Exploiting heterogeneous time scale of dynamics to enhance 2D HETCOR solid-state NMR sensitivity

Exploring the Higher Order Structure and Conformational Transitions in Insulin Microcrystalline Biopharmaceuticals by Proton-Detected Solid-State Nuclear Magnetic Resonance at Natural Abundance

The integrity of a higher order structure (HOS) is an essential requirement to ensure the efficacy, stability, and safety of protein therapeutics. Solution-state nuclear magnetic resonance (NMR) occupies a unique niche as one of the most promising methods to access atomic-level structural information on soluble biopharmaceutical formulations. Another major class of drugs is poorly soluble, such as microcrystalline suspensions, which poses significant challenges for the characterization of the active ingredient in its native state. Here, we have demonstrated a solid-state NMR method for HOS characterization of biopharmaceutical suspensions employing a selective excitation scheme under fast magic angle spinning (MAS). The applicability of the method is shown on commercial insulin suspensions at natural isotopic abundance. Selective excitation aided with proton detection and non-uniform sampling (NUS) provides improved sensitivity and resolution. The enhanced resolution enabled us to demonstrate the first experimental evidence of a phenol-escaping pathway in insulin, leading to conformational transitions to different hexameric states. This approach has the potential to serve as a valuable means for meticulously examining microcrystalline biopharmaceutical suspensions, which was previously not attainable in their native formulation states and can be seamlessly extended to other classes of biopharmaceuticals such as mAbs and other microcrystalline proteins.

Insights into the Structure of Sucralfate by Advanced Solid- and Liquid-State NMR

Sucralfate, which is a sucrose octasulfate aluminum complex, is an active pharmaceutical ingredient (API) falling in the category of cytoprotective agents which are very effective for gastric and duodenal ulcers. On interaction with stomach acid, it ionizes into aluminum and sucrose octasulfate ions to form a protective layer over the ulcerated region inhibiting further attack from acid. The mechanism of action of sucralfate in the context of its structure is not well understood. Considering that at least two forms of this API are available in the market, there are no reports on the various forms of sucralfate and differences in their pharmacological action. We characterized the two forms of sucralfate using multinuclear, multidimensional solid-state NMR, and the results show significant structural differences between them arising from variation in the aluminum environment and the level of hydration. The impact of structural differences on pharmacological action was examined by studying acid-induced Al release by 27Al liquid-state NMR. The sucralfate, European pharmaceutical standard, Form I, undergoes faster disruption in acid compared to Form II. The difference is explained on the basis of structural differences in the two forms which gives significant insights into the action of sucralfate in relation to its structure.

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.

Measurement of spin-lattice relaxation times in multiphase polymer systems

Solid-state Nuclear Magnetic Resonance (NMR) has emerged as a pivotal technique for unraveling the microstructure and dynamics of intricate polymer and biological materials. Within this context, site-specific proton spin-lattice relaxation times in the laboratory frame (T1) and rotating frame (T1ρ) have become indispensable tools for investigating phase separation structures and molecular dynamics in multiphase polymer systems. Notably, the site-specific measurement of proton T1 and T1ρ is usually achieved via 13C detection in polymers, where 1H polarization is typically transferred to 13C via cross polarization (CP). Nevertheless, CP relies on the 1H-13C heteronuclear dipolar couplings, and thus it does not work well for the mobile components. In this study, via the integration of CP and RINEPT (refocused insensitive nuclei enhanced by polarization transfer), we propose a robust approach for the measurement of site-specific proton T1 and T1ρ in multiphase polymers. It overcomes the limitation of CP on transferring 1H polarization to 13C in mobile components, and thus enables simultaneous determination of site-specific proton T1 and T1ρ in rigid and mobile components in multiphase polymers in a single experiment. Such experiment can also be used for dynamics-based spectral editing due to the dynamic selectivity of CP- and RINEPT-based polarization transfer process. The proposed experiments are well demonstrated on three typical multiphase polymer systems, poly(methyl methacrylate)/polybutadiene (PMMA/PB) polymer blend, polyurethane (PU) and polystyrene-polybutadiene-polystyrene (SBS) elastomers. We envisage the proposed experiments can be a universal avenue for structural and dynamic elucidation of multiphase polymers containing both rigid and mobile components.

Specific Signal Enhancement on an RNA-Protein Interface by Dynamic Nuclear Polarization

Sensitivity and specificity are both crucial for the efficient solid-state NMR structure determination of large biomolecules. We present an approach that features both advantages by site-specific enhancement of NMR spectroscopic signals from the protein-RNA binding site within a ribonucleoprotein (RNP) by dynamic nuclear polarization (DNP). This approach uses modern biochemical techniques for sparse isotope labeling and exploits the molecular dynamics of ¹³ C-labeled methyl groups exclusively present in the protein. These dynamics drive heteronuclear cross relaxation and thus allow specific hyperpolarization transfer across the biomolecular complex's interface. For the example of the L7Ae protein in complex with a 26mer guide RNA minimal construct from the box C/D complex in archaea, we demonstrate that a single methyl-nucleotide contact is responsible for most of the polarization transfer to the RNA, and that this specific transfer can be used to boost both NMR spectral sensitivity and specificity by DNP.

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 the characterization of bioconjugations and protein-material interactions

Protein solid-state NMR has evolved dramatically over the last two decades, with the development of new hardware and sample preparation methodologies. This technique is now ripe for complex applications, among which one can count bioconjugation, protein chemistry and functional biomaterials. In this review, we provide our account on this aspect of protein solid-state NMR.

Field and magic angle spinning frequency dependence of proton resonances in rotating solids

Proton detection in solid state NMR is continuously developing and allows one to gain new insights in structural biology. Overall, this progress is a result of the synergy between hardware development, new NMR methodology and new isotope labeling strategies, to name a few factors. Even though current developments are rapid, it is worthwhile to summarize what can currently be achieved employing proton detection in biological solids. We illustrate this by analysing the signal-to-noise ratio (SNR) for spectra obtained for a microcrystalline α-spectrin SH3 domain protein sample by (i) employing different degrees of chemical dilution to replace protons by incorporating deuterons in different sites, by (ii) variation of the magic angle spinning (MAS) frequencies between 20 and 110 kHz, and by (iii) variation of the static magnetic field B₀. The experimental SNR values are validated with numerical simulations employing up to 9 proton spins. Although in reality a protein would contain far more than 9 protons, in a deuterated environment this is a sufficient number to achieve satisfactory simulations consistent with the experimental data. The key results of this analysis are (i) with current hardware, deuteration is still necessary to record spectra of optimum quality; (ii) 13CH3 isotopomers for methyl groups yield the best SNR when MAS frequencies above 100 kHz are available; and (iii) sensitivity increases with a factor beyond B0 3/2 with the static magnetic field due to a transition of proton-proton dipolar interactions from a strong to a weak coupling limit.

Dynamic Nuclear Polarization for Sensitivity Enhancement in Biomolecular Solid-State NMR

Solid-state NMR with magic-angle spinning (MAS) is an important method in structural biology. While NMR can provide invaluable information about local geometry on an atomic scale even for large biomolecular assemblies lacking long-range order, it is often limited by low sensitivity due to small nuclear spin polarization in thermal equilibrium. Dynamic nuclear polarization (DNP) has evolved during the last decades to become a powerful method capable of increasing this sensitivity by two to three orders of magnitude, thereby reducing the valuable experimental time from weeks or months to just hours or days; in many cases, this allows experiments that would be otherwise completely unfeasible. In this review, we give an overview of the developments that have opened the field for DNP-enhanced biomolecular solid-state NMR including state-of-the-art applications at fast MAS and high magnetic field. We present DNP mechanisms, polarizing agents, and sample constitution methods suitable for biomolecules. A wide field of biomolecular NMR applications is covered including membrane proteins, amyloid fibrils, large biomolecular assemblies, and biomaterials. Finally, we present perspectives and recent developments that may shape the field of biomolecular DNP in the future.

Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning

Solid-state nuclear magnetic resonance (NMR) often suffers from significant limitations due to the inherent low signal sensitivity when low-γ nuclei are involved. Herein, we report an elegant solid-state NMR approach for rapid structural analysis of minute amounts of organic solids. By encoding staggered chemical shift evolution in the indirect dimension and staggered acquisition in the ¹H dimension, a proton-detected homonuclear ¹H/¹H and heteronuclear ¹³C/¹H chemical shift correlation (HETCOR) spectrum can be obtained simultaneously in a single experiment at a fast magic-angle-spinning (MAS) condition with barely increasing the experimental time. We further show that during the conventional ¹H-detected HETCOR experimental time, multiple homonuclear ¹H/¹H correlation spectra can be recorded in addition to the HETCOR spectrum, enabling the determination of ¹H-¹H distances. We establish that abundant ¹H polarization can be efficiently manipulated and fully utilized in proton-detected solid-state NMR spectroscopy for extraction of more critical structural information and thus reduction of the total experimental time.

Multiplexing experiments in NMR and multi-nuclear MRI

Multiplexing NMR experiments by direct detection of multiple free induction decays (FIDs) in a single experiment offers a dramatic increase in the spectral information content and often yields significant improvement in sensitivity per unit time. Experiments with multi-FID detection have been designed with both homonuclear and multinuclear acquisition, and the advent of multiple receivers on commercial spectrometers opens up new possibilities for recording spectra from different nuclear species in parallel. Here we provide an extensive overview of such techniques, designed for applications in liquid- and solid-state NMR as well as in hyperpolarized samples. A brief overview of multinuclear MRI is also provided, to stimulate cross fertilization of ideas between the two areas of research (NMR and MRI). It is shown how such techniques enable the design of experiments that allow structure elucidation of small molecules from a single measurement. Likewise, in biomolecular NMR experiments multi-FID detection allows complete resonance assignment in proteins. Probes with multiple RF microcoils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems, effectively reducing the cost of NMR analysis and increasing the information content at the same time. Solid-state NMR experiments have also benefited immensely from both parallel and sequential multi-FID detection in a variety of multi-dimensional pulse schemes. We are confident that multi-FID detection will become an essential component of future NMR methodologies, effectively increasing the sensitivity and information content of NMR measurements.