Efficiency of heteronuclear dipolar decoupling schemes in solid-state NMR: investigation of effective transverse relaxation times

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Solid-State NMR: Methods for Biological Solids

In the last two decades, solid-state nuclear magnetic resonance (ssNMR) spectroscopy has transformed from a spectroscopic technique investigating small molecules and industrial polymers to a potent tool decrypting structure and underlying dynamics of complex biological systems, such as membrane proteins, fibrils, and assemblies, in near-physiological environments and temperatures. This transformation can be ascribed to improvements in hardware design, sample preparation, pulsed methods, isotope labeling strategies, resolution, and sensitivity. The fundamental engagement between nuclear spins and radio-frequency pulses in the presence of a strong static magnetic field is identical between solution and ssNMR, but the experimental procedures vastly differ because of the absence of molecular tumbling in solids. This review discusses routinely employed state-of-the-art static and MAS pulsed NMR methods relevant for biological samples with rotational correlation times exceeding 100's of nanoseconds. Recent developments in signal filtering approaches, proton methodologies, and multiple acquisition techniques to boost sensitivity and speed up data acquisition at fast MAS are also discussed. Several examples of protein structures (globular, membrane, fibrils, and assemblies) solved with ssNMR spectroscopy have been considered. We also discuss integrated approaches to structurally characterize challenging biological systems and some newly emanating subdisciplines in ssNMR spectroscopy.

Quantifying Polymer Chain Orientation in Strong and Tough Nanofibers with Low Crystallinity: Toward Next Generation Nanostructured Superfibers

Advanced fibers revolutionized structural materials in the second half of the 20th century. However, all high-strength fibers developed to date are brittle. Recently, pioneering simultaneous ultrahigh strength and toughness were discovered in fine (<250 nm) individual electrospun polymer nanofibers (NFs). This highly desirable combination of properties was attributed to high macromolecular chain alignment coupled with low crystallinity. Quantitative analysis of the degree of preferred chain orientation will be crucial for control of NF mechanical properties. However, quantification of supramolecular nanoarchitecture in NFs with low crystallinity in the ultrafine diameter range is highly challenging. Here, we discuss the applicability of traditional as well as emerging methods for quantification of polymer chain orientation in nanoscale one-dimensional samples. Advantages and limitations of different techniques are critically evaluated on experimental examples. It is shown that straightforward application of some of the techniques to sub-wavelength-diameter NFs can lead to severe quantitative and even qualitative artifacts. Sources of such size-related artifacts, stemming from instrumental, materials, and geometric phenomena at the nanoscale, are analyzed on the example of polarized Raman method but are relevant to other spectroscopic techniques. A proposed modified, artifact-free method is demonstrated. Outstanding issues and their proposed solutions are discussed. The results provide guidance for accurate nanofiber characterization to improve fundamental understanding and accelerate development of nanofibers and related nanostructured materials produced by electrospinning or other methods. We expect that the discussion in this review will also be useful to studies of many biological systems that exhibit nanofilamentary architectures and combinations of high strength and toughness.

A robust heteronuclear dipolar recoupling method comparable to TEDOR for proteins in magic-angle spinning solid-state NMR

In this letter, we propose a robust heteronuclear dipolar recoupling method for proteins in magic-angle spinning (MAS) solid-state NMR. This method is as simple, robust and efficient as the well-known TEDOR in the aspect of magnetization transfer between ¹⁵N and ¹³C. Deriving from our recent band-selective dual back-to-back pulses (DBP) (Zhang et al., 2016), this method uses new phase-cycling schemes to realize broadband DBP (Bro-DBP). For broadband ¹⁵N-¹³C magnetization transfer (simultaneous ¹⁵N→¹³C' and ¹⁵N→¹³Cα), Bro-DBP has almost the same ¹⁵N→¹³Cα efficiency while offers 30-40% enhancement on ¹⁵N→¹³C' transfer, compared to TEDOR. Besides, Bro-DBP can also be used as a carbonyl (¹³C')-selected method, whose ¹⁵N→¹³C' efficiency is up to 1.7 times that of TEDOR and is also higher than that of band-selective DBP. The performance of Bro-DBP is demonstrated on the N-formyl-[U-¹³C,¹⁵N]-Met-Leu-Phe-OH (fMLF) peptide and the U-¹³C, ¹⁵N labeled β1 immunoglobulin binding domain of protein G (GB1) microcrystalline protein. Since Bro-DBP is as robust, simple and efficient as TEDOR, we believe it is very useful for protein studies in MAS solid-state NMR.

Peptide and Protein Dynamics and Low-Temperature/DNP Magic Angle Spinning NMR

In DNP MAS NMR experiments at ∼80-110 K, the structurally important -¹³CH₃ and -¹⁵NH₃⁺ signals in MAS spectra of biological samples disappear due to the interference of the molecular motions with the ¹H decoupling. Here we investigate the effect of these dynamic processes on the NMR line shapes and signal intensities in several typical systems: (1) microcrystalline APG, (2) membrane protein bR, (3) amyloid fibrils PI3-SH3, (4) monomeric alanine-CD₃, and (5) the protonated and deuterated dipeptide N-Ac-VL over 78-300 K. In APG, the three-site hopping of the Ala-C_β peak disappears completely at 112 K, concomitant with the attenuation of CP signals from other ¹³C's and ¹⁵N's. Similarly, the ¹⁵N signal from Ala-NH₃⁺ disappears at ∼173 K, concurrent with the attenuation in CP experiments of other ¹⁵N's as well as ¹³C's. In bR and PI3-SH3, the methyl groups are attenuated at ∼95 K, while all other ¹³C's remain unaffected. However, both systems exhibit substantial losses of intensity at ∼243 K. Finally, with spectra of Ala and N-Ac-VL, we show that it is possible to extract site specific dynamic data from the temperature dependence of the intensity losses. Furthermore, ²H labeling can assist with recovering the spectral intensity. Thus, our study provides insight into the dynamic behavior of biological systems over a wide range of temperatures, and serves as a guide to optimizing the sensitivity and resolution of structural data in low temperature DNP MAS NMR spectra.

Rationalising Heteronuclear Decoupling in Refocussing Applications of Solid-State NMR Spectroscopy

Factors affecting the performance of 1 H heteronuclear decoupling sequences for magic-angle spinning (MAS) NMR spectroscopy of organic solids are explored, as observed by time constants for the decay of nuclear magnetisation under a spin-echo (T2' ). By using a common protocol over a wide range of experimental conditions, including very high magnetic fields and very high radio-frequency (RF) nutation rates, decoupling performance is observed to degrade consistently with increasing magnetic field. Inhomogeneity of the RF field is found to have a significant impact on T2' values, with differences of about 20 % observed between probes with different coil geometries. Increasing RF nutation rates dramatically improve robustness with respect to RF offset, but the performance of phase-modulated sequences degrades at the very high nutation rates achievable in microcoils as a result of RF transients. The insights gained provide better understanding of the factors limiting decoupling performance under different conditions, and the high values of T2' observed (which generally exceed previous literature values) provide reference points for experiments involving spin magnetisation refocussing, such as 2D correlation spectra and measuring small spin couplings.