Sensitivity and resolution in proton solid-state NMR at intermediate deuteration levels: quantitative linewidth characterization and applications to correlation spectroscopy

Field and magic angle spinning frequency dependence of proton resonances in rotating solids

Proton detection in solid state NMR is continuously developing and allows one to gain new insights in structural biology. Overall, this progress is a result of the synergy between hardware development, new NMR methodology and new isotope labeling strategies, to name a few factors. Even though current developments are rapid, it is worthwhile to summarize what can currently be achieved employing proton detection in biological solids. We illustrate this by analysing the signal-to-noise ratio (SNR) for spectra obtained for a microcrystalline α-spectrin SH3 domain protein sample by (i) employing different degrees of chemical dilution to replace protons by incorporating deuterons in different sites, by (ii) variation of the magic angle spinning (MAS) frequencies between 20 and 110 kHz, and by (iii) variation of the static magnetic field B₀. The experimental SNR values are validated with numerical simulations employing up to 9 proton spins. Although in reality a protein would contain far more than 9 protons, in a deuterated environment this is a sufficient number to achieve satisfactory simulations consistent with the experimental data. The key results of this analysis are (i) with current hardware, deuteration is still necessary to record spectra of optimum quality; (ii) 13CH3 isotopomers for methyl groups yield the best SNR when MAS frequencies above 100 kHz are available; and (iii) sensitivity increases with a factor beyond B0 3/2 with the static magnetic field due to a transition of proton-proton dipolar interactions from a strong to a weak coupling limit.

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.

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.

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.

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

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

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.

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.

Magic angle spinning spheres

Magic angle spinning (MAS) is commonly used in nuclear magnetic resonance of solids to improve spectral resolution. Rather than using cylindrical rotors for MAS, we demonstrate that spherical rotors can be spun stably at the magic angle. Spherical rotors conserve valuable space in the probe head and simplify sample exchange and microwave coupling for dynamic nuclear polarization. In this current implementation of spherical rotors, a single gas stream provides bearing gas to reduce friction, drive propulsion to generate and maintain angular momentum, and variable temperature control for thermostating. Grooves are machined directly into zirconia spheres, thereby converting the rotor body into a robust turbine with high torque. We demonstrate that 9.5-mm-outside diameter spherical rotors can be spun at frequencies up to 4.6 kHz with N₂(g) and 10.6 kHz with He(g). Angular stability of the spinning axis is demonstrated by observation of ⁷⁹Br rotational echoes out to 10 ms from KBr packed within spherical rotors. Spinning frequency stability of ±1 Hz is achieved with resistive heating feedback control. A sample size of 36 μl can be accommodated in 9.5-mm-diameter spheres with a cylindrical hole machined along the spinning axis. We further show that spheres can be more extensively hollowed out to accommodate 161 μl of the sample, which provides superior signal-to-noise ratio compared to traditional 3.2-mm-diameter cylindrical rotors.

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.

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.

1H-Detected REDOR with Fast Magic-Angle Spinning of a Deuterated Protein

Rotational echo double resonance (REDOR) is a highly successful method for heteronuclear distance determination in biological solid-state NMR, and 1H detection methods have emerged in recent years as a powerful approach to improving sensitivity and resolution for small sample quantities by utilizing fast magic-angle spinning (>30 kHz) and deuteration strategies. In theory, involving 1H as one of the spins for measuring REDOR effects can greatly increase the distance measurement range, but few experiments of this type have been reported. Here we introduce a pulse sequence that combines frequency-selective REDOR (FSR) with 1H detection. We demonstrate this method with applications to samples of uniformly 13C,15N,2H-labeled alanine and uniformly 13C,2H,15N-labeled GB1 protein, back-exchanged with 30% H2O and 70% D2O, employing a variety of frequency-selective 13C pulses to highlight unique spectral features. The resulting, robust REDOR effects provide (1) tools for resonance assignment, (2) restraints of secondary structure, (3) probes of tertiary structure, and (4) approaches to determine the preferred orientation of aromatic rings in the protein core.

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.

Insights into the structure and dynamics of measles virus nucleocapsids by 1H-detected solid-state NMR

(1)H-detected solid-state nuclear magnetic resonance (NMR) experiments are recorded on both intact and trypsin-cleaved sedimented measles virus (MeV) nucleocapsids under ultra-fast magic-angle spinning. High-resolution (1)H,(15)N-fingerprints allow probing the degree of molecular order and flexibility of individual capsid proteins, providing an exciting atomic-scale complement to electro microscopy (EM) studies of the same systems.

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.

Heteronuclear proton double quantum-carbon single quantum scalar correlation in solids

A new NMR experiment that exploits the advantages of proton double quantum (DQ) NMR through a proton DQ-carbon single quantum (SQ) correlation experiment in the solid state is proposed. Analogous to the previously proposed 2D (1)H (DQ)-(13)C refocused INEPT experiment (Webber et al., 2010), the correlation between (1)H and (13)C is achieved through scalar coupling evolution, while the double quantum coherence among protons is generated through dipolar couplings. However, the new experiment relies on (13)C transverse coherence for scalar transfer. The new experiment dubbed MAS-J-(1)H (DQ)-(13)C-HMQC, is particularly suited for unlabeled molecules and can provide higher sensitivity than its INEPT counterpart. The experiment is applied to four different samples.

Dynamic nuclear polarization of 17O: direct polarization

Dynamic nuclear polarization of (17)O was studied using four different polarizing agents: the biradical TOTAPOL and the monoradicals trityl and SA-BDPA, as well as a mixture of the latter two. Field profiles, DNP mechanisms, and enhancements were measured to better understand and optimize directly polarizing this low-gamma quadrupolar nucleus using both mono- and biradical polarizing agents. Enhancements were recorded at <88 K and were >100 using the trityl (OX063) radical and <10 with the other polarizing agents. The >10,000-fold savings in acquisition time enabled a series of biologically relevant small molecules to be studied with small sample sizes and the measurement of various quadrupolar parameters. The results are discussed with comparison to room temperature studies and GIPAW quantum chemical calculations. These experimental results illustrate the strength of high field DNP and the importance of radical selection for studying low-gamma nuclei.

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

When applied to biomolecules, solid-state NMR suffers from low sensitivity and resolution. The major obstacle to applying proton detection in the solid state is the proton dipolar network, and deuteration can help avoid this problem. In the past, researchers had primarily focused on the investigation of exchangeable protons in these systems. In this Account, we review NMR spectroscopic strategies that allow researchers to observe aliphatic non-exchangeable proton resonances in proteins with high sensitivity and resolution. Our labeling scheme is based on u-[(2)H,(13)C]-glucose and 5-25% H2O (95-75% D2O) in the M9 bacterial growth medium, known as RAP (reduced adjoining protonation). We highlight spectroscopic approaches for obtaining resonance assignments, a prerequisite for any study of structure and dynamics of a protein by NMR spectroscopy. Because of the dilution of the proton spin system in the solid state, solution-state NMR (1)HCC(1)H type strategies cannot easily be transferred to these experiments. Instead, we needed to pursue ((1)H)CC(1)H, CC(1)H, (1)HCC or ((2)H)CC(1)H type experiments. In protonated samples, we obtained distance restraints for structure calculations from samples grown in bacteria in media containing [1,3]-(13)C-glycerol, [2]-(13)C-glycerol, or selectively enriched glucose to dilute the (13)C spin system. In RAP-labeled samples, we obtained a similar dilution effect by randomly introducing protons into an otherwise deuterated matrix. This isotopic labeling scheme allows us to measure the long-range contacts among aliphatic protons, which can then serve as restraints for the three-dimensional structure calculation of a protein. Due to the high gyromagnetic ratio of protons, longer range contacts are more easily accessible for these nuclei than for carbon nuclei in homologous experiments. Finally, the RAP labeling scheme allows access to dynamic parameters, such as longitudinal relaxation times T1, and order parameters S(2) for backbone and side chain carbon resonances. We expect that these measurements will open up new opportunities to obtain a more detailed description of protein backbone and side chain dynamics.

High-resolution proton CRAMPS NMR using narrowband analog filters and postponed data acquisition

Proton linewidths decrease with increasing magic-angle spinning (MAS) rates. However, without spin dilution by deuteration, even with the fastest MAS rates available today, the narrowest proton linewidths are obtained by using the combined rotation and multiple pulse spectroscopy (CRAMPS) method. Direct observation under windowed CRAMPS typically introduces several tens of times more noise, partly because wideband analog filters (e.g. 5 MHz) must be used or sometimes even bypassed. Here we report that it is possible to keep using narrowband analog filters (about 50 kHz cutoff frequency) in CRAMPS by taking advantage of the time delay caused by the filters, which is inversely proportional to the cutoff frequency. This delay coincides with typical CRAMPS cycle times, enabling acquisition of the data point in the next detection window. The noise of such CRAMPS spectra is only about 5 times larger than MAS-only spectra. This new method allows CRAMPS to be performed on systems that lack wideline hardware (wideband filters and fast ADCs), for example, older spectrometers originally intended for solution NMR.

Solid-state NMR analysis of membrane proteins and protein aggregates by proton detected spectroscopy

Solid-state NMR has emerged as an important tool for structural biology and chemistry, capable of solving atomic-resolution structures for proteins in membrane-bound and aggregated states. Proton detection methods have been recently realized under fast magic-angle spinning conditions, providing large sensitivity enhancements for efficient examination of uniformly labeled proteins. The first and often most challenging step of protein structure determination by NMR is the site-specific resonance assignment. Here we demonstrate resonance assignments based on high-sensitivity proton-detected three-dimensional experiments for samples of different physical states, including a fully-protonated small protein (GB1, 6 kDa), a deuterated microcrystalline protein (DsbA, 21 kDa), a membrane protein (DsbB, 20 kDa) prepared in a lipid environment, and the extended core of a fibrillar protein (α-synuclein, 14 kDa). In our implementation of these experiments, including CONH, CO(CA)NH, CANH, CA(CO)NH, CBCANH, and CBCA(CO)NH, dipolar-based polarization transfer methods have been chosen for optimal efficiency for relatively high protonation levels (full protonation or 100 % amide proton), fast magic-angle spinning conditions (40 kHz) and moderate proton decoupling power levels. Each H-N pair correlates exclusively to either intra- or inter-residue carbons, but not both, to maximize spectral resolution. Experiment time can be reduced by at least a factor of 10 by using proton detection in comparison to carbon detection. These high-sensitivity experiments are especially important for membrane proteins, which often have rather low expression yield. Proton-detection based experiments are expected to play an important role in accelerating protein structure elucidation by solid-state NMR with the improved sensitivity and resolution.

Ultra-high resolution in MAS solid-state NMR of perdeuterated proteins: implications for structure and dynamics

High resolution proton spectra are obtained in MAS solid-state NMR in case samples are prepared using perdeuterated protein and D(2)O in the recrystallization buffer. Deuteration reduces drastically (1)H, (1)H dipolar interactions and allows to obtain amide proton line widths on the order of 20 Hz. Similarly, high-resolution proton spectra of aliphatic groups can be obtained if specifically labeled precursors for biosynthesis of methyl containing side chains are used, or if limited amounts of H(2)O in the bacterial growth medium is employed. This review summarizes recent spectroscopic developments to access structure and dynamics of biomacromolecules in the solid-state, and shows a number of applications to amyloid fibrils and membrane proteins.

Applications of high-resolution 1H solid-state NMR

This article reviews the large increase in applications of high-resolution (1)H magic-angle spinning (MAS) solid-state NMR, in particular two-dimensional heteronuclear and homonuclear (double-quantum and spin-diffusion NOESY-like exchange) experiments, in the last five years. These applications benefit from faster MAS frequencies (up to 80 kHz), higher magnetic fields (up to 1 GHz) and pulse sequence developments (e.g., homonuclear decoupling sequences applicable under moderate and fast MAS). (1)H solid-state NMR techniques are shown to provide unique structural insight for a diverse range of systems including pharmaceuticals, self-assembled supramolecular structures and silica-based inorganic-organic materials, such as microporous and mesoporous materials and heterogeneous organometallic catalysts, for which single-crystal diffraction structures cannot be obtained. The power of NMR crystallography approaches that combine experiment with first-principles calculations of NMR parameters (notably using the GIPAW approach) are demonstrated, e.g., to yield quantitative insight into hydrogen-bonding and aromatic CH-π interactions, as well as to generate trial three-dimensional packing arrangements. It is shown how temperature-dependent changes in the (1)H chemical shift, linewidth and DQ-filtered signal intensity can be analysed to determine the thermodynamics and kinetics of molecular level processes, such as the making and breaking of hydrogen bonds, with particular application to proton-conducting materials. Other applications to polymers and biopolymers, inorganic compounds and bioinorganic systems, paramagnetic compounds and proteins are presented. The potential of new technological advances such as DNP methods and new microcoil designs is described.

Deuterated peptides and proteins: structure and dynamics studies by MAS solid-state NMR

Perdeuteration and back substitution of exchangeable protons in microcrystalline proteins, in combination with recrystallization from D(2)O-containing buffers, significantly reduce (1)H, (1)H dipolar interactions. This way, amide proton line widths on the order of 20 Hz are obtained. Aliphatic protons are accessible either via specifically protonated precursors or by using low amounts of H(2)O in the bacterial growth medium. The labeling scheme enables characterization of structure and dynamics in the solid-state without dipolar truncation artifacts.

Dynamic nuclear polarization at 9T using a novel 250 gyrotron microwave source

In the 1990's we initiated development of high frequency gyrotron microwave sources with the goal of performing dynamic nuclear polarization at magnetic fields (∼5-23 T) used in contemporary NMR experiments. This article describes the motivation for these efforts and the developments that led to the operation of a gyrotron source for DNP operating at 250 GHz. We also mention results obtained with this instrument that would have been otherwise impossible absent the increased sensitivity. Finally, we describe recent efforts that have extended DNP to 460 GHz and 700 MHz (1)H frequencies.

Progress in correlation spectroscopy at ultra-fast magic-angle spinning: basic building blocks and complex experiments for the study of protein structure and dynamics

Recent progress in multi-dimensional solid-state NMR correlation spectroscopy at high static magnetic fields and ultra-fast magic-angle spinning is discussed. A focus of the review is on applications to protein resonance assignment and structure determination as well as on the characterization of protein dynamics in the solid state. First, the consequences of ultra-fast spinning on sensitivity and sample heating are considered. Recoupling and decoupling techniques at ultra-fast MAS are then presented, as well as more complex experiments assembled from these basic building blocks. Furthermore, we discuss new avenues in biomolecular solid-state NMR spectroscopy that become feasible in the ultra-fast spinning regime, such as sensitivity enhancement based on paramagnetic doping, and the prospect of direct proton detection.

1H-detected 1H-1H correlation spectroscopy of a stereo-array isotope labeled amino acid under fast magic-angle spinning

The combined use of selective deuteration, stereo-array isotope labeling (SAIL), and fast magic-angle spinning effectively suppresses the 1H-1H dipolar couplings in organic solids. This method provided the high-field 1H NMR linewidths comparable to those achieved by combined rotation and multiple-pulse spectroscopy. This technique was applied to two-dimensional 1H-detected 1H-1H polarization transfer CHH experiments of valine. The signal sensitivity for the 1H-detected CHH experiments was greater than that for the 13C-detected 1H-1H polarization transfer experiments by a factor of 2-4. We obtained the 1H-1H distances in SAIL valine by CHH experiments with an accuracy of about 0.2A by using a theory developed for 1H-1H polarization transfer in 13C-labeled organic compounds.

Proton-detected heteronuclear single quantum correlation NMR spectroscopy in rigid solids with ultra-fast MAS

In this article, we show the potential for utilizing proton-detected heteronuclear single quantum correlation (HSQC) NMR in rigid solids under ultra-fast magic angle spinning (MAS) conditions. The indirect detection of carbon-13 from coupled neighboring hydrogen nuclei provides a sensitivity enhancement of 3- to 4-fold in crystalline amino acids over direct-detected versions. Furthermore, the sensitivity enhancement is shown to be significantly larger for disordered solids that display inhomogeneously broadened carbon-13 spectra. Latrodectus hesperus (Black Widow) dragline silk is given as an example where the sample is mass-limited and the sensitivity enhancement for the proton-detected experiment is 8- to 13-fold. The ultra-fast MAS proton-detected HSQC solid-state NMR technique has the added advantage that no proton homonuclear decoupling is applied during the experiment. Further, well-resolved, indirectly observed carbon-13 spectra can be obtained in some cases without heteronuclear proton decoupling.

Optimum levels of exchangeable protons in perdeuterated proteins for proton detection in MAS solid-state NMR spectroscopy

We present a systematic study of the effect of the level of exchangeable protons on the observed amide proton linewidth obtained in perdeuterated proteins. Decreasing the amount of D(2)O employed in the crystallization buffer from 90 to 0%, we observe a fourfold increase in linewidth for both (1)H and (15)N resonances. At the same time, we find a gradual increase in the signal-to-noise ratio (SNR) for (1)H-(15)N correlations in dipolar coupling based experiments for H(2)O concentrations of up to 40%. Beyond 40%, a significant reduction in SNR is observed. Scalar-coupling based (1)H-(15)N correlation experiments yield a nearly constant SNR for samples prepared with < or =30% H(2)O. Samples in which more H(2)O is employed for crystallization show a significantly reduced NMR intensity. Calculation of the SNR by taking into account the reduction in (1)H T (1) in samples containing more protons (SNR per unit time), yields a maximum SNR for samples crystallized using 30 and 40% H(2)O for scalar and dipolar coupling based experiments, respectively. A sensitivity gain of 3.8 is obtained by increasing the H(2)O concentration from 10 to 40% in the CP based experiment, whereas the linewidth only becomes 1.5 times broader. In general, we find that CP is more favorable compared to INEPT based transfer when the number of possible (1)H,(1)H interactions increases. At low levels of deuteration (> or =60% H(2)O in the crystallization buffer), resonances from rigid residues are broadened beyond detection. All experiments are carried out at MAS frequency of 24 kHz employing perdeuterated samples of the chicken alpha-spectrin SH3 domain.

Proton-detected separated local field spectroscopy

PISEMO, a separated local field experiment that can be performed with either direct (15)N (or (13)C) detection or indirect (1)H detection, is demonstrated on a single crystal of a model peptide. The (1)H signals modulated by (1)H-(15)N heteronuclear dipole-dipole couplings are observed stroboscopically in the windows of the multiple-pulse sequence used to attenuate (1)H-(1)H homonuclear dipole-dipole couplings. (1)H-detection yields spectra with about 2.5 times the signal to noise ratio observed with (15)N-detection under equivalent conditions. Resolution in both the (15)N chemical shift and (1)H-(15)N heteronuclear dipole-dipole coupling dimensions is similar to that observed with PISEMA, however, since only on-resonance pulses are utilized, the bandwidth is better.

NMR crystallography: the effect of deuteration on high resolution 13C solid state NMR spectra of a 7-TM protein

The effect of deuteration on the 13C linewidths of U-13C, 15N 2D crystalline bacteriorhodopsin (bR) from Halobacterium salinarium, a 248-amino acid protein with seven-transmembrane (7TM) spanning regions, has been studied in purple membranes as a prelude to potential structural studies. Spectral doubling of resonances was observed for receptor expressed in 2H medium (for both 50:50% 1H:2H, and a more highly deuterated form) with the resonances being of similar intensities and separated by <0.3 ppm in the methyl spectral regions in which they were readily distinguished. Line-widths of the methyl side chains were not significantly altered when the protein was expressed in highly deuterated medium compared to growth in fully protonated medium (spectral line widths were about 0.5 ppm on average for receptor expressed both in the fully protonated and highly deuterated media from the C delta, C gamma 1, and C gamma 2 Ile 13C signals observed in the direct, 21-39 ppm, and indirect, 9-17 ppm, dimensions). The measured 13C NMR line-widths observed for both protonated and deuterated form of the receptor are sufficiently narrow, indicating that this crystalline protein morphology is suitable for structural studies. 1) decoupling comparison of the protonated and deuterated bR imply that deuteration may be advantageous for samples in which low power 1H decoupling is required.

Origins of linewidth in 1H magic-angle spinning NMR

A detailed study of the factors determining the linewidth (and hence resolution) in 1H solid-state magic-angle spinning NMR is described. Although it has been known from the early days of magic-angle spinning (MAS) that resolution of spectra from abundant nuclear spins, such as 1H, increases approximately linearly with increasing sample rotation rate, the difficulty of describing the dynamics of extended networks of coupled spins has made it difficult to predict a priori the resolution expected for a given sample. Using recently developed, highly efficient methods of numerical simulation, together with experimental measurements on a variety of test systems, we propose a comprehensive picture of 1H resolution under MAS. The "homogeneous" component of the linewidth is shown to depend primarily on the ratio between an effective local coupling strength and the spin rate, modified by geometrical factors which loosely correspond to the "dimensionality" of the coupling network. The remaining "inhomogeneous" component of the natural linewidth is confirmed to have the same properties as in dilute-spin NMR. Variations in the NMR frequency due to chemical shift effects are shown to have minimal impact on 1H resolution. The implications of these results for solid-state NMR experiments involving 1H and other abundant-spin nuclei are discussed.