Sensitivity and resolution of proton detected spectra of a deuterated protein at 40 and 60 kHz magic-angle-spinning

Deuteration of proteins boosted by cell lysates: high-resolution amide and Hα magic-angle-spinning (MAS) NMR without the reprotonation bottleneck

Amide-proton-detected magic-angle-spinning NMR of deuterated proteins has become a main technique in NMR-based structural biology. In standard deuteration protocols that rely on D2 O-based culture media, non-exchangeable amide sites remain deuterated, making these sites unobservable. Here we demonstrate that proteins produced with a H2 O-based culture medium doped with deuterated cell lysate allow scientists to overcome this "reprotonation bottleneck" while retaining a high level of deuteration (ca. 80 %) and narrow linewidths. We quantified coherence lifetimes of several proteins prepared with this labeling pattern over a range of magic-angle-spinning (MAS) frequencies (40-100 kHz). We demonstrate that under commonly used conditions (50-60 kHz MAS), the amide1 H linewidths with our labeling approach are comparable to those of perdeuterated proteins and better than those of protonated samples at 100 kHz. For three proteins in the 33-50 kDa size range, many previously unobserved amides become visible. We report how to prepare the deuterated cell lysate for our approach from fractions of perdeuterated cultures which are usually discarded, and we show that such media can be used identically to commercial media. The residual protonation of H α sites allows for well-resolved H α -detected spectra and H α resonance assignment, exemplified by the de novo assignment of 168 H α sites in a 39 kDa protein. The approach based on this H2 O/cell-lysate deuteration and MAS frequencies compatible with 1.3 or 1.9 mm rotors presents a strong sensitivity benefit over 0.7 mm 100 kHz MAS experiments.

3D-Printable centrifugal devices for biomolecular solid state NMR rotors

The advent of magic angle spinning (MAS) rates exceeding 100 kHz has facilitated the acquisition of 1H-detected solid-state NMR spectra of biomolecules with high resolution. However, challenges can arise when preparing rotors for these experiments, due to the physical properties of biomolecular solid samples and the small dimensions of the rotors. In this study, we have designed 3D-printable centrifugal devices that facilitate efficient and consistent packing of crystalline protein slurries or viscous phospholipids into 0.7 mm rotors. We demonstrate the efficacy of these packing devices using 1H-detected solid state NMR at 105 kHz. In addition to devices for 0.7 mm rotors, we have also developed devices for other frequently employed rotor sizes and styles. We have made all our designs openly accessible, and we encourage their usage and ongoing development as a shared effort within the solid state NMR community.

Site-specific protein backbone deuterium 2Hα quadrupolar patterns by proton-detected quadruple-resonance 3D 2HαcαNH MAS NMR spectroscopy

A novel deuterium-excited and proton-detected quadruple-resonance three-dimensional (3D) ²H_αc_αNH MAS nuclear magnetic resonance (NMR) method is presented to obtain site-specific ²H_α deuterium quadrupolar couplings from protein backbone, as an extension to the 2D version of the experiment reported earlier. Proton-detection results in high sensitivity compared to the heteronuclei detection methods. Utilizing four independent radiofrequency (RF) channels (quadruple-resonance), we managed to excite the ²H_α, then transfer deuterium polarization to its attached C_α, followed by polarization transfers to the neighboring backbone nitrogen and then to the amide proton for detection. This experiment results in an easy to interpret HSQC-like 2D ¹H-¹⁵N fingerprint NMR spectrum, which contains site-specific deuterium quadrupolar patterns in the indirect third dimension. Provided that four-channel NMR probe technology is available, the setup of the ²H_αc_αNH experiment is relatively straightforward, by using low power deuterium excitation and polarization transfer schemes we have been developing. To our knowledge, this is the first demonstration of a quadruple-resonance MAS NMR experiment to link ²H_α quadrupolar couplings to proton-detection, extending our previous triple-resonance demonstrations. Distortion-free excitation and polarization transfer of ∼160-170 kHz ²H_α quadrupolar coupling were presented by using a deuterium RF strength of ∼20 kHz. From these ²H_α patterns, an average backbone order parameter of S = 0.92 was determined on a deuterated SH3 sample, with an average η = 0.22. These indicate that SH3 backbone represents sizable dynamics in the microsecond timescale where the ²H_α lineshape is sensitive. Moreover, site-specific ²H_α T₁ relaxation times were obtained for a proof of concept. This 3D ²H_αc_αNH NMR experiment has the potential to determine structure and dynamics of perdeuterated proteins by utilizing deuterium as a novel reporter.

Site-specific protein methyl deuterium quadrupolar patterns by proton-detected 3D 2H-13C-1H MAS NMR spectroscopy

Determination of protein structure and dynamics is key to understand the mechanism of protein action. Perdeuterated proteins have been used to obtain high resolution/sensitivty NMR experiments via proton-detection. These methods utilizes ¹H, ¹³C and ¹⁵N nuclei for chemical shift dispersion or relaxation probes, despite the existing abundant deuterons. However, a high-sensitivity NMR method to utilize deuterons and e.g. determine site-specific deuterium quadrupolar pattern information has been lacking due to technical difficulties associated with deuterium's large quadrupolar couplings. Here, we present a novel deuterium-excited and proton-detected three-dimensional ²H-¹³C-¹H MAS NMR experiment to utilize deuterons and to obtain site-specific methyl ²H quadrupolar patterns on detuterated proteins for the first time. A high-resolution fingerprint ¹H-¹⁵N HSQC-spectrum is correlated with the anisotropic deuterium quadrupolar tensor in the third dimension. Results from a model perdeuterated protein has been shown.

Dynamics of uniformly labelled solid proteins between 100 and 300 K: A 2D 2H-13C MAS NMR approach

We describe a ²H based MAS nuclear magnetic resonance (NMR) method to obtain site-specific molecular dynamics of biomolecules. The method utilizes the use of deuterium nucleus as a spin label that is proven to be very useful in dynamics studies of solid biological and functional materials. The aim is to understand overall characteristics of protein backbone and side-chain motions for CD₃, CD₂ and CD groups, in terms of timescale, type and activation energy of the underlying processes. Variable temperature two-dimensional (2D) ²H-¹³C correlation MAS NMR spectra were recorded for the uniformly ²H,¹³C,¹⁵N labelled Alanine and microcrystalline SH3 at a broad temperature range, from 320 K down to 100 K. First, the deuterium quadrupolar-coupling constant from specific D-C sites is obtained with the 2D experiment by utilizing carbon chemical shifts. Second, the static quadrupolar patterns are obtained at 100 K. Third, variable temperature approach enabled the observation of quadrupolar pattern over different motional regimes; slow, intermediate and fast. And finally, the apparent activation energies for C-D sites are determined and compared, by evaluating the temperature induced signal intensities. This information led to the determination of the dynamic processes for different D-C sites at a broad range of temperature and motional timescales. This is a first representation of 2D ²H-¹³C MAS NMR approach applied to fully isotope labelled deuterated protein covering 220 K temperature range.

Proton Detected Solid-State NMR of Membrane Proteins at 28 Tesla (1.2 GHz) and 100 kHz Magic-Angle Spinning

The available magnetic field strength for high resolution NMR in persistent superconducting magnets has recently improved from 23.5 to 28 Tesla, increasing the proton resonance frequency from 1 to 1.2 GHz. For magic-angle spinning (MAS) NMR, this is expected to improve resolution, provided the sample preparation results in homogeneous broadening. We compare two-dimensional (2D) proton detected MAS NMR spectra of four membrane proteins at 950 and 1200 MHz. We find a consistent improvement in resolution that scales superlinearly with the increase in magnetic field for three of the four examples. In 3D and 4D spectra, which are now routinely acquired, this improvement indicates the ability to resolve at least 2 and 2.5 times as many signals, respectively.

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.

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.

Collective exchange processes reveal an active site proton cage in bacteriorhodopsin

Proton translocation across membranes is vital to all kingdoms of life. Mechanistically, it relies on characteristic proton flows and modifications of hydrogen bonding patterns, termed protonation dynamics, which can be directly observed by fast magic angle spinning (MAS) NMR. Here, we demonstrate that reversible proton displacement in the active site of bacteriorhodopsin already takes place in its equilibrated dark-state, providing new information on the underlying hydrogen exchange processes. In particular, MAS NMR reveals proton exchange at D85 and the retinal Schiff base, suggesting a tautomeric equilibrium and thus partial ionization of D85. We provide evidence for a proton cage and detect a preformed proton path between D85 and the proton shuttle R82. The protons at D96 and D85 exchange with water, in line with ab initio molecular dynamics simulations. We propose that retinal isomerization makes the observed proton exchange processes irreversible and delivers a proton towards the extracellular release site.

Daniel Friedrich et al. show that reversible proton translocation occurs in the dark–state of bacteriorhodopsin, involving the retinal Schiff base and D85 exchanging protons with H₂O. They find evidence of an active site proton cage and possible proton transfer via R82.

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.

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.

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.

Advances in instrumentation and methodology for solid-state NMR of biological assemblies

Many advances in instrumentation and methodology have furthered the use of solid-state NMR as a technique for determining the structures and studying the dynamics of molecules involved in complex biological assemblies. Solid-state NMR does not require large crystals, has no inherent size limit, and with appropriate isotopic labeling schemes, supports solving one component of a complex assembly at a time. It is complementary to cryo-EM, in that it provides local, atomic-level detail that can be modeled into larger-scale structures. This review focuses on the development of high-field MAS instrumentation and methodology; including probe design, benchmarking strategies, labeling schemes, and experiments that enable the use of quadrupolar nuclei in biomolecular NMR. Current challenges facing solid-state NMR of biological assemblies and new directions in this dynamic research area are also discussed.

Rapid access to RNA resonances by proton-detected solid-state NMR at >100 kHz MAS

Fast (>100 kHz) magic angle spinning solid-state NMR allows combining high-sensitive proton detection with the absence of an intrinsic molecular weight limit. Here we apply this technique to RNA and assign nucleotide spin systems through highly sensitive multidimensional experiments.

Fast (>100 kHz) magic angle spinning solid-state NMR allows combining high-sensitive proton detection with the absence of an intrinsic molecular weight limit. Using this technique we observe for the first time narrow ¹H RNA resonances and assign nucleotide spin systems with only 200 μg of uniformly ¹³C,¹⁵N-labelled RNA.

Solid-state MAS NMR resonance assignment methods for proteins

The prerequisite to structural or functional studies of proteins by NMR is generally the assignment of resonances. Since the first assignment of proteins by solid-state MAS NMR was conducted almost two decades ago, a wide variety of different pulse sequences and methods have been proposed and continue to be developed. Traditionally, a variety of 2D and 3D ¹³C-detected experiments have been used for the assignment of backbone and side-chain ¹³C and ¹⁵N resonances. These methods have found widespread use across the field. But as the hardware has changed and higher spinning frequencies and magnetic fields are becoming available, the ability to use direct proton detection is opening up a new set of assignment methods based on triple-resonance experiments. This review describes solid-state MAS NMR assignment methods using carbon detection and proton detection at different deuteration levels. The use of different isotopic labelling schemes as an aid to assignment in difficult cases is discussed as well as the increasing number of software packages that support manual and automated resonance assignment.

New applications of solid-state NMR in structural biology

Various recent developments in solid-state nuclear magnetic resonance (ssNMR) spectroscopy have enabled an array of new insights regarding the structure, dynamics, and interactions of biomolecules. In the ever more integrated world of structural biology, ssNMR studies provide structural and dynamic information that is complementary to the data accessible by other means. ssNMR enables the study of samples lacking a crystalline lattice, featuring static as well as dynamic disorder, and does so independent of higher-order symmetry. The present study surveys recent applications of biomolecular ssNMR and examines how this technique is increasingly integrated with other structural biology techniques, such as (cryo) electron microscopy, solution-state NMR, and X-ray crystallography. Traditional ssNMR targets include lipid bilayer membranes and membrane proteins in a lipid bilayer environment. Another classic application has been in the area of protein misfolding and aggregation disorders, where ssNMR has provided essential structural data on oligomers and amyloid fibril aggregates. More recently, the application of ssNMR has expanded to a growing array of biological assemblies, ranging from non-amyloid protein aggregates, protein–protein complexes, viral capsids, and many others. Across these areas, multidimensional magic angle spinning (MAS) ssNMR has, in the last decade, revealed three-dimensional structures, including many that had been inaccessible by other structural biology techniques. Equally important insights in structural and molecular biology derive from the ability of MAS ssNMR to probe information beyond comprehensive protein structures, such as dynamics, solvent exposure, protein–protein interfaces, and substrate–enzyme interactions.

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.

Interaction of Monomeric Interleukin-8 with CXCR1 Mapped by Proton-Detected Fast MAS Solid-State NMR

The human chemokine interleukin-8 (IL-8; CXCL8) is a key mediator of innate immune and inflammatory responses. This small, soluble protein triggers a host of biological effects upon binding and activating CXCR1, a G protein-coupled receptor, located in the cell membrane of neutrophils. Here, we describe ¹H-detected magic angle spinning solid-state NMR studies of monomeric IL-8 (1-66) bound to full-length and truncated constructs of CXCR1 in phospholipid bilayers under physiological conditions. Cross-polarization experiments demonstrate that most backbone amide sites of IL-8 (1-66) are immobilized and that their chemical shifts are perturbed upon binding to CXCR1, demonstrating that the dynamics and environments of chemokine residues are affected by interactions with the chemokine receptor. Comparisons of spectra of IL-8 (1-66) bound to full-length CXCR1 (1-350) and to N-terminal truncated construct NT-CXCR1 (39-350) identify specific chemokine residues involved in interactions with binding sites associated with N-terminal residues (binding site-I) and extracellular loop and helical residues (binding site-II) of the receptor. Intermolecular paramagnetic relaxation enhancement broadening of IL-8 (1-66) signals results from interactions of the chemokine with CXCR1 (1-350) containing Mn²⁺ chelated to an unnatural amino acid assists in the characterization of the receptor-bound form of the chemokine.

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.

Measurement of amide proton chemical shift anisotropy in perdeuterated proteins using CSA amplification

Measuring ¹H chemical shift anisotropy (CSA) is useful for probing proton environments and dynamics but remains a challenge due to strong homonuclear interaction and relatively small shift anisotropy, especially in proteins with multiple proton sites. Here the extended chemical shift anisotropy amplification (xCSA) method is applied for amide proton CSA measurement in uniformly ²H enriched proteins under fast magic angle spinning. The xCSA method is capable of distinguishing the sign of the CSA asymmetry parameter, complimenting other multiple-pulse recoupling methods. A three-dimensional xCSA experiment is demonstrated for measuring the proton CSA of amide sites in aGB1 protein sample and the possible correlation of amide proton CSA with protein secondary structure is discussed.

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.

Backbone assignment of perdeuterated proteins by solid-state NMR using proton detection and ultrafast magic-angle spinning

Solid-state NMR (ssNMR) is a technique that allows the study of protein structure and dynamics at atomic detail. In contrast to X-ray crystallography and cryo-electron microscopy, proteins can be studied under physiological conditions-for example, in a lipid bilayer and at room temperature (0-35 °C). However, ssNMR requires considerable amounts (milligram quantities) of isotopically labeled samples. In recent years, ¹H-detection of perdeuterated protein samples has been proposed as a method of alleviating the sensitivity issue. Such methods are, however, substantially more demanding to the spectroscopist, as compared with traditional ¹³C-detected approaches. As a guide, this protocol describes a procedure for the chemical shift assignment of the backbone atoms of proteins in the solid state by ¹H-detected ssNMR. It requires a perdeuterated, uniformly ¹³C- and ¹⁵N-labeled protein sample with subsequent proton back-exchange to the labile sites. The sample needs to be spun at a minimum of 40 kHz in the NMR spectrometer. With a minimal set of five 3D NMR spectra, the protein backbone and some of the side-chain atoms can be completely assigned. These spectra correlate resonances within one amino acid residue and between neighboring residues; taken together, these correlations allow for complete chemical shift assignment via a 'backbone walk'. This results in a backbone chemical shift table, which is the basis for further analysis of the protein structure and/or dynamics by ssNMR. Depending on the spectral quality and complexity of the protein, data acquisition and analysis are possible within 2 months.

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.

Structure of fully protonated proteins by proton-detected magic-angle spinning NMR

Protein structure determination by proton-detected magic-angle spinning (MAS) NMR has focused on highly deuterated samples, in which only a small number of protons are introduced and observation of signals from side chains is extremely limited. Here, we show in two fully protonated proteins that, at 100-kHz MAS and above, spectral resolution is high enough to detect resolved correlations from amide and side-chain protons of all residue types, and to reliably measure a dense network of (1)H-(1)H proximities that define a protein structure. The high data quality allowed the correct identification of internuclear distance restraints encoded in 3D spectra with automated data analysis, resulting in accurate, unbiased, and fast structure determination. Additionally, we find that narrower proton resonance lines, longer coherence lifetimes, and improved magnetization transfer offset the reduced sample size at 100-kHz spinning and above. Less than 2 weeks of experiment time and a single 0.5-mg sample was sufficient for the acquisition of all data necessary for backbone and side-chain resonance assignment and unsupervised structure determination. We expect the technique to pave the way for atomic-resolution structure analysis applicable to a wide range of proteins.

Atomic-resolution structure of cytoskeletal bactofilin by solid-state NMR

Bactofilins are a recently discovered class of cytoskeletal proteins of which no atomic-resolution structure has been reported thus far. The bacterial cytoskeleton plays an essential role in a wide range of processes, including morphogenesis, cell division, and motility. Among the cytoskeletal proteins, the bactofilins are bacteria-specific and do not have a eukaryotic counterpart. The bactofilin BacA of the species Caulobacter crescentus is not amenable to study by x-ray crystallography or solution nuclear magnetic resonance (NMR) because of its inherent noncrystallinity and insolubility. We present the atomic structure of BacA calculated from solid-state NMR-derived distance restraints. We show that the core domain of BacA forms a right-handed β helix with six windings and a triangular hydrophobic core. The BacA structure was determined to 1.0 Å precision (heavy-atom root mean square deviation) on the basis of unambiguous restraints derived from four-dimensional (4D) HN-HN and 2D C-C NMR spectra.

Protein residue linking in a single spectrum for magic-angle spinning NMR assignment

Here we introduce a new pulse sequence for resonance assignment that halves the number of data sets required for sequential linking by directly correlating sequential amide resonances in a single diagonal-free spectrum. The method is demonstrated with both microcrystalline and sedimented deuterated proteins spinning at 60 and 111 kHz, and a fully protonated microcrystalline protein spinning at 111 kHz, with as little as 0.5 mg protein sample. We find that amide signals have a low chance of ambiguous linkage, which is further improved by linking in both forward and backward directions. The spectra obtained are amenable to automated resonance assignment using general-purpose software such as UNIO-MATCH.

High-resolution proton-detected NMR of proteins at very fast MAS

When combined with high-frequency (currently ∼60 kHz) magic-angle spinning (MAS), proton detection boosts sensitivity and increases coherence lifetimes, resulting in narrow ((1))H lines. Herein, we review methods for efficient proton detected techniques and applications in highly deuterated proteins, with an emphasis on 100% selected ((1))H site concentration for the purpose of sensitivity. We discuss the factors affecting resolution and sensitivity that have resulted in higher and higher frequency MAS. Next we describe the various methods that have been used for backbone and side-chain assignment with proton detection, highlighting the efficient use of scalar-based ((13))C-((13))C transfers. Additionally, we show new spectra making use of these schemes for side-chain assignment of methyl ((13))C-((1))H resonances. The rapid acquisition of resolved 2D spectra with proton detection allows efficient measurement of relaxation parameters used as a measure of dynamic processes. Under rapid MAS, relaxation times can be measured in a site-specific manner in medium-sized proteins, enabling the investigation of molecular motions at high resolution. Additionally, we discuss methods for measurement of structural parameters, including measurement of internuclear ((1))H-((1))H contacts and the use of paramagnetic effects in the determination of global structure.