A Test for the Number of Coupled Spins I=1/2 in Magic‐Angle‐Spinning Solids: Zero‐Quantum Recoupling of Multiple‐Quantum Coherences

Current methodologies for estimating the number of coupled spins I=1/2 in solids are based upon the maximum multiple‐quantum order that can be observed. This strategy establishes a clear lower bound on the number of coupled spins I=1/2. However, it is difficult to ascertain the exact number of coupled spins, since the absence of a peak could be due either to the limited size of the spin system or to the experimental difficulty of exciting high‐quantum orders and recovering those coherences into detectable signals. Herein, a supplementary test is proposed that allows one to determine whether a given coherence has the highest possible order in the spin system. The sample is subjected to magic‐angle spinning and the behaviour of the coherence under a rotor‐synchronised spin‐echo sequence is compared to its behaviour under a zero‐quantum recoupling sequence. A similar decay of the coherence in these two experiments is strong evidence for the coherence order being the maximum possible. We propose applications to biomolecular solid‐state NMR spectroscopy.     

Enhancing NMR Sensitivity of Natural‐Abundance Low‐γ Nuclei by Ultrafast Magic‐Angle‐Spinning Solid‐State NMR Spectroscopy

Although magic‐angle‐spinning (MAS) solid‐state NMR spectroscopy has been able to provide piercing atomic‐level insights into the structure and dynamics of various solids, the poor sensitivity has limited its widespread application, especially when the sample amount is limited. Herein, we demonstrate the feasibility of acquiring high S/N ratio natural‐abundance ¹³C NMR spectrum of a small amount of sample (≈2.0 mg) by using multiple‐contact cross polarization (MCP) under ultrafast MAS. As shown by our data from pharmaceutical compounds, the signal enhancement achieved depends on the number of CP contacts employed within a single scan, which depends on the T_1ρ of protons. The use of MCP for fast 2D ¹H/¹³C heteronuclear correlation experiments is also demonstrated. The significant signal enhancement can be greatly beneficial for the atomic‐resolution characterization of many types of crystalline solids including polymorphic drugs and nanomaterials.     

Quadruple‐Resonance Magic‐Angle Spinning NMR Spectroscopy of Deuterated Solid Proteins

¹H‐detected magic‐angle spinning NMR experiments facilitate structural biology of solid proteins, which requires using deuterated proteins. However, often amide protons cannot be back‐exchanged sufficiently, because of a possible lack of solvent exposure. For such systems, using ²H excitation instead of ¹H excitation can be beneficial because of the larger abundance and shorter longitudinal relaxation time, T₁, of deuterium. A new structure determination approach, “quadruple‐resonance NMR spectroscopy”, is presented which relies on an efficient ²H‐excitation and ²H‐¹³C cross‐polarization (CP) step, combined with ¹H detection. We show that by using ²H‐excited experiments better sensitivity is possible on an SH3 sample recrystallized from 30 % H₂O. For a membrane protein, the ABC transporter ArtMP in native lipid bilayers, different sets of signals can be observed from different initial polarization pathways, which can be evaluated further to extract structural properties.     

Hexameric Capsules Studied by Magic Angle Spinning Solid‐State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules

Powders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid‐state NMR (MAS ssNMR). Evaporation selectively removed non‐encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, ¹H/¹³C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state.     

Fast Acquisition of Proton-Detected HETCOR Solid-State NMR Spectra of Quadrupolar Nuclei and Rapid Measurement of NH Bond Lengths by Frequency Selective HMQC and RESPDOR Pulse Sequences

Fast magic-angle spinning (MAS), frequency selective (FS) heteronuclear multiple quantum coherence (HMQC) experiments which function in an analogous manner to solution SOFAST HMQC NMR experiments, are demonstrated. Fast MAS enables efficient FS excitation of ¹H solid-state NMR signals. Selective excitation and observation preserves ¹H magnetization, leading to a significant shortening of the optimal inter-scan delay. Dipolar and scalar ¹H{¹⁴N} FS HMQC solid-state NMR experiments routinely provide 4- to 9-fold reductions in experiment times as compared to conventional ¹H{¹⁴N} HMQC solid-state NMR experiments. ¹H{¹⁴N} FS resonance-echo saturation-pulse double-resonance (RESPDOR) allowed dipolar dephasing curves to be obtained in minutes, enabling the rapid determination of NH dipolar coupling constants and internuclear distances. ¹H{¹⁴N} FS RESPDOR was used to assign multicomponent active pharmaceutical ingredients (APIs) as salts or cocrystals. FS HMQC also provided enhanced sensitivity for ¹H{¹⁷O} and ¹H{³⁵Cl} HMQC experiments on ¹⁷O-labeled Fmoc-alanine and histidine hydrochloride monohydrate, respectively. FS HMQC and FS RESPDOR experiments will provide access to valuable structural constraints from materials that are challenging to study due to unfavorable relaxation times or dilution of the nuclei of interest.     

二维双量子魔角旋转核磁共振技术在功能材料研究中的应用

本文简要介绍了二维双量子魔角旋转核磁共振（DQ-MAS NMR）新技术的基本原理,详细综述了1H、19F、29Si、31P、19F和27Al DQ-MAS NMR技术在各种固体功能材料中的应用,并展望了该技术的应用前景.     

51V magic angle spinning NMR in VOPO4 phases

51V magic angle spinning NMR was applied to the alpha(II), beta and gamma phases of VOPO4 at three magnetic field strengths (4.7, 7.1, and 11.7 T). The 51V quadrupole and chemical shift tensors were determined by iterative fitting of the NMR lineshapes at the three magnetic field strengths. The applicability of the method is illustrated by comparison with literature data. Although determined chemical shift tensors are completely axially symmetric and of the same magnitude, all studied phases can clearly be distinguished by their quadrupole coupling tensor. Relationships between the 51V NMR data and structural characteristics such as crystal symmetries are discussed.     

Proton‐Detected Solid‐State NMR Spectroscopy of Natural‐Abundance Peptide and Protein Pharmaceuticals

The natural way : A sensitive NMR spectroscopic method is developed to obtain well‐resolved two‐dimensional spectra (¹⁵N–¹H and ¹³C–¹H) for natural‐abundance (that is, without the need for isotopic enrichment) large‐molecule samples, such as biopharmaceuticals. This method gives structural insights on two lyophilized aprotinin samples and three insulin samples in lyophilized, microcrystalline suspension formulation (red; see picture) and fibril (green) forms.

     

Assessing the potential of fast amplitude modulation pulses for improving triple‐quantum magic angle spinning NMR spectra of half‐integer quadrupolar nuclei

A comprehensive experimental and numerical study of the potential of fast amplitude (FAM) irradiation for improving the triple‐quantum (3Q) magic angle spinning (MAS) NMR spectra of half‐integer nuclei (²³Na, ²⁷Al, ⁴⁵Sc, ⁹³Nb) was carried out. Materials of academic and industrial importance, such as infrared‐emitter Na₃YSi₃O₉, microporous aluminophosphate VPI‐5, mineral andalusite, calcined kaolinite, Sc₂O₃ and relaxor ferroelectric PMN, were investigated. It was found that FAM pulses are indeed of practical relevance and particularly useful for the observation of the NMR resonances given by nuclei in distorted local environments (large quadrupole coupling constants). In addition, a novel strategy for the optimization of the FAM‐II MQ MAS NMR experiment, which improves the multiple‐ to single‐quantum coherence transfer efficiency, is also reported. Copyright © 2003 John Wiley & Sons, Ltd.     

Powder‐XRD and 14N magic angle‐spinning solid‐state NMR spectroscopy of some metal nitrides

Some metal nitrides (TiN, ZrN, InN, GaN, Ca₃N₂, Mg₃N₂, and Ge₃N₄) have been studied by powder X‐ray diffraction (XRD) and ¹⁴N magic angle‐spinning (MAS) solid‐state NMR spectroscopy. For Ca₃N₂, Mg₃N₂, and Ge₃N₄, no ¹⁴N NMR signal was observed. Low speed (ν_r = 2 kHz for TiN, ZrN, and GaN; ν_r = 1 kHz for InN) and ‘high speed’ (ν_r = 15 kHz for TiN; ν_r = 5 kHz for ZrN; ν_r = 10 kHz for InN and GaN) MAS NMR experiments were performed. For TiN, ZrN, InN, and GaN, powder‐XRD was used to identify the phases present in each sample. The number of peaks observed for each sample in their ¹⁴N MAS solid‐state NMR spectrum matches perfectly well with the number of nitrogen‐containing phases identified by powder‐XRD. The ¹⁴N MAS solid‐state NMR spectra are symmetric and dominated by the quadrupolar interaction. The envelopes of the spinning sidebands manifold are Lorentzian, and it is concluded that there is a distribution of the quadrupolar coupling constants Q_cc's arising from structural defects in the compounds studied. Copyright © 2015 John Wiley & Sons, Ltd.     

Solid‐state NMR characterization of oxygen sites in organically modified aluminosilicate xerogels

Solid‐state NMR characterization of hybrid aluminosilicate xerogels, by ¹⁷O magic angle spinning (MAS) and triple quantum magic angle spinning (MQMAS) techniques, evidences Si—O—Si and Si—O—Al oxygen sites, spectrally separated in MQMAS experiments. Inversion of the MQMAS spectra allows the measurement of quadrupolar parameters, isotropic chemical shifts, distribution of chemical shift and discussion of the mobility of the structural units. Copyright © 2003 John Wiley & Sons, Ltd.     

Site‐Resolved Backbone and Side‐Chain Intermediate Dynamics in a Carbohydrate‐Binding Module Protein Studied by Magic‐Angle Spinning NMR Spectroscopy

Magic‐angle spinning solid‐state NMR spectroscopy has been applied to study the dynamics of CBM3b–Cbh9A from Clostridium thermocellum (ctCBM3b), a cellulose binding module protein. This 146‐residue protein has a nine‐stranded β‐sandwich fold, in which 35 % of the residues are in the β‐sheet and the remainder are composed of loops and turns. Dynamically averaged ¹H‐¹³C dipolar coupling order parameters were extracted in a site‐specific manner by using a pseudo‐three‐dimensional constant‐time recoupled separated‐local‐field experiment (dipolar‐chemical shift correlation experiment; DIPSHIFT). The backbone‐Cα and Cβ order parameters indicate that the majority of the protein, including turns, is rigid despite having a high content of loops; this suggests that restricted motions of the turns stabilize the loops and create a rigid structure. Water molecules, located in the crystalline interface between protein units, induce an increased dynamics of the interface residues thereby lubricating crystal water‐mediated contacts, whereas other crystal contacts remain rigid.     

13C cross‐polarization magic angle spinning NMR of ferrocene derivatives

The 13C chemical shifts of the CP/MAS NMR for ferrocene derivatives have been measured. Copyright © 2002 John Wiley & Sons, Ltd.