Chemical shift anisotropy amplification with high amplification factor and improved sensitivity

An improved version of the recently proposed chemical shift anisotropy amplification experiment is described. The original experiment correlates a fast magic angle spinning spectrum in the omega2 dimension with a sideband pattern in omega1 in which the intensities mimic those for a sample spinning at a fraction of the rate omegar/N. Advantages of the experiment include the use of standard methods to extract the principal tensor components from the omega1 sideband patterns and the small number of t1 increments required. The improved version described here permits large amplification factors N to be obtained without resort to prohibitively long sequences of pi-pulses and allows sensitivity to be maximized by eliminating the need to store the magnetization along the z-axis for t1. Amplification factors up to 32 are demonstrated experimentally.     

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.     

Correlation of fast and slow chemical shift spinning sideband patterns under fast magic-angle spinning

A new two-dimensional solid-state NMR experiment, which correlates slow and fast chemical shift anisotropy sideband patterns is proposed. The experiment, dubbed ROSES, is performed under fast magic-angle spinning and leads to an isotropic spectrum in the directly detected omega(2) dimension. In the evolution dimension omega(1), the isotropic chemical shift is reduced by a factor S, and spinning sidebands are observed spaced by a scaled effective spinning speed omega(R)/S. These spinning sidebands patterns are not identical to those observed with standard slow magic-angle spinning experiments. Chemical shift anisotropy parameters can be accurately extracted with standard methods from these spinning sideband patterns. The experiment is demonstrated with carbon-13 experiments on powdered samples of a dipeptide and a cyclic undecapeptide, cyclosporin-A.     

Applications of the CSA-amplified PASS experiment

The recently reported CSA-amplified PASS experiment correlates the spinning sidebands at the true spinning frequency omega(r) with the spinning sidebands that would be obtained at the effective spinning frequency omega(r)/N, where N is termed the scaling factor. The experiment is useful for the measurement of small chemical shift anisotropies, for which slow magic-angle spinning frequencies, required to measure several spinning sidebands, can be unstable. We have experimentally evaluated the reliability of this experiment for this application. In particular we have demonstrated that large scaling factors of the order of N=27 may be used, whilst still obtaining accurate chemical shift sideband intensities at the effective spinning frequency from the F(1) projection. Moreover, the sideband intensities are accurately obtained even in the presence of significant pulse imperfections. A second application of the CSA-amplified PASS experiment is the measurement of the chemical shift anisotropy of sites that experience homonuclear dipolar coupling, as may be found in uniformly labelled biological molecules, or for nuclei with a high natural abundance. The effects of homonuclear dipolar coupling on CSA-amplified PASS spectra has been investigated by numerical simulations and are demonstrated using uniformly (13)C enriched l-histidine monohydrochloride monohydrate.     

Correlating fast and slow chemical shift spinning sideband patterns in solid-state NMR

An experiment is presented that enables the measurement of small chemical shift anisotropy tensors under fast magic-angle spinning (MAS). The two-dimensional spectra obtained give a fast MAS sideband pattern in the directly observed dimension with the spinning sideband intensities equivalent to the chemical shift anisotropy scaled by a factor of N, or equivalently the sample spinning frequency scaled by 1/N, in the indirectly observed dimension. The scaling factor may be arbitrarily varied by changing the number and timings of the rotor synchronized pi-pulses used. Desirable features of the experiment include a fixed length pulse sequence and efficient sampling of the indirectly observed dimension. In addition, neither quadrature detection in the indirect dimension nor storage periods are required, consequently no signal intensity is discarded by the pulse sequence. The experiment is demonstrated using (31)P NMR of sodium phosphate and (13)C NMR of fumaric acid monoethyl ester for which a scaling factor of N=10.2 was employed.     

Characterization of 15N chemical shift tensors via 15N-13C REDOR and 1N-1H dipolar-shift CPMAS NMR spectroscopy

As part of our studies on the characterization of 15N chemical shift anisotropy (CSA) via magic angle spinning (MAS) NMR spectroscopy, we have investigated via numerical simulations the sensitivity of two different REDOR experimental protocols to the angles defining the orientation of the 15N-13C' bond vector in the principal axis system of the 15N CSA tensor of the amide nitrogen in a peptide bond. Additionally, employing polycrystalline samples of 15N and 13C', 15N-labeled acetanilide, we have obtained, in a first study of this type, the orientation of the 15N CSA tensor in the molecular frame by orienting the tensor with respect to the 15N-3C' and 15N-1H dipolar vectors via 15N-13C' REDOR and 15N-1H dipolar-shift MAS experiments, respectively.     

59Co chemical shift anisotropy and quadrupole coupling for K3Co(CN)6 from MQMAS and MAS NMR spectroscopy

59Co triple-quantum (3Q) MAS and single-pulse MAS NMR spectra of K3Co(CN)6 have been obtained at 14.1 T and used in a comparison of these methods for determination of small chemical shift anisotropies for spin I = 7/2 nuclei. From the 3QMAS NMR spectrum a spinning sideband manifold in the isotropic dimension with high resolution is reconstructed from the intensities of all spinning sidebands in the 3QMAS spectrum. The chemical shift anisotropy (CSA) parameters determined from this spectrum are compared with those obtained from MAS NMR spectra of (i) the complete manifold of spinning sidebands for the central and satellite transitions and of (ii) the second-order quadrupolar lineshapes for the centerband and spinning sidebands from the central transition. A good agreement between the three data sets, all of high precision, is obtained for the shift anisotropy (delta(sigma) = delta(iso) - delta(zz)) whereas minor deviations are observed for the CSA asymmetry parameter (eta(sigma)). The temperature dependence of the isotropic 59Co chemical shift has been studied over a temperature range from -28 to +76 degrees C. A linear and positive temperature dependence of 0.97 ppm/degree C is observed.     

Solid state 31P and 207Pb MAS NMR studies on polycrystalline O,O'-dialkyldithiophosphate lead(II) complexes

A number of lead(II) O,O'-dialkyldithiophosphate complexes were studied by (13)C, (31)P, and (207)Pb MAS NMR. Simulations of (31)P chemical shift anisotropy using spinning sideband analysis reveal a linear relationship between the SPS bond angle and the principal values delta(22) and delta(33) of the (31)P chemical shift tensor. The (31)P CSA data were used to assign ligands with different structural functions. In the cases of diethyldithiophosphate and di-iso-butyldithiophosphate lead(II) complexes, (2)J((31)P, (207)Pb)-couplings were resolved and used to confirm the suggested assignment of the ligands. The SIMPSON computer program was used to calculate (31)P and (207)Pb spectral sideband patterns.     

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 15N chemical shift anisotropy in a protein dissolved in a dilute liquid crystalline medium with the application of magic angle sample spinning

The chemical shifts of nuclei that have chemical shielding anisotropy, such as the 15N amide in a protein, show significant changes in their chemical shifts when the sample is altered from an isotropic state to an aligned state. Such orientation-dependent chemical shift changes provide information on the magnitudes and orientation of the chemical shielding tensors relative to the molecule's alignment frame. Because of the extremely high sensitivity of the chemical shifts to the sample conditions, the changes in chemical shifts induced by adding aligned bicelles do not arise only from the protein alignment but should also include the accumulated effects of environmental changes including protein-bicelle interactions. With the aim of determining accurate 15N chemical shielding tensor values for solution proteins, here we have used magic angle sample spinning (MAS) to observe discriminately the orientation-dependent changes in the 15N chemical shift. The application of MAS to an aligned bicelle solution removes the torque that aligns the bicelles against the magnetic field. Thus, the application of MAS to a protein in a bicelle solution eliminates only the molecular alignment effect, while keeping all other sample conditions the same. The observed chemical shift differences between experiments with and without MAS therefore provide accurate values of the orientation-dependent 15N chemical shifts. From the values for ubiquitin in a 7.5% (w/v) bicelle medium, we determined the 15N chemical shielding anisotropy (CSA) tensor. For this evaluation, we considered uncertainties in measuring the 1H-15N dipolar couplings and the 15N chemical shifts and also structural noise present in the reference X-ray structure, assuming a random distribution of each NH bond vector in a cone with 5 degrees deviation from the original orientation. Taking into account these types of noise, we determined the average 15N CSA tensor for the residues in ubiquitin as Delta sigma=-162.0+/-4.3 ppm, eta=0.18+/-0.02, and beta=18.6+/-0.5 degrees, assuming a 1H-15N bond length of 1.02 A. These tensor values are consistent with those obtained from solid-state NMR experiments.     

Errors in chemical shift tensor components and orientation in the molecular frame as obtained from MAS NMR spinning sideband analysis

It is shown how to calculate random errors in chemical shift tensor components and in the Euler angles which fix the orientation of the σ tensor in the molecular frame, as obtained from spinning sideband analysis of MAS NMR spectra of powdered solids, when heteronuclear dipolar coupling interactions occur in a two spin system. The procedure was applied to experimental data corresponding to the chemical shift tensor of a carbon-13 bonded to a phosphorus-31 nucleus. Clues are given concerning the experimental variables to be set in order to obtain the desired accuracy in the orientation angles.     

Small 51V chemical shift anisotropy for LaVO4 from MQMAS and MAS NMR spectroscopy

51V MQMAS NMR of the triple-quantum transitions is shown to be particularly useful in the determination of the sign and magnitude of the chemical shift anisotropy (CSA) parameter delta(sigma)(= delta(iso)-delta(zz)) along with the asymmetry parameter (eta(sigma)) for a vanadium environment with a small CSA and a rather strong quadrupole coupling. This is demonstrated for the orthovanadate LaVO(4) for which 51V magic-angle spinning (MAS) NMR of the central and satellite transitions at 14.1T gives precise values for the quadrupole coupling parameters, however, an ambiguous sign for delta(sigma). The CSA parameters are reliably obtained from analysis of the spinning sidebands observed in a 51V triple-quantum MAS experiment. Combining these data with least-squares analysis of the manifold of spinning sidebands in the single-pulse MAS NMR spectrum results in a precise determination of the magnitudes and relative orientation of the 51V quadrupole coupling and CSA tensors for LaVO(4).     

2H chemical shift anisotropies from high-field 2H MAS NMR spectroscopy

2H chemical shift anisotropies (CSAs) have been determined for the first time for polycrystalline samples employing 2H MAS NMR spectroscopy at high magnetic field strength (14.1 T). The 2H CSA is reflected as distinct asymmetries in the manifold of spinning sidebands (ssbs) observed for the two overlapping single-quantum transitions. Least-squares fitting to the manifold of ssbs allows determination of the 2H CSA parameters along with the quadrupole coupling parameters. This is demonstrated for KD2PO4, ND4D2PO4, KDSO4, KDCO3, alpha-(COOD)2, alpha-(COOD)2.2D2O, and boehmite (AlOOD) which exhibit 2H shift anisotropies in the range 13< or =deltasigma< or =27 ppm. For fixed values of the shift anisotropy and the 2H quadrupole coupling it is shown that the precision of the CSA parameters depends strongly on the asymmetry parameter (etaQ) for the quadrupole coupling tensor, giving the highest precision for etaQ approximately 0. The 2H CSA parameters (deltasigma and etasigma) are in good agreement with 1H CSA data reported in the literature for the corresponding protonated samples from 1H NMR spectra employing various homonuclear decoupling techniques. The determination of 2H quadrupole coupling parameters and 2H (1H) CSAs from the same 2H MAS NMR experiment may be particularly useful in studies of hydrogen bonding since the 2H quadrupole coupling constant and the CSA appear to characterize bond lengths in a hydrogen bond in a different manner.     

Multiple-rotor-cycle 2D PASS experiments with applications to (207)Pb NMR spectroscopy

Thetwo-dimensional phase-adjusted spinning sidebands (2D PASS) experiment is a useful technique for simplifying magic-angle spinning (MAS) NMR spectra that contain overlapping or complicated spinning sideband manifolds. The pulse sequence separates spinning sidebands by their order in a two-dimensional experiment. The result is an isotropic/anisotropic correlation experiment, in which a sheared projection of the 2D spectrum effectively yields an isotropic spectrum with no sidebands. The original 2D PASS experiment works best at lower MAS speeds (1-5 kHz). At higher spinning speeds (8-12 kHz) the experiment requires higher RF power levels so that the pulses do not overlap. In the case of nuclei such as (207)Pb, a large chemical shift anisotropy often yields too many spinning sidebands to be handled by a reasonable 2D PASS experiment unless higher spinning speeds are used. Performing the experiment at these speeds requires fewer 2D rows and a correspondingly shorter experimental time. Therefore, we have implemented PASS pulse sequences that occupy multiple MAS rotor cycles, thereby avoiding pulse overlap. These multiple-rotor-cycle 2D PASS sequences are intended for use in high-speed MAS situations such as those required by (207)Pb. A version of the multiple-rotor-cycle 2D PASS sequence that uses composite pulses to suppress spectral artifacts is also presented. These sequences are demonstrated on (207)Pb test samples, including lead zirconate, a perovskite-phase compound that is representative of a large class of interesting materials.     

A new sensitive isotropic-anisotropic separation experiment-SPEED MAS

A new sensitive 2D isotropic-anisotropic separation experiment that utilizes stroboscopic phase encoding in the evolution dimension (SPEED) under magic angle sample spinning is presented. This 2D experiment consists of a train of 2N - 1 pi pulses that are applied over 2N rotor periods. The pi pulse train effectively reduces the apparent spinning speed in the evolution dimension by a factor of 1 / (2N) from the mechanical spinning speed. Thus, problems commonly associated with magic angle turning such as stable slow spinning, different matching and TPPM proton decoupling conditions are avoided. Data replication similar to the five pi replicated magic angle turning (FIREMAT) and pseudo 2D sideband suppression (P2DSS) experiments transfers resolution from the acquisition dimension to the evolution dimension. Hence, large spectral windows with good digital resolution are obtained with a few evolution increments. Here, slow spinning sideband patterns are extracted from the replicated 2D dataset with TIGER processing. Nevertheless, 2D Fourier transformation is also applicable. The extracted sideband patterns are identical to magic angle turning sideband pattern allowing for easy extraction of principal shift components. Accurate (13)C principal shift components are obtained for 3-methylglutaric acid using SPEED and FIREMAT experiments to validate the method. Furthermore, SPEED spectra for calcium acetate and alpha santonin are reported to show the wide applicability of this new experiment.     

An approximate method of sideband intensity calculation in magic angle spinning NMR

A theory of sideband intensity is derived by expanding into a Taylor series the free induction decay observed under magic angle spinning (MAS). According to this procedure, the free induction decay signal is completely represented by a basis of irreducible tensors from rank zero to rank infinity. After averaging over all orientations, only the zero-order irreducible tensors contribute to the sideband intensities. Symmetry properties of the sidebands can be seen clearly in this expansion and an approximate formula up to ninth order is obtained by truncating the series. Sideband intensities can be calculated rapidly with this formula. The results agree satisfactorily with the exact sideband intensities obtained by numerical simulation if the ratio of the anisotropy to the spinning speed, ω₀δ/ω_r, is smaller than 3. The relationship of the sideband intensities with the moments of a MAS spectrum shows that the proposed method is an alternative to moment analysis when the spinning speed is not very slow. Anisotropic information about the chemical shift anisotropy interaction therefore can be extracted efficiently from experimental spectra by this approximate method.     

Chemical shift anisotropy selective inversion

Magic angle spinning (MAS) is used in solid-state NMR to remove the broadening effects of the chemical shift anisotropy (CSA). In this work we investigate a technique that can reintroduce the CSA in order to selectively invert transverse magnetization. The technique involves an amplitude sweep of the radio frequency field through a multiple of the spinning frequency. The selectivity of this inversion mechanism is determined by the size of the CSA. We develop a theoretical framework to describe this process and demonstrate the CSA selective inversion with numerical simulations and experimental data. We combine this approach with cross-polarization (CP) for potential applications in multi-dimensional MAS NMR.     

Orientational information from TEDOR spectral sidebands.

In a dipolar-coupled spin-1/2 network of the type 15N1-(13)C-15N2, an assessment of the sensitivity of the N --> C and C --> N TEDOR sideband intensities to the Euler angles defining the orientation of the two heteronuclear dipolar vectors in the 13C and 15N chemical shift (CS) tensor principal axes system has been carried out via numerical calculations. The results clearly indicate the potential of TEDOR MAS NMR spectroscopy for the characterization of the CS tensor orientation in the molecular frame. The efficacy of the method has been experimentally illustrated by TEDOR studies on a polycrystalline sample of [1, 3-(15)N2, 2-(13)C]uracil, which is one of the four bases in RNA.     

6Li and 7li solid-state NMR spectroscopy of nitrogen ceramic phases

Two nitrogen ceramic phases, the oxynitride LiSiON and the nitride LiSi2N3, have been studied by 6Li and 7Li NMR. Magic angle spinning (MAS) NMR experiments have been carried out at two magnetic field strengths (7.05 and 14.1 T). The spectra give evidence of the relative effects of the quadrupolar and chemical shift interactions. The electric field gradient tensor of both phases has been determined accurately by iterative fitting of the 6Li and 7Li MAS NMR line shapes at the two magnetic field strengths. Due to the fact that for 7Li the quadrupolar interaction is much larger than the chemical shift interaction, it is shown that neither the small chemical shift anisotropy nor the relative orientation of the two interaction tensors can be determined accurately by 7Li MAS NMR. For 6Li, the two interactions are comparable and the value of these parameters obtained from the fits of the 6Li experimental MAS line shapes are therefore much more reliable.     

Chemical bonding of lead in glasses through isotropic vs anisotropic correlation: PASS shifted echo

When observing spin I = (1/2) nuclei with important chemical shift anisotropy in disordered materials, the distribution of isotropic shift can become so large that no accessible spinning rate is able to provide a resolved spectrum. This is the case of 207Pb in glasses where static and high-speed MAS spectra are nearly identical. It is still possible in such a case to rebuild a spinning sideband free spectrum using a shifted echo modified PASS sequence. This makes it possible to discuss isotropic and anisotropic chemical shifts of lead in phosphate glasses, to characterize its structural role and its chemical bonding state.