STARTMAS: A MAS-based method for acquiring isotropic NMR spectra of spin I=3/2 nuclei in real time

NMR of Macromolecular Assemblies and Machines at 1 GHz and Beyond: New Transformative Opportunities for Molecular Structural Biology

As a result of profound gains in sensitivity and resolution afforded by ultrahigh magnetic fields, transformative applications in the fields of structural biology and materials science are being realized. The development of dual low temperature superconducting (LTS)/high-temperature superconducting (HTS) magnets has enabled the achievement of magnetic fields above 1 GHz (23.5 T), which will open doors to an unprecedented new range of applications. In this contribution, we discuss the promise of ultrahigh field magnetic resonance. We highlight several methodological developments pertinent at high-magnetic fields including measurement of 1H-1H distances and 1H chemical shift anisotropy in the solid state as well as studies of quadrupolar nuclei such as 17O. Higher magnetic fields have advanced heteronuclear detection in solution NMR, valuable for applications including metabolomics and disordered proteins, as well as expanded use of proton detection in the solid state in conjunction with ultrafast magic angle spinning. We also present several recent applications to structural studies of the AP205 bacteriophage, the M2 channel from Influenza A, and biomaterials such as human bone. Gains in sensitivity and resolution from increased field strengths will enable advanced applications of NMR spectroscopy including in vivo studies of whole cells and intact virions.

Development and application of novel NMR methodologies for the in situ characterization of crystallization processes of metastable crystalline materials

In this contribution we report on the development and application of modern NMR approaches for the in situ characterization of the crystallization of metastable materials. The work was performed within the framework of the DFG priority programme SPP 1415 “Crystalline Non-Equilibrium Phases”. As one of the goals of this project, the development of a NMR methodology which enables an analysis of local structural motifs on short (1–2 Å) and extended (2–6 Å) length scales without the need for fast magic angle spinning (MAS) has been defined, since the enormous centripetal forces which occur during fast sample rotation (up to 107 g) may intervene with the chemical or physical process which is being monitored. To achieve this goal, we developed a magic angle turning probe and pulse sequences allowing to trace the isotropic chemical shifts and heteronuclear dipolar couplings and hence the determination of structural motifs on short and intermediate length scales. With the implementation of novel inductive heating approaches the range of accessible rotation frequencies for in situ high temperature NMR measurements has been enlarged, now covering the ν MAS range of 0–10 kHz with an accessible temperature of up to 700°C. Application of NMR methodologies for the characterization of crystallization processes and the structure and dynamics of novel phases, partially in joint collaborations within the priority program, are also reported.

Removal of sidebands in double-rotation NMR in real time

Double-rotation (DOR) is the only technique generally capable of yielding high-resolution NMR spectra of half-integer quadrupolar nuclei in one dimension for solids without the need for sophisticated coherence pathway selection. Unfortunately, due to the low outer rotor spinning frequencies currently available, the spectra often contain a large number of spinning sidebands which may overlap with the resonances of interest. We implement a simple, robust, and easy to use family of pulse sequences, which in practice are fully analogous to the 'total suppression of sidebands' (TOSS) sequences, to suppress all sidebands arising from the spinning of the outer rotor in DOR experiments. By removing the rotor phase dependence of the evolution of the sidebands, the sidebands destructively interfere with one another during the course of signal averaging to yield 'solution-like' spectra of half-integer quadrupolar nuclei in solids. Advantages and shortcomings of the method compared to other DOR sideband suppression methods are explored with the aid of simulations.

Probing quadrupolar nuclei by solid-state NMR spectroscopy: recent advances

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first- and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single- and multiple-quantum coherences, and acquisition of spectra under low- and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

A broad overview is given of some key recent developments in solid state NMR techniques that have driven enhanced applications to inorganic materials science. Reference is made to advances in hardware, pulse sequences and associated computational methods (e.g. first principles calculations, spectral simulation), along with their combination to provide more information about solid phases. The resulting methodology has allowed more nuclei to be observed and more structural information to be extracted. Cross referencing between experimental parameters and their calculation from the structure has given an added dimension to NMR as a characterisation probe of materials. Emphasis is placed on the progress made in the last decade especially from those nuclei that were little studied previously. The general points about technique development and the increased range of nuclei observed are illustrated through some specific exemplars from inorganic materials science.

Determination of NMR interaction parameters from double rotation NMR

It is shown that the anisotropic NMR parameters for half-integer quadrupolar nuclei can be determined using double rotation (DOR) NMR at a single magnetic field with comparable accuracy to multi-field static and MAS experiments. The (17)O nuclei in isotopically enriched l-alanine and OPPh(3) are used as illustrations. The anisotropic NMR parameters are obtained from spectral simulation of the DOR spinning sideband intensities using a computer program written with the GAMMA spin-simulation libraries. Contributions due to the quadrupolar interaction, chemical shift anisotropy, dipolar coupling and J coupling are included in the simulations. In l-alanine the oxygen chemical shift span is 455 +/- 20 ppm and 350 +/- 20 ppm for the O1 and O2 sites, respectively, and the Euler angles are determined to an accuracy of +/- 5-10 degrees . For cases where effects due to heteronuclear J and dipolar coupling are observed, it is possible to determine the angle between the internuclear vector and the principal axis of the electric field gradient (EFG). Thus, the orientation of the major components of both the EFG and chemical shift tensors (i.e., V(33) and delta(33)) in the molecular frame may be obtained from the relative intensity of the split DOR peaks. For OPPh(3) the principal axis of the (17)O EFG is found to be close to the O-P bond, and the (17)O-(31)P one-bond J coupling ((1)J(OP)=161 +/- 2 Hz) is determined to a much higher accuracy than previously.