Sensitivity enhancement in (13)C solid-state NMR of protein microcrystals by use of paramagnetic metal ions for optimizing (1)H T(1) relaxation

Solvent paramagnetic relaxation enhancement as a versatile method for studying structure and dynamics of biomolecular systems

Solvent paramagnetic relaxation enhancement (sPRE) is a versatile nuclear magnetic resonance (NMR)-based method that allows characterization of the structure and dynamics of biomolecular systems through providing quantitative experimental information on solvent accessibility of NMR-active nuclei. Addition of soluble paramagnetic probes to the solution of a biomolecule leads to paramagnetic relaxation enhancement in a concentration-dependent manner. Here we review recent progress in the sPRE-based characterization of structural and dynamic properties of biomolecules and their complexes, and aim to deliver a comprehensive illustration of a growing number of applications of the method to various biological systems. We discuss the physical principles of sPRE measurements and provide an overview of available co-solute paramagnetic probes. We then explore how sPRE, in combination with complementary biophysical techniques, can further advance biomolecular structure determination, identification of interaction surfaces within protein complexes, and probing of conformational changes and low-population transient states, as well as deliver insights into weak, nonspecific, and transient interactions between proteins and co-solutes. In addition, we present examples of how the incorporation of solvent paramagnetic probes can improve the sensitivity of NMR experiments and discuss the prospects of applying sPRE to NMR metabolomics, drug discovery, and the study of intrinsically disordered proteins.

19F Solid-state NMR characterization of pharmaceutical solids

Solid-state NMR has been increasingly recognized as a high-resolution and versatile spectroscopic tool to characterize drug substances and products. However, the analysis of pharmaceutical materials is often carried out at natural isotopic abundance and a relatively low drug loading in multi-component systems and therefore suffers from challenges of low sensitivity. The fact that fluorinated therapeutics are well represented in pipeline drugs and commercial products offers an excellent opportunity to utilize fluorine as a molecular probe for pharmaceutical analysis. We aim to review recent advancements of ¹⁹F magic angle spinning NMR methods in modern drug research and development. Applications to polymorph screening at the micromolar level, structural elucidation, and investigation of molecular interactions at the Ångström to submicron resolution in drug delivery, stability, and quality will be discussed.

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.

On-Site Measurement of Fat and Protein Contents in Milk Using Mobile NMR Technology

Robust and easy-to-use NMR sensor technology is proposed for accurate, on-site determination of fat and protein contents in milk. The two parameters are determined using fast consecutive ¹H and ³⁵Cl low-field NMR experiments on milk samples upon the 1:1 addition of a low-cost contrast solution. Reliable and accurate measurements are obtained without tedious calibrations and the need for extensive database information and may readily be conducted by non-experts in production site environments. This enables on-site application at farms or dairies, or use in laboratories harvesting significant reductions in costs and time per analysis as compared to wet-chemistry analysis. The performance is demonstrated for calibration samples, various supermarket milk products, and raw milk samples, of which some were analyzed directly in the milking room. To illustrate the wide application range, the supermarket milk products included both conventionally/organically produced, lactose-free milk, cow's, sheep's and goat's milk, homogenized and unhomogenized milk, and a broad nutrient range (0.1-9% fat, 1-6% protein). Excellent agreement between NMR measurements and reference values, without corrections or changes in calibration for various products and during extensive periods of experiment conduction (4 months) demonstrates the robustness of the procedure and instrumentation. For the raw milk samples, correlations between NMR and IR, NMR and wet-chemistry, as well as IR and wet-chemistry results, show that NMR, in terms of accuracy, compares favorably with the other methods.

Solid-state 17O NMR study of α-d-glucose: exploring new frontiers in isotopic labeling, sensitivity enhancement, and NMR crystallography

We report the first “total synthesis” of 17O-labeled d-glucose and its solid-state 17O NMR characterization with unprecedented sensitivity and resolution.

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.

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

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

Paramagnetic solid-state NMR of proteins

The paramagnetic properties of metal ions and stable radicals can affect NMR spectra, which can lead to changes in peak intensities, relaxation times and chemical shifts. The changes from paramagnetic effects provide intriguing opportunities for solid-state NMR studies of proteins. In this review, we summarized the trends and progress of paramagnetic solid-state NMR of proteins in the past decade, and showed that paramagnetic effects have great potential applications for sensitivity enhancement, structure determination and topological analysis for microcrystalline proteins, protein complexes, protein aggregates and membrane proteins.

Use of paramagnetic systems to speed-up NMR data acquisition and for structural and dynamic studies

NMR spectroscopy is a powerful experimental technique to study biological systems at the atomic resolution. However, its intrinsic low sensitivity results in long acquisition times that in extreme cases lasts for days (or even weeks) often exceeding the lifetime of the sample under investigation. Different paramagnetic agents have been used in an effort to decrease the spin-lattice (T1) relaxation times of the studied nuclei, which are the main cause for long acquisition times necessary for signal averaging to enhance the signal-to-noise ratio of NMR spectra. Consequently, most of the experimental time is "wasted" in waiting for the magnetization to recover between successive scans. In this review, we discuss how to set up an optimal paramagnetic relaxation enhancement (PRE) system to effectively reduce the T1 relaxation times avoiding significant broadening of NMR signals. Additionally, we describe how PRE-agents can be used to provide structural and dynamic information and can even be used to follow the intermediates of chemical reactions and to speed-up data acquisition. We also describe the unique challenges and benefits associated with the application of PRE to solid-state NMR spectroscopy, explaining how the use of PREs is more complex for membrane mimetic systems as PREs can also be exploited to change the alignment of oriented membrane systems. Functionalization of membrane mimetics, such as bicelles, can provide a controlled region of paramagnetic effect that has the potential, together with the desired alignment, to provide crucial biologically relevant structural information. And finally, we discuss how paramagnetic metals can be utilized to further increase the dynamic nuclear polarization (DNP) effects and how to preserve the enhancements when dissolution DNP is implemented.

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.

Quantifying Microsecond Exchange in Large Protein Complexes with Accelerated Relaxation Dispersion Experiments in the Solid State

Solid state NMR is a powerful method to obtain information on the structure and dynamics of protein complexes that, due to solubility and size limitations, cannot be achieved by other methods. Here, we present an approach that allows the quantification of microsecond conformational exchange in large protein complexes by using a paramagnetic agent to accelerate ¹⁵N R_1ρ relaxation dispersion measurements and overcome sensitivity limitations. The method is validated on crystalline GB1 and then applied to a >300 kDa precipitated complex of GB1 with full length human immunoglobulin G (IgG). The addition of a paramagnetic agent increased the signal to noise ratio per time unit by a factor of 5, which allowed full relaxation dispersion curves to be recorded on a sample containing less than 50 μg of labelled material in 5 and 10 days on 850 and 700 MHz spectrometers, respectively. We discover a similar exchange process across the β-sheet in GB1 in crystals and in complex with IgG. However, the slow motion observed for a number of residues in the α-helix of crystalline GB1 is not detected in the complex.

Cysteine-ethylation of tissue-extracted membrane proteins as a tool to detect conformational states by solid-state NMR spectroscopy

Solid-state NMR (ssNMR) is an ideal tool to study structure and dynamics of membrane proteins in their native lipid environment. In principle, ssNMR has no size limitations. However, this feature is rarely exploited as large membrane proteins display severe resonance overlap. In addition, dismal yields from recombinant bacterial expression systems limit severely spectroscopic characterization of membrane proteins. For very large mammalian membrane proteins, extraction from the original organism remains the most viable approach. In this case, NMR-observable nuclei must be introduced post-translationally, but the approaches developed so far are rather scarce. Here, we detail the synthesis and engineering of a reactive ¹³C-ethylmethanethiosulfonate (¹³C-EMTS) reagent for the post-translational alkylation of cysteine sidechains of a 110kDa sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) extracted from rabbit skeletal muscle tissue. When reconstituted into liposomes, it is possible to resolve the resonances of the engineered ethyl groups by magic-angle spinning (MAS) 2D [¹³C,¹³C]-DARR experiments. Notably, the ethyl-group modification does not perturb the function of SERCA, yielding well-resolved ¹³C-¹³C fingerprints that are used to image its structural states in the catalytic cycle and filtering out overwhelming naturally-abundant ¹³C nuclei signals arising from the enzyme and lipids. We anticipate that this approach will be used together with ¹⁹F NMR to monitor conformational transitions of enzymes and proteins that are difficult to express recombinantly.

Monitoring fast chemical processes by reaction-interrupted excitation transfer (ExTra) NMR spectroscopy

ExTra NMR utilises selectively inverted magnetisation for in situ monitoring of fast chemical processes.

Optorelaxers: Achieving real-time control of NMR relaxation

We present an approach to increase the detection sensitivity of NMR by shortening the spin-lattice relaxation time using transient paramagnetic species created by light irradiation of "optorelaxer" molecules. In the ultimate implementation of this concept, not yet realized here, these transient species are absent during the detection period, thereby avoiding the loss of spectral resolution caused by inhomogeneous broadening from paramagnetic species. Real-time control of NMR relaxation by visible light is demonstrated with Fe(II)(ptz)₆(BF₄)₂, (ptz = 1-propyltetrazole), abbreviated FePTZ. Illumination of FePTZ at 30 K results in a decrease of the ¹H NMR spin-lattice relaxation time T₁ due to formation of a high spin photoexcited state. The ¹H NMR of polystyrene containing a low concentration of FePTZ molecules shows a similar reduction in T₁, establishing that FePTZ can act as an optorelaxer for the protons of a matrix. Numerical modeling of the spin-diffusion processes from the protons in a FePTZ core to those in a shell of polystyrene accounts for the observed T₁ effects under both dark and light conditions. Additionally, ¹H MAS (magic-angle spinning) NMR results for pure FePTZ provide information on the isotropic and anisotropic portions of the electron-nuclear hyperfine interactions.

Reduction of the 13C cross-polarization experimental time for pharmaceutical samples with long T1 by ball milling in solid-state NMR

Many pharmaceutical samples have notably long ¹H T₁ (proton spin-lattice relaxation time), leading to lengthy experiments lasting several days in solid-state NMR studies. In this work, we propose the use of ball milling on the pharmaceutical samples to reduce the ¹H T₁, which also leads to enhanced sensitivity in {¹H}-¹³C Cross-Polarization (CP) experiments due to reduced particle sizes and increased surface areas of the samples. Experimentally, we determined that depending on the substrates and milling time, the signal-to-noise ratio (S/N) of a 1D ¹³C CP spectrum can be increased by a factor of 3-6, which means that the experimental time can be shortened by a factor of 9-36. Furthermore, the application of simple ball-milling within a short time avoids the amorphization of the studied samples such that no signal due to amorphous state is observed in the ¹³C CP spectrum. This simple ball milling method used for sensitivity enhancement can be further applied in the SS-NMR studies of pharmaceutical samples.

13C → 1H transfer of light-induced hyperpolarization allows for selective detection of protons in frozen photosynthetic reaction center

In the present study, we exploit the light-induced hyperpolarization occurring on ¹³C nuclei due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect to boost the NMR signal intensity of selected protons via inverse cross-polarization. Such hyperpolarization transfer is implemented into ¹H-detected two-dimensional ¹³C-¹H correlation magic-angle-spinning (MAS) NMR experiment to study protons in frozen photosynthetic reaction centers (RCs). As a first trial, the performance of such an experiment is tested on selectively ¹³C labeled RCs from the purple bacteria of Rhodobacter sphaeroides. We observed response from the protons belonging to the photochemically active cofactors in their native protein environment. Such an approach is a potential heteronuclear spin-torch experiment which could be complementary to the classical heteronuclear correlation (HETCOR) experiments for mapping proton chemical shifts of photosynthetic cofactors and to understand the role of the proton pool around the electron donors in the electron transfer process occurring during photosynthesis.

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.

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

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

Assignment of solid-state 13C and 1H NMR spectra of paramagnetic Ni(II) acetylacetonate complexes aided by first-principles computations

Recent advances in computational methodology allowed for first-principles calculations of the nuclear shielding tensor for a series of paramagnetic nickel(II) acetylacetonate complexes, [Ni(acac)₂L₂] with L = H₂O, D₂O, NH₃, ND₃, and PMe₂Ph have provided detailed insight into the origin of the paramagnetic contributions to the total shift tensor. This was employed for the assignment of the solid-state ^1,2H and ¹³C MAS NMR spectra of these compounds. The two major contributions to the isotropic shifts are by orbital (diamagnetic-like) and contact mechanism. The orbital shielding, contact, as well as dipolar terms all contribute to the anisotropic component. The calculations suggest reassignment of the ¹³C methyl and carbonyl resonances in the acac ligand [Inorg. Chem.53, 2014, 399] leading to isotropic paramagnetic shifts of δ(¹³C) ≈ 800-1100 ppm and ≈180-300 ppm for ¹³C for the methyl and carbonyl carbons located three and two bonds away from the paramagnetic Ni(II) ion, respectively. Assignment using three different empirical correlations, i.e., paramagnetic shifts, shift anisotropy, and relaxation (T₁) were ambiguous, however the latter two support the computational results. Thus, solid-state NMR spectroscopy in combination with modern quantum-chemical calculations of paramagnetic shifts constitutes a promising tool for structural investigations of metal complexes and materials.

Characterization of Protein-Protein Interfaces in Large Complexes by Solid-State NMR Solvent Paramagnetic Relaxation Enhancements

Solid-state NMR is becoming a viable alternative for obtaining information about structures and dynamics of large biomolecular complexes, including ones that are not accessible to other high-resolution biophysical techniques. In this context, methods for probing protein–protein interfaces at atomic resolution are highly desirable. Solvent paramagnetic relaxation enhancements (sPREs) proved to be a powerful method for probing protein–protein interfaces in large complexes in solution but have not been employed toward this goal in the solid state. We demonstrate that ¹H and ¹⁵N relaxation-based sPREs provide a powerful tool for characterizing intermolecular interactions in large assemblies in the solid state. We present approaches for measuring sPREs in practically the entire range of magic angle spinning frequencies used for biomolecular studies and discuss their benefits and limitations. We validate the approach on crystalline GB1, with our experimental results in good agreement with theoretical predictions. Finally, we use sPREs to characterize protein–protein interfaces in the GB1 complex with immunoglobulin G (IgG). Our results suggest the potential existence of an additional binding site and provide new insights into GB1:IgG complex structure that amend and revise the current model available from studies with IgG fragments. We demonstrate sPREs as a practical, widely applicable, robust, and very sensitive technique for determining intermolecular interaction interfaces in large biomolecular complexes in the solid state.

Gd3+-chelated lipid accelerates solid-state NMR spectroscopy of seven-transmembrane proteins

Solid-state NMR (SSNMR) is an attractive technique for studying large membrane proteins in membrane-mimetic environments. However, SSNMR experiments often suffer from low efficiency, due to the inherent low sensitivity and the long recycle delays needed to recover the magnetization. Here we demonstrate that the incorporation of a small amount of a Gd³⁺-chelated lipid, Gd³⁺-DMPE-DTPA, into proteoliposomes greatly shortens the spin-lattice relaxation time (¹H-T ₁) of lipid-reconstituted membrane proteins and accelerates the data collection. This effect has been evaluated on a 30 kDa, seven-transmembrane protein, Leptosphaeria rhodopsin. With the Gd³⁺-chelated lipid, we can perform 2D SSNMR experiments 3 times faster than by diamagnetic control. By combining this paramagnetic relaxation-assisted data collection with non-uniform sampling, the 3D experimental times are reduced eightfold with respect to traditional 3D experiments on diamagnetic samples. A comparison between the paramagnetic relaxation enhancement (PRE) effects of Cu²⁺- and Gd³⁺-chelated lipids indicates the much higher relaxivity of the latter. Hence, a tenfold lower concentration is needed for Gd³⁺-chelated lipids to achieve comparable PRE effects to Cu²⁺-chelated lipids. In addition, Gd³⁺-chelated lipids neither alter the protein structures nor induce significant line-width broadening of the protein signals. This work is expected to be beneficial for structural and dynamic studies of large membrane proteins by SSNMR.

Exploring the salt-cocrystal continuum with solid-state NMR using natural-abundance samples: implications for crystal engineering

There has been significant recent interest in differentiating multicomponent solid forms, such as salts and cocrystals, and, where appropriate, in determining the position of the proton in the X-H⋯A-YX-⋯H-A+-Y continuum in these systems, owing to the direct relationship of this property to the clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In the present study, solid forms of simple cocrystals/salts were investigated by high-field (700 MHz) solid-state NMR (ssNMR) using samples with naturally abundant 15N nuclei. Four model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting {PyN⋯H-O-}/{PyN+-H⋯O-} hydrogen bonds (Py is pyridine) were selected and prepared. The crystal structures were determined at both low and room temperature using X-ray diffraction. The H-atom positions were determined by measuring the 15N-1H distances through 15N-1H dipolar interactions using two-dimensional inversely proton-detected cross polarization with variable contact-time (invCP-VC) 1H→15N→1H experiments at ultrafast (νR ≥ 60-70 kHz) magic angle spinning (MAS) frequency. It is observed that this method is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises. This work, while carried out on simple systems, has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of APIs is considered seriously.

Sensitivity enhancement for membrane proteins reconstituted in parallel and perpendicular oriented bicelles obtained by using repetitive cross-polarization and membrane-incorporated free radicals

Multidimensional separated local-field and spin-exchange experiments employed by oriented-sample solid-state NMR are essential for structure determination and spectroscopic assignment of membrane proteins reconstituted in macroscopically aligned lipid bilayers. However, these experiments typically require a large number of scans in order to establish interspin correlations. Here we have shown that a combination of optimized repetitive cross polarization (REP-CP) and membrane-embedded free radicals allows one to enhance the signal-to-noise ratio by factors 2.4-3.0 in the case of Pf1 coat protein reconstituted in magnetically aligned bicelles with their normals being either parallel or perpendicular to the main magnetic field. Notably, spectral resolution is not affected at the 2:1 radical-to-protein ratio. Spectroscopic assignment of Pf1 coat protein in the parallel bicelles has been established as an illustration of the method. The proposed methodology will advance applications of oriented-sample NMR technique when applied to samples containing smaller quantities of proteins and three-dimensional experiments.

Fast magic-angle sample spinning solid-state NMR at 60-100kHz for natural abundance samples

In spite of tremendous progress made in pulse sequence designs and sophisticated hardware developments, methods to improve sensitivity and resolution in solid-state NMR (ssNMR) are still emerging. The rate at which sample is spun at magic angle determines the extent to which sensitivity and resolution of NMR spectra are improved. To this end, the prime objective of this article is to give a comprehensive theoretical and experimental framework of fast magic angle spinning (MAS) technique. The engineering design of fast MAS rotors based on spinning rate, sample volume, and sensitivity is presented in detail. Besides, the benefits of fast MAS citing the recent progress in methodology, especially for natural abundance samples are also highlighted. The effect of the MAS rate on (1)H resolution, which is a key to the success of the (1)H inverse detection methods, is described by a simple mathematical factor named as the homogeneity factor k. A comparison between various (1)H inverse detection methods is also presented. Moreover, methods to reduce the number of spinning sidebands (SSBs) for the systems with huge anisotropies in combination with (1)H inverse detection at fast MAS are discussed.

Heating and temperature gradients of lipid bilayer samples induced by RF irradiation in MAS solid-state NMR experiments

The MAS solid-state NMR has been a powerful technique for studying membrane proteins within the native-like lipid bilayer environment. In general, RF irradiation in MAS NMR experiments can heat and potentially destroy expensive membrane protein samples. However, under practical MAS NMR experimental conditions, detailed characterization of RF heating effect of lipid bilayer samples is still lacking. Herein, using ¹ H chemical shift of water for temperature calibration, we systematically study the dependence of RF heating on hydration levels and salt concentrations of three lipids in MAS NMR experiments. Under practical ¹ H decoupling conditions used in biological MAS NMR experiments, three lipids show different dependence of RF heating on hydration levels as well as salt concentrations, which are closely associated with the properties of lipids. The maximum temperature elevation of about 10 °C is similar for the three lipids containing 200% hydration, which is much lower than that in static solid-state NMR experiments. The RF heating due to salt is observed to be less than that due to hydration, with a maximum temperature elevation of less than 4 °C in the hydrated samples containing 120 mmol l^-1 of salt. Upon RF irradiation, the temperature gradient across the sample is observed to be greatly increased up to 20 °C, as demonstrated by the remarkable broadening of ¹ H signal of water. Based on detailed characterization of RF heating effect, we demonstrate that RF heating and temperature gradient can be significantly reduced by decreasing the hydration levels of lipid bilayer samples from 200% to 30%. Copyright © 2016 John Wiley & Sons, Ltd.

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

One of the fundamental challenges in the application of solid-state NMR is its limited sensitivity, yet a majority of experiments do not make efficient use of the limited polarization available. The loss in polarization in a single acquisition experiment is mandated by the need to select out a single coherence pathway. In contrast, sequential acquisition strategies can encode more than one pathway in the same experiment or recover unused polarization to supplement a standard experiment. In this article, we present pulse sequences that implement sequential acquisition strategies on one and two radiofrequency channels with a combination of proton and carbon detection to record multiple experiments under magic-angle spinning. We show that complementary 2D experiments such as [Formula: see text] and [Formula: see text] or DARR and [Formula: see text], and 3D experiments such as [Formula: see text] and [Formula: see text], or [Formula: see text] and [Formula: see text]  can be combined in a single experiment to ensure time savings of at least 40 %. These experiments can be done under fast or slow-moderate magic-angle spinning frequencies aided by windowed [Formula: see text] acquisition and homonulcear decoupling. The pulse sequence suite is further expanded by including pathways that allow the recovery of residual polarization, the so-called 'afterglow' pathways, to encode a number of pulse sequences to aid in assignments and chemical-shift mapping.

Evolution of CPMAS under fast magic-angle-spinning at 100 kHz and beyond

This article describes recent trends of high-field solid-state NMR (SSNMR) experiments for small organic molecules and biomolecules using (13)C and (15)N CPMAS under ultra-fast MAS at a spinning speed (νR) of 80-100kHz. First, we illustrate major differences between a modern low-power RF scheme using UFMAS in an ultra-high field and a traditional CPMAS scheme using a moderate sample spinning in a lower field. Features and sensitivity advantage of a low-power RF scheme using UFMAS and a small sample coil are summarized for CPMAS-based experiments. Our 1D (13)C CPMAS experiments for uniformly (13)C- and (15)N-labeled alanine demonstrated that the sensitivity per given sample amount obtained at νR of 100kHz and a (1)H NMR frequency (νH) of 750.1MHz is ~10 fold higher than that of a traditional CPMAS experiment obtained at νR of 20kHz and νH of 400.2MHz. A comparison of different (1)H-decoupling schemes in CPMAS at νR of 100kHz for the same sample demonstrated that low-power WALTZ-16 decoupling unexpectedly displayed superior performance over traditional low-power schemes designed for SSNMR such as TPPM and XiX in a range of decoupling field strengths of 5-20kHz. Excellent (1)H decoupling performance of WALTZ-16 was confirmed on a protein microcrystal sample of GB1 at νR of 80kHz. We also discuss the feasibility of a SSNMR microanalysis of a GB1 protein sample in a scale of 1nmol to 80nmol by (1)H-detected 2D (15)N/(1)H SSNMR by a synergetic use of a high field, a low-power RF scheme, a paramagnetic-assisted condensed data collection (PACC), and UFMAS.

Increasing the sensitivity of NMR diffusion measurements by paramagnetic longitudinal relaxation enhancement, with application to ribosome-nascent chain complexes

The translational diffusion of macromolecules can be examined non-invasively by stimulated echo (STE) NMR experiments to accurately determine their molecular sizes. These measurements can be important probes of intermolecular interactions and protein folding and unfolding, and are crucial in monitoring the integrity of large macromolecular assemblies such as ribosome-nascent chain complexes (RNCs). However, NMR studies of these complexes can be severely constrained by their slow tumbling, low solubility (with maximum concentrations of up to 10 μM), and short lifetimes resulting in weak signal, and therefore continuing improvements in experimental sensitivity are essential. Here we explore the use of the paramagnetic longitudinal relaxation enhancement (PLRE) agent NiDO2A on the sensitivity of (15)N XSTE and SORDID heteronuclear STE experiments, which can be used to monitor the integrity of these unstable complexes. We exploit the dependence of the PLRE effect on the gyromagnetic ratio and electronic relaxation time to accelerate recovery of (1)H magnetization without adversely affecting storage on N z during diffusion delays or introducing significant transverse relaxation line broadening. By applying the longitudinal relaxation-optimized SORDID pulse sequence together with NiDO2A to 70S Escherichia coli ribosomes and RNCs, NMR diffusion sensitivity enhancements of up to 4.5-fold relative to XSTE are achieved, alongside ~1.9-fold improvements in two-dimensional NMR sensitivity, without compromising the sample integrity. We anticipate these results will significantly advance the use of NMR to probe dynamic regions of ribosomes and other large, unstable macromolecular assemblies.

¹³C- and ¹H-detection under fast MAS for the study of poorly available proteins: application to sub-milligram quantities of a 7 trans-membrane protein

We demonstrate that (13)C-detected spectra recorded using fast (60 kHz) magic angle spinning on sub-milligram (<10 μmol) quantities of a protonated 7 trans-membrane helix protein (bacteriorhodopsin) in its native lipid environment are comparable in sensitivity and resolution to those recorded using 15-fold larger sample volumes with conventional solid state NMR methodology. We demonstrate the utility of proton-detected measurements which yield narrow (1)H linewidths under these conditions, and that no structural alterations are observed. We propose that these methods will prove useful to gain structural information on membrane proteins with poor availability, which can be studied in their native lipid environments.

Multiple acquisition of magic angle spinning solid-state NMR experiments using one receiver: application to microcrystalline and membrane protein preparations

Solid-state NMR spectroscopy of proteins is a notoriously low-throughput technique. Relatively low-sensitivity and poor resolution of protein samples require long acquisition times for multidimensional NMR experiments. To speed up data acquisition, we developed a family of experiments called Polarization Optimized Experiments (POE), in which we utilized the orphan spin operators that are discarded in classical multidimensional NMR experiments, recovering them to allow simultaneous acquisition of multiple 2D and 3D experiments, all while using conventional probes with spectrometers equipped with one receiver. POE allow the concatenation of multiple 2D or 3D pulse sequences into a single experiment, thus potentially combining all of the aforementioned advances, boosting the capability of ssNMR spectrometers at least two-fold without the addition of any hardware. In this perspective, we describe the first generation of POE, such as dual acquisition MAS (or DUMAS) methods, and then illustrate the evolution of these experiments into MEIOSIS, a method that enables the simultaneous acquisition of multiple 2D and 3D spectra. Using these new pulse schemes for the solid-state NMR investigation of biopolymers makes it possible to obtain sequential resonance assignments, as well as distance restraints, in about half the experimental time. While designed for acquisition of heteronuclei, these new experiments can be easily implemented for proton detection and coupled with other recent advancements, such as dynamic nuclear polarization (DNP), to improve signal to noise. Finally, we illustrate the application of these methods to microcrystalline protein preparations as well as single and multi-span membrane proteins reconstituted in lipid membranes.

Structural studies of proteins by paramagnetic solid-state NMR spectroscopy

Paramagnetism-based nuclear pseudocontact shifts and spin relaxation enhancements contain a wealth of information in solid-state NMR spectra about electron-nucleus distances on the ∼20 Å length scale, far beyond that normally probed through measurements of nuclear dipolar couplings. Such data are especially vital in the context of structural studies of proteins and other biological molecules that suffer from a sparse number of experimentally-accessible atomic distances constraining their three-dimensional fold or intermolecular interactions. This perspective provides a brief overview of the recent developments and applications of paramagnetic magic-angle spinning NMR to biological systems, with primary focus on the investigations of metalloproteins and natively diamagnetic proteins modified with covalent paramagnetic tags.

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

The use of small rotors capable of very fast magic-angle spinning (MAS) in conjunction with proton dilution by perdeuteration and partial reprotonation at exchangeable sites has enabled the acquisition of resolved, proton detected, solid-state NMR spectra on samples of biological macromolecules. The ability to detect the high-gamma protons, instead of carbons or nitrogens, increases sensitivity. In order to achieve sufficient resolution of the amide proton signals, rotors must be spun at the maximum rate possible given their size and the proton back-exchange percentage tuned. Here we investigate the optimal proton back-exchange ratio for triply labeled SH3 at 40 kHz MAS. We find that spectra acquired on 60 % back-exchanged samples in 1.9 mm rotors have similar resolution at 40 kHz MAS as spectra of 100 % back-exchanged samples in 1.3 mm rotors spinning at 60 kHz MAS, and for (H)NH 2D and (H)CNH 3D spectra, show 10-20 % higher sensitivity. For 100 % back-exchanged samples, the sensitivity in 1.9 mm rotors is superior by a factor of 1.9 in (H)NH and 1.8 in (H)CNH spectra but at lower resolution. For (H)C(C)NH experiments with a carbon-carbon mixing period, this sensitivity gain is lost due to shorter relaxation times and less efficient transfer steps. We present a detailed study on the sensitivity of these types of experiments for both types of rotors, which should enable experimentalists to make an informed decision about which type of rotor is best for specific applications.

Observation of 1H-13C and 1H-1H proximities in a paramagnetic solid by NMR at high magnetic field under ultra-fast MAS

The assignment of NMR signals in paramagnetic solids is often challenging since: (i) the large paramagnetic shifts often mask the diamagnetic shifts specific to the local chemical environment, and (ii) the hyperfine interactions with unpaired electrons broaden the NMR spectra and decrease the coherence lifetime, thus reducing the efficiency of usual homo- and hetero-nuclear NMR correlation experiments. Here we show that the assignment of (1)H and (13)C signals in isotopically unmodified paramagnetic compounds with moderate hyperfine interactions can be facilitated by the use of two two-dimensional (2D) experiments: (i) (1)H-(13)C correlations with (1)H detection and (ii) (1)H-(1)H double-quantum↔single-quantum correlations. These methods are experimentally demonstrated on isotopically unmodified copper (II) complex of l-alanine at high magnetic field (18.8 T) and ultra-fast Magic Angle Spinning (MAS) frequency of 62.5 kHz. Compared to (13)C detection, we show that (1)H detection leads to a 3-fold enhancement in sensitivity for (1)H-(13)C 2D correlation experiments. By combining (1)H-(13)C and (1)H-(1)H 2D correlation experiments with the analysis of (13)C longitudinal relaxation times, we have been able to assign the (1)H and (13)C signals of each l-alanine ligand.

Visualizing transient dark states by NMR spectroscopy

Myriad biological processes proceed through states that defy characterization by conventional atomic-resolution structural biological methods. The invisibility of these 'dark' states can arise from their transient nature, low equilibrium population, large molecular weight, and/or heterogeneity. Although they are invisible, these dark states underlie a range of processes, acting as encounter complexes between proteins and as intermediates in protein folding and aggregation. New methods have made these states accessible to high-resolution analysis by nuclear magnetic resonance (NMR) spectroscopy, as long as the dark state is in dynamic equilibrium with an NMR-visible species. These methods - paramagnetic NMR, relaxation dispersion, saturation transfer, lifetime line broadening, and hydrogen exchange - allow the exploration of otherwise invisible states in exchange with a visible species over a range of timescales, each taking advantage of some unique property of the dark state to amplify its effect on a particular NMR observable. In this review, we introduce these methods and explore two specific techniques - paramagnetic relaxation enhancement and dark state exchange saturation transfer - in greater detail.

Recent advances in magic angle spinning solid state NMR of membrane proteins

Membrane proteins mediate many critical functions in cells. Determining their three-dimensional structures in the native lipid environment has been one of the main objectives in structural biology. There are two major NMR methodologies that allow this objective to be accomplished. Oriented sample NMR, which can be applied to membrane proteins that are uniformly aligned in the magnetic field, has been successful in determining the backbone structures of a handful of membrane proteins. Owing to methodological and technological developments, Magic Angle Spinning (MAS) solid-state NMR (ssNMR) spectroscopy has emerged as another major technique for the complete characterization of the structure and dynamics of membrane proteins. First developed on peptides and small microcrystalline proteins, MAS ssNMR has recently been successfully applied to large membrane proteins. In this review we describe recent progress in MAS ssNMR methodologies, which are now available for studies of membrane protein structure determination, and outline a few examples, which highlight the broad capability of ssNMR spectroscopy.

Acceleration of natural-abundance solid-state MAS NMR measurements on bone by paramagnetic relaxation from gadolinium-DTPA

Reducing the data collection time without affecting the signal intensity and spectral resolution is one of the major challenges for the widespread application of multidimensional nuclear magnetic resonance (NMR) spectroscopy, especially in experiments conducted on complex heterogeneous biological systems such as bone. In most of these experiments, the NMR data collection time is ultimately governed by the proton spin-lattice relaxation times (T1). For over two decades, gadolinium(III)-DTPA (Gd-DTPA, DTPA=Diethylene triamine pentaacetic acid) has been one of the most widely used contrast-enhancement agents in magnetic resonance imaging (MRI). In this study, we demonstrate that Gd-DTPA can also be effectively used to enhance the longitudinal relaxation rates of protons in solid-state NMR experiments conducted on bone without significant line-broadening and chemical-shift-perturbation side effects. Using bovine cortical bone samples incubated in different concentrations of Gd-DTPA complex, the (1)H T1 values were calculated from data collected by (1)H spin-inversion recovery method detected in natural-abundance (13)C cross-polarization magic angle spinning (CPMAS) NMR experiments. Our results reveal that the (1)H T1 values can be successfully reduced by a factor of 3.5 using as low as 10mM Gd-DTPA without reducing the spectral resolution and thus enabling faster data acquisition of the (13)C CPMAS spectra. These results obtained from (13)C-detected CPMAS experiments were further confirmed using (1)H-detected ultrafast MAS experiments on Gd-DTPA doped bone samples. This approach considerably improves the signal-to-noise ratio per unit time of NMR experiments applied to bone samples by reducing the experimental time required to acquire the same number of scans.

Capturing a reactive state of amyloid aggregates: NMR-based characterization of copper-bound Alzheimer disease amyloid β-fibrils in a redox cycle

The interaction of redox-active copper ions with misfolded amyloid β (Aβ) is linked to production of reactive oxygen species (ROS), which has been associated with oxidative stress and neuronal damages in Alzheimer disease. Despite intensive studies, it is still not conclusive how the interaction of Cu(+)/Cu(2+) with Aβ aggregates leads to ROS production even at the in vitro level. In this study, we examined the interaction between Cu(+)/Cu(2+) and Aβ fibrils by solid-state NMR (SSNMR) and other spectroscopic methods. Our photometric studies confirmed the production of ~60 μM hydrogen peroxide (H2O2) from a solution of 20 μM Cu(2+) ions in complex with Aβ(1-40) in fibrils ([Cu(2+)]/[Aβ] = 0.4) within 2 h of incubation after addition of biological reducing agent ascorbate at the physiological concentration (~1 mM). Furthermore, SSNMR (1)H T1 measurements demonstrated that during ROS production the conversion of paramagnetic Cu(2+) into diamagnetic Cu(+) occurs while the reactive Cu(+) ions remain bound to the amyloid fibrils. The results also suggest that O2 is required for rapid recycling of Cu(+) bound to Aβ back to Cu(2+), which allows for continuous production of H2O2. Both (13)C and (15)N SSNMR results show that Cu(+) coordinates to Aβ(1-40) fibrils primarily through the side chain Nδ of both His-13 and His-14, suggesting major rearrangements from the Cu(2+) coordination via Nε in the redox cycle. (13)C SSNMR chemical shift analysis suggests that the overall Aβ conformations are largely unaffected by Cu(+) binding. These results present crucial site-specific evidence of how the full-length Aβ in amyloid fibrils offers catalytic Cu(+) centers.

Paramagnetic doping of a 7TM membrane protein in lipid bilayers by Gd³⁺-complexes for solid-state NMR spectroscopy

A considerable limitation of NMR spectroscopy is its inherent low sensitivity. Approximately 90 % of the measuring time is used by the spin system to return to its Boltzmann equilibrium after excitation, which is determined by (1)H-T1 in cross-polarized solid-state NMR experiments. It has been shown that sample doping by paramagnetic relaxation agents such as Cu(2+)-EDTA accelerates this process considerably resulting in enhanced sensitivity. Here, we extend this concept to Gd(3+)-complexes. Their effect on (1)H-T1 has been assessed on the membrane protein proteorhodopsin, a 7TM light-driven proton pump. A comparison between Gd(3+)-DOTA, Gd(3+)-TTAHA, covalently attached Cu(2+)-EDTA-tags and Cu(2+)-EDTA reveals a 3.2-, 2.6-, 2.4- and 2-fold improved signal-to-noise ratio per unit time due to longitudinal paramagnetic relaxation enhancement. Furthermore, Gd(3+)-DOTA shows a remarkably high relaxivity, which is 77-times higher than that of Cu(2+)-EDTA. Therefore, an order of magnitude lower dopant concentration can be used. In addition, no line-broadening effects or peak shifts have been observed on proteorhodopsin in the presence of Gd(3+)-DOTA. These favourable properties make it very useful for solid-state NMR experiments on membrane proteins.

High-resolution paramagnetically enhanced solid-state NMR spectroscopy of membrane proteins at fast magic angle spinning

Magic angle spinning nuclear magnetic resonance (MAS NMR) is well suited for the study of membrane proteins in membrane mimetic and native membrane environments. These experiments often suffer from low sensitivity, due in part to the long recycle delays required for magnetization and probe recovery, as well as detection of low gamma nuclei. In ultrafast MAS experiments sensitivity can be enhanced through the use of low power sequences combined with paramagnetically enhanced relaxation times to reduce recycle delays, as well as proton detected experiments. In this work we investigate the sensitivity of (13)C and (1)H detected experiments applied to 27 kDa membrane proteins reconstituted in lipids and packed in small 1.3 mm MAS NMR rotors. We demonstrate that spin diffusion is sufficient to uniformly distribute paramagnetic relaxation enhancement provided by either covalently bound or dissolved CuEDTA over 7TM alpha helical membrane proteins. Using paramagnetic enhancement and low power decoupling in carbon detected experiments we can recycle experiments ~13 times faster than under traditional conditions. However, due to the small sample volume the overall sensitivity per unit time is still lower than that seen in the 3.2 mm probe. Proton detected experiments, however, showed increased efficiency and it was found that the 1.3 mm probe could achieve sensitivity comparable to that of the 3.2 mm in a given amount of time. This is an attractive prospect for samples of limited quantity, as this allows for a reduction in the amount of protein that needs to be produced without the necessity for increased experimental time.

Solid state 13C and 2H NMR investigations of paramagnetic [Ni(II)(acac)2L2] complexes

Nine structurally related paramagnetic acetylacetonato nickel(II) complexes: [Ni(acac)2] and trans-[Ni(acac)2(X)2]nH/D2O, X = H2O, D2O, NH3, MeOH, PMePh2, PMe2Ph, or [dppe]1/2, n = 0 or 1, dppe = 1,2-bis(diphenylphosphino)ethane, as well as cis-[Ni(F6-acac)2(D2O)2], F6-acac = hexafluoroacetylonato, have been characterized by solid state (13)C MAS NMR spectroscopy. (2)H MAS NMR was used to probe the local hydrogen bonding network in [Ni(acac)2(D2O)2]D2O and cis-[Ni(F6-acac)2(D2O)2]. The complexes serve to benchmark the paramagnetic shift, which can be associated with the resonances of atoms of the coordinated ligands. The methine (CH) and methyl (CH3) have characteristic combinations of the isotropic shift (δ) and anisotropy parameters (d, η). The size of the anisotropy (d), which is the sum of the chemical shift anisotropy (CSA) and the paramagnetic electron-nuclei dipolar coupling, is much more descriptive than the isotropic shift. Moreover, the CSA is found to constitute up to one-third of the total anisotropy and should be taken into consideration when (13)C anisotropies are used for structure determination of paramagnetic materials. The (13)C MAS NMR spectra of trans-[Ni(acac)2(PMe2Ph)2], trans-[Ni(acac)2(PMePh2)2], and the noncrystallographically characterized trans-[Ni(acac)2(dppe)]n were assigned using these correlations. The complexes with L = H2O, D2O, NH3, and MeOH can be prepared by a series of solid state desorption and sorption reactions. Crystal structures for trans-[Ni(acac)2(NH3)2] and trans-[Ni(acac)2(PMePh2)2] are reported.

Shortening spin-lattice relaxation using a copper-chelated lipid at low-temperatures - A magic angle spinning solid-state NMR study on a membrane-bound protein

Inherent low sensitivity of NMR spectroscopy has been a major disadvantage, especially to study biomolecules like membrane proteins. Recent studies have successfully demonstrated the advantages of performing solid-state NMR experiments at very low and ultralow temperatures to enhance the sensitivity. However, the long spin-lattice relaxation time, T1, at very low temperatures is a major limitation. To overcome this difficulty, we demonstrate the use of a copper-chelated lipid for magic angle spinning solid-state NMR measurements on cytochrome-b5 reconstituted in multilamellar vesicles. Our results on multilamellar vesicles containing as small as 0.5mol% of a copper-chelated lipid can significantly shorten T1 of protons, which can be used to considerably reduce the data collection time or to enhance the signal-to-noise ratio. We also monitored the effect of slow cooling on the resolution and sensitivity of (13)C and (15)N signals from the protein and (13)C signals from lipids.

Sensitivity and resolution enhanced solid-state NMR for paramagnetic systems and biomolecules under very fast magic angle spinning

Recent research in fast magic angle spinning (MAS) methods has drastically improved the resolution and sensitivity of NMR spectroscopy of biomolecules and materials in solids. In this Account, we summarize recent and ongoing developments in this area by presenting (13)C and (1)H solid-state NMR (SSNMR) studies on paramagnetic systems and biomolecules under fast MAS from our laboratories. First, we describe how very fast MAS (VFMAS) at the spinning speed of at least 20 kHz allows us to overcome major difficulties in (1)H and (13)C high-resolution SSNMR of paramagnetic systems. As a result, we can enhance both sensitivity and resolution by up to a few orders of magnitude. Using fast recycling (∼ms/scan) with short (1)H T1 values, we can perform (1)H SSNMR microanalysis of paramagnetic systems on the microgram scale with greatly improved sensitivity over that observed for diamagnetic systems. Second, we discuss how VFMAS at a spinning speed greater than ∼40 kHz can enhance the sensitivity and resolution of (13)C biomolecular SSNMR measurements. Low-power (1)H decoupling schemes under VFMAS offer excellent spectral resolution for (13)C SSNMR by nominal (1)H RF irradiation at ∼10 kHz. By combining the VFMAS approach with enhanced (1)H T1 relaxation by paramagnetic doping, we can achieve extremely fast recycling in modern biomolecular SSNMR experiments. Experiments with (13)C-labeled ubiquitin doped with 10 mM Cu-EDTA demonstrate how effectively this new approach, called paramagnetic assisted condensed data collection (PACC), enhances the sensitivity. Lastly, we examine (13)C SSNMR measurements for biomolecules under faster MAS at a higher field. Our preliminary (13)C SSNMR data of Aβ amyloid fibrils and GB1 microcrystals acquired at (1)H NMR frequencies of 750-800 MHz suggest that the combined use of the PACC approach and ultrahigh fields could allow for routine multidimensional SSNMR analyses of proteins at the 50-200 nmol level. Also, we briefly discuss the prospects for studying bimolecules using (13)C SSNMR under ultrafast MAS at the spinning speed of ∼100 kHz.

Magic angle spinning NMR of paramagnetic proteins

Metal ions are ubiquitous in biochemical and cellular processes. Since many metal ions are paramagnetic due to the presence of unpaired electrons, paramagnetic molecules are an important class of targets for research in structural biology and related fields. Today, NMR spectroscopy plays a central role in the investigation of the structure and chemical properties of paramagnetic metalloproteins, linking the observed paramagnetic phenomena directly to electronic and molecular structure. A major step forward in the study of proteins by solid-state NMR came with the advent of ultrafast magic angle spinning (MAS) and the ability to use (1)H detection. Combined, these techniques have allowed investigators to observe nuclei that previously were invisible in highly paramagnetic metalloproteins. In addition, these techniques have enabled quantitative site-specific measurement of a variety of long-range paramagnetic effects. Instead of limiting solid-state NMR studies of biological systems, paramagnetism provides an information-rich phenomenon that can be exploited in these studies. This Account emphasizes state-of-the-art methods and applications of solid-state NMR in paramagnetic systems in biological chemistry. In particular, we discuss the use of ultrafast MAS and (1)H-detection in perdeuterated paramagnetic metalloproteins. Current methodology allows us to determine the structure and dynamics of metalloenzymes, and, as an example, we describe solid-state NMR studies of microcrystalline superoxide dismutase, a 32 kDa dimer. Data were acquired with remarkably short times, and these experiments required only a few milligrams of sample.

Designing dipolar recoupling and decoupling experiments for biological solid-state NMR using interleaved continuous wave and RF pulse irradiation

Rapid developments in solid-state NMR methodology have boosted this technique into a highly versatile tool for structural biology. The invention of increasingly advanced rf pulse sequences that take advantage of better hardware and sample preparation have played an important part in these advances. In the development of these new pulse sequences, researchers have taken advantage of analytical tools, such as average Hamiltonian theory or lately numerical methods based on optimal control theory. In this Account, we focus on the interplay between these strategies in the systematic development of simple pulse sequences that combines continuous wave (CW) irradiation with short pulses to obtain improved rf pulse, recoupling, sampling, and decoupling performance. Our initial work on this problem focused on the challenges associated with the increasing use of fully or partly deuterated proteins to obtain high-resolution, liquid-state-like solid-state NMR spectra. Here we exploit the overwhelming presence of (2)H in such samples as a source of polarization and to gain structural information. The (2)H nuclei possess dominant quadrupolar couplings which complicate even the simplest operations, such as rf pulses and polarization transfer to surrounding nuclei. Using optimal control and easy analytical adaptations, we demonstrate that a series of rotor synchronized short pulses may form the basis for essentially ideal rf pulse performance. Using similar approaches, we design (2)H to (13)C polarization transfer experiments that increase the efficiency by one order of magnitude over standard cross polarization experiments. We demonstrate how we can translate advanced optimal control waveforms into simple interleaved CW and rf pulse methods that form a new cross polarization experiment. This experiment significantly improves (1)H-(15)N and (15)N-(13)C transfers, which are key elements in the vast majority of biological solid-state NMR experiments. In addition, we demonstrate how interleaved sampling of spectra exploiting polarization from (1)H and (2)H nuclei can substantially enhance the sensitivity of such experiments. Finally, we present systematic development of (1)H decoupling methods where CW irradiation of moderate amplitude is interleaved with strong rotor-synchronized refocusing pulses. We show that these sequences remove residual cross terms between dipolar coupling and chemical shielding anisotropy more effectively and improve the spectral resolution over that observed in current state-of-the-art methods.

Monitoring cartilage tissue engineering using magnetic resonance spectroscopy, imaging, and elastography

A key technical challenge in cartilage tissue engineering is the development of a noninvasive method for monitoring the composition, structure, and function of the tissue at different growth stages. Due to its noninvasive, three-dimensional imaging capabilities and the breadth of available contrast mechanisms, magnetic resonance imaging (MRI) techniques can be expected to play a leading role in assessing engineered cartilage. In this review, we describe the new MR-based tools (spectroscopy, imaging, and elastography) that can provide quantitative biomarkers for cartilage tissue development both in vitro and in vivo. Magnetic resonance spectroscopy can identify the changing molecular structure and alternations in the conformation of major macromolecules (collagen and proteoglycans) using parameters such as chemical shift, relaxation rates, and magnetic spin couplings. MRI provides high-resolution images whose contrast reflects developing tissue microstructure and porosity through changes in local relaxation times and the apparent diffusion coefficient. Magnetic resonance elastography uses low-frequency mechanical vibrations in conjunction with MRI to measure soft tissue mechanical properties (shear modulus and viscosity). When combined, these three techniques provide a noninvasive, multiscale window for characterizing cartilage tissue growth at all stages of tissue development, from the initial cell seeding of scaffolds to the development of the extracellular matrix during construct incubation, and finally, to the postimplantation assessment of tissue integration in animals and patients.