A robust technique for two-dimensional separation of undistorted chemical-shift anisotropy powder patterns in magic-angle-spinning NMR

Composite-180° pulse-based symmetry sequences to recouple proton chemical shift anisotropy tensors under ultrafast MAS solid-state NMR spectroscopy

There is considerable interest in the measurement of proton ((1)H) chemical shift anisotropy (CSA) tensors to obtain deeper insights into H-bonding interactions which find numerous applications in chemical and biological systems. However, the presence of strong (1)H/(1)H dipolar interaction makes it difficult to determine small size (1)H CSAs from the homogeneously broadened NMR spectra. Previously reported pulse sequences for (1)H CSA recoupling are prone to the effects of radio frequency field (B1) inhomogeneity. In the present work we have carried out a systematic study using both numerical and experimental approaches to evaluate γ-encoded radio frequency (RF) pulse sequences based on R-symmetries that recouple (1)H CSA in the indirect dimension of a 2D (1)H/(1)H anisotropic/isotropic chemical shift correlation experiment under ultrafast magic angle spinning (MAS) frequencies. The spectral resolution and sensitivity can be significantly improved in both frequency dimensions of the 2D (1)H/(1)H correlation spectrum without decoupling (1)H/(1)H dipolar couplings but by using ultrafast MAS rates up to 70 kHz. We successfully demonstrate that with a reasonable RF field requirement (<200 kHz) a set of symmetry-based recoupling sequences, with a series of phase-alternating 270°0-90°180 composite-180° pulses, are more robust in combating B1 inhomogeneity effects. In addition, our results show that the new pulse sequences render remarkable (1)H CSA recoupling efficiency and undistorted CSA lineshapes. Experimental results on citric acid and malonic acid comparing the efficiencies of these newly developed pulse sequences with that of previously reported CSA recoupling pulse sequences are also reported under ultrafast MAS conditions.

NMR of bent-core nematogens: a mini-review

This review focuses on two of our studied bent-core liquid crystals 2-methyl-3-[4-(4-octylbenzoyloxy)-benzylidene]-amino-benzoic acid 4-(4-dodecylphenylazo)-phenyl ester A131, and a shape-persistent fluorenone mesogen, F1a by means of temperature-dependent (13)C NMR spectra and deuterium NMR spectra, respectively. Orientational order parameters, molecular packing, and/or conformations in the nematic phases or nematic glass can be extracted from the observed NMR spectra. These will be discussed in terms of certain motional models. In particular, F1a has been found to form a biaxial nematic glass. The derived orientational order parameters would put the biaxial system of phase symmetry lower than orthorhombic.

Molecular dynamics of neutral polymer bonding agent (NPBA) as revealed by solid-state NMR spectroscopy

Neutral polymer bonding agent (NPBA) is one of the most promising polymeric materials, widely used in nitrate ester plasticized polyether (NEPE) propellant as bonding agent. The structure and dynamics of NPBA under different conditions of temperatures and sample processing are comprehensively investigated by solid state NMR (SSNMR). The results indicate that both the main chain and side chain of NPBA are quite rigid below its glass transition temperature (Tg). In contrast, above the Tg, the main chain remains relatively immobilized, while the side chains become highly flexible, which presumably weakens the interaction between bonding agent and the binder or oxidant fillers and in turn destabilizes the high modulus layer formed around the oxidant fillers. In addition, no obvious variation is found for the microstructure of NPBA upon aging treatment or soaking with acetone. These experimental results provide useful insights for understanding the structural properties of NPBA and its interaction with other constituents of solid composite propellants under different processing and working conditions.

Unique molecular dynamics of structural elements in an asymmetric Janus bisamide supramolecule characterized by solid-state NMR

An asymmetric tapered Janus bisamide supramolecule consisting of 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido]benzene bisamide (abbreviated as C22PhBAEO3) can possess three-dimensional (3D) long-range order under mild thermal treatment conditions. To understand its structural formation and unique phase-transition processes, the locally detailed structure and molecular dynamics of its structural elements in disordered and ordered phases of C22PhBAEO3 were investigated using various solid-state (SS) NMR techniques at the atomic level. On the basis of the determined conformations and packing structures of the alkyl chains in ordered and disordered crystalline phases, along with the geometry and kinetic parameters of the structural elements' dynamics, this study addresses the self-assembly, the phase-transition mechanisms, and the relationship between the structure and dynamics of these asymmetric Janus bisamide supramolecules.

Structural analysis of a banana-liquid crystal in the B4 phase by solid-state NMR

In this paper, we present a structural investigation of 1,3-phenylene bis[4-((4-10-decyloxyphenyl)iminomethyl)-benzoate], known as a banana-liquid crystal, in the B4 phase, which was performed by solid-state nuclear magnetic resonance (NMR) methodology combined with quantum chemical calculations. The present solid-state NMR measurements including (13)C CPMAS, 2D TOSS-deTOSS, dipole-dephase, 1D and 2D EXSY, and MAS-j-HMQC provided accurate spectral assignments and unambiguous NMR parameters such as (13)C chemical shift tensors, which were used for construction of the three-dimensional structure with the aid of density functional theory calculations. In the obtained molecular structure, two arms of the bent-core molecule are asymmetrically expanded such that the direction of the dipole moment is off alignment with respect to the middle line of the center benzene ring. It is this antisymmetric structure that is the origin of the twisted helical system in the B4 phase.

Recoupling of chemical shift anisotropy by R-symmetry sequences in magic angle spinning NMR spectroscopy

(13)C and (15)N chemical shift (CS) interaction is a sensitive probe of structure and dynamics in a wide variety of biological and inorganic systems, and in the recent years several magic angle spinning NMR approaches have emerged for residue-specific measurements of chemical shift anisotropy (CSA) tensors in uniformly and sparsely enriched proteins. All of the currently existing methods are applicable to slow and moderate magic angle spinning (MAS) regime, i.e., MAS frequencies below 20 kHz. With the advent of fast and ultrafast MAS probes capable of spinning frequencies of 40-100 kHz, and with the superior resolution and sensitivity attained at such high frequencies, development of CSA recoupling techniques working under such conditions is necessary. In this work, we present a family of R-symmetry based pulse sequences for recoupling of (13)C∕(15)N CSA interactions that work well in both natural abundance and isotopically enriched systems. We demonstrate that efficient recoupling of either first-rank (σ(1)) or second-rank (σ(2)) spatial components of CSA interaction is attained with appropriately chosen γ-encoded RN(n)(v) symmetry sequences. The advantage of these γ-encoded RN(n)(v)-symmetry based CSA (RNCSA) recoupling schemes is that they are suitable for CSA recoupling under a wide range of MAS frequencies, including fast MAS regime. Comprehensive analysis of the recoupling properties of these RN(n)(v) symmetry sequences reveals that the σ(1)-CSA recoupling symmetry sequences exhibit large scaling factors; however, the partial homonuclear dipolar Hamiltonian components are symmetry allowed, which makes this family of sequences suitable for CSA measurements in systems with weak homonuclear dipolar interactions. On the other hand, the γ-encoded symmetry sequences for σ(2)-CSA recoupling have smaller scaling factors but they efficiently suppress the homonuclear dipole-dipole interactions. Therefore, the latter family of sequences is applicable for measurements of CSA parameters in systems with strong homonuclear dipolar couplings, such as uniformly-(13)C labeled biological solids. We demonstrate RNCSA NMR experiments and numerical simulations establishing the utility of this approach to the measurements of (13)C and (15)N CSA parameters in model compounds, [(15)N]-N-acetyl-valine (NAV), [U-(13)C, (15)N]-alanine, [U-(13)C,(15)N]-histidine, and present the application of this approach to [U-(13)C∕(15)N]-Tyr labeled C-terminal domain of HIV-1 CA protein.

Chemical shift tensor determination using magnetically oriented microcrystal array (MOMA): 13C solid-state CP NMR without MAS

Chemical shift tensors for the carboxyl and methyl carbons of L-alanine crystals were determined using a magnetically oriented microcrystal array (MOMA) prepared from a microcrystalline powder sample of L-alanine. A MOMA is a single-crystal-like composite in which microcrystals are aligned three-dimensionally in a matrix resin. The single-crystal rotation method was applied to the MOMA to determine the principal values and axes of the chemical shift tensors. The result showed good agreement with the literature data for the single crystal of L-alanine. This demonstrates that the present technique is a powerful tool for determining the chemical shift tensor of a crystal from a microcrystal powder sample.

Fast and slow dynamics in a discotic liquid crystal with regions of columnar order and disorder

Aromatic disk-shaped molecules tend to self-organize into a herringbone packing where the disks are inclined at angles ±θ with respect to the axis of the column. In discotic liquid crystals this can introduce defects between stacks of limited length. In a C(3)-symmetric hexa-peri-hexabenzocoronene, solid-state NMR, x-ray scattering, and rheology identifies such a packing with θ=43° and stacks of about seven disks. Disordered regions containing defects fill the space in between the ordered stacks. Biaxial intra- and intercolumnar dynamics differing by eight decades are identified.

An efficient amplification pulse sequence for measuring chemical shift anisotropy under fast magic-angle spinning

A two-dimensional experiment for measuring chemical shift anisotropy (CSA) under fast magic-angle spinning (MAS) is presented. The chemical shift anisotropy evolution is amplified by a sequence of π-pulses that repetitively interrupt MAS averaging. The amplification generates spinning sideband manifolds in the indirect dimension separated by the isotropic shift along the direct dimension. The basic unit of the pulse sequence is designed based on the magic-angle turning experiment and can be concatenated for larger amplification factors.

Technical aspects of fast magic-angle turning NMR for dilute spin-1/2 nuclei with broad spectra

For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000 ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan's modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8γB(1); methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple-π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb(2)Te(3) and Ag(0.53)Pb(18)Sb(1.2)Te(20), with spectra spanning up to 190 kHz, at 22 kHz MAS. We have also determined the (125)Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation.

The refocused discrimination induced by variable amplitude minipulses (DIVAM) experiment — Improved domain selection in semicrystalline fluoropolymers by 19F solid state nuclear magnetic resonance spectroscopy

The discrimination induced by variable amplitude minipulses (DIVAM) filter can be tuned to select for signals from a particular domain, therefore it is possible to obtain signals specific to different domains using only one experiment. An early description of the DIVAM sequence, where the filter terminates with cross-polarization, explains this tune ability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes. Recently, a more complete description of the selection behaviour was offered for the DIVAM filter, when it was directly applied to the observed nucleus (direct DIVAM), taking into account both the incoherent and coherent terms. Direct DIVAM experiments on poly(vinylidenefluoride) (PVDF) show significant phase distortions when large excitation angles were used. The signal from the amorphous domain is seen to nutate in a normal fashion with respect to the excitation angle, while those from the crystalline and defect units did not. The refocused DIVAM sequence is introduced to restore normal nutation for all signals. The selection behaviour is investigated using SIMPSON (simulation program for solid-state nuclear magnetic resonance (NMR) spectroscopy) simulations. These illustrate that the isotropic shift terms have been effectively removed and the dipolar term attenuated, such that the chemical shift anisotropy (CSA) leads to domain selection; however, in a different manner than seen in direct DIVAM. Therefore, this sequence provides a method to select on the basis of the CSA term in the presence of both strong dipolar couplings and a large range of isotropic shifts.

Measurement and calculation of 13C chemical shift tensors in α-glucose and α-glucose monohydrate

The 13C chemical shift tensors of two crystalline forms of glucose (α-glucose and α-glucose·H2O) were determined from one-dimensional (1D) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy experiments. The experimental values determined from 1D and 2D methods are in very good agreement. Quantum chemical calculations were also carried out using the gauge-including projector augmented wave (GIPAW) method for plane-wave density functional theory (DFT) as implemented in the CAmbridge Serial Total Energy Package (CASTEP). The calculated 13C chemical shifts were found to be in excellent agreement with experimental values for crystal structures that had their hydrogen atoms optimized and after an appropriate calibration was applied to convert calculated chemical shieldings into chemical shifts. The work presented here lays an important foundation for future solid-state NMR and quantum chemical calculation investigations of the various crystalline forms of cellulose.

Local and global dynamics of the G protein-coupled receptor CXCR1

The local and global dynamics of the chemokine receptor CXCR1 are characterized using a combination of solution NMR and solid-state NMR experiments. In isotropic bicelles (q = 0.1), only 13% of the expected number of backbone amide resonances is observed in (1)H/(15)N HSQC solution NMR spectra of uniformly (15)N-labeled samples; extensive deuteration and the use of TROSY made little difference in the 800 MHz spectra. The limited number of observed amide signals is ascribed to mobile backbone sites and assigned to specific residues in the protein; 19 of the signals are from residues at the N-terminus and 25 from residues at the C-terminus. The solution NMR spectra display no evidence of local backbone motions from residues in the transmembrane helices or interhelical loops of CXCR1. This finding is reinforced by comparisons of solid-state NMR spectra of both magnetically aligned and unoriented bilayers containing either full-length or doubly N- and C-terminal truncated CXCR1 constructs. CXCR1 undergoes rapid rotational diffusion about the normal of liquid crystalline phospholipid bilayers; reductions in the frequency span and a change to axial symmetry are observed for both carbonyl carbon and amide nitrogen chemical shift powder patterns of unoriented samples containing (13)C- and (15)N-labeled CXCR1. In contrast, when the phospholipids are in the gel phase, CXCR1 does not undergo rapid global reorientation on the 10(4) Hz time scale defined by the carbonyl carbon and amide nitrogen chemical shift powder patterns.

Protonation, tautomerization, and rotameric structure of histidine: a comprehensive study by magic-angle-spinning solid-state NMR

Histidine structure and chemistry lie at the heart of many enzyme active sites, ion channels, and metalloproteins. While solid-state NMR spectroscopy has been used to study histidine chemical shifts, the full pH dependence of the complete panel of (15)N, (13)C, and (1)H chemical shifts and the sensitivity of these chemical shifts to tautomeric structure have not been reported. Here we use magic-angle-spinning solid-state NMR spectroscopy to determine the (15)N, (13)C, and (1)H chemical shifts of histidine from pH 4.5 to 11. Two-dimensional homonuclear and heteronuclear correlation spectra indicate that these chemical shifts depend sensitively on the protonation state and tautomeric structure. The chemical shifts of the rare π tautomer were observed for the first time, at the most basic pH used. Intra- and intermolecular hydrogen bonding between the imidazole nitrogens and the histidine backbone or water was detected, and N-H bond length measurements indicated the strength of the hydrogen bond. We also demonstrate the accurate measurement of the histidine side-chain torsion angles χ(1) and χ(2) through backbone-side chain (13)C-(15)N distances; the resulting torsion angles were within 4° of the crystal structure values. These results provide a comprehensive set of benchmark values for NMR parameters of histidine over a wide pH range and should facilitate the study of functionally important histidines in proteins.

Solid-state NMR techniques for the structural determination of amyloid fibrils

This review discusses the solid-state NMR techniques developed for the study of amyloid fibrils. Literature up to the end of 2010 has been surveyed and the materials are organized according to five categories, viz. homonuclear dipolar recoupling and polarization transfer via J-coupling, heteronuclear dipolar recoupling, correlation spectroscopy, recoupling of chemical shift anisotropy, and tensor correlation. Our emphasis is on the NMR techniques and their practical aspects. The biological implications of the results obtained for amyloid fibrils are only briefly discussed. Our main objective is to showcase the power of NMR in the study of biological unoriented solids.

A simple one-dimensional method of chemical shift anisotropy determination under MAS conditions

A method of determination of chemical shift anisotropy (CSA) tensor principal components under MAS condition is presented. It is a simple, one-dimensional, and robust alternative to the commonly exploited, but more complicated 2D-PASS. The required CSA components are delivered by simultaneous numerical analysis of a few regular MAS spectra acquired under different spinning rates.

Measurement of the principal values of the chemical-shift tensors of overlapping protonated and unprotonated carbons with the 2D-SUPER technique and dipolar dephasing (DD-SUPER)

A modified 2D-SUPER technique is demonstrated to allow independent measurement of the principal values of the chemical-shift tensors of overlapping protonated and unprotonated carbons. The insertion of a dipolar-dephasing period into the sequence causes loss of signal from protonated carbons. The spectrum obtained with this modification allows one to determine the principal values of the unprotonated carbons with high precision. Subsequent fitting of the usual 2D-SUPER spectrum, with the chemical-shift parameters of the unprotonated carbons fixed, gives the parameters of the overlapped resonances of the protonated carbons. As an example, we report the determination of the (13)C chemical-shift parameters of the carbons of form II of piroxicam. The experimental results are compared with those obtained from calculations using the DFT/GIAO method. Potential applications of this method are discussed.

Dipolar-coupling-mediated total correlation spectroscopy in solid-state 13C NMR: selection of individual 13C-13C dipolar interactions

Herein is described a useful approach in solid-state NMR, for selecting homonuclear (13)C-(13)C spin pairs in a multiple-(13)C homonuclear dipolar coupled spin system. This method builds upon the zero-quantum (ZQ) dipolar recoupling method introduced by Levitt and coworkers (Marin-Montesinos et al., 2006) by extending the originally introduced one-dimensional (1D) experiment into a two-dimensional (2D) method with selective irradiation scheme, while moving the (13)C-(13)C mixing scheme from the transverse to the longitudinal mode, together with a dramatic improvement in the proton decoupling efficiency. Selective spin-pair recoupling experiments incorporating Gaussian and cosine-modulated Gaussian pulses for inverting specific spins were performed, demonstrating the ability to detect informative, simplified/individualized, long-range (13)C-(13)C homonuclear dipolar coupling interactions more accurately by removing less informative, stronger, short-range (13)C-(13)C interactions from 2D correlation spectra. The capability of this new approach was demonstrated experimentally on uniformly (13)C-labeled Glutamine and a tripeptide sample, GAL.

On the magic-angle turning and phase-adjusted spinning sidebands experiments

The underlying relation between the magic-angle turning (MAT) and phase-adjusted spinning sidebands (PASS) experiments is examined. The MAT experiment satisfies the PASS conditions for separating spinning sidebands with a non-constant total evolution time and only requires linear t(1) increments of up to one rotor period. The time-domain data of the two experiments are related by a shearing transformation. A combination of the linear evolution-time increments of MAT and simple data processing of PASS are particularly attractive for the implementation of MAT for measuring chemical shift anisotropy.

Fate of the amino acid in glucose-glycine melanoidins investigated by solid-state nuclear magnetic resonance (NMR)

The fate of the amino acid in the model Maillard reaction between glucose and glycine in a 1:1 molar ratio has been investigated by applying advanced 13C and 15N solid-state nuclear magnetic resonance (NMR) techniques to 13C- and 15N-labeled melanoidins formed in dry and solution reactions. Quantitative 13C NMR shows that approximately 23% of carbon is from glycine; the approximately 2% loss compared to the 25% glycine C in the reactants is due to the COO moiety being liberated as CO2 (Strecker degradation). 13C J-modulation experiments on melanoidins made from doubly 13C-labeled glycine show that the C-C backbone bond of about two-thirds of the incorporated amino acid stays intact, and about half of all glycine is incorporated as N-CH2-COO without fragmentation. Degradation processes without CO2 loss affect about one-eighth of glycine in dry reaction and about one-fourth in solution. These results indicate that Strecker degradation affects about one-fourth (dry reaction) to one-third (in solution) of all glycine but is not the main pathway of glycine incorporation. Spectra of Strecker degradation products show that C2 of glycine reacts to form N-CH3, C-CHn-C, or aromatic units, but not pyrazines or pyridines. The gycine-C1 carbon incorporated into the melanoidins remains>or=90% part of COO moieties; approximately 5% of amides have also been detected. The C2-N bond stays intact for approximately 70% of the incorporated glycine. The 15N spectra show many peaks, over a 200 ppm range, documenting a multitude of different chemical environments of nitrogen, but no enamines or imines. The majority (>78%) of nitrogen, in particular most pyrrolic N, is not protonated. Because N-H predominates in amino acids and proteins, nonprotonated nitrogen may be a characteristic marker of Maillard reaction products.

Determining the helical tilt of membrane peptides using electron paramagnetic resonance spectroscopy

Theoretical calculations of hyperfine splitting values derived from the EPR spectra of TOAC spin-labeled rigid aligned alpha-helical membrane peptides reveal a unique periodic variation. In the absence of helical motion, a plot of the corresponding hyperfine splitting values as a function of residue number results in a sinusoidal curve that depends on the helical tilt angle that the peptide makes with respect to the magnetic field. Motion about the long helical axis reduces the amplitude of the curve and averages out the corresponding hyperfine splitting values. The corresponding spectra can be used to determine the director axis tilt angle from the TOAC spin label, which can be used to calculate the helical tilt angle due to the rigidity of the TOAC spin label. Additionally, this paper describes a method to experimentally determine this helical tilt angle from the hyperfine splitting values of three consecutive residues.

Effects of L-spin longitudinal quadrupolar relaxation in S[L] heteronuclear recoupling and S-spin magic-angle spinning NMR

In experiments on S-L heteronuclear spin systems with evolution of the S-spin magnetization under the influence of a quadrupolar nucleus (L-spin), effects of longitudinal quadrupolar (T(1Q)) relaxation of the L-spin coherence on the sub-millisecond time scale have been documented and explored, and methods for minimizing their effect have been demonstrated. The longitudinal relaxation results in heteronuclear dephasing even in the reference signal S(0) of S[L] REDOR, REAPDOR, RIDER, or SPIDER experiments, due to T(1Q)-relaxation of the transiently generated S(y)L(z) coherence, reducing or even eliminating the observable dephasing DeltaS. Pulse sequences for measuring an improved reference signal S(00) with minimal heteronuclear recoupling but the same number of pulses as for S(0) and S have been demonstrated. From the observed intensity DeltaS(0)=S(00)-S(0) and the SPIDER signal DeltaS/S(0), T(1Q) can be estimated. Accelerated decays analogous to the dipolar S(0) curves will occur in T(2) measurements for J-coupled S-L spin pairs. Even in the absence of recoupling pulses, fast T(1Q) relaxation of the unobserved nucleus shortens the transverse relaxation time T(2S,MAS) of the observed nucleus, in particular at low spinning frequencies, due to unavoidable heteronuclear dipolar evolution during a rotation period. The observed spinning-frequency dependence of T(2S,MAS) matches the theoretical prediction and may be used to estimate T(1Q). The effects are demonstrated on several (13)C[(14)N] spin systems, including an arginine derivative, the natural N-acetylated polysaccharide chitin, and a model peptide, (POG)(10).

Arginine dynamics in a membrane-bound cationic beta-hairpin peptide from solid-state NMR

The site-specific motion of Arg residues in a membrane-bound disulfide-linked antimicrobial peptide, protegrin-1 (PG-1), was investigated by using magic-angle-spinning solid-state NMR spectroscopy to better understand the membrane insertion and lipid interaction of this cationic membrane-disruptive peptide. The C-H and N-H dipolar couplings and 13C chemical shift anisotropies were measured in the anionic POPE/POPG membrane, and were found to be reduced from the rigid-limit values by varying extents; this indicates the presence of segmental motion. An Arg residue at the beta-turn region of the peptide showed much weaker spin interactions, which indicates larger amplitudes of motion than an Arg residue in the beta-strand region of the peptide. This is consistent with the exposure of the beta turn to the membrane surface and the immersion of the beta strand in the hydrophobic middle of the membrane, and supports the previously proposed oligomerization of the peptide into beta barrels in the anionic membrane. The 13C T2 and 1H T(1rho) relaxation times indicate that the beta-turn backbone undergoes large-amplitude intermediate-timescale motion in the fluid phase of the membrane; this causes significant line broadening and loss of spectral intensity. This study illustrates the strong correlation between the dynamics and structure of membrane proteins, and the capability of solid-state NMR spectroscopy to provide detailed information on site-specific dynamics in complex membrane-protein assemblies.