Proton-detected solid-state NMR spectroscopy at aliphatic sites: application to crystalline systems

Dipolar Recoupling in Rotating Solids

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) has evolved significantly over the past three decades and established itself as a vital tool for the structural analysis of biological macromolecules and materials. This review delves into the development and application of dipolar recoupling techniques in MAS NMR, which are crucial for obtaining detailed structural and dynamic information. We discuss a variety of homonuclear and heteronuclear recoupling methods which are essential for measuring spatial restraints and explain in detail the spin dynamics that these sequences generate. We also explore recent developments in high spinning frequency MAS, proton detection, and dynamic nuclear polarization, underscoring their importance in advancing biomolecular NMR. Our aim is to provide a comprehensive account of contemporary dipolar recoupling methods, their principles, and their application to structural biology and materials, highlighting significant contributions to the field and emerging techniques that enhance resolution and sensitivity in MAS 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.

1H-detected characterization of carbon-carbon networks in highly flexible protonated biomolecules using MAS NMR

In the last three decades, the scope of solid-state NMR has expanded to exploring complex biomolecules, from large protein assemblies to intact cells at atomic-level resolution. This diversity in macromolecules frequently features highly flexible components whose insoluble environment precludes the use of solution NMR to study their structure and interactions. While High-resolution Magic-Angle Spinning (HR-MAS) probes offer the capacity for gradient-based 1H-detected spectroscopy in solids, such probes are not commonly used for routine MAS NMR experiments. As a result, most exploration of the flexible regime entails either 13C-detected experiments, the use of partially perdeuterated systems, or ultra-fast MAS. Here we explore proton-detected pulse schemes probing through-bond 13C-13C networks to study mobile protein sidechains as well as polysaccharides in a broadband manner. We demonstrate the use of such schemes to study a mixture of microtubule-associated protein (MAP) tau and human microtubules (MTs), and the cell wall of the fungus Schizophyllum commune using 2D and 3D spectroscopy, to show its viability for obtaining unambiguous correlations using standard fast-spinning MAS probes at high and ultra-high magnetic fields.

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.

Fast magic angle spinning for the characterization of milligram quantities of organic and biological solids at natural isotopic abundance by 13C-13C correlation DNP-enhanced NMR

We show that multidimensional solid-state NMR ¹³C-¹³C correlation spectra of biomolecular assemblies and microcrystalline organic molecules can be acquired at natural isotopic abundance with only milligram quantities of sample. These experiments combine fast Magic Angle Spinning of the sample, low-power dipolar recoupling, and dynamic nuclear polarization performed with AsymPol biradicals, a recently introduced family of polarizing agents. Such experiments are essential for structural characterization as they provide short- and long-range distance information. This approach is demonstrated on diverse sample types, including polyglutamine fibrils implicated in Huntington's disease and microcrystalline ampicillin, a small antibiotic molecule.

Cell-free synthesis of amyloid fibrils with infectious properties and amenable to sub-milligram magic-angle spinning NMR analysis

Structural investigations of amyloid fibrils often rely on heterologous bacterial overexpression of the protein of interest. Due to their inherent hydrophobicity and tendency to aggregate as inclusion bodies, many amyloid proteins are challenging to express in bacterial systems. Cell-free protein expression is a promising alternative to classical bacterial expression to produce hydrophobic proteins and introduce NMR-active isotopes that can improve and speed up the NMR analysis. Here we implement the cell-free synthesis of the functional amyloid prion HET-s(218-289). We present an interesting case where HET-s(218-289) directly assembles into infectious fibril in the cell-free expression mixture without the requirement of denaturation procedures and purification. By introducing tailored 13C and 15N isotopes or CF3 and 13CH2F labels at strategic amino-acid positions, we demonstrate that cell-free synthesized amyloid fibrils are readily amenable to high-resolution magic-angle spinning NMR at sub-milligram quantity.

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.

NMR Assignment of Methyl Groups in Immobilized Proteins Using Multiple-Bond 13C Homonuclear Transfers, Proton Detection, and Very Fast MAS

In nuclear magnetic resonance spectroscopy of proteins, methyl protons play a particular role as extremely sensitive reporters on dynamics, allosteric effects, and protein–protein interactions, accessible even in high-molecular-weight systems approaching 1 MDa. The notorious issue of their chemical shift assignment is addressed here by a joint use of solid-state 1H-detected methods at very fast (nearly 100 kHz) magic-angle spinning, partial deuteration, and high-magnetic fields. The suitability of a series of RF schemes is evaluated for the efficient coherence transfer across entire 13C side chains of methyl-containing residues, which is key for establishing connection between methyl and backbone 1H resonances. The performance of ten methods for recoupling of either isotropic 13C–13C scalar or anisotropic dipolar interactions (five variants of TOBSY, FLOPSY, DIPSI, WALTZ, RFDR, and DREAM) is evaluated experimentally at two state-of-the-art magic-angle spinning (55 and 94.5 kHz) and static magnetic field conditions (18.8 and 23.5 T). Model isotopically labeled compounds (alanine and Met-Leu-Phe tripeptide) and ILV-methyl and amide-selectively protonated, and otherwise deuterated chicken α-spectrin SH3 protein are used as convenient reference systems. Spin dynamics simulations in SIMPSON are performed to determine optimal parameters of these RF schemes, up to recently experimentally attained spinning frequencies (200 kHz) and B0 field strengths (28.2 T). The concept of linearization of 13C side chain by appropriate isotope labeling is revisited and showed to significantly increase sensitivity of methyl-to-backbone correlations. A resolution enhancement provided by 4D spectroscopy with non-uniform (sparse) sampling is demonstrated to remove ambiguities in simultaneous resonance assignment of methyl proton and carbon chemical shifts.

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.

Investigating Crystalline Protein Suspension Formulations of Pembrolizumab from MAS NMR Spectroscopy

Developing biological formulations to maintain the chemical and structural integrity of therapeutic antibodies remains a significant challenge. Monoclonal antibody (mAb) crystalline suspension formulation is a promising alternative for high concentration subcutaneous drug delivery. It demonstrates many merits compared to the solution formulation to reach a high concentration at the reduced viscosity and enhanced stability. One main challenge in drug development is the lack of high-resolution characterization of the crystallinity and stability of mAb microcrystals in the native formulations. Conventional analytical techniques often cannot evaluate structural details of mAb microcrystals in the native suspension due to the presence of visible particles, relatively small crystal size, high protein concentration, and multicomponent nature of a liquid formulation. This study demonstrates the first high-resolution characterization of mAb microcrystalline suspension using magic angle spinning (MAS) NMR spectroscopy. Crystalline suspension formulation of pembrolizumab (Keytruda, Merck & Co., Inc., Kenilworth, NJ 07033, U.S.) is utilized as a model system. Remarkably narrow ¹³C spectral linewidth of approximately 29 Hz suggests a high order of crystallinity and conformational homogeneity of pembrolizumab crystals. The impact of thermal stress and dehydration on the structure, dynamics, and stability of these mAb crystals in the formulation environment is evaluated. Moreover, isotopic labeling and heteronuclear ¹³C and ¹⁵N spectroscopies have been utilized to identify the binding of caffeine in the pembrolizumab crystal lattice, providing molecular insights into the cocrystallization of the protein and ligand. Our study provides valuable structural details for facilitating the design of crystalline suspension formulation of Keytruda and demonstrates the high potential of MAS NMR as an advanced tool for biophysical characterization of biological therapeutics.

Atomic-resolution chemical characterization of (2x)72-kDa tryptophan synthase via four- and five-dimensional 1H-detected solid-state NMR

NMR chemical shifts provide detailed information on the chemical properties of molecules, thereby complementing structural data from techniques like X-ray crystallography and electron microscopy. Detailed analysis of protein NMR data, however, often hinges on comprehensive, site-specific assignment of backbone resonances, which becomes a bottleneck for molecular weights beyond 40 to 45 kDa. Here, we show that assignments for the (2x)72-kDa protein tryptophan synthase (665 amino acids per asymmetric unit) can be achieved via higher-dimensional, proton-detected, solid-state NMR using a single, 1-mg, uniformly labeled, microcrystalline sample. This framework grants access to atom-specific characterization of chemical properties and relaxation for the backbone and side chains, including those residues important for the catalytic turnover. Combined with first-principles calculations, the chemical shifts in the β-subunit active site suggest a connection between active-site chemistry, the electrostatic environment, and catalytically important dynamics of the portal to the β-subunit from solution.

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.

Towards a native environment: structure and function of membrane proteins in lipid bilayers by NMR

Solid-state NMR (ssNMR) is a versatile technique that can be used for the characterization of various materials, ranging from small molecules to biological samples, including membrane proteins. ssNMR can probe both the structure and dynamics of membrane proteins, revealing protein function in a near-native lipid bilayer environment. The main limitation of the method is spectral resolution and sensitivity, however recent developments in ssNMR hardware, including the commercialization of 28 T magnets (1.2 GHz proton frequency) and ultrafast MAS spinning (<100 kHz) promise to accelerate acquisition, while reducing sample requirement, both of which are critical to membrane protein studies. Here, we review recent advances in ssNMR methodology used for structure determination of membrane proteins in native and mimetic environments, as well as the study of protein functions such as protein dynamics, and interactions with ligands, lipids and cholesterol.

Selective 1H-1H recoupling via symmetry sequences in fully protonated samples at fast magic angle spinning

Proton-detected solid-state NMR at fast Magic Angle Spinning (MAS) is becoming the norm to characterize molecules. Routinely ¹H-¹H and ¹H-X dipolar couplings are used to characterize the structure and dynamics of molecules. Selective proton recoupling techniques are emerging as a method for structural characterization via estimation of qualitative and quantitative distances. In the present study, we demonstrate through numerical simulations and experiments that the well-characterized CN^v_n sequences can also be tailored for selective recoupling of proton spins by employing C elements of the type (β)_Φ(4β)_Φ+π(3β)_Φ. Herein, several CN^v_n sequences were examined through numerical simulations and experiments. C6¹₄ recoupling sequence with a modified POST-element ((β)_Φ(4β)_Φ+π(3β)_Φ) shows selective polarization transfer efficiencies on the order of 40-50% between various proton spin pairs in fully protonated samples at rf amplitudes ranging from 0.3 to 0.8 times the MAS frequency. These selective recoupling sequences have been labeled as frequency-selective-CN^v_n sequences. The extent of selectivity, polarization transfer efficiency and the feasibility of experimentally measuring proton-proton distances in fully protonated samples are explored here. The development of efficient and robust selective ¹H-¹H recoupling experiments is required to structurally characterize molecules without artificial isotope enrichment or the need for diffracting crystals.

Recent developments in deuterium solid-state NMR for the detection of slow motions in proteins

Slow timescale dynamics in proteins are essential for a variety of biological functions spanning ligand binding, enzymatic catalysis, protein folding and misfolding regulations, as well as protein-protein and protein-nucleic acid interactions. In this review, we focus on the experimental and theoretical developments of 2H static NMR methods applicable for studies of microsecond to millisecond motional modes in proteins, particularly rotating frame relaxation dispersion (R1ρ), quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) relaxation dispersion, and quadrupolar chemical exchange saturation transfer NMR experiments (Q-CEST). With applications chosen from amyloid-β fibrils, we show the complementarity of these approaches for elucidating the complexities of conformational ensembles in disordered domains in the non-crystalline solid state, with the employment of selective deuterium labels. Combined with recent advances in relaxation dispersion backbone measurements for 15N/13C/1H nuclei, these techniques provide powerful tools for studies of biologically relevant timescale dynamics in disordered domains in the solid state.

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.

Recent advances in solid-state relaxation dispersion techniques

This review describes two rotating-frame (R_1ρ) relaxation dispersion methods, namely the Bloch-McConnell Relaxation Dispersion and the Near-rotary Resonance Relaxation Dispersion, which enable the study of microsecond time-scale conformational fluctuations in the solid state using magic-angle-spinning nuclear magnetic resonance spectroscopy. The goal is to provide the reader with key ideas, experimental descriptions, and practical considerations associated with R_1ρ measurements that are needed for analyzing relaxation dispersion and quantifying conformational exchange. While the focus is on protein motion, many presented concepts can be equally well adapted to study the microsecond time-scale dynamics of other bio- (e.g. lipids, polysaccharides, nucleic acids), organic (e.g. pharmaceutical compounds), or inorganic molecules (e.g., metal organic frameworks). This article summarizes the essential contributions made by recent theoretical and experimental solid-state NMR studies to our understanding of protein motion. Here we discuss recent advances in fast MAS applications that enable the observation and atomic level characterization of sparsely populated conformational states which are otherwise inaccessible for other experimental methods. Such high-energy states are often associated with protein functions such as molecular recognition, ligand binding, or enzymatic catalysis, as well as with disease-related properties such as misfolding and amyloid formation.

Accessing Methyl Groups in Proteins via 1H-detected MAS Solid-state NMR Spectroscopy Employing Random Protonation

We recently introduced RAP (reduced adjoining protonation) labelling as an easy to implement and cost-effective strategy to yield selectively methyl protonated protein samples. We show here that even though the amount of H2O employed in the bacterial growth medium is rather low, the intensities obtained in MAS solid-state NMR 1H,13C correlation spectra are comparable to spectra obtained for samples in which α-ketoisovalerate was employed as precursor. In addition to correlations for Leu and Val residues, RAP labelled samples yield also resonances for all methyl containing side chains. The labelling scheme has been employed to quantify order parameters, together with the respective asymmetry parameters. We obtain a very good correlation between the order parameters measured using a GlcRAP (glucose carbon source) and a α-ketoisovalerate labelled sample. The labelling scheme holds the potential to be very useful for the collection of long-range distance restraints among side chain atoms. Experiments are demonstrated using RAP and α-ketoisovalerate labelled samples of the α-spectrin SH3 domain, and are applied to fibrils formed from the Alzheimer's disease Aβ1-40 peptide.

MAS dependent sensitivity of different isotopomers in selectively methyl protonated protein samples in solid state NMR

Sensitivity and resolution together determine the quality of NMR spectra in biological solids. For high-resolution structure determination with solid-state NMR, proton-detection emerged as an attractive strategy in the last few years. Recent progress in probe technology has extended the range of available MAS frequencies up to above 100 kHz, enabling the detection of resolved resonances from sidechain protons, which are important reporters of structure. Here we characterise the interplay between MAS frequency in the newly available range of 70-110 kHz and proton content on the spectral quality obtainable on a 1 GHz spectrometer for methyl resonances. Variable degrees of proton densities are tested on microcrystalline samples of the α-spectrin SH3 domain with selectively protonated methyl isotopomers (CH₃, CH₂D, CHD₂) in a perdeuterated matrix. The experimental results are supported by simulations that allow the prediction of the sensitivity outside this experimental frequency window. Our results facilitate the selection of the appropriate labelling scheme at a given MAS rotation frequency.

Determination of methyl order parameters using solid state NMR under off magic angle spinning

Quantification of dipolar couplings in biological solids is important for the understanding of dynamic processes. Under Magic Angle Spinning (MAS), order parameters are normally obtained by recoupling of anisotropic interactions involving the application of radio frequency pulses. We have recently shown that amide backbone order parameters can be estimated accurately in a spin-echo experiment in case the rotor spinning angle is slightly mis-calibrated. In this work, we apply this method to determine methyl order parameters in a deuterated sample of the SH3 domain of chicken α-spectrin in which the methyl containing side chains valine and leucine are selectively protonated.

Resolution enhancement and proton proximity probed by 3D TQ/DQ/SQ proton NMR spectroscopy under ultrafast magic-angle-spinning beyond 70 kHz

Proton nuclear magnetic resonance (NMR) in solid state has gained significant attention in recent years due to the remarkable resolution and sensitivity enhancement afforded by ultrafast magic-angle-spinning (MAS). In spite of the substantial suppression of ¹H-¹H dipolar couplings, the proton spectral resolution is still poor compared to that of ¹³C or ¹⁵N NMR, rendering it challenging for the structural and conformational analysis of complex chemicals or biological solids. Herein, by utilizing the benefits of double-quantum (DQ) and triple-quantum (TQ) coherences, we propose a 3D single-channel pulse sequence that correlates proton triple-quantum/double-quantum/single-quantum (TQ/DQ/SQ) chemical shifts. In addition to the two-spin proximity information, this 3D TQ/DQ/SQ pulse sequence enables more reliable extraction of three-spin proximity information compared to the regular 2D TQ/SQ correlation experiment, which could aid in revealing the proton network in solids. Furthermore, the TQ/DQ slice taken at a specific SQ chemical shift only reveals the local correlations to the corresponding SQ chemical shift, and thus it enables accurate assignments of the proton peaks along the TQ and DQ dimensions and simplifies the interpretation of proton spectra especially for dense proton networks. The high performance of this 3D pulse sequence is well demonstrated on small compounds, L-alanine and a tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (MLF). We expect that this new methodology can inspire the development of multidimensional solid-state NMR pulse sequences using the merits of TQ and DQ coherences and enable high-throughput investigations of complex solids using abundant protons.

Quo Vadis Biomolecular NMR Spectroscopy?

In-cell nuclear magnetic resonance (NMR) spectroscopy offers the possibility to study proteins and other biomolecules at atomic resolution directly in cells. As such, it provides compelling means to complement existing tools in cellular structural biology. Given the dominance of electron microscopy (EM)-based methods in current structure determination routines, I share my personal view about the role of biomolecular NMR spectroscopy in the aftermath of the revolution in resolution. Specifically, I focus on spin-off applications that in-cell NMR has helped to develop and how they may provide broader and more generally applicable routes for future NMR investigations. I discuss the use of ‘static’ and time-resolved solution NMR spectroscopy to detect post-translational protein modifications (PTMs) and to investigate structural consequences that occur in their response. I argue that available examples vindicate the need for collective and systematic efforts to determine post-translationally modified protein structures in the future. Furthermore, I explain my reasoning behind a Quinary Structure Assessment (QSA) initiative to interrogate cellular effects on protein dynamics and transient interactions present in physiological environments.

Accurate Determination of 1 H-15 N Dipolar Couplings Using Inaccurate Settings of the Magic Angle in Solid-State NMR Spectroscopy

Magic-angle spinning (MAS) is an essential ingredient in a wide variety of solid-state NMR experiments. The standard procedures to adjust the rotor angle are not highly accurate, resulting in a slight misadjustment of the rotor from the magic angle ( θ R L = tan - 1 2 ) on the order of a few millidegrees. This small missetting has no significant impact on the overall spectral resolution, but is sufficient to reintroduce anisotropic interactions. Shown here is that site-specific ¹ H-¹⁵ N dipolar couplings can be accurately measured in a heavily deuterated protein. This method can be applied at arbitrarily high MAS frequencies, since neither rotor synchronization nor particularly high radiofrequency field strengths are required. The off-MAS method allows the quantification of order parameters for very dynamic residues, which often escape an analysis using existing methods.

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.

Formation of the β-barrel assembly machinery complex in lipid bilayers as seen by solid-state NMR

The β-barrel assembly machinery (BAM) is a pentameric complex (BamA–E), which catalyzes the essential process of β-barrel protein insertion into the outer membrane of E. coli. Thus far, a detailed understanding of the insertion mechanism has been elusive but recent results suggest that local protein motion, in addition to the surrounding membrane environment, may be of critical relevance. We have devised a high-sensitivity solid-state NMR approach to directly probe protein motion and the structural changes associated with BAM complex assembly in lipid bilayers. Our results reveal how essential BamA domains, such as the interface formed by the polypeptide transport associated domains P4 and P5 become stabilized after complex formation and suggest that BamA β-barrel opening and P5 reorientation is directly related to complex formation in membranes. Both the lateral gate, as well as P5, exhibit local dynamics, a property that could play an integral role in substrate recognition and insertion.

The β-barrel assembly machinery (BAM) catalyzes β-barrel protein insertion into the outer membrane of E.coli. Here authors employ high-sensitivity solid-state NMR to reveal how the lipid environment and formation of the BamA-BamCDE complex affect BamA structure and dynamics with regards to the lateral gate and the β-barrel associated domains.

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.

Combined 1H-Detected Solid-State NMR Spectroscopy and Electron Cryotomography to Study Membrane Proteins across Resolutions in Native Environments

Membrane proteins remain challenging targets for structural biology, despite much effort, as their native environment is heterogeneous and complex. Most methods rely on detergents to extract membrane proteins from their native environment, but this removal can significantly alter the structure and function of these proteins. Here, we overcome these challenges with a hybrid method to study membrane proteins in their native membranes, combining high-resolution solid-state nuclear magnetic resonance spectroscopy and electron cryotomography using the same sample. Our method allows the structure and function of membrane proteins to be studied in their native environments, across different spatial and temporal resolutions, and the combination is more powerful than each technique individually. We use the method to demonstrate that the bacterial membrane protein YidC adopts a different conformation in native membranes and that substrate binding to YidC in these native membranes differs from purified and reconstituted systems.

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CryoET and ssNMR give complementary information about proteins in native membranes

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One sample can be prepared for both methods without the use of detergents

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Hybrid method shows differences between purified and native preparations of YidC

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Sample preparation reduces costs and time and suggests new strategy for assignment

Rapid Quantitative Measurements of Paramagnetic Relaxation Enhancements in Cu(II)-Tagged Proteins by Proton-Detected Solid-State NMR Spectroscopy

We demonstrate rapid quantitative measurements of site-resolved paramagnetic relaxation enhancements (PREs), which are a source of valuable structural restraints corresponding to electron-nucleus distances in the ∼10-20 Å regime, in solid-state nuclear magnetic resonance (NMR) spectra of proteins containing covalent Cu²⁺-binding tags. Specifically, using protein GB1 K28C-EDTA-Cu²⁺ mutant as a model, we show the determination of backbone amide ¹⁵N longitudinal and ¹H transverse PREs within a few hours of experiment time based on proton-detected 2D or 3D correlation spectra recorded with magic-angle spinning frequencies ≥ ∼ 60 kHz for samples containing ∼10-50 nanomoles of ²H,¹³C,¹⁵N-labeled protein back-exchanged in H₂O. Additionally, we show that the electron relaxation time for the Cu²⁺ center, needed to convert PREs into distances, can be estimated directly from the experimental data. Altogether, these results are important for establishing solid-state NMR based on paramagnetic-tagging as a routine tool for structure determination of natively diamagnetic proteins.

Design and construction of a quadruple-resonance MAS NMR probe for investigation of extensively deuterated biomolecules

Extensive deuteration is frequently used in solid-state NMR studies of biomolecules because it dramatically reduces both homonuclear (¹H-¹H) and heteronuclear (¹H-¹³C and ¹H-¹⁵N) dipolar interactions. This approach greatly improves resolution, enables low-power rf decoupling, and facilitates ¹H-detected experiments even in rigid solids at moderate MAS rates. However, the resolution enhancement is obtained at some cost due the reduced abundance of protons available for polarization transfer. Although deuterium is a useful spin-1 NMR nucleus, in typical experiments the deuterons are not directly utilized because the available probes are usually triple-tuned to ¹H,¹³C and ¹⁵N. Here we describe a ¹H/¹³C/²H/¹⁵N MAS ssNMR probe designed for solid-state NMR of extensively deuterated biomolecules. The probe utilizes coaxial coils, with a modified Alderman-Grant resonator for the ¹H channel, and a multiply resonant solenoid for ¹³C/²H/¹⁵N. A coaxial tuning-tube design is used for all four channels in order to efficiently utilize the constrained physical space available inside the magnet bore. Isolation among the channels is likewise achieved using short, adjustable transmission line elements. We present benchmarks illustrating the tuning of each channel and isolation among them and the magnetic field profiles at each frequency of interest. Finally, representative NMR data are shown demonstrating the performance of both the detection and decoupling circuits.

Electrostatic Constraints Assessed by 1H MAS NMR Illuminate Differences in Crystalline Polymorphs

Atomically resolved crystal structures not only suffer from the inherent uncertainty in accurately locating H atoms but also are incapable of fully revealing the underlying forces enabling the formation of final structures. Therefore, the development and application of novel techniques to illuminate intermolecular forces in crystalline solids are highly relevant to understand the role of hydrogen atoms in structure adoption. Novel developments in ¹H NMR MAS methodology can now achieve robust measurements of ¹H chemical shift anisotropy (CSA) tensors which are highly sensitive to electrostatics. Herein, we use ¹H CSA tensors, measured by MAS experiments and characterized using DFT calculations, to reveal the structure-driving factors between the two polymorphic forms of acetaminophen (aka Tylenol or paracetamol) including differences in hydrogen bonding and the role of aromatic interactions. We demonstrate how the ¹H CSAs can provide additional insights into the static picture provided by diffraction to elucidate rigid molecules.

Limits of Resolution and Sensitivity of Proton Detected MAS Solid-State NMR Experiments at 111 kHz in Deuterated and Protonated Proteins

MAS solid-state NMR is capable of determining structures of protonated solid proteins using proton-detected experiments. These experiments are performed at MAS rotation frequency of around 110 kHz, employing 0.5 mg of material. Here, we compare ¹H, ¹³C correlation spectra obtained from protonated and deuterated microcrystalline proteins at MAS rotation frequency of 111 kHz, and show that the spectral quality obtained from deuterated samples is superior to those acquired using protonated samples in terms of resolution and sensitivity. In comparison to protonated samples, spectra obtained from deuterated samples yield a gain in resolution on the order of 3 and 2 in the proton and carbon dimensions, respectively. Additionally, the spectrum from the deuterated sample yields approximately 2–3 times more sensitivity compared to the spectrum of a protonated sample. This gain could be further increased by a factor of 2 by making use of stereospecific precursors for biosynthesis. Although the overall resolution and sensitivity of ¹H, ¹³C correlation spectra obtained using protonated solid samples with rotation frequencies on the order of 110 kHz is high, the spectral quality is still poor when compared to the deuterated samples. We believe that experiments involving large protein complexes in which sensitivity is limiting will benefit from the application of deuteration schemes.

Structural biology of supramolecular assemblies by magic-angle spinning NMR spectroscopy

In recent years, exciting developments in instrument technology and experimental methodology have advanced the field of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) to new heights. Contemporary MAS NMR yields atomic-level insights into structure and dynamics of an astounding range of biological systems, many of which cannot be studied by other methods. With the advent of fast MAS, proton detection, and novel pulse sequences, large supramolecular assemblies, such as cytoskeletal proteins and intact viruses, are now accessible for detailed analysis. In this review, we will discuss the current MAS NMR methodologies that enable characterization of complex biomolecular systems and will present examples of applications to several classes of assemblies comprising bacterial and mammalian cytoskeleton as well as human immunodeficiency virus 1 and bacteriophage viruses. The body of work reviewed herein is representative of the recent advancements in the field, with respect to the complexity of the systems studied, the quality of the data, and the significance to the biology.

Five decades of homonuclear dipolar decoupling in solid-state NMR: Status and outlook

It has been slightly more than fifty years since the first homonuclear spin decoupling scheme, Lee-Goldburg decoupling, was proposed for removing homonuclear dipolar interactions in solid-state nuclear magnetic resonance. A family of such schemes has made observation of high-resolution NMR spectra of abundant spins possible in various applications in solid state. This review outlines the strategies used in this field and the future prospects of homonuclear spin decoupling in solid-state NMR.

Nuclear magnetic resonance (NMR) applied to membrane–protein complexes

Increasing evidence suggests that most proteins occur and function in complexes rather than as isolated entities when embedded in cellular membranes. Nuclear magnetic resonance (NMR) provides increasing possibilities to study structure, dynamics and assembly of such systems. In our review, we discuss recent methodological progress to study membrane–protein complexes (MPCs) by NMR, starting with expression, isotope-labeling and reconstitution protocols. We review approaches to deal with spectral complexity and limited spectral spectroscopic sensitivity that are usually encountered in NMR-based studies of MPCs. We highlight NMR applications in various classes of MPCs, including G-protein-coupled receptors, ion channels and retinal proteins and extend our discussion to protein–protein complexes that span entire cellular compartments or orchestrate processes such as protein transport across or within membranes. These examples demonstrate the growing potential of NMR-based studies of MPCs to provide critical insight into the energetics of protein–ligand and protein–protein interactions that underlie essential biological functions in cellular membranes.

(1)H-detected solid-state NMR of proteins entrapped in bioinspired silica: a new tool for biomaterials characterization

Proton-detection in solid-state NMR, enabled by high magnetic fields (>18 T) and fast magic angle spinning (>50 kHz), allows for the acquisition of traditional (1)H-(15)N experiments on systems that are too big to be observed in solution. Among those, proteins entrapped in a bioinspired silica matrix are an attractive target that is receiving a large share of attention. We demonstrate that (1)H-detected SSNMR provides a novel approach to the rapid assessment of structural integrity in proteins entrapped in bioinspired silica.

Immobilization of soluble protein complexes in MAS solid-state NMR: Sedimentation versus viscosity

In recent years, MAS solid-state NMR has emerged as a technique for the investigation of soluble protein complexes. It was found that high molecular weight complexes do not need to be crystallized in order to obtain an immobilized sample for solid-state NMR investigations. Sedimentation induced by sample rotation impairs rotational diffusion of proteins and enables efficient dipolar coupling based cross polarization transfers. In addition, viscosity contributes to the immobilization of the molecules in the sample. Natural Deep Eutectic Solvents (NADES) have very high viscosities, and can replace water in living organisms. We observe a considerable amount of cross polarization transfers for NADES solvents, even though their molecular weight is too low to yield significant sedimentation. We discuss how viscosity and sedimentation both affect the quality of the obtained experimental spectra. The FROSTY/sedNMR approach holds the potential to study large protein complexes, which are otherwise not amenable for a structural characterization using NMR. We show that using this method, backbone assignments of the symmetric proteasome activator complex (1.1MDa), and high quality correlation spectra of non-symmetric protein complexes such as the prokaryotic ribosome 50S large subunit binding to trigger factor (1.4MDa) are obtained.

Proton-detected 3D (15)N/(1)H/(1)H isotropic/anisotropic/isotropic chemical shift correlation solid-state NMR at 70kHz MAS

Chemical shift anisotropy (CSA) tensors offer a wealth of information for structural and dynamics studies of a variety of chemical and biological systems. In particular, CSA of amide protons can provide piercing insights into hydrogen-bonding interactions that vary with the backbone conformation of a protein and dynamics. However, the narrow span of amide proton resonances makes it very difficult to measure (1)H CSAs of proteins even by using the recently proposed 2D (1)H/(1)H anisotropic/isotropic chemical shift (CSA/CS) correlation technique. Such difficulties due to overlapping proton resonances can in general be overcome by utilizing the broad span of isotropic chemical shifts of low-gamma nuclei like (15)N. In this context, we demonstrate a proton-detected 3D (15)N/(1)H/(1)H CS/CSA/CS correlation experiment at fast MAS frequency (70kHz) to measure (1)H CSA values of unresolved amide protons of N-acetyl-(15)N-l-valyl-(15)N-l-leucine (NAVL).

Constant-time 2D and 3D through-bond correlation NMR spectroscopy of solids under 60 kHz MAS

Establishing connectivity and proximity of nuclei is an important step in elucidating the structure and dynamics of molecules in solids using magic angle spinning (MAS) NMR spectroscopy. Although recent studies have successfully demonstrated the feasibility of proton-detected multidimensional solid-state NMR experiments under ultrafast-MAS frequencies and obtaining high-resolution spectral lines of protons, assignment of proton resonances is a major challenge. In this study, we first re-visit and demonstrate the feasibility of 2D constant-time uniform-sign cross-peak correlation (CTUC-COSY) NMR experiment on rigid solids under ultrafast-MAS conditions, where the sensitivity of the experiment is enhanced by the reduced spin-spin relaxation rate and the use of low radio-frequency power for heteronuclear decoupling during the evolution intervals of the pulse sequence. In addition, we experimentally demonstrate the performance of a proton-detected pulse sequence to obtain a 3D (1)H/(13)C/(1)H chemical shift correlation spectrum by incorporating an additional cross-polarization period in the CTUC-COSY pulse sequence to enable proton chemical shift evolution and proton detection in the incrementable t1 and t3 periods, respectively. In addition to through-space and through-bond (13)C/(1)H and (13)C/(13)C chemical shift correlations, the 3D (1)H/(13)C/(1)H experiment also provides a COSY-type (1)H/(1)H chemical shift correlation spectrum, where only the chemical shifts of those protons, which are bonded to two neighboring carbons, are correlated. By extracting 2D F1/F3 slices ((1)H/(1)H chemical shift correlation spectrum) at different (13)C chemical shift frequencies from the 3D (1)H/(13)C/(1)H spectrum, resonances of proton atoms located close to a specific carbon atom can be identified. Overall, the through-bond and through-space homonuclear/heteronuclear proximities determined from the 3D (1)H/(13)C/(1)H experiment would be useful to study the structure and dynamics of a variety of chemical and biological solids.

Analysis of local molecular motions of aromatic sidechains in proteins by 2D and 3D fast MAS NMR spectroscopy and quantum mechanical calculations

We report a new multidimensional magic angle spinning NMR methodology, which provides an accurate and detailed probe of molecular motions occurring on timescales of nano- to microseconds, in sidechains of proteins. The approach is based on a 3D CPVC-RFDR correlation experiment recorded under fast MAS conditions (ν(R) = 62 kHz), where (13)C-(1)H CPVC dipolar lineshapes are recorded in a chemical shift resolved manner. The power of the technique is demonstrated in model tripeptide Tyr-(d)Ala-Phe and two nanocrystalline proteins, GB1 and LC8. We demonstrate that, through numerical simulations of dipolar lineshapes of aromatic sidechains, their detailed dynamic profile, i.e., the motional modes, is obtained. In GB1 and LC8 the results unequivocally indicate that a number of aromatic residues are dynamic, and using quantum mechanical calculations, we correlate the molecular motions of aromatic groups to their local environment in the crystal lattice. The approach presented here is general and can be readily extended to other biological systems.

Proton-detected 3D (1)H/(13)C/(1)H correlation experiment for structural analysis in rigid solids under ultrafast-MAS above 60 kHz

A proton-detected 3D (1)H/(13)C/(1)H chemical shift correlation experiment is proposed for the assignment of chemical shift resonances, identification of (13)C-(1)H connectivities, and proximities of (13)C-(1)H and (1)H-(1)H nuclei under ultrafast magic-angle-spinning (ultrafast-MAS) conditions. Ultrafast-MAS is used to suppress all anisotropic interactions including (1)H-(1)H dipolar couplings, while the finite-pulse radio frequency driven dipolar recoupling (fp-RFDR) pulse sequence is used to recouple dipolar couplings among protons and the insensitive nuclei enhanced by polarization transfer technique is used to transfer magnetization between heteronuclear spins. The 3D experiment eliminates signals from non-carbon-bonded protons and non-proton-bonded carbons to enhance spectral resolution. The 2D (F1/F3) (1)H/(1)H and 2D (13)C/(1)H (F2/F3) chemical shift correlation spectra extracted from the 3D spectrum enable the identification of (1)H-(1)H proximity and (13)C-(1)H connectivity. In addition, the 2D (F1/F2) (1)H/(13)C chemical shift correlation spectrum, incorporated with proton magnetization exchange via the fp-RFDR recoupling of (1)H-(1)H dipolar couplings, enables the measurement of proximities between (13)C and even the remote non-carbon-bonded protons. The 3D experiment also gives three-spin proximities of (1)H-(1)H-(13)C chains. Experimental results obtained from powder samples of L-alanine and L-histidine ⋅ H2O ⋅ HCl demonstrate the efficiency of the 3D experiment.

Proton chemical shift tensors determined by 3D ultrafast MAS double-quantum NMR spectroscopy

Proton NMR spectroscopy in the solid state has recently attracted much attention owing to the significant enhancement in spectral resolution afforded by the remarkable advances in ultrafast magic angle spinning (MAS) capabilities. In particular, proton chemical shift anisotropy (CSA) has become an important tool for obtaining specific insights into inter/intra-molecular hydrogen bonding. However, even at the highest currently feasible spinning frequencies (110-120 kHz), (1)H MAS NMR spectra of rigid solids still suffer from poor resolution and severe peak overlap caused by the strong (1)H-(1)H homonuclear dipolar couplings and narrow (1)H chemical shift (CS) ranges, which render it difficult to determine the CSA of specific proton sites in the standard CSA/single-quantum (SQ) chemical shift correlation experiment. Herein, we propose a three-dimensional (3D) (1)H double-quantum (DQ) chemical shift/CSA/SQ chemical shift correlation experiment to extract the CS tensors of proton sites whose signals are not well resolved along the single-quantum chemical shift dimension. As extracted from the 3D spectrum, the F1/F3 (DQ/SQ) projection provides valuable information about (1)H-(1)H proximities, which might also reveal the hydrogen-bonding connectivities. In addition, the F2/F3 (CSA/SQ) correlation spectrum, which is similar to the regular 2D CSA/SQ correlation experiment, yields chemical shift anisotropic line shapes at different isotropic chemical shifts. More importantly, since the F2/F1 (CSA/DQ) spectrum correlates the CSA with the DQ signal induced by two neighboring proton sites, the CSA spectrum sliced at a specific DQ chemical shift position contains the CSA information of two neighboring spins indicated by the DQ chemical shift. If these two spins have different CS tensors, both tensors can be extracted by numerical fitting. We believe that this robust and elegant single-channel proton-based 3D experiment provides useful atomistic-level structural and dynamical information for a variety of solid systems that possess high proton density.

(15)N CSA tensors and (15)N-(1)H dipolar couplings of protein hydrophobic core residues investigated by static solid-state NMR

In this work, we assess the usefulness of static (15)N NMR techniques for the determination of the (15)N chemical shift anisotropy (CSA) tensor parameters and (15)N-(1)H dipolar splittings in powder protein samples. By using five single labeled samples of the villin headpiece subdomain protein in a hydrated lyophilized powder state, we determine the backbone (15)N CSA tensors at two temperatures, 22 and -35 °C, in order to get a snapshot of the variability across the residues and as a function of temperature. All sites probed belonged to the hydrophobic core and most of them were part of α-helical regions. The values of the anisotropy (which include the effect of the dynamics) varied between 130 and 156 ppm at 22 °C, while the values of the asymmetry were in the 0.32-0.082 range. The Leu-75 and Leu-61 backbone sites exhibited high mobility based on the values of their temperature-dependent anisotropy parameters. Under the assumption that most differences stem from dynamics, we obtained the values of the motional order parameters for the (15)N backbone sites. While a simple one-dimensional line shape experiment was used for the determination of the (15)N CSA parameters, a more advanced approach based on the "magic sandwich" SAMMY pulse sequence (Nevzorov and Opella, 2003) was employed for the determination of the (15)N-(1)H dipolar patterns, which yielded estimates of the dipolar couplings. Accordingly, the motional order parameters for the dipolar interaction were obtained. It was found that the order parameters from the CSA and dipolar measurements are highly correlated, validating that the variability between the residues is governed by the differences in dynamics. The values of the parameters obtained in this work can serve as reference values for developing more advanced magic-angle spinning recoupling techniques for multiple labeled samples.