Solid-state protein-structure determination with proton-detected triple-resonance 3D magic-angle-spinning NMR spectroscopy

5D solid-state NMR spectroscopy for facilitated resonance assignment

1H-detected solid-state NMR spectroscopy has been becoming increasingly popular for the characterization of protein structure, dynamics, and function. Recently, we showed that higher-dimensionality solid-state NMR spectroscopy can aid resonance assignments in large micro-crystalline protein targets to combat ambiguity (Klein et al., Proc. Natl. Acad. Sci. U.S.A. 2022). However, assignments represent both, a time-limiting factor and one of the major practical disadvantages within solid-state NMR studies compared to other structural-biology techniques from a very general perspective. Here, we show that 5D solid-state NMR spectroscopy is not only justified for high-molecular-weight targets but will also be a realistic and practicable method to streamline resonance assignment in small to medium-sized protein targets, which such methodology might not have been expected to be of advantage for. Using a combination of non-uniform sampling and the signal separating algorithm for spectral reconstruction on a deuterated and proton back-exchanged micro-crystalline protein at fast magic-angle spinning, direct amide-to-amide correlations in five dimensions are obtained with competitive sensitivity compatible with common hardware and measurement time commitments. The self-sufficient backbone walks enable efficient assignment with very high confidence and can be combined with higher-dimensionality sidechain-to-backbone correlations from protonated preparations into minimal sets of experiments to be acquired for simultaneous backbone and sidechain assignment. The strategies present themselves as potent alternatives for efficient assignment compared to the traditional assignment approaches in 3D, avoiding user misassignments derived from ambiguity or loss of overview and facilitating automation. This will ease future access to NMR-based characterization for the typical solid-state NMR targets at fast MAS.

Simplified Preservation of Equivalent Pathways Spectroscopy

Inspired by the recently proposed transverse mixing optimal control pulses (TROP) approach for improving signal in multidimensional magic-angle spinning (MAS) NMR experiments, we present simplified preservation of equivalent pathways spectroscopy (SPEPS). It transfers both transverse components of magnetization that occur during indirect evolutions, theoretically enabling a √2 improvement in sensitivity for each such dimension. We compare SPEPS transfer with TROP and cross-polarization (CP) using membrane protein and fibril samples at MAS of 55 and 100 kHz. In three-dimensional (3D) (H)CANH spectra, SPEPS outperformed TROP and CP by factors of on average 1.16 and 1.69, respectively, for the membrane protein, but only a marginal improvement of 1.09 was observed for the fibril. These differences are discussed, making note of the longer transfer time used for CP, 14 ms, as compared with 2.9 and 3.6 ms for SPEPS and TROP, respectively. Using SPEPS for two transfers in the 3D (H)CANCO experiment resulted in an even larger benefit in signal intensity, with an average improvement of 1.82 as compared with CP. This results in multifold time savings, in particular considering the weaker peaks that are observed to benefit the most from SPEPS.

Nuclear spin diffusion under fast magic-angle spinning in solid-state NMR

Solid-state nuclear spin diffusion is the coherent and reversible process through which spin order is transferred via dipolar couplings. With the recent increases in magic-angle spinning (MAS) frequencies and magnetic fields becoming routinely applied in solid-state nuclear magnetic resonance, understanding how the increased 1H resolution obtained affects spin diffusion is necessary for interpretation of several common experiments. To investigate the coherent contributions to spin diffusion with fast MAS, we have developed a low-order correlation in Liouville space model based on the work of Dumez et al. (J. Chem. Phys. 33, 224501, 2010). Specifically, we introduce a new method for basis set selection, which accounts for the resonance-offset dependence at fast MAS. Furthermore, we consider the necessity of including chemical shift, both isotropic and anisotropic, in the modeling of spin diffusion. Using this model, we explore how different experimental factors change the nature of spin diffusion. Then, we show case studies to exemplify the issues that arise in using spin diffusion techniques at fast spinning. We show that the efficiency of polarization transfer via spin diffusion occurring within a deuterated and 100% back-exchanged protein sample at 60 kHz MAS is almost entirely dependent on resonance offset. We additionally identify temperature-dependent magnetization transfer in beta-aspartyl L-alanine, which could be explained by the influence of an incoherent relaxation-based nuclear Overhauser effect.

Windowed cross polarization at 55 kHz magic-angle spinning

Cross polarization (CP) transfers via Hartmann-Hahn matching conditions are one of the cornerstones of solid-state magic-angle spinning NMR experiments. Here we investigate a windowed sequence for cross polarization (wCP) at 55 kHz magic-angle spinning, placing one window (and one pulse) per rotor period on one or both rf channels. The wCP sequence is known to have additional matching conditions. We observe a striking similarity between wCP and CP transfer conditions when considering the flip angle of the pulse rather than the rf-field strength applied during the pulse. Using fictitious spin-1/2 formalism and average Hamiltonian theory, we derive an analytical approximation that matches these observed transfer conditions. We recorded data at spectrometers with different external magnetic fields up to 1200 MHz, for strong and weak heteronuclear dipolar couplings. These transfers, and even the selectivity of CP were again found to relate to flip angle (average nutation).

Solid-State NMR 19F-1H-15N Correlation Experiments for Resonance Assignment and Distance Measurements of Multifluorinated Proteins

Several solid-state NMR techniques have been introduced recently to measure nanometer distances involving 19F, whose high gyromagnetic ratio makes it a potent nuclear spin for structural investigation. These solid-state NMR techniques either use 19F correlation with 1H or 13C to obtain qualitative interatomic contacts or use the rotational-echo double-resonance (REDOR) pulse sequence to measure quantitative distances. However, no NMR technique is yet available for disambiguating 1H-19F distances in multiply fluorinated proteins and protein-ligand complexes. Here, we introduce a three-dimensional (3D) 19F-15N-1H correlation experiment that resolves the distances of multiple fluorines to their adjacent amide protons. We show that optimal polarization transfer between 1H and 19F spins is achieved using an out-and-back 1H-19F REDOR sequence. We demonstrate this 3D correlation experiment on the model protein GB1 and apply it to the multidrug-resistance transporter, EmrE, complexed to a tetrafluorinated substrate. This technique should be useful for resolving and assigning distance constraints in multiply fluorinated proteins, leading to significant savings of time and precious samples compared to producing several singly fluorinated samples. Moreover, the method enables structural determination of protein-ligand complexes for ligands that contain multiple fluorines.

Nucleic acid-protein interfaces studied by MAS solid-state NMR spectroscopy

Solid-state NMR (ssNMR) has become a well-established technique to study large and insoluble protein assemblies. However, its application to nucleic acid-protein complexes has remained scarce, mainly due to the challenges presented by overlapping nucleic acid signals. In the past decade, several efforts have led to the first structure determination of an RNA molecule by ssNMR. With the establishment of these tools, it has become possible to address the problem of structure determination of nucleic acid-protein complexes by ssNMR. Here we review first and more recent ssNMR methodologies that study nucleic acid-protein interfaces by means of chemical shift and peak intensity perturbations, direct distance measurements and paramagnetic effects. At the end, we review the first structure of an RNA-protein complex that has been determined from ssNMR-derived intermolecular restraints.

Selective Nuclear Magnetic Resonance Method for Enhancing Long-Range Heteronuclear Correlations in Solids

The long-range heteronuclear correlation remains a significant challenge in solid-state nuclear magnetic resonance (NMR), which is critical in the structural elucidation of biomolecular, material, and pharmaceutical solids. We propose a selective NMR method, heteronuclear selective phase-optimized recoupling (hetSPR), to selectively enhance long-range correlations of interest by utilizing characteristic chemical shifts. Compared to conventional methods, hetSPR can selectively enhance desired heteronuclear correlations (e.g., ¹H-¹³C and ¹H-¹⁹F) by factors up to 5 and largely suppress the unwanted ones. The method proves useful by enhancing the long-range correlation from an intermolecular ¹H-¹⁹F distance of 4.8 Å by a factor of 2.4 in a fluorinated pharmaceutical drug, bicalutamide, under fast magic-angle spinning. It does not use selective pulses and is thus user-friendly even for nonexperts. The new method is expected to boost solid-state NMR to elucidate the structures of various solids.

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.

PHRONESIS: a one-shot approach for sequential assignment of protein resonances by ultrafast MAS solid-state NMR spectroscopy

Solid‐state NMR (ssNMR) spectroscopy has emerged as the method of choice to analyze the structural dynamics of fibrillar, membrane‐bound, and crystalline proteins that are recalcitrant to other structural techniques. Recently, ¹H detection under fast magic angle spinning and multiple acquisition ssNMR techniques have propelled the structural analysis of complex biomacromolecules. However, data acquisition and resonance‐specific assignments remain a bottleneck for this technique. Here, we present a comprehensive multi‐acquisition experiment (PHRONESIS) that simultaneously generates up to ten 3D ¹H‐detected ssNMR spectra. PHRONESIS utilizes broadband transfer and selective pulses to drive multiple independent polarization pathways. High selectivity excitation and de‐excitation of specific resonances were achieved by high‐fidelity selective pulses that were designed using a combination of an evolutionary algorithm and artificial intelligence. We demonstrated the power of this approach with microcrystalline U‐¹³C,¹⁵N GB1 protein, reaching 100 % of the resonance assignments using one data set of ten 3D experiments. The strategy outlined in this work opens up new avenues for implementing novel ¹H‐detected multi‐acquisition ssNMR experiments to speed up and expand the application to larger biomolecular systems.

Unambiguous Side-Chain Assignments for Solid-State NMR Structure Elucidation of Nondeuterated Proteins via a Combined 5D/4D Side-Chain-to-Backbone Experiment

Owing to fast-magic-angle-spinning technology, proton-detected solid-state NMR has been facilitating the analysis of insoluble, crystalline, sedimented, and membrane proteins. However, potential applications have been largely restricted by limited access to side-chain resonances. The recent availability of spinning frequencies exceeding 100 kHz in principle now allows direct probing of all protons without the need for partial deuteration. This potentiates both the number of accessible target proteins and possibilities to exploit side-chain protons as reporters on distances and interactions. Their low dispersion, however, has severely compromised their chemical-shift assignment, which is a prerequisite for their use in downstream applications. Herein, we show that unambiguous correlations are obtained from 5D methodology by which the side-chain resonances are directly connected with the backbone. When further concatenated with simultaneous 4D intra-side-chain correlations, this yields comprehensive assignments in the side chains and hence allows a high density of distance restraints for high-resolution structure calculation from minimal amounts of protein.

Modest Offset Difference Internuclear Selective Transfer via Homonuclear Dipolar Coupling

Homonuclear dipolar recoupling is routinely used for magic-angle spinning NMR-based structure determination. In fully protonated samples, only short proton-proton distances are accessible to broadband recoupling approaches because of high proton density. Selective methods allow detection of longer distances by directing polarization to a subset of spins. Here we introduce the selective pulse sequence MODIST, which recouples spins that have a modest chemical shift offset difference, and demonstrate it to selectively record correlations between amide protons. The sequence was selected for good retention of total signal, leading to up to twice the intensity for proton-proton correlations compared with other selective methods. The sequence is effective across a range of spinning conditions and magnetic fields, here tested at 55.555 and 100 kHz magic-angle spinning and at proton Larmor frequencies from 600 to 1200 MHz. For influenza A M2 in lipid bilayers, cross-peaks characteristic of a helical conformation are observed.

Solid-State NMR Dipolar and Chemical Shift Anisotropy Recoupling Techniques for Structural and Dynamical Studies in Biological Systems

With the development of NMR methodology and technology during the past decades, solid-state NMR (ssNMR) has become a particularly important tool for investigating structure and dynamics at atomic scale in biological systems, where the recoupling techniques play pivotal roles in modern high-resolution MAS NMR. In this review, following a brief introduction on the basic theory of recoupling in ssNMR, we highlight the recent advances in dipolar and chemical shift anisotropy recoupling methods, as well as their applications in structural determination and dynamical characterization at multiple time scales (i.e., fast-, intermediate-, and slow-motion). The performances of these prevalent recoupling techniques are compared and discussed in multiple aspects, together with the representative applications in biomolecules. Given the recent emerging advances in NMR technology, new challenges for recoupling methodology development and potential opportunities for biological systems are also discussed.

Dipolar Order Parameters in Large Systems With Fast Spinning

Order parameters are a useful tool for quantifying amplitudes of molecular motions. Here we measure dipolar order parameters by recoupling heteronuclear dipole-dipole couplings under fast spinning. We apply symmetry based recoupling methods to samples spinning under magic angle at 60 kHz by employing a variable flip angle compound inversion pulse. We validate the methods by measuring site-specific ¹⁵N-¹H order parameters of a microcrystalline protein over a small temperature range and the same protein in a large, precipitated complex with antibody. The measurements of the order parameters in the complex are consistent with the observed protein undergoing overall motion within the assembly.

Deuteration for High-Resolution Detection of Protons in Protein Magic Angle Spinning (MAS) Solid-State NMR

Proton detection developed in the last 20 years as the method of choice to study biomolecules in the solid state. In perdeuterated proteins, proton dipolar interactions are strongly attenuated, which allows yielding of high-resolution proton spectra. Perdeuteration and backsubstitution of exchangeable protons is essential if samples are rotated with MAS rotation frequencies below 60 kHz. Protonated samples can be investigated directly without spin dilution using proton detection methods in case the MAS frequency exceeds 110 kHz. This review summarizes labeling strategies and the spectroscopic methods to perform experiments that yield assignments, quantitative information on structure, and dynamics using perdeuterated samples. Techniques for solvent suppression, H/D exchange, and deuterium spectroscopy are discussed. Finally, experimental and theoretical results that allow estimation of the sensitivity of proton detected experiments as a function of the MAS frequency and the external B₀ field in a perdeuterated environment are compiled.

Theory of frequency-selective homonuclear dipolar recoupling in solid-state NMR

In solid-state nuclear magnetic resonance, frequency-selective homonuclear dipolar recoupling is key to quantitative distance measurement or selective enhancement of correlations between atoms of interest in multiple-spin systems, which are not amenable to band-selective or broadband recoupling. Previous frequency-selective recoupling is mostly based on the so-called rotational resonance (R²) condition that restricts the application to spin pairs with resonance frequencies differing in integral multiples of the magic-angle spinning (MAS) frequency. Recently, we have proposed a series of frequency-selective homonuclear recoupling sequences called SPR (short for Selective Phase-optimized Recoupling), which have been successfully applied for selective ¹H-¹H or ¹³C-¹³C recoupling under from moderate (∼10 kHz) to ultra-fast (150 kHz) MAS frequencies. In this study, we fully analyze the average Hamiltonian theory of SPR sequences and reveal the origin of frequency selectivity in recoupling. The theoretical description, as well as numerical simulations and experiments, demonstrates that the frequency selectivity can be easily controlled by the flip angle (p) in the (p)_ϕk(p)_ϕk+π unit in the pSPR-N_n sequences. Small flip angles lead to frequency-selective recoupling, while large flip angles may lead to broadband recoupling in principle. The result shall shed new light on the design of homonuclear recoupling sequences with arbitrary frequency bandwidths.

From Angstroms to Nanometers: Measuring Interatomic Distances by Solid-State NMR

Internuclear distances represent one of the main structural constraints in molecular structure determination using solid-state NMR spectroscopy, complementing chemical shifts and orientational restraints. Although a large number of magic-angle-spinning (MAS) NMR techniques have been available for distance measurements, traditional ¹³C and ¹⁵N NMR experiments are inherently limited to distances of a few angstroms due to the low gyromagnetic ratios of these nuclei. Recent development of fast MAS triple-resonance ¹⁹F and ¹H NMR probes has stimulated the design of MAS NMR experiments that measure distances in the 1-2 nm range with high sensitivity. This review describes the principles and applications of these multiplexed multidimensional correlation distance NMR experiments, with an emphasis on ¹⁹F- and ¹H-based distance experiments. Representative applications of these long-distance NMR methods to biological macromolecules as well as small molecules are reviewed.

Identification of RNA base pairs and complete assignment of nucleobase resonances by 1H-detected solid-state NMR spectroscopy at 100 kHz MAS

Knowledge of RNA structure, either in isolation or in complex, is fundamental to understand the mechanism of cellular processes. Solid‐state NMR (ssNMR) is applicable to high molecular‐weight complexes and does not require crystallization; thus, it is well‐suited to study RNA as part of large multicomponent assemblies. Recently, we solved the first structures of both RNA and an RNA‐protein complex by ssNMR using conventional ¹³C‐ and ¹⁵N‐detection. This approach is limited by the severe overlap of the RNA peaks together with the low sensitivity of multidimensional experiments. Here, we overcome the limitations in sensitivity and resolution by using ¹H‐detection at fast MAS rates. We develop experiments that allow the identification of complete nucleobase spin‐systems together with their site‐specific base pair pattern using sub‐milligram quantities of one uniformly labelled RNA sample. These experiments provide rapid access to RNA secondary structure by ssNMR in protein‐RNA complexes of any size.

Transferred-Rotational-Echo Double Resonance

Internuclear distance determination is the foundation for NMR-based structure calculation. However, high-precision distance measurement is a laborious process requiring lengthy data acquisitions due to the large set of multidimensional spectra needed at different mixing times. This prevents application to large or challenging molecular systems. Here, we present a new approach, transferred-rotational-echo double resonance (TREDOR), a heteronuclear transfer method in which we simultaneously detect both starting and transferred signals in a single spectrum. This co-acquisition is used to compensate for coherence decay, resulting in accurate and precise distance determination by a single parameter fit using a single spectrum recorded at an ideal mixing time. We showcase TREDOR with the microcrystalline SH3 protein using 3D spectra to resolve resonances. By combining the measured N-C and H-C distances, we calculate the structure of SH3, which converges to the correct fold, with a root-mean-square deviation of 2.1 Å compared to a reference X-ray structure. The TREDOR data used in the structure calculation were acquired in only 4 days on a 600 MHz instrument. This is achieved due to the more than 2-fold time saving afforded by co-acquisition of additional information and demonstrates TREDOR as a fast and straightforward method for determining structures via magic-angle spinning NMR.

Combining fast magic angle spinning dynamic nuclear polarization with indirect detection to further enhance the sensitivity of solid-state NMR spectroscopy

Dynamic nuclear polarization (DNP) and indirect detection are two commonly applied approaches for enhancing the sensitivity of solid-state NMR spectroscopy. However, their use in tandem has not yet been investigated. With the advent of low-temperature fast magic angle spinning (MAS) probes with 1.3-mm diameter rotors capable of MAS at 40 ​kHz it becomes feasible to combine these two techniques. In this study, we performed DNP-enhanced 2D indirectly detected heteronuclear correlation (idHETCOR) experiments on ¹³C, ¹⁵N, ¹¹³Cd and ⁸⁹Y nuclei in functionalized mesoporous silica, CdS nanoparticles, and Y₂O₃ nanoparticles. The sensitivity of the 2D idHETCOR experiments was compared with those of DNP-enhanced directly-detected 1D cross polarization (CP) and 2D HETCOR experiments performed with a standard 3.2-mm rotor. Due to low CP polarization transfer efficiencies and large proton linewidth, the sensitivity gains achieved by indirect detection alone were lower than in conventional (non-DNP) experiments. Nevertheless, despite the smaller sample volume the 2D idHETCOR experiments showed better absolute sensitivities than 2D HETCOR experiments for nuclei with the lowest gyromagnetic ratios. For ⁸⁹Y, 2D idHETCOR provided 8.2 times better sensitivity than the 1 D⁸⁹Y-detected CP experiment performed with a 3.2-mm rotor.

Selectively Enhanced 1H-1H Correlations in Proton-Detected Solid-State NMR under Ultrafast MAS Conditions

Proton-detected solid-state NMR has emerged as a powerful analytical technique in structural elucidation via ¹H-¹H correlations, which are mostly established by broadband methods. We propose a new class of frequency-selective homonuclear recoupling methods to selectively enhance ¹H-¹H correlations of interest under ultrafast magic-angle spinning (MAS). These methods, dubbed as selective phase-optimized recoupling (SPR), can provide a sensitivity enhancement by a factor of ∼3 over the widely used radio-frequency-driven recoupling (RFDR) to observe ¹H_N-¹H_N contacts in a protonated tripeptide N-formyl-Met-Leu-Phe (fMLF) under 150 kHz MAS and are successfully utilized to probe a long-range ¹H-¹H contact in a pharmaceutical molecule, the hydrochloride form of pioglitazone (PIO-HCl). SPR is not only highly efficient in frequency-selective recoupling but also easy to implement, imparting to it great potential to probe ¹H-¹H contacts for the structural elucidation of organic solids such as proteins and pharmaceuticals under ultrafast MAS conditions.

A modification of γ-encoded RN symmetry pulses for increasing the scaling factor and more accurate measurements of the strong heteronuclear dipolar couplings

Symmetry based γ-encoded RN_n^ν elements are broadly used in magic-angle spinning solid-state NMR experiments to achieve selective recoupling of the heteronuclear dipolar interactions. The recoupled dipolar couplings in such experiments are scaled by a factor, K_sc, which theoretically depends on the chosen symmetry numbers N, n, and ν. However, the maximum theoretical value of K_sc for γ-encoded RN_n^ν pulses is limited to ~0.25, resulting in long RN_n^ν experiment times. Also, the dependence of K_sc on the experimental parameters can result in systematic errors in the experimental determination of the dipolar couplings, especially at low and moderate MAS rates. In this manuscript, we investigate the use of MODifiEd RN_n^ν symmetry (MODERN_n^ν(ϕ_M)) pulses that increase the dipolar scaling factor by at least 1.45 fold compared to γ-encoded RN_n^ν. The second advantage of MODERN_n^ν(ϕ_M) pulses with respect to traditional RN_n^ν pulses is the reduced influence of experimental parameters on K_sc, which allows for more accurate measurement of short-range distances. The robustness of MODERN_n^ν(ϕ_M) is compared with γ-encoded R14₂³ symmetry pulses. The enhanced performance is demonstrated on two uniformly-¹³C-enriched samples, N-acetyl valine and the microcrystalline protein GB1, at a 31.111 kHz MAS rate.

Can proton-proton recoupling in fully protonated solids provide quantitative, selective and efficient polarization transfer?

Dipolar recoupling sequences have been used to probe spatial proximity of nuclear spins and were traditionally designed to probe rare spins such as ¹³C and/or ¹⁵N nuclei. The multi-spin dipolar-coupling network of the rare spins is weak due to smaller couplings and large chemical shift dispersion. Therefore, the recoupling approaches were tailored to design offset compensated or broadband sequences. In contrast, protons have a substantially stronger dipolar-coupling network and much narrower chemical shift range. Broadband recoupling sequences such as radio-frequency driven recoupling (RFDR), back-to-back (BABA), and lab frame proton-proton spin diffusion have been routinely used to characterize the structures of protein/macromolecules and small molecules. Recently selective ¹H-¹H recoupling sequences have been proposed that combine chemical shift offset of the resolved proton spectrum (at fast MAS) with first- and second-order dipolar recoupling Hamiltonians to obtain quantitative and qualitative proton distances, respectively. Herein, we evaluate the performances of broadband and selective proton recoupling sequences such as finite pulse RFDR (fp-RFDR), band-selective spectral spin diffusion (BASS-SD), second-order cross-polarization (SOCP), and selective recoupling of proton (SERP) in terms of the selectivity and efficiency of ¹H-¹H polarization transfers in a dense network of proton spins and explore the possibility of measuring ¹H-¹H distances. We use theoretical considerations, numerical simulations, and experiments to support the distinct advantages and disadvantages of each recoupling sequence. Experiments were performed on L-histidine.HCl.H₂O at a MAS frequency of 71.43 kHz. This study rationalizes the proper selection of ¹H-¹H recoupling sequences when working with fully protonated solids.

MAS NMR detection of hydrogen bonds for protein secondary structure characterization

Hydrogen bonds are essential for protein structure and function, making experimental access to long-range interactions between amide protons and heteroatoms invaluable. Here we show that measuring distance restraints involving backbone hydrogen atoms and carbonyl- or α-carbons enables the identification of secondary structure elements based on hydrogen bonds, provides long-range contacts and validates spectral assignments. To this end, we apply specifically tailored, proton-detected 3D (H)NCOH and (H)NCAH experiments under fast magic angle spinning (MAS) conditions to microcrystalline samples of SH3 and GB1. We observe through-space, semi-quantitative correlations between protein backbone carbon atoms and multiple amide protons, enabling us to determine hydrogen bonding patterns and thus to identify β-sheet topologies and α-helices in proteins. Our approach shows the value of fast MAS and suggests new routes in probing both secondary structure and the role of functionally-relevant protons in all targets of solid-state MAS NMR.

Improved Protocol for the Production of the Low-Expression Eukaryotic Membrane Protein Human Aquaporin 2 in Pichia pastoris for Solid-State NMR

Solid-state nuclear magnetic resonance (SSNMR) is a powerful biophysical technique for studies of membrane proteins; it requires the incorporation of isotopic labels into the sample. This is usually accomplished through over-expression of the protein of interest in a prokaryotic or eukaryotic host in minimal media, wherein all (or some) carbon and nitrogen sources are isotopically labeled. In order to obtain multi-dimensional NMR spectra with adequate signal-to-noise ratios suitable for in-depth analysis, one requires high yields of homogeneously structured protein. Some membrane proteins, such as human aquaporin 2 (hAQP2), exhibit poor expression, which can make producing a sample for SSNMR in an economic fashion extremely difficult, as growth in minimal media adds additional strain on expression hosts. We have developed an optimized growth protocol for eukaryotic membrane proteins in the methylotrophic yeast Pichia pastoris. Our new growth protocol uses the combination of sorbitol supplementation, higher cell density, and low temperature induction (LT-SEVIN), which increases the yield of full-length, isotopically labeled hAQP2 ten-fold. Combining mass spectrometry and SSNMR, we were able to determine the nature and the extent of post-translational modifications of the protein. The resultant protein can be functionally reconstituted into lipids and yields excellent resolution and spectral coverage when analyzed by two-dimensional SSNMR spectroscopy.

Structural and dynamic origins of ESR lineshapes in spin-labeled GB1 domain: the insights from spin dynamics simulations based on long MD trajectories

Site-directed spin labeling (SDSL) ESR is a valuable tool to probe protein systems that are not amenable to characterization by x-ray crystallography, NMR or EM. While general principles that govern the shape of SDSL ESR spectra are known, its precise relationship with protein structure and dynamics is still not fully understood. To address this problem, we designed seven variants of GB1 domain bearing R1 spin label and recorded the corresponding MD trajectories (combined length 180 μs). The MD data were subsequently used to calculate time evolution of the relevant spin density matrix and thus predict the ESR spectra. The simulated spectra proved to be in good agreement with the experiment. Further analysis confirmed that the spectral shape primarily reflects the degree of steric confinement of the R1 tag and, for the well-folded protein such as GB1, offers little information on local backbone dynamics. The rotameric preferences of R1 side chain are determined by the type of the secondary structure at the attachment site. The rotameric jumps involving dihedral angles χ1 and χ2 are sufficiently fast to directly influence the ESR lineshapes. However, the jumps involving multiple dihedral angles tend to occur in (anti)correlated manner, causing smaller-than-expected movements of the R1 proxyl ring. Of interest, ESR spectra of GB1 domain with solvent-exposed spin label can be accurately reproduced by means of Redfield theory. In particular, the asymmetric character of the spectra is attributable to Redfield-type cross-correlations. We envisage that the current MD-based, experimentally validated approach should lead to a more definitive, accurate picture of SDSL ESR experiments.

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.

Non-uniform sampling in quantitative assessment of heterogeneous solid-state NMR line shapes

Non-uniform sampling has been successfully used for solution and solid-state NMR of homogeneous samples. In the solid state, protein samples are often dominated by inhomogeneous contributions to the homogeneous line widths. In spite of different technical strategies for peak reconstruction by different methods, we validate that NUS can generally be used also for such situations where spectra are made up of complex peak shapes rather than Lorentian lines. Using the RMSD between subsampled and reconstructed data and those spectra obtained with uniform sampling for a sample comprising a wide conformational distribution, we quantitatively evaluate the identity of inhomogeneous peak patterns. The evaluation comprises Iterative Soft Thresholding (hmsIST implementation) as a method explicitly not assuming Lorentian lineshapes, as well as Sparse Multidimensional Iterative Lineshape Enhanced (SMILE) algorithm and Signal Separation Algorithm (SSA) reconstruction, which do work on the basis of Lorentian lineshape models, with different sampling densities. Even though individual peculiarities are apparent, all methods turn out principally viable to reconstruct the heterogeneously broadened peak shapes.

Conformational investigation of peptides using solid-state NMR spectroscopy-A study of polymorphism of β-turn peptides containing diprolines

The construction of complex protein folds relies on the precise conversion of a linear polypeptide chain into a compact 3-dimensional structure. In this context, study of isolated secondary structural modules containing short stretches of amino acids assumes significance. Additionally, peptides, both natural and synthetic, play a major role as potential drugs. With a view to understand the local conformations adopted by peptides in the solid state, we propose a multinuclear NMR approach utilizing spectra of nuclei in their natural isotopic abundance. Various solid-state NMR experiments have been utilized for assignment of the spectra. Additionally, the gauge-including projector augmented-wave (GIPAW) calculations were used to confirm the assignments. Particularly, the utility of the double-quantum-single-quantum correlation experiments is highlighted for the purpose of assignment and for inferring the conformation across the peptide bond. The methodology is illustrated for the case of designed peptides containing diproline residues occurring at the β-turns for identifying their cis-trans conformational polymorphism. The proposed method promises to be of use in the study of conformations of small- to medium-sized peptides such as antimicrobial peptides and in the study of polymorphism leading to applications in drug development protocols.

Fast electron paramagnetic resonance magic angle spinning simulations using analytical powder averaging techniques

Simulations describing the spin physics underpinning nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy play an important role in the design of new experiments. When experiments are performed in the solid state, samples are commonly composed of powders or glasses, with molecules oriented at a large number of angles with respect to the laboratory frame. These powder angles must be represented in simulations to account for anisotropic interactions. Numerical techniques are typically used to accurately compute such powder averages. A large number of Euler angles are usually required, leading to lengthy simulation times. This is particularly true in broad spectra, such as those observed in EPR. The combination of the traditionally separate techniques of EPR and magic angle spinning (MAS) NMR could play an important role in future electron detected experiments, combined with dynamic nuclear polarization, which will allow for exceptional detection sensitivity of NMR spin coherences. Here, we present a method of reducing the required number of Euler angles in magnetic resonance simulations by analytically performing the powder average over one of the Euler angles in the static and MAS cases for the TEMPO nitroxide radical in a 7 T field. In the static case, this leads to a 97.5% reduction in simulation time over the fully numerical case and reproduces the expected spinning sideband manifold when simulated with a MAS frequency of 150 kHz. This technique is applicable to more traditional NMR experiments as well, such as those involving quadrupolar nuclei or multiple dimensions.

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.

Exact distance measurements for structure and dynamics in solid proteins by fast-magic-angle-spinning NMR

Fast-magic-angle-spinning solid-state NMR is a developing technique for determination of protein structure and dynamics. Proton-proton correlations usually lead to rough distance restraints, a serious hurdle towards high-resolution structures. Analogous to the "eNOE" concept in solution, an integrative approach for more accurate restraints enables improved structural accuracy with minimal analytical effort.

Alpha protons as NMR probes in deuterated proteins

We describe a new labeling method that allows for full protonation at the backbone Hα position, maintaining protein side chains with a high level of deuteration. We refer to the method as alpha proton exchange by transamination (α-PET) since it relies on transaminase activity demonstrated here using Escherichia coli expression. We show that α-PET labeling is particularly useful in improving structural characterization of solid proteins by introduction of an additional proton reporter, while eliminating many strong dipolar couplings. The approach benefits from the high sensitivity associated with 1.3 mm samples, more abundant information including Hα resonances, and the narrow proton linewidths encountered for highly deuterated proteins. The labeling strategy solves amide proton exchange problems commonly encountered for membrane proteins when using perdeuteration and backexchange protocols, allowing access to alpha and all amide protons including those in exchange-protected regions. The incorporation of Hα protons provides new insights, as the close Hα-Hα and Hα-H^N contacts present in β-sheets become accessible, improving the chance to determine the protein structure as compared with H^N-H^N contacts alone. Protonation of the Hα position higher than 90% is achieved for Ile, Leu, Phe, Tyr, Met, Val, Ala, Gln, Asn, Thr, Ser, Glu, Asp even though LAAO is only active at this degree for Ile, Leu, Phe, Tyr, Trp, Met. Additionally, the glycine methylene carbon is labeled preferentially with a single deuteron, allowing stereospecific assignment of glycine alpha protons. In solution, we show that the high deuteration level dramatically reduces R₂ relaxation rates, which is beneficial for the study of large proteins and protein dynamics. We demonstrate the method using two model systems, as well as a 32 kDa membrane protein, hVDAC1, showing the applicability of the method to study membrane proteins.

Proton-detected 3D 1H anisotropic/14N/1H isotropic chemical shifts correlation NMR under fast magic angle spinning on solid samples without isotopic enrichment

The chemical shift anisotropy (CSA) interaction of a nucleus is an important indicator of the local electronic environment particularly for the contributions arising from hydrogen (H)-bonding, electrostatic and π-π interactions. CSAs of protons bonded to nitrogen atoms are of significant interest due to their common role as H-bonding partners in many chemical, pharmaceutical and biological systems. Although very fast (∼100 kHz) magic angle sample spinning (MAS) experiments have enabled the measurement of proton CSAs directly from solids, due to a narrow chemical shift (CS) distribution, overlapping NH proton resonances are common and necessitate the introduction of an additional frequency dimension to the regular 2D ¹H CSA/¹H CS correlation method to achieve sufficient resolution. While this can be accomplished by using the isotropic shift frequency of ¹⁴N or ¹⁵N nuclei, the use of the naturally-abundant ¹⁴N nucleus avoids ¹⁵N isotopic labeling and therefore would be useful for a variety of solids. To this end, we propose a proton-detected 3D ¹H CSA/¹⁴N/¹H CS correlation method under fast MAS (90 kHz) to determine the CSA tensors of NH protons in samples without isotopic enrichment. Our experimental results demonstrate that the proposed 3D NMR experiment is capable of resolving the overlapping ¹H resonances of amide (NH) groups through the ¹⁴N isotropic shift frequency dimension and enables the accurate measurement of site-specific ¹H CSAs directly from powder samples under fast MAS conditions. In addition to the 3D ¹H CSA/¹⁴N/¹H CS experiment, an approach employing ¹⁴N-edited 2D ¹H CSA/¹H CS experiment is also demonstrated as an additional means to address spectral overlap of NH resonances with aliphatic and other proton resonances in solids.

Probing Membrane Protein Insertion into Lipid Bilayers by Solid-State NMR

Determination of the environment surrounding a protein is often key to understanding its function and can also be used to infer the structural properties of the protein. By using proton-detected solid-state NMR, we show that reduced spin diffusion within the protein under conditions of fast magic-angle spinning, high magnetic field, and sample deuteration allows the efficient measurement of site-specific exposure to mobile water and lipids. We demonstrate this site specificity on two membrane proteins, the human voltage dependent anion channel, and the alkane transporter AlkL from Pseudomonas putida. Transfer from lipids is observed selectively in the membrane spanning region, and an average lipid-protein transfer rate of 6 s^-1 was determined for residues protected from exchange. Transfer within the protein, as tracked in the ¹⁵ N-¹ H 2D plane, was estimated from initial rates and found to be in a similar range of about 8 to 15 s^-1 for several resolved residues, explaining the site specificity.

Automated projection spectroscopy in solid-state NMR

Given that solid-state NMR is being used for protein samples of increasing molecular weight and complexity, higher-dimensionality methods are likely to be more and more indispensable for unambiguous chemical shift assignments in the near future. In addition, solid-state NMR spectral properties are increasingly comparable with solution NMR, allowing adaptation of more sophisticated solution NMR strategies for the solid state in addition to the conventional methodology. Assessing first principles, here we demonstrate the application of automated projection spectroscopy for a micro-crystalline protein in the solid state.

Structure determination of supra-molecular assemblies by solid-state NMR: Practical considerations

In the cellular environment, biomolecules assemble in large complexes which can act as molecular machines. Determining the structure of intact assemblies can reveal conformations and inter-molecular interactions that are only present in the context of the full assembly. Solid-state NMR (ssNMR) spectroscopy is a technique suitable for the study of samples with high molecular weight that allows the atomic structure determination of such large protein assemblies under nearly physiological conditions. This review provides a practical guide for the first steps of studying biological supra-molecular assemblies using ssNMR. The production of isotope-labeled samples is achievable via several means, which include recombinant expression, cell-free protein synthesis, extraction of assemblies directly from cells, or even the study of assemblies in whole cells in situ. Specialized isotope labeling schemes greatly facilitate the assignment of chemical shifts and the collection of structural data. Advanced strategies such as mixed, diluted, or segmental subunit labeling offer the possibility to study inter-molecular interfaces. Detailed and practical considerations are presented with respect to first setting up magic-angle spinning (MAS) ssNMR experiments, including the selection of the ssNMR rotor, different methods to best transfer the sample and prepare the rotor, as well as common and robust procedures for the calibration of the instrument. Diagnostic spectra to evaluate the resolution and sensitivity of the sample are presented. Possible improvements that can reduce sample heterogeneity and improve the quality of ssNMR spectra are reviewed.

Conformational Dynamics in the Core of Human Y145Stop Prion Protein Amyloid Probed by Relaxation Dispersion NMR

Microsecond to millisecond timescale backbone dynamics of the amyloid core residues in Y145Stop human prion protein (PrP) fibrils were investigated by using ¹⁵ N rotating frame (R_1ρ ) relaxation dispersion solid-state nuclear magnetic resonance spectroscopy over a wide range of spin-lock fields. Numerical simulations enabled the experimental relaxation dispersion profiles for most of the fibril core residues to be modelled by using a two-state exchange process with a common exchange rate of 1000 s^-1 , corresponding to protein backbone motion on the timescale of 1 ms, and an excited-state population of 2 %. We also found that the relaxation dispersion profiles for several amino acids positioned near the edges of the most structured regions of the amyloid core were better modelled by assuming somewhat higher excited-state populations (∼5-15 %) and faster exchange rate constants, corresponding to protein backbone motions on the timescale of ∼100-300 μs. The slow backbone dynamics of the core residues were evaluated in the context of the structural model of human Y145Stop PrP amyloid.

A one-dimensional solid-state NMR approach for 14NH/14NH overtone correlation through 1H/1H mixing under fast MAS

Homonuclear correlations are key to structural studies using solid-state NMR. In this contribution, using 14N overtone transition (OT) as a selective excitation approach, we propose a proton-detected one-dimensional (1D) 14NOT/14NOT/1H correlation solid-state NMR method mediated through 1H/1H mixing at fast magic angle spinning to achieve NH/NH proximities in naturally abundant samples. The proposed method is time efficient by a factor of ∼7.5 in comparison to the existing fundamental 14N frequency-based three-dimensional (3D) 14N/14N/1H correlation method.

Simple and Robust Study of Backbone Dynamics of Crystalline Proteins Employing 1H-15N Dipolar Coupling Dispersion

We report a new solid-state multidimensional NMR approach based on the cross-polarization with variable-contact pulse sequence [ Paluch , P. ; Pawlak , T. ; Amoureux , J.-P. ; Potrzebowski , M. J. J. Magn. Reson. 233 , 2013 , 56 ], with ¹H inverse detection and very fast magic angle spinning (ν_R = 60 kHz), dedicated to the measurement of local molecular motions of ¹H-¹⁵N vectors. The introduced three-dimensional experiments, ¹H-¹⁵N-¹H and hCA(N)H, are particularly useful for the study of molecular dynamics of proteins and other complex structures. The applicability and power of this methodology have been revealed by employing as a model sample the GB-1 small protein doped with Na₂CuEDTA. The results clearly prove that the dispersion of ¹H-¹⁵N dipolar coupling constants well correlates with higher order structure of the protein. Our approach complements the conventional studies and offers a fast and reasonably simple method.

Studying intact bacterial peptidoglycan by proton-detected NMR spectroscopy at 100 kHz MAS frequency

The bacterial cell wall is composed of the peptidoglycan (PG), a large polymer that maintains the integrity of the bacterial cell. Due to its multi-gigadalton size, heterogeneity, and dynamics, atomic-resolution studies are inherently complex. Solid-state NMR is an important technique to gain insight into its structure, dynamics and interactions. Here, we explore the possibilities to study the PG with ultra-fast (100 kHz) magic-angle spinning NMR. We demonstrate that highly resolved spectra can be obtained, and show strategies to obtain site-specific resonance assignments and distance information. We also explore the use of proton-proton correlation experiments, thus opening the way for NMR studies of intact cell walls without the need for isotope labeling.

Protons as Versatile Reporters in Solid-State NMR Spectroscopy

Solid-state nuclear magnetic resonance (ssNMR) is a spectroscopic technique that is used for characterization of molecular properties in the solid phase at atomic resolution. In particular, using the approach of magic-angle spinning (MAS), ssNMR has seen widespread applications for topics ranging from material sciences to catalysis, metabolomics, and structural biology, where both isotropic and anisotropic parameters can be exploited for a detailed assessment of molecular properties. High-resolution detection of protons long represented the holy grail of the field. With its high natural abundance and high gyromagnetic ratio, ¹H has naturally been the most important nucleus type for the solution counterpart of NMR spectroscopy. In the solid state, similar benefits are obtained over detection of heteronuclei, however, a rocky road led to its success as their high gyromagnetic ratio has also been associated with various detrimental effects. Two exciting approaches have been developed in recent years that enable proton detection: After partial deuteration of the sample to reduce the proton spin density, the exploitation of protons could begin. Also, faster MAS, nowadays using tiny rotors with frequencies up to 130 kHz, has relieved the need for expensive deuteration. Apart from the sheer gain in sensitivity from choosing protons as the detection nucleus, the proton chemical shift and several other useful aspects of protons have revolutionized the field. In this Account, we are describing the fundamentals of proton detection as well as the arising possibilities for characterization of biomolecules as associated with the developments in our own lab. In particular, we focus on facilitated chemical-shift assignment, structure calculation based on protons, and on assessment of dynamics in solid proteins. For example, the proton chemical-shift dimension adds additional information for resonance assignments in the protein backbone and side chains. Chemical shifts and high gyromagnetic ratio of protons enable direct readout of spatial information over large distances. Dynamics in the protein backbone or side chains can be characterized efficiently using protons as reporters. For all of this, the sample amounts necessary for a given signal-to-noise have drastically shrunk, and new methodology enables assessment of molecules with increasing monomer molecular weight and complexity. Taken together, protons are able to overcome previous limitations, by speeding up processes, enhancing accuracies, and increasing the accessible ranges of ssNMR spectroscopy, as we shall discuss in detail in the following. In particular, these methodological developments have been pushing solid-state NMR into a new regime of biological topics as they realistically allow access to complex cellular molecules, elucidating their functions and interactions in a multitude of ways.

Detection of side-chain proton resonances of fully protonated biosolids in nano-litre volumes by magic angle spinning solid-state NMR

We present a new solid-state NMR proton-detected three-dimensional experiment dedicated to the observation of protein proton side chain resonances in nano-liter volumes. The experiment takes advantage of very fast magic angle spinning and double quantum 13C-13C transfer to establish efficient (H)CCH correlations detected on side chain protons. Our approach is demonstrated on the HET-s prion domain in its functional amyloid fibrillar form, fully protonated, with a sample amount of less than 500 µg using a MAS frequency of 70 kHz. The majority of aliphatic and aromatic side chain protons (70%) are observable, in addition to Hα resonances, in a single experiment providing a complementary approach to the established proton-detected amide-based multidimensional solid-state NMR experiments for the study and resonance assignment of biosolid samples, in particular for aromatic side chain resonances.

1H magic-angle spinning NMR evolves as a powerful new tool for membrane proteins

Building on a decade of continuous advances of the community, the recent development of very fast (60 kHz and above) magic-angle spinning (MAS) probes has revolutionised the field of solid-state NMR. This new spinning regime reduces the ¹H-¹H dipolar couplings, so that direct detection of the larger magnetic moment available from ¹H is now possible at high resolution, not only in deuterated molecules but also in fully-protonated substrates. Such capabilities allow rapid "fingerprinting" of samples with a ten-fold reduction of the required sample amounts with respect to conventional approaches, and permit extensive, robust and expeditious assignment of small-to-medium sized proteins (up to ca. 300 residues), and the determination of inter-nuclear proximities, relative orientations of secondary structural elements, protein-cofactor interactions, local and global dynamics. Fast MAS and ¹H detection techniques have nowadays been shown to be applicable to membrane-bound systems. This paper reviews the strategies underlying this recent leap forward in sensitivity and resolution, describing its potential for the detailed characterization of membrane proteins.

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.

Progress in proton-detected solid-state NMR (SSNMR): Super-fast 2D SSNMR collection for nano-mole-scale proteins

Proton-detected solid-state NMR (SSNMR) spectroscopy has attracted much attention due to its excellent sensitivity and effectiveness in the analysis of trace amounts of amyloid proteins and other important biological systems. In this perspective article, we present the recent sensitivity limit of 1H-detected SSNMR using "ultra-fast" magic-angle spinning (MAS) at a spinning rate (νR) of 80-100 kHz. It was demonstrated that the high sensitivity of 1H-detected SSNMR at νR of 100 kHz and fast recycling using the paramagnetic-assisted condensed data collection (PACC) approach permitted "super-fast" collection of 1H-detected 2D protein SSNMR. A 1H-detected 2D 1H-15N correlation SSNMR spectrum for ∼27 nmol of a uniformly 13C- and 15N-labeled GB1 protein sample in microcrystalline form was acquired in only 9 s with 50% non-uniform sampling and short recycle delays of 100 ms. Additional data suggests that it is now feasible to detect as little as 1 nmol of the protein in 5.9 h by 1H-detected 2D 1H-15N SSNMR at a nominal signal-to-noise ratio of five. The demonstrated sensitivity is comparable to that of modern solution protein NMR. Moreover, this article summarizes the influence of ultra-fast MAS and 1H-detection on the spectral resolution and sensitivity of protein SSNMR. Recent progress in signal assignment and structural elucidation by 1H-detected protein SSNMR is outlined with both theoretical and experimental aspects.

Is protein deuteration beneficial for proton detected solid-state NMR at and above 100 kHz magic-angle spinning?

¹H-detection in solid-state NMR of proteins has been traditionally combined with deuteration for both resolution and sensitivity reasons, with the optimal level of proton dilution being dependent on MAS rate. Here we present ¹H-detected ¹⁵N and ¹³C CP-HSQC spectra on two microcrystalline samples acquired at 60 and 111 kHz MAS and at ultra-high field. We critically compare the benefits of three labeling schemes yielding different levels of proton content in terms of resolution, coherence lifetimes and feasibility of scalar-based 2D correlations under these experimental conditions. We observe unexpectedly high resolution and sensitivity of aromatic resonances in 2D ¹³C-¹H correlation spectra of protonated samples. Ultrafast MAS reduces or even removes the necessity of ¹H dilution for high-resolution ¹H-detection in biomolecular solid-state NMR. It yields ¹⁵N,¹H and ¹³C,¹H fingerprint spectra of exceptional resolution for fully protonated samples, with notably superior ¹H and ¹³C lineshapes for side-chain resonances.

Solid-state NMR spectroscopic trends for supramolecular assemblies and protein aggregates

Solid-state NMR is able to generate structural data on sample preparations that are explicitly non-crystalline. In particular, for amyloid fibril samples, which can comprise significant degrees of sample disorder, solid-state NMR has been used very successfully. But also solid-state NMR studies of other supramolecular assemblies that have resisted assessment by more standard methods are being performed with increasing ease and biological impact, many of which are briefly reviewed here. New technical trends with respect to structure calculation, protein dynamics and smaller sample amounts have reshaped the field of solid-state NMR recently. In particular, proton-detected approaches based on fast Magic-Angle Spinning (MAS) were demonstrated for crystalline systems initially. Currently, such approaches are being expanded to the above-mentioned non-crystalline targets, the characterization of which can now be pursued with sample amounts on the order of a milligram. In this Trends article, I am giving a brief overview about achievements of the last years as well as the directions that the field has been heading into and delineate some satisfactory perspectives for solid-state NMR's future striving.