The utility of phase alternated pulses for the measurement of dipolar couplings in 2D-SLF experiments

The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.     

The chemical shift interaction under weak radio frequency pulses

The chemical shift anisotropy of a nuclear spin system in a strong magnetic field can be comparable to the strength of the rf pulse (weak pulse condition). In this case, the commonly used assumption that the chemical shift interaction Hamiltonian in the duration of a rf pulse can be neglected is no longer effective. The rf response characteristics of a spin-1/2 system under a weak pulse condition is studied in detail. The relationships between the distortion of the chemical shift powder spectrum and the relative rf field amplitude under various conditions are given. The suppression of sidebands of a MAS spectrum is analysed as an example of solid multiple pulse experiments. The experimental results are in good agreement with computer simulations.     

Asymmetric MRI magnet design using a hybrid numerical method

Two-dimensional 1H/13C polarization inversion spin exchange at the magic angle experiments were applied to single crystal samples of amino acids to demonstrate their potential utility on oriented samples of peptides and proteins. High resolution is achieved and structural information obtained on backbone and side chain sites from these spectra. A triple-resonance experiment that correlates the 1H-13Calpha dipolar coupling frequency with the chemical shift frequencies of the alpha-carbon, as well as the directly bonded amide 15N site, is also demonstrated. In this experiment the large 1H-13Calpha heteronuclear dipolar interaction provides an independent frequency dimension that significantly improves the resolution among overlapping 13C resonances of oriented polypeptides, while simultaneously providing measurements of the 13Calpha chemical shift, 1H-13C dipolar coupling, and 15N chemical shift frequencies and angular restraints for backbone structure determination.     

Slice profile improvement for a clinical MRI system

Optimal control methods have been recently introduced to improve the design of selective radio frequency pulses and several optimized selective pulses that can produce excellent slice profiles have been reported. These pulses usually require high peak rf amplitudes to implement and thus can not be widely utilized because of the limitations of the specific absorption rate and the rf power amplifier of a clinical system. We have a Siemens 1.5 T MRI clinical system. Several pulse files which consist of the bandwith matched 90° and 180° selective pulses are provided. Some of these can produce excellent slice profiles. However, they can only be used in the pulse sequences with the pulse length of 5.12 msec. The purpose of this paper is to improve the slice profiles produced by the pulse file in the pulse sequences with the shorter 2.56 msec pulse length. A pulse file optimized by the conjugate gradient method is proposed to substitute the 2.56 msec Siemens pulse file. Our experimental results confirm that the slice profiles and images are improved by the optimized pulse file with a lower peak voltage. The proposed pulse file can also be applied in other clinical MRI systems.     

Optimal control of coupled spin dynamics: design of NMR pulse sequences by gradient ascent algorithms

In this paper, we introduce optimal control algorithm for the design of pulse sequences in NMR spectroscopy. This methodology is used for designing pulse sequences that maximize the coherence transfer between coupled spins in a given specified time, minimize the relaxation effects in a given coherence transfer step or minimize the time required to produce a given unitary propagator, as desired. The application of these pulse engineering methods to design pulse sequences that are robust to experimentally important parameter variations, such as chemical shift dispersion or radiofrequency (rf) variations due to imperfections such as rf inhomogeneity is also explained.     

The roles of homonuclear line narrowing and the1H amide chemical shift tensor in structure determination of proteins by solid-state NMR spectroscopy

The seminal contributions of Ulrich Haeberlen to homonuclear line narrowing and the determination of¹H chemical shift tensors are crucial for protein structure determination by solid-state nuclear magnetic resonance spectroscopy. The¹H chemical shift is particularly important in spectra obtained on oriented samples of membrane proteins as a mechanism for providing dispersion among resonances that are not resolved with the¹H-¹⁵N dipolar coupling and¹⁵N chemical shift frequencies. This is demonstrated with three-dimensional experiments on uniformly¹⁵N-labeled samples of Magainin antibiotic peptide and the protein Vpu from HIV-1 in oriented lipid bilayers. These experiments enable resonances in two-dimensional¹H-¹⁵N dipolar coupling/¹⁵N chemical shift planes separated by¹H chemical shift frequencies to be resolved and analyzed. These three-dimensional spectra are compared to one-dimensional spectra of full-length Vpu, the cytoplasmic domain of Vpu, and Magainin, as well as to two-dimensional spectra of fd coat protein and Colicin El polypeptide. The¹H amide chemical shift tensor provides valuable structural information, and this is demonstrated with its contributions to orientational restrictions to one of the in-plane helical residues of Magainin.     

改进的远程C—H化学位移相关谱脉冲序列

本文在实验上比较了COLOC和近期发表的几个改进的远程C-H化学位移相关谱脉冲序列,综合各脉冲序列优点提出了一个改进的COLOCH序列。用一键(~1J_ch)相关峰压制率和远程相关峰成功率作为判断脉冲序列的依据。研究结果表明,Martin,COLOCS和COLOCH是较成功的脉冲序列,而且实验参数设置对实验的成功十分重要。     

New methods for the acquisition of ultra-wideline solid-state NMR spectra of spin-1/2 nuclides

The Wideband Uniform Rate Smooth Truncation - Carr-Purcell Meiboom-Gill (WURST-CPMG) pulse sequence was recently introduced as a new method of acquiring ultra-wideline solid-state NMR (SSNMR) patterns of quadrupolar nuclei (Chem. Phys. Lett. 464 (2008) 97). Herein, we describe the application of the WURST-CPMG pulse sequence to stationary samples (i.e., non-spinning or "static" samples) of various spin-1/2 nuclides ((119)Sn, (207)Pb, (199)Hg and (195)Pt) in order to examine its effectiveness for acquiring ultra-wideline SSNMR patterns. WURST-CPMG is compared to the CPMG and Cross Polarization (CP)-CPMG pulse sequences in select cases ((119)Sn and (207)Pb, respectively), and its usefulness in obtaining ultra-wideline SSNMR spectra in a piecewise fashion is explored. In addition, a preliminary investigation of pulses generated using optimal control theory (OCT) for the purpose of wideline excitation is presented; spectra acquired using these pulses are compared with standard, rectangular pulses of similar pulse powers. Both methods show much promise for acquiring high quality wideline patterns dominated by chemical shift anisotropy, with minimal distortions and significantly reduced experimental times.     

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.     

Heteronuclear decoupling under fast MAS by a rotor-synchronized Hahn-echo pulse train

A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-gamma nuclei ((13)C, (15)N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small (1)H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger (1)H chemical shifts.     

Three-dimensional 13C shift/1H-15N coupling/15N shift solid-state NMR correlation spectroscopy.

Triple-resonance experiments capable of correlating directly bonded and proximate carbon and nitrogen backbone sites of uniformly 13C- and 15N-labeled peptides in stationary oriented samples are described. The pulse sequences integrate cross-polarization from 1H to 13C and from 13C to 15N with flip-flop (phase and frequency switched) Lee-Goldburg irradiation for both 13C homonuclear decoupling and 1H-15N spin exchange at the magic angle. Because heteronuclear decoupling is applied throughout, the three-dimensional pulse sequence yields 13C shift/1H-15N coupling/15N shift correlation spectra with single-line resonances in all three frequency dimensions. Not only do the three-dimensional spectra correlate 13C and 15N resonances, they are well resolved due to the three independent frequency dimensions, and they can provide up to four orientationally dependent frequencies as input for structure determination. These experiments have the potential to make sequential backbone resonance assignments in uniformly 13C- and 15N-labeled proteins.     

A general scheme for suppression of ABX strong coupling signals in heteronuclear scalar and dipolar correlation experiments.

Enhanced versions of heteronuclear chemical shift correlation experiments which yield high-quality spectra with efficient suppression of extra peaks arising from strong coupling effects are proposed. The enhanced pulse sequences feature properly designed filtering schemes inserted during preparation, or prior to acquisition, or at both places depending on the particular experiment. These modifications extend the applicability of existing methods, since they exclude misinterpretation of spurious peaks and allow unambiguous assignment of the desired correlations. The general applicability of the filtering method is noteworthy; both scalar- and dipolar-correlated experiments with both X and 1H detection using phase cycling or gradient pulses for coherence selection can be freed of strong coupling artifacts.     

Multiple pulse NMR imaging of polymers and chemistry.

Multiple pulse line narrowing techniques can be used to improve resolution and sensitivity in solid state NMR imaging. For example, pulse sequences which remove homonuclear dipolar broadening have been used to image proton-containing materials. Further enhancements in resolution and sensitivity are obtained by removing inhomogeneous interactions such as chemical shift, susceptibility, and heteronuclear dipolar broadening. Pulse sequences have been designed which provide efficient line narrowing over large spectral widths by taking into account the experimenter's control over the amplitude and time dependence of the gradient-induced resonance offset. These methods have been applied to centimeter sized samples to obtain images of polymers, composite materials, and gas-solid chemical reactions. T1 and T2 contrast allows differentiation between materials.     

Three-dimensional solid-state NMR spectroscopy is essential for resolution of resonances from in-plane residues in uniformly (15)N-labeled helical membrane proteins in oriented lipid bilayers

Uniformly (15)N-labeled samples of membrane proteins with helices aligned parallel to the membrane surface give two-dimensional PISEMA spectra that are highly overlapped due to limited dispersions of (1)H-(15)N dipolar coupling and (15)N chemical shift frequencies. However, resolution is greatly improved in three-dimensional (1)H chemical shift/(1)H-(15)N dipolar coupling/(15)N chemical shift correlation spectra. The 23-residue antibiotic peptide magainin and a 54-residue polypeptide corresponding to the cytoplasmic domain of the HIV-1 accessory protein Vpu are used as examples. Both polypeptides consist almost entirely of alpha-helices, with their axes aligned parallel to the membrane surface. The measurement of three orientationally dependent frequencies for Val17 of magainin enabled the three-dimensional orientation of this helical peptide to be determined in the lipid bilayer.     

多脉冲序列可编程辅助控制器及其应用

本文描述了一种以单板机为基础的多脉冲序列可编程辅助控制器(PCMP),在常规液体NMR谱仪上实现多脉冲实验。应用PCMP获得了固体的J-耦合化学位移各向同性谱、化学位移缩窄谱,同时也可以完成时间平均旋进频率交叉极化实验以及组合脉冲宽带去耦实验,进一步开拓了常规液体谱仪的功能。     

Fluorine-19 Solid-State NMR Magic-Angle-Turning Experiments Using Multiple-Pulse Homonuclear Decoupling

For compounds giving “crowded” 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as ¹³C and ¹⁵N and under ¹H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as ¹H, ¹⁹F, and ³¹P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F₁ and F₂ dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence.     

Optimal control in NMR spectroscopy: Numerical implementation in SIMPSON

We present the implementation of optimal control into the open source simulation package SIMPSON for development and optimization of nuclear magnetic resonance experiments for a wide range of applications, including liquid- and solid-state NMR, magnetic resonance imaging, quantum computation, and combinations between NMR and other spectroscopies. Optimal control enables efficient optimization of NMR experiments in terms of amplitudes, phases, offsets etc. for hundreds-to-thousands of pulses to fully exploit the experimentally available high degree of freedom in pulse sequences to combat variations/limitations in experimental or spin system parameters or design experiments with specific properties typically not covered as easily by standard design procedures. This facilitates straightforward optimization of experiments under consideration of rf and static field inhomogeneities, limitations in available or desired rf field strengths (e.g., for reduction of sample heating), spread in resonance offsets or coupling parameters, variations in spin systems etc. to meet the actual experimental conditions as close as possible. The paper provides a brief account on the relevant theory and in particular the computational interface relevant for optimization of state-to-state transfer (on the density operator level) and the effective Hamiltonian on the level of propagators along with several representative examples within liquid- and solid-state NMR spectroscopy.     

Truncated sinc slice excitation for 31P spectroscopic imaging

The shortest possible delay (Td) between slice selection and data acquisition is important for producing high quality 31P spectra. In single slice multivoxel spectroscopic imaging, conventional excitation using sinc-shaped rf pulses within typical gradient limitations can have values of Td that lead to significant spectral distortion and loss of signal. Truncated sinc excitation, which ends the excitation close to the center of the main rf lobe has been suggested for MR angiographic applications to produce short values of Td. In this work, the slice profiles, spectral signal-to-noise ratio (SNR) and spectral distortions are compared using the minimum delay achievable on a commercial MRI system for conventional 'sinc' rf excitation and truncated sinc excitation. Slice profiles are calculated using the Bloch equations and measured with a phantom. SNR and spectral distortions are evaluated from whole slice spectra on a human volunteer. On an MRI system with 1 G/cm gradients (0.5 msec risetime), for a 2.5-cm slice at 31P frequencies, conventional excitation can be adjusted to achieve Td = 2.5 msec while truncated sinc excitation yields Td = 1.5 msec. The truncated sinc excitation's shorter value of Td leads to much smaller spectral distortions, but its slice profile has "dispersive tails" which increase as more of the rf is truncated. Slice profile corrected SNR for the beta-ATP peak of 31P on a human volunteer is equivalent for both sequences while, qualitatively, in the PDE region the truncated sinc approach has improved SNR.     

A 2D MAS solid-state NMR method to recover the amplified heteronuclear dipolar and chemical shift anisotropic interactions

A two-dimensional solid-state NMR method for the measurement of chemical shift anisotropy tensors of X nuclei (15N or 13C) from multiple sites of a polypeptide powder sample is presented. This method employs rotor-synchronized pi pulses to amplify the magnitude of the inhomogeneous X-CSA and 1H-X dipolar coupling interactions. A combination of on-resonance and magic angle rf irradiation of protons is used to vary the ratio of the magnitudes of the 1H-X dipolar and X-CSA interactions which are recovered under MAS, in addition to suppressing the 1H-1H dipolar interactions. The increased number of spinning sidebands in the recovered anisotropic interactions is useful to determine the CSA tensors accurately. The performance of this method is examined for powder samples of N-acetyl-(15)N-L-valine (NAV), N-acetyl-15N-L-valyl-15N-L-leucine (NAVL), and alpha-13C-L-leucine. The sources of experimental errors in the measurement of CSA tensors and the application of the pulse sequences under high-field fast MAS operations are discussed.     

Magnetic field mapping in NMR imaging

The Hahn spin preparation sequence provides a practical means for rapid and sensitive mapping of magnetic field inhomogeneity in NMR imaging applications. Choice of the rf pulse delay times tau 1 and tau 2 as well as conditions and limitations on the proposed use of this sequence for chemical shift imaging are discussed.