Sensitivity-enhanced static 15N NMR of solids by 1h indirect detection

1H detection of heteronuclear dipolar oscillations with water suppression in single crystal peptide and oriented protein samples

Oriented sample solid-state NMR is a complementary approach to protein structure determination with the distinct advantage that it can be applied to supramolecular assemblies, such as viruses and membrane proteins, under near-native conditions, which generally include high levels of hydration as found in living systems. Thus, in order to perform ¹H detected versions of multi-dimensional experiments water suppression techniques must be integrated into the pulse sequences. For example, ¹H-windowed detection of ¹H-¹⁵N dipolar couplings enable multi-dimensional NMR experiments to be performed. Here we show that the addition of a solvent suppression pulse during the z-filter interval greatly improves the sensitivity of the experiments by suppressing the ¹H signals from water present. This is demonstrated here with a crystal sample submerged in water and then extended to proteins. The combination of solvent-suppressed ¹H detected PISEMO and the use of a strip shield-solenoid coil probe configuration provides a two-fold sensitivity enhancement in both the crystal sample and Pf1 coat protein sample compared to the ¹⁵N direct detection method. Here we also examine protein NMR line-widths and sensitivity enhancements in the context of window detected separated local field experiments for protein samples.

Pulsed Third-Spin-Assisted Recoupling NMR for Obtaining Long-Range 13C-13C and 15N-13C Distance Restraints

We present a class of pulsed third-spin-assisted recoupling (P-TSAR) magic-angle-spinning solid-state NMR techniques that achieve efficient polarization transfer over long distances to provide important restraints for structure determination. These experiments utilize second-order cross terms between strong 1H-13C and 1H-15N dipolar couplings to achieve 13C-13C and 15N-13C polarization transfer, similar to the principle of continuous-wave (CW) TSAR experiments. However, in contrast to the CW-TSAR experiments, these P-TSAR experiments require much less radiofrequency (rf) energy and allow a much simpler routine for optimizing the rf field strength. We call the technique PULSAR (pulsed proton-assisted recoupling) for homonuclear spin pairs. For heteronuclear spin pairs, we improve the recently introduced PERSPIRATIONCP (proton-enhanced rotor-echo short pulse irradiation cross-polarization) experiment by shifting the pulse positions and removing the z-filters, which significantly broaden the bandwidth and increase the efficiency of polarization transfer. We demonstrate the PULSAR and PERSPIRATIONCP techniques on the model protein GB1 and found cross peaks for distances as long as 10 and 8 Å for 13C-13C and 15N-13C spin pairs, respectively. We then apply these methods to the amyloid fibrils formed by the peptide hormone glucagon and show that long-range correlation peaks are readily observed to constrain intermolecular packing in this cross-β fibril. We provide an analytical model for the PULSAR and PERSPIRATIONCP experiments to explain the measured and simulated chemical shift dependence and pulse flip angle dependence of polarization transfer. These two techniques are useful for measuring long-range distance restraints to determine the three-dimensional structures of proteins and other biological macromolecules.

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.

Noise Reduction in Solid-State NMR Spectra Using Principal Component Analysis

A noise reduction method was developed for solid-state nuclear magnetic resonance spectroscopy using multivariate analysis. Principal component analysis was first applied for cross-polarization/magic angle spinning and ¹³C spin-lattice relaxation measurements of solid-state nuclear magnetic resonance array spectra. The contact time of cross-polarization/magic angle spinning and the delay time in spin-lattice relaxation measurements were continuously changed to obtain a series of spectra, which were used for noise reduction using principal component analysis. The noise reduction method successfully produced spectra with improved signal-to-noise ratios. This noise reduction method shortens the measurement time and allows for detection of components with minute signals.

Toward high-resolution NMR spectroscopy of microscopic liquid samples

A longstanding limitation of high-resolution NMR spectroscopy is the requirement for samples to have macroscopic dimensions. Commercial probes, for example, are designed for volumes of at least 5 μL, in spite of decades of work directed toward the goal of miniaturization. Progress in miniaturizing inductive detectors has been limited by a perceived need to meet two technical requirements: (1) minimal separation between the sample and the detector, which is essential for sensitivity, and (2) near-perfect magnetic-field homogeneity at the sample, which is typically needed for spectral resolution. The first of these requirements is real, but the second can be relaxed, as we demonstrate here. By using pulse sequences that yield high-resolution spectra in an inhomogeneous field, we eliminate the need for near-perfect field homogeneity and the accompanying requirement for susceptibility matching of microfabricated detector components. With this requirement removed, typical imperfections in microfabricated components can be tolerated, and detector dimensions can be matched to those of the sample, even for samples of volume ≪5 μL. Pulse sequences that are robust to field inhomogeneity thus enable small-volume detection with optimal sensitivity. We illustrate the potential of this approach to miniaturization by presenting spectra acquired with a flat-wire detector that can easily be scaled to subnanoliter volumes. In particular, we report high-resolution NMR spectroscopy of an alanine sample of volume 500 pL.

Thin-film solid-state proton NMR measurements using a synthetic mica substrate: polymer blends

Solid-state proton nuclear magnetic resonance (NMR) measurements are performed successfully on polymer blend thin films through the use of synthetic mica as a substrate. When used as a substrate, synthetic fluorophlogopite mica with its proton-free, diamagnetic character, allows for adequate measurement sensitivity while minimally perturbing the proton thin-film spectra, especially relative to more commonly available natural micas. Specifically, we use multiple-pulse techniques in the presence of magic-angle spinning to measure the degree of mixing in two different polymer blend thin films, polystyrene/poly(xylylene ether) and poly(1-methyladamantyl methacrylate) (PMAdMA)/triphenylsulfonium perfluorobutanesulfonate (TPS-PFBS), spin-coated onto mica substrates. Our earlier studies had focused on bulk systems where NMR signals are stronger, but may not be representative of thin films of the same systems that are relevant to many applications such as photoresist formulations in the electronics industry. The superiority of synthetic over natural paramagnetic mica is demonstrated by the maintenance of resolution and spinning sideband intensities (relative to bulk samples) for the synthetic mica samples. In contrast, degraded resolution and large spinning sidebands are shown to typify spectra of the natural mica samples. This approach can be applied to many other proton measurements of solid thin films, thereby greatly extending the types of systems to be investigated. Magnetic susceptibility measurements are also reported for all micas used.

Shiftless nuclear magnetic resonance spectroscopy

The acquisition and analysis of high resolution one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectra without chemical shift frequencies are described. Many variations of shiftless NMR spectroscopy are feasible. A two-dimensional experiment that correlates the dipole-dipole and dipole-dipole couplings in the model peptide , (15)N labeled N-acetyl-leucine is demonstrated. In addition to the resolution of resonances from individual sites in a single crystal sample, the bond lengths and angles are characterized by the two-dimensional powder pattern obtained from a polycrystalline sample.

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.

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

We present a systematic study of proton linewidths in rigid solids as a function of sample spinning frequency and proton density, with the latter controlled by the ratio of protonated and perdeuterated model compounds. We find that the linewidth correlates more closely with the overall proton density (rho(H)) than the size of local clusters of (1)H spins. At relatively high magic-angle spinning (MAS) rates, the linewidth depends linearly upon the inverse MAS rate. In the limit of infinite spinning rate and/or zero proton concentration, the linewidth extrapolates to a non-zero value, owing to contributions from scalar couplings, chemical shift dispersion, and B(0) field inhomogeneity. The slope of this line depends on the overall concentration of unexchangeable protons in the sample and the spinning rate. At up to 30% protonation levels ( approximately 2 (1)H/100A(3)), proton detection experiments are demonstrated to have a substantial (2- to 3-fold) sensitivity gain over corresponding (13)C-detected experiments. Within this range, the absolute sensitivity increases with protonation level; the optimal compromise between sensitivity and resolution is in the range of 20-30% protonation. We illustrate the use of dilute protons for polarization transfer to and from low-gamma spins within 5A, and to be utilized as both magnetization source and detection spins. The intermediate protonation regime enhances relaxation properties, which we expect will enable new types of (1)H correlation pulse sequences to be implemented with improved resolution and sensitivity.

Hadamard NMR spectroscopy in solids

The duration of 2D and 3D NMR experiments in solids can be reduced by several orders of magnitude by using frequency domain Hadamard encoding with long selective pulses. We demonstrate Hadamard encoded experiments in (13)C enriched solids samples. To avoid multiple quantum interferences, the Hadamard encoding pulses are applied sequentially rather than simultaneously in this study. Among other possible applications, dipolar assisted rotational resonance experiments and measurement of NOESY type build-up rates in proton driven spin diffusion are demonstrated.

Sensitive high resolution inverse detection NMR spectroscopy of proteins in the solid state

A new indirect detection scheme for obtaining (15)N/(1)H shift correlation spectra in crystalline proteins is described. Excellent water suppression is achieved without the need for pulsed field gradients, and using only a 2-step phase cycle. Careful attention to overall NMR instrument stability was found critical for obtaining the best resolution and sensitivity. Magnetic dilution by deuteration of the protein in combination with high-speed magic angle spinning produces (1)H resonances averaging only 0.22 ppm in width, and in some cases lines as narrow as 0.17 ppm are obtained. In application to two different polymorphs of ubiquitin, structure dependent differences in both (15)N and (1)H amide chemical shifts are observed. In one case, distinct shifts for different molecules in the asymmetric unit are seen, and all differ substantially from solution NMR shifts. A gain of 7 in sensitivity makes the method competitive with solution NMR as long as nanocrystalline samples are available.

1H detected 1H,15N correlation spectroscopy in rotating solids

We describe new correlation experiments suitable for determining long-range 1H-1H distances in 2H,15N-labeled peptides and proteins. The approach uses perdeuteration together with back substitution of exchangeable protons during sample preparation as a means of attenuating the strong 1H-1H dipolar couplings that broaden 1H magic angle spinning (MAS) spectra of solids. In the approach described here, we retain 100% of the 1H sensitivity by labeling and detecting all exchangeable sites. This is in contrast to homonuclear multiple pulse decoupling sequences that are applied during detection and that compromise sensitivity because of the requirement of sampling between pulses. As a result 1H detection provides a gain in sensitivity of >5 compared to the 15N detected version of the experiment (at a MAS frequency of 13.5kHz). The pulse schemes make use of the favorable dispersion of the amide 15Ns resonances in the protein backbone. The experiments are demonstrated on a sample of the uniformly 2H,15N-labeled dipeptide N-Ac-Val-Leu-OH and are analogous to the solution-state suite of HSQC-NOESY experiments. In this compound the 1H amide linewidths at 750MHz vary from approximately 0.67 ppm at omega(r)/2pi approximately 5kHz to approximately 0.20 ppm at omega(r)/2pi approximately 30kHz, indicating that useful resolution is available in the 1H spectrum via this approach. Since the experiments circumvent the problem of dipolar truncation in the 1H-1H spin system, they should make it possible to measure long-range distances in a uniformly labeled environment. Thus, we expect the experiments to be useful in constraining the global fold of a protein.

(15)N-(1)H bond length determination in natural abundance by inverse detection in fast-MAS solid-state NMR spectroscopy

A solid-state 15N-1H correlation NMR experiment is presented, which provides a substantial gain in signal sensitivity by 1H inverse detection under fast MAS conditions and allows for the precise determination of NH bond lengths via heteronuclear 1H-15N dipole-dipole couplings on samples naturally abundant in 15N. Pulsed-field gradients or, alternatively, radio frequency pulses ensure suppression of unwanted 1H signals. In this way, natural-abundance 15N-1H correlation NMR spectroscopy becomes feasible in the solid state with experiment times of a few hours. The dipole-dipole coupling constants are extracted from spinning sideband patterns generated by recently developed recoupling strategies. The information on 15N/1H chemical shifts and quantitative 15N-1H couplings can readily be combined in a single two-dimensional spectrum using a split-t1 approach.

REDOR-based heteronuclear dipolar correlation experiments in multi-spin systems: rotor-encoding, directing, and multiple distance and angle determination

We review a variety of recently developed 1H-X heteronuclear recoupling techniques, which rely only on the homonuclear decoupling efficiency of very-fast magic-angle spinning. All these techniques, which are based on the simple rotational-echo, double-resonance (REDOR) approach for heteronuclear recoupling, are presented in a common context. Advantages and possibilities with respect to the complementary application of conventionally X and 1H-inversely detected variants are discussed in relation to the separability and analysis of multiple couplings. We present an improved and more sensitive approach to the determination of 1H-X dipolar couplings by spinning-sideband analysis, termed REREDOR, which is applicable to XHn groups in rigid and mobile systems and bears some similarity to more elaborate separated local-field methods. The estimation of medium-range 1H-X distances by analyzing signal intensities in two-dimensional REDOR correlation spectra in a model-free way is also discussed. More specifically, we demonstrate the possibility of combined distance and angle determination in H-X-H or X-H-X three-spin systems by asymmetric recoupling schemes and spinning-sideband analysis. Finally, an 1H-X correlation experiment is introduced which accomplishes high sensitivity by inverse (1H) detection and is therefore applicable to samples with 15N in natural abundance.

Chemical shift anisotropy and offset effects in cross polarization solid-state NMR spectroscopy

The effect of an offset term in the cross-polarization (CP) Hamiltonian of a heteronuclear spin-12 pair due to off-resonant radio frequency (rf) irradiation and/or chemical shift anisotropy on one of the rf channels is investigated. Analytical solutions, simulations, and experimental results are presented. Formulating the CP spin dynamics in terms of an explicit unitary evolution operator enables the CP period to be inserted as a module in a given pulse scheme regardless of the initial density matrix present. The outcome of post-CP manipulation via pulses can be calculated on the resulting density matrix as the phases and amplitudes of all coherence modes are available. Using these tools it is shown that the offset can be used to reduce the rf power on that channel and the performance is further improved by a post-CP pulse whose flip angle matches and compensates the tilt of the effective field on the offset channel. Experimental investigations on single crystalline and polycrystalline samples of peptides confirm the oscillatory nature of CP dynamics and prove the slowing down of the dynamics under offset and/or mismatch conditions.

Sensitivity enhancement in structural measurements by solid state NMR through pulsed spin locking

Free induction decay (FID) signals in solid state NMR measurements performed with magic angle spinning can often be extended in time by factors on the order of 10 by a simple pulsed spin locking technique. The sensitivity of a structural measurement in which the structural information is contained in the dependence of the integrated FID amplitude on a preceding evolution period can therefore be enhanced substantially by pulsed spin locking in the signal detection period. We demonstrate sensitivity enhancements in a variety of solid state NMR techniques that are applicable to selectively isotopically labeled samples, including 13C-15N rotational echo double resonance (REDOR), 13C-13C dipolar recoupling measurements using the constant-time finite-pulse radio-frequency-driven recoupling (fpRFDR-CT) and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) techniques, and torsion angle measurements using the double quantum chemical shift anisotropy (DQCSA) technique. Further, we demonstrate that the structural information in the solid state NMR data is not distorted by pulsed spin locking in the detection period.

Determination of membrane Peptide orientation by 1H-detected 2H NMR spectroscopy

We demonstrate the application of the proton inverse detected deuteron (PRIDE) NMR technique to the measurement of the orientation of membrane-bound peptides with enhanced sensitivity. Gramicidin D, a transmembrane peptide, and ovispirin, a surface-bound peptide, were used as model systems. The peptides were 2H-labeled by 1H/2H exchange and oriented uniaxially on glass plates. The directly detected 2H spectra of both peptides showed only a strong D(2)O signal and no large quadrupolar splittings. In contrast, the PRIDE spectrum of gramicidin exhibited quadrupolar splittings as large as 281 kHz, consistent with its transmembrane orientation. Moreover, the large D(2)O signal in the directly detected 2H spectra was cleanly suppressed in the PRIDE spectrum. For ovispirin, the 1H indirectly detected 2H spectrum revealed a 104 kHz splitting and a zero-frequency peak. The former reflects the in-plane orientation of most of the helix axis, while the latter results from residues with a magic-angle orientation of the N-D bonds. These are consistent with previous 15N NMR results on ovispirin. The combination of PRIDE and exchange labeling provides an economical and sensitive method of studying membrane peptide orientations in lipid bilayers without the influence of D(2)O and with the ability to detect N-D bonds at the magic angle from the bilayer normal.

Orientation and dynamics of an antimicrobial peptide in the lipid bilayer by solid-state NMR spectroscopy

The orientation and dynamics of an 18-residue antimicrobial peptide, ovispirin, has been investigated using solid-state NMR spectroscopy. Ovispirin is a cathelicidin-like model peptide (NH(2)-KNLRRIIRKIIHIIKKYG-COOH) with potent, broad-spectrum bactericidal activity. (15)N NMR spectra of oriented ovispirin reconstituted into synthetic phospholipids show that the helical peptide is predominantly oriented in the plane of the lipid bilayer, except for a small portion of the helix, possibly at the C-terminus, which deviates from the surface orientation. This suggests differential insertion of the peptide backbone into the lipid bilayer. (15)N spectra of both oriented and unoriented peptides show a reduced (15)N chemical shift anisotropy at room temperature compared with that of rigid proteins, indicating that the peptide undergoes uniaxial rotational diffusion around the bilayer normal with correlation times shorter than 10(-4) s. This motion is frozen below the gel-to-liquid crystalline transition temperature of the lipids. Ovispirin interacts strongly with the lipid bilayer, as manifested by the significantly reduced (2)H quadrupolar splittings of perdeuterated palmitoyloleoylphosphatidylcholine acyl chains upon peptide binding. Therefore, ovispirin is a curved helix residing in the membrane-water interface that executes rapid uniaxial rotation. These structural and dynamic features are important for understanding the antimicrobial function of this peptide.

Indirect NMR detection in solids with multiple cross-polarization periods

The use of indirect detection for signal enhancement in solids is much less common than in liquids, but has attracted renewed interest recently. In this work we describe an indirect detection scheme that offers a large signal enhancement for rare spins in solids. The method uses multiple periods of cross polarization, each followed by an evolution period. The latter is increased stepwise in a pseudo 2D experiment, in which the signal of the rare spin is detected as modulation of the abundant spin. As an illustration of this method, the natural abundance deuterium NMR spectrum of a static powder sample of 1,2,4,5-tetramethylbenzene is presented.