under fast MAS (90 kHz) to determine the CSA tensors of NH protons in samples without isotopic enrichment. Our experimental results demonstrate that the proposed 3D NMR experiment is capable of resolving the overlapping ¹H resonances of amide (NH) groups through the ¹⁴N isotropic shift frequency dimension and enables the accurate measurement of site-specific ¹H CSAs directly from powder samples under fast MAS conditions. In addition to the 3D ¹H CSA/¹⁴N/¹H CS experiment, an approach employing ¹⁴N-edited 2D ¹H CSA/¹H CS experiment is also demonstrated as an additional means to address spectral overlap of NH resonances with aliphatic and other proton resonances in solids.

Spatial reorientation experiments for NMR of solids and partially oriented liquids

Motional reorientation experiments are extensions of Magic Angle Spinning (MAS) where the rotor axis is changed in order to average out, reintroduce, or scale anisotropic interactions (e.g. dipolar couplings, quadrupolar interactions or chemical shift anisotropies). This review focuses on Variable Angle Spinning (VAS), Switched Angle Spinning (SAS), and Dynamic Angle Spinning (DAS), all of which involve spinning at two or more different angles sequentially, either in successive experiments or during a multidimensional experiment. In all of these experiments, anisotropic terms in the Hamiltonian are scaled by changing the orientation of the spinning sample relative to the static magnetic field. These experiments vary in experimental complexity and instrumentation requirements. In VAS, many one-dimensional spectra are collected as a function of spinning angle. In SAS, dipolar couplings and/or chemical shift anisotropies are reintroduced by switching the sample between two different angles, often 0° or 90° and the magic angle, yielding a two-dimensional isotropic-anisotropic correlation spectrum. Dynamic Angle Spinning (DAS) is a related experiment that is used to simultaneously average out the first- and second-order quadrupolar interactions, which cannot be accomplished by spinning at any unique rotor angle in physical space. Although motional reorientation experiments generally require specialized instrumentation and data analysis schemes, some are accessible with only minor modification of standard MAS probes. In this review, the mechanics of each type of experiment are described, with representative examples. Current and historical probe and coil designs are discussed from the standpoint of how each one accomplishes the particular objectives of the experiment(s) it was designed to perform. Finally, applications to inorganic materials and liquid crystals, which present very different experimental challenges, are discussed. The review concludes with perspectives on how motional reorientation experiments can be applied to current problems in chemistry, molecular biology, and materials science, given the many advances in high-field NMR magnets, fast spinning, and sample preparation realized in recent years.

Carbon-13 chemical-shift tensors in indigo: A two-dimensional NMR-ROCSA and DFT Study

The principal components of the (13)C NMR chemical-shift tensors for the eight unique carbon sites of crystalline indigo have been measured using the ROCSA pulse sequence. The chemical shifts have been assigned unambiguously to their respective nuclear sites through comparison of the experimental data to the results of density-functional calculations employing a refined X-ray diffraction structure. These measurements expand the database of measured aromatic (13)C chemical-shift tensors to the indole ring. Magnetic shielding calculations for hypoxanthine and adenosine are also reported. Comparisons of calculations that include the effect of the crystalline lattice with calculations that model indigo as an isolated molecule give an estimate of the intermolecular contribution to the magnetic shielding.

Applications of NMR crystallography to problems in biomineralization: refinement of the crystal structure and 31P solid-state NMR spectral assignment of octacalcium phosphate

By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its (31)P NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques.

A simple one-dimensional method of chemical shift anisotropy determination under MAS conditions

A method of determination of chemical shift anisotropy (CSA) tensor principal components under MAS condition is presented. It is a simple, one-dimensional, and robust alternative to the commonly exploited, but more complicated 2D-PASS. The required CSA components are delivered by simultaneous numerical analysis of a few regular MAS spectra acquired under different spinning rates.

Correlating sideband patterns with powder patterns for accurate determination of chemical shift parameters in solid-state NMR

Powder patterns and sideband patterns have different strengths when it comes to using them to determine chemical shift parameters. Here, we show that chemical shift parameters can be determined with high accuracy by analysing the correlation pattern from a 2D experiment which correlates a powder pattern in the indirect dimension with a sideband pattern in the direct dimension. The chemical shift parameters so determined have greater accuracy than those obtained by analysing a sideband or powder pattern alone, for the same signal-to-noise ratio. This method can be applied for both resolved correlation patterns and to cases where two components share similar isotropic chemical shifts. The methodology is demonstrated in this paper, both theoretically and experimentally, on the (31)P signals of the bis-phosphonate drug, pamidronate.

Multidimensional solid state NMR of anisotropic interactions in peptides and proteins

Accurate determinations of chemical shift anisotropy (CSA) tensors are valuable for NMR of biological systems. In this review we describe recent developments in CSA measurement techniques and applications, particularly in the context of peptides and proteins. These techniques include goniometeric measurements of single crystals, slow magic-angle spinning studies of powder samples, and CSA recoupling under moderate to fast MAS. Experimental CSA data can be analyzed by comparison with ab initio calculations for structure determination and refinement. This approach has particularly high potential for aliphatic (13)C analysis, especially Calpha tensors which are directly related to structure. Carbonyl and (15)N CSA tensors demonstrate a more complex dependence upon hydrogen bonding and electrostatics, in addition to conformational dependence. The improved understanding of these tensors and the ability to measure them quantitatively provide additional opportunities for structure determination, as well as insights into dynamics.

Using chemical shift anisotropy to resolve isotropic signals in solid-state NMR

A key problem in solid-state NMR is resolving overlapping isotropic signals. We present here a two-dimensional method which can enable sites with the same isotropic chemical shift to be distinguished according to their chemical shift anisotropy and asymmetry. The method involves correlating sideband spectra at different effective spinning rates using CSA-amplification pulse sequences. The resulting two-dimensional correlation pattern allows very accurate determination of the chemical shift principal values in addition to the recovery of parameters for two overlapping patterns which allows the resolution of overlapping signals.

Determination of chemical shift anisotropies of unresolved carbonyl sites by C-alpha detection under magic-angle spinning

We demonstrate that the static powder pattern line shape of chemical shift anisotropy (CSA) can be obtained for unresolved carbonyl sites of polypeptides under magic-angle spinning. The CSA interaction is first recoupled at the carbonyl site. The phase factors associated with the CSA recoupling are then transferred to the adjacent alpha carbon by an isotropic polarization transfer based on scalar spin-spin coupling. Because alpha carbons of polypeptides are usually better resolved, we can then obtain the CSA static powder pattern line shapes of the carbonyl sites after Fourier transformation in the second dimension. We validate our approach experimentally by measurements on [U-(13)C, (15)N]-l-alanine, [U-(13)C, (15)N]-l-valine and prion fibrils with uniform (13)C and (15)N labels on selected residues.