A suite of pulse sequences based on multiple sequential acquisitions at one and two radiofrequency channels for solid-state magic-angle spinning NMR studies of proteins

Simultaneous recoupling of chemical shift tensors of two nuclei by R-symmetry sequences

Chemical shift tensors (CSTs) are sensitive probes of structure and dynamics. R-symmetry pulse sequences (RNCSA) can efficiently recouple CSTs of varying magnitudes in magic angle spinning (MAS) NMR experiments, for a broad range of conditions and MAS frequencies. Herein, we introduce dual-channel R-symmetry pulse sequences for simultaneously recording CSTs of two different nuclei in a single experiment (DORNE-CSA). We demonstrate the performance of DORNE-CSA sequences for simultaneous measurement of 13C and 15N CSTs, on a U-13C,15N-labeled microcrystalline l-histidine. We show that the DORNE-CSA method is robust, provides accurate CST parameters, and takes only half of the measurement time compared to a pair of RNCSA experiments otherwise required for recording the CSTs of individual nuclei. DORNE-CSA approach is broadly applicable to a wide range of biological and inorganic systems.

Slice & Dice: nested spin-lattice relaxation measurements

Spin-lattice relaxation rate (R₁) measurements are commonly used to characterize protein dynamics. However, the time needed to collect the data can be quite long due to long relaxation times of the low-gamma nuclei, especially in the solid state. We present a method to collect backbone heavy atom relaxation data by nesting the collection of datasets in the solid state. This method results in a factor of 2 to 2.5 times faster data acquisition for backbone R₁ relaxation data for the ¹³C and ¹⁵N sites of proteins.

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.

1H-Detected Biomolecular NMR under Fast Magic-Angle Spinning

Since the first pioneering studies on small deuterated peptides dating more than 20 years ago, 1H detection has evolved into the most efficient approach for investigation of biomolecular structure, dynamics, and interactions by solid-state NMR. The development of faster and faster magic-angle spinning (MAS) rates (up to 150 kHz today) at ultrahigh magnetic fields has triggered a real revolution in the field. This new spinning regime reduces the 1H-1H dipolar couplings, so that a direct detection of 1H signals, for long impossible without proton dilution, has become possible at high resolution. The switch from the traditional MAS NMR approaches with 13C and 15N detection to 1H boosts the signal by more than an order of magnitude, accelerating the site-specific analysis and opening the way to more complex immobilized biological systems of higher molecular weight and available in limited amounts. This paper reviews the concepts underlying this recent leap forward in sensitivity and resolution, presents a detailed description of the experimental aspects of acquisition of multidimensional correlation spectra with fast MAS, and summarizes the most successful strategies for the assignment of the resonances and for the elucidation of protein structure and conformational dynamics. It finally outlines the many examples where 1H-detected MAS NMR has contributed to the detailed characterization of a variety of crystalline and noncrystalline biomolecular targets involved in biological processes ranging from catalysis through drug binding, viral infectivity, amyloid fibril formation, to transport across lipid membranes.

CURD: a Single-Shot Strategy to Obtain Assignments and Distance Restraints for Proteins Using Solid-State MAS NMR Spectroscopy

We present a strategy dubbed CURD (correlations using recycle delays) to acquire chemical-shift assignments and distance restraints for proteins in a single experimental block under slow-moderate magic-angle spinning conditions. This is done by concatenating the 3D-CCC and 3D-NNC experiments, both of which individually require long experimental times for sufficient resolution and sensitivity to be realized. Unlike previous approaches, the CURD strategy does not increase the amount of radio-frequency deposition on the sample and does not require lengthy procedures to optimize any of the pulse sequence elements. Instead, time savings is obtained by using the hitherto unused recycle delay of one of the experiments (2D-CC/3D-CCC) to establish inter-residue correlations for the second experiment (2D-NN/3D-NNC). Experiments are demonstrated on a model protein at the MAS frequency of 12.5 kHz and are shown to result in time savings of the order of days for most of the routine cases.

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.

Correction of field instabilities in biomolecular solid-state NMR by simultaneous acquisition of a frequency reference

With the advent of faster magic-angle spinning (MAS) and higher magnetic fields, the resolution of biomolecular solid-state nuclear magnetic resonance (NMR) spectra has been continuously increasing. As a direct consequence, the always narrower spectral lines, especially in proton-detected spectroscopy, are also becoming more sensitive to temporal instabilities of the magnetic field in the sample volume. Field drifts in the order of tenths of parts per million occur after probe insertion or temperature change, during cryogen refill, or are intrinsic to the superconducting high-field magnets, particularly in the months after charging. As an alternative to a field-frequency lock based on deuterium solvent resonance rarely available for solid-state NMR, we present a strategy to compensate non-linear field drifts using simultaneous acquisition of a frequency reference (SAFR). It is based on the acquisition of an auxiliary 1D spectrum in each scan of the experiment. Typically, a small-flip-angle pulse is added at the beginning of the pulse sequence. Based on the frequency of the maximum of the solvent signal, the field evolution in time is reconstructed and used to correct the raw data after acquisition, thereby acting in its principle as a digital lock system. The general applicability of our approach is demonstrated on 2D and 3D protein spectra during various situations with a non-linear field drift. SAFR with small-flip-angle pulses causes no significant loss in sensitivity or increase in experimental time in protein spectroscopy. The correction leads to the possibility of recording high-quality spectra in a typical biomolecular experiment even during non-linear field changes in the order of 0.1 ppm h- 1 without the need for hardware solutions, such as stabilizing the temperature of the magnet bore. The improvement of linewidths and peak shapes turns out to be especially important for 1 H-detected spectra under fast MAS, but the method is suitable for the detection of carbon or other nuclei as well.

Accelerating 15N and 13C R1 and R1ρ relaxation measurements by multiple pathway solid-state NMR experiments

Magic angle spinning (MAS) Solid-state NMR is a powerful technique to probe dynamics of biological systems at atomic resolution. R₁ and R_1ρ relaxation measurements can provide detailed insight on amplitudes and time scales of motions, especially when information from several different site-specific types of probes is combined. However, such experiments are time-consuming to perform. Shortening the time necessary to record relaxation data for different nuclei will greatly enhance practicality of such approaches. Here, we present staggered acquisition experiments to acquire multiple relaxation experiments from a single excitation to reduce the overall experimental time. Our strategy enables one to collect ¹⁵N and ¹³C relaxation data in a single experiment in a fraction of the time necessary for two separate experiments, with the same signal to noise ratio.

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.

Simultaneous MQMAS NMR Experiments for Two Half-Integer Quadrupolar Nuclei

A procedure to acquire two Multiple-Quantum Magic Angle Spinning (MQMAS) NMR experiments with the same instrument time is presented. A triply tuned probe is utilized with multiple receivers to collect data with staggered acquisitions and thus more efficiently use the instrument time. The data for one nucleus is collected during the recovery delay of the other nucleus, and vice versa. The instrument time is reduced to 60-80% of the time needed for the single acquisition collection Specifically our approach is presented for recording triple-quantum (3Q) ¹⁷O and either 3Q or quintuple-quantum (5Q) ²⁷Al MAS NMR spectra of a 1.18Na₂O•5SiO₂•Al₂O₃ glass gel.

Multi-receiver solid-state NMR using polarization optimized experiments (POE) at ultrafast magic angle spinning

Ultrafast magic angle spinning (MAS) technology and ¹H detection have dramatically enhanced the sensitivity of solid-state NMR (ssNMR) spectroscopy of biopolymers. We previously showed that, when combined with polarization optimized experiments (POE), these advancements enable the simultaneous acquisition of multi-dimensional ¹H- or ¹³C-detected experiments using a single receiver. Here, we propose a new sub-class within the POE family, namely HC-DUMAS, HC-MEIOSIS, and HC-MAeSTOSO, that utilize dual receiver technology for the simultaneous detection of ¹H and ¹³C nuclei. We also expand this approach to record ¹H-, ¹³C-, and ¹⁵N-detected homonuclear 2D spectra simultaneously using three independent receivers. The combination of POE and multi-receiver technology will further shorten the total experimental time of ssNMR experiments for biological solids.

Simultaneous recording of intra- and inter-residue linking experiments for backbone assignments in proteins at MAS frequencies higher than 60 kHz

Obtaining site-specific assignments for the NMR spectra of proteins in the solid state is a significant bottleneck in deciphering their biophysics. This is primarily due to the time-intensive nature of the experiments. Additionally, the low resolution in the [Formula: see text]-dimension requires multiple complementary experiments to be recorded to lift degeneracies in assignments. We present here an approach, gleaned from the techniques used in multiple-acquisition experiments, which allows the recording of forward and backward residue-linking experiments in a single experimental block. Spectra from six additional pathways are also recovered from the same experimental block, without increasing the probe duty cycle. These experiments give intra- and inter residue connectivities for the backbone [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] resonances and should alone be sufficient to assign these nuclei in proteins at MAS frequencies > 60 kHz. The validity of this approach is tested with experiments on a standard tripeptide N-formyl methionyl-leucine-phenylalanine (f-MLF) at a MAS frequency of 62.5 kHz, which is also used as a test-case for determining the sensitivity of each of the experiments. We expect this approach to have an immediate impact on the way assignments are obtained at MAS frequencies [Formula: see text].

Proton-detected polarization optimized experiments (POE) using ultrafast magic angle spinning solid-state NMR: Multi-acquisition of membrane protein spectra

Proton-detected solid-state NMR (ssNMR) spectroscopy has dramatically improved the sensitivity and resolution of fast magic angle spinning (MAS) methods. While relatively straightforward for fibers and crystalline samples, the routine application of these techniques to membrane protein samples is still challenging. This is due to the low sensitivity of these samples, which require high lipid:protein ratios to maintain the structural and functional integrity of membrane proteins. We previously introduced a family of novel polarization optimized experiments (POE) that enable to make the best of nuclear polarization and obtain multiple-acquisitions from a single pulse sequence and one receiver. Here, we present the ¹H-detected versions of POE using ultrafast MAS ssNMR. Specifically, we implemented proton detection into our three main POE strategies, H-DUMAS, H-MEIOSIS, and H-MAeSTOSO, achieving the acquisition of up to ten different experiments using a single pulse sequence. We tested these experiments on a model compound N-Acetyl-Val-Leu dipeptide and applied to a six transmembrane acetate transporter, SatP, reconstituted in lipid membranes. These new methods will speed up the spectroscopy of challenging biomacromolecules such as membrane proteins.

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

Multidimensional solid-state NMR spectroscopy plays a significant role in offering atomic-level insights into molecular systems. In particular, heteronuclear chemical shift correlation (HETCOR) experiments could provide local chemical and structural information in terms of spatial heteronuclear proximity and through-bond connectivity. In solid state, the transfer of magnetization between heteronuclei, a key step in HETCOR experiments, is usually achieved using cross-polarization (CP) or insensitive nuclei enhanced by polarization transfer (INEPT) depending on the sample characteristics and magic-angle-spinning (MAS) frequency. But, for a multiphase system constituting molecular components that differ in their time scales of mobilities, CP efficiency is pretty low for mobile components because of the averaging of heteronuclear dipolar couplings whereas INEPT is inefficient for immobile components due to the short T₂ and can yield through-space connectivity due to strong proton spin diffusion for immobile components especially under moderate spinning speeds. Herein, in this study we present two 2D pulse sequences that enable the sequential acquisition of ¹³C/¹H HETCOR NMR spectra for the rigid and mobile components by taking full advantage of the abundant proton magnetization in a single experiment with barely increasing the overall experimental time. In particular, the ¹³C-detected HETCOR experiment could be applied under slow MAS conditions, where a multiple-pulse sequence is typically employed to enhance ¹H spectral resolution in the indirect dimension. In contrast, the ¹H-detected HETCOR experiment should be applied under ultrafast MAS, where CP and heteronuclear nuclear Overhauser effect (NOE) polarization transfer are combined to enhance ¹³C signal intensities for mobile components. These pulse sequences are experimentally demonstrated on two model systems to obtain 2D ¹³C/¹H chemical shift correlation spectra of rigid and mobile components independently and separately. These pulse sequences can be used for dynamics based spectral editing and resonance assignments. Therefore, we believe the proposed 2D HETCOR NMR pulse sequences will be beneficial for the structural studies of heterogeneous systems containing molecular components that differ in their time scale of motions for understanding the interplay of structures and properties.

A suite of solid-state NMR experiments to utilize orphaned magnetization for assignment of proteins using parallel high and low gamma detection

We present a suite of two-receiver solid-state NMR experiments for backbone and side chain resonance assignment. The experiments rely on either dipolar coupling or scalar coupling for polarization transfer and are devised to acquire a ¹H-detected 3D experiment AND a nested ¹³C-detected 2D from a shared excitation pulse. In order to compensate for the lower sensitivity of detection on ¹³C nucleus, 2D rows are signal averaged during 3D planes. The 3D dual receiver experiments do not suffer from any appreciable signal loss compared to their single receiver versions and require no extra optimization. The resulting data is higher in information content with no additional experiment time. The approach is expected to become widespread as multiple receivers become standard for new NMR spectrometers.

Experiments with direct detection of multiple FIDs

Pulse schemes with direct observation of multiple free induction decays (FIDs) offer a dramatic increase in the spectral information content of NMR experiments and often yield substantial improvement in measurement sensitivity per unit time. Availability of multiple receivers on the state-of-the-art commercial spectrometers allows spectra from different nuclear species to be recorded in parallel routinely. Experiments with multi-FID detection have been designed with both, homonuclear and multinuclear acquisition. We provide a brief overview of such techniques designed for applications in liquid- and solid- state NMR as well as in hyperpolarized samples. Here we show how these techniques have led to design of experiments that allow structure elucidation of small molecules and resonance assignment in proteins from a single measurement. Probes with multiple RF micro-coils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems. Solid-state NMR experiments have also benefited immensely from both parallel and simultaneous FID acquisition in a variety of multi-dimensional pulse schemes. We believe that multi-FID detection will become an essential component of the future NMR methodologies effectively increasing the information content of NMR experiments and reducing the cost of NMR analysis.

Taking Simultaneous Snapshots of Intrinsically Disordered Proteins in Action

Intrinsically disordered proteins (IDPs) as well as intrinsically disordered regions (IDRs) of complex protein machineries have recently been recognized as key players in many cellular functions. NMR represents a unique tool to access atomic resolution structural and dynamic information on highly flexible IDPs/IDRs. Improvements in instrumental sensitivity made heteronuclear direct detection possible for biomolecular NMR applications. The CON experiment has become one of the most useful NMR experiments to get a snapshot of an IDP/IDR in conditions approaching physiological ones. The availability of NMR spectrometers equipped with multiple receivers now enables the acquisition of several experiments simultaneously instead of one after the other. Here, we propose several variants of the CON experiment in which, during the recovery delay, a second two-dimensional experiment is acquired, either based on ¹H detection (CON//HN) or on ¹⁵N detection (CON//btNH, CON//(H)CAN). The possibility to collect simultaneous snapshots of an IDP/IDR through different two-dimensional spectra provides a novel tool to follow chemical reactions, such as the occurrence of posttranslational modifications, as well as to study samples of limited lifetime such as cell lysates or whole cells.

Hybridization of TEDOR and NCX MAS solid-state NMR experiments for simultaneous acquisition of heteronuclear correlation spectra and distance measurements

Magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy is a major technique for the characterization of the structural dynamics of biopolymers at atomic resolution. However, the intrinsic low sensitivity of this technique poses significant limitations to its routine application in structural biology. Here we achieve substantial savings in experimental time using a new subclass of Polarization Optimized Experiments (POEs) that concatenate TEDOR and SPECIFIC-CP transfers into a single pulse sequence. Specifically, we designed new 2D and 3D experiments (2D TEDOR-NCX, 3D TEDOR-NCOCX, and 3D TEDOR-NCACX) to obtain distance measurements and heteronuclear chemical shift correlations for resonance assignments using only one experiment. We successfully tested these experiments on N-Acetyl-Val-Leu dipeptide, microcrystalline U-13C,15N ubiquitin, and single- and multi-span membrane proteins reconstituted in lipid membranes. These pulse sequences can be implemented on any ssNMR spectrometer equipped with standard solid-state hardware using only one receiver. Since these new POEs speed up data acquisition considerably, we anticipate their broad application to fibrillar, microcrystalline, and membrane-bound proteins.

Practical aspects of the simultaneous collection of COSY and TOCSY spectra

The practical aspects of some NMR experiments designed for the simultaneous acquisition of 2D COSY and 2D TOCSY spectra are presented and discussed. Several techniques involving afterglow-based, coherence transfer pathway (CTP)-based, and NMR by Ordered Acquisition using ¹ H-detection (NOAH)-based strategies for the collection of different free-induction signal decays (FIDs) within the same scan are evaluated and compared. These methods offer a faster recording of these spectra in small-molecule NMR when sensitivity is not a limiting factor, with a reduction in spectrometer time about 45-60% when compared with the conventional sequential acquisition of the parent experiments. It is also shown how the optimized design of an extended three-FID approach yields one COSY and two TOCSY spectra simultaneously by combining CTP and NOAH principles in the same experiment, affording substantial sensitivity enhancements per time unit.

Probing membrane protein ground and conformationally excited states using dipolar- and J-coupling mediated MAS solid state NMR experiments

The intrinsic conformational plasticity of membrane proteins directly influences the magnitude of the orientational-dependent NMR interactions such as dipolar couplings (DC) and chemical shift anisotropy (CSA). As a result, the conventional cross-polarization (CP)-based techniques mainly capture the more rigid regions of membrane proteins, while the most dynamic regions are essentially invisible. Nonetheless, dynamic regions can be detected using experiments in which polarization transfer takes place via J-coupling interactions. Here, we review our recent efforts to develop single and dual acquisition pulse sequences with either 1H or 13C detection that utilize both DC and J-coupling mediated transfer to detect both rigid and mobile regions of membrane proteins in native-like lipid environments. We show the application of these new methods for studying the conformational equilibrium of a single-pass membrane protein, phospholamban, which regulates the calcium transport across the sarcoplasmic reticulum (SR) membrane by interacting with the SR Ca2+-ATPase. We anticipate that these methods will be ideal to portray the complex dynamics of membrane proteins in their native environments.

1H-detected MAS solid-state NMR experiments enable the simultaneous mapping of rigid and dynamic domains of membrane proteins

Magic angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy is emerging as a unique method for the atomic resolution structure determination of native membrane proteins in lipid bilayers. Although 13C-detected ssNMR experiments continue to play a major role, recent technological developments have made it possible to carry out 1H-detected experiments, boosting both sensitivity and resolution. Here, we describe a new set of 1H-detected hybrid pulse sequences that combine through-bond and through-space correlation elements into single experiments, enabling the simultaneous detection of rigid and dynamic domains of membrane proteins. As proof-of-principle, we applied these new pulse sequences to the membrane protein phospholamban (PLN) reconstituted in lipid bilayers under moderate MAS conditions. The cross-polarization (CP) based elements enabled the detection of the relatively immobile residues of PLN in the transmembrane domain using through-space correlations; whereas the most dynamic region, which is in equilibrium between folded and unfolded states, was mapped by through-bond INEPT-based elements. These new 1H-detected experiments will enable one to detect not only the most populated (ground) states of biomacromolecules, but also sparsely populated high-energy (excited) states for a complete characterization of protein free energy landscapes.