固体核磁共振魔角旋转条件下的定量交叉极化技术(英文)

交叉极化与魔角旋转结合（CP/MAS）的方法已经成为增强固体核磁共振（NMR）检测灵敏度最重要的技术之一.CP/MAS技术的应用大大提高了固体NMR谱图的采集效率.然而,I-S偶极耦合作用、旋转坐标系下的自旋-晶格弛豫、分子运动,以及样品中丰核的分布情况等因素,通常会导致CP/MAS谱图失去定量作用.近年来,多个研究组通过改进或者设计新型固体NMR脉冲序列,获得了基于CP的可用于定量分析的固体NMR谱图.本综述首先简要介绍了CP及CP动力学,随后介绍了一系列基于CP的定量固体NMR信号增强技术,具体包括ramped-amplitude CP（RAMP-CP）、multiple-contact CP、quantification of CP（QCP）、Lee-Goldburg frequency modulated CP（LG-FMCP）和quantitative CP（QUCP）.     

Heteronuclear Polarization Transfer by Radiofrequency-Driven Dipolar Recoupling Under Magic-Angle Spinning

At high spinning speeds, standard cross polarization (CP) can be difficult due to the narrow sideband structure of the matching condition. Recent proposals have been made to broaden the matching condition with respect to the applied spin-locking field strengths through the use of simultaneous, rotor-synchronized phase inversions of the spin-locking fields (the SPICP pulse sequence), specifically as relevant to CP between ¹H and ¹³C at spinning speeds sufficient to modulate the relevant dipolar couplings. This has been shown to effectively reduce sensitivity to RF inhomogeneity and fluctuations in overall RF power; however, the sensitivity to chemical shift (offset and anisotropy) remains. Application of the technique to the problem of polarization transfer between low-γ nuclei with large bandwidths (e.g., ¹³C and ¹⁵N) is therefore problematic: the transfer efficiency varies across the spectrum and is optimized for only a narrow region around resonance. A solution to this problem is presented in the form of the RFDRCP (RF-driven recoupling in CP) sequence. In this new method, rotor-synchronized (composite) π pulses are placed between the simultaneous phase inversions of the SPICP sequence to periodically invert the chemical-shift terms in the Hamiltonian, thereby removing their effects to zero order. The zero-order average Hamiltonian for this sequence is analogous to the average Hamiltonian for the homonuclear RFDR sequence. The technique has successfully been used to acquire 2D ¹³C-¹⁵N correlation spectra of uniformly labeled amino acids.     

固体核磁共振中膜蛋白双交叉极化效率与动力学参数相关的定量分析

固体核磁共振（NMR）中双交叉极化（DCP）是用于膜蛋白信号指认的多维异核相关实验的基本技术模块.DCP的效率在很大程度上决定了多维异核相关实验的效率.本文分析了3种典型的膜环境中的膜蛋白（AQPZ、DAGK和EV71 2B）的DCP效率及其影响因素.结果显示,在相同的实验条件下,3种蛋白样品的DCP效率存在明显差异：其中AQPZ的DCP效率最高（31%）,DAGK的效率次之（23%）,EV71 2B的效率最低（14%）.通过测量它们在旋转坐标下的自旋-晶格弛豫时间（T_1ρ）和偶极耦合常数（D_HN）,发现膜蛋白的运动会明显缩短T_1ρ,但对D_HN的影响较小.在实验的基础上,建立了T_1ρ与DCP效率相关的模型,并基于DCP动力学的定量分析,证明了运动导致的T_1ρ缩短是降低DCP效率的主要原因.因此,可以通过定量分析未知样品的T_1ρ来预测其DCP的最优效率,为DCP实验的优化提供依据.     

Anomalous Rotational Resonance Spectra in Magic-Angle Spinning NMR

Magic-angle spinning NMR spectra of samples containing dilute spin-1/2 pairs display broadenings or splittings when a rotational resonance condition is satisfied, meaning that a small integer multiple of the spinning frequency matches the difference in the two isotropic shift frequencies. We show experimental rotational resonance NMR spectra of a 13C2-labeled retinal which are in qualitative disagreement with existing theory. We propose an explanation of these anomalous rotational spectra involving residual heteronuclear couplings between the 13C nuclei and the neighboring 1H nuclei. These couplings strongly influence the rotational resonance 13C spectrum, despite the presence of a strong radiofrequency decoupling field at the 1H Larmor frequency. We model the residual heteronuclear couplings by differential transverse relaxation of the 13C single-quantum coherences. We present a superoperator theory of the phenomenon and describe a numerical algorithm for rapid Liouville space simulations in periodic systems. Good agreement with experimental results is obtained by using a biexponential transverse relaxation model for each spin site.     

Magic-Angle Spinning NMR and Molecular Mobility in Heterogeneous Systems

Magic-angle sample spinning (MAS) nuclear magnetic resonance (NMR) measurements are treated for heterogeneous systems with nanometer dimensions. An appreciable line narrowing in the MAS NMR spectra of the embedded molecules may be achieved also in the cases when the molecules still possess an appreciable local mobility. It appears that the MAS frequencies are of comparable order of magnitude as the frequencies which characterize the random molecular motional processes and which compete with MAS. It will be shown that this behavior may occur if inhomogeneous local magnetic fields due to susceptibility effects have a dominating influence on the widths and shapes of the resonance NMR lines. Properties of these local fields are described. Spectra simulations are carried for molecules embedded in these heterogeneous systems when the coherent averaging by MAS is superimposed by random local motions. This situation may occur for molecules contained in nanoporous solids and also for heterogeneous systems like membranes and biological tissues with flexible components like water, lipids, and small peptides. Several examples are treated which reveal advantages and limitations of these experiments and their theoretical interpretation.     

Application of Rotor-Synchronized Amplitude-Modulated Cross-Polarization in aC–H Spin Pair under Fast Magic-Angle Spinning

Rotor-synchronized amplitude-modulated Hartmann–Hahn cross-polarization has been applied under fast magic-angle spinning to a powder sample of ferrocene. The influence on the cross-polarization process of the heteronuclear double quantum (flop–flop) transitions induced by the amplitude modulation for low ratios of the radiofrequency-field strength to the spinning speed is studied in details. The experimental data are in good agreement with theoretical calculations for a two-spinsystem, although the intensity of the double quantum transitions is observed to be significantly smaller than expected. Moreover, it is shown that an efficient polarization transfer at the Hartmann–Hahn centerband matching condition as well as a broadening of the matching conditions is obtained by an appropriate partial scaling of the effective radiofrequency fields which minimizes the destructive effect of double quantum cross-polarization.     

Control of Spinning Sidebands in High Resolution NMR Spectroscopy

The presence of spinning sidebands can severely compromise the detection of low molarity analytes. Spinning sidebands have traditionally been minimized by improving the magnetic field homogeneity and by varying the spinning of the sample in a linear fashion during data acquisition. The effect of the latter is to spread the spinning sideband intensity over a range of frequencies so that the final result is a spinning sideband whose shape reflects the distribution of spinning speeds. We have designed a customized profile of spinner speed variation that optimizes the reduction of spinning sidebands. The customized profile is based on theoretical considerations of how the intensity of sidebands vary with the rate of sample rotation and also compensates for the mechanical design of the spinner mechanism. The result is a unique combination of an exponential increase in gas flow rate to balance the theoretical considerations coupled with a strategically placed rapid change in air flow to annul the sluggish response of the spinning mechanism to acceleration. The resulting sideband shape is a broad, flat, square step in the baseline that is least likely to interfere with low molarity analyte peaks.     

Magic Angle Spinning NMR Spectroscopy with Composite RF Pulses

Using numerical optimization procedures it is shown that it is possible to design composite 180 degrees RF pulses for MAS NMR spectroscopy by explicitly taking into account the variation of the resonance offset of each crystallite during the application of the RF pulses. When using composite RF pulses in experiments such as TOSS, where the delays between the RF pulses have to be critically adjusted, an optimization of these delays can lead to the desired performance characteristics.     

Selective Homonuclear Polarization Transfer in the Tilted Rotating Frame under Magic Angle Spinning in Solids

An application of the R2TR method (1995,Chem. Phys. Lett.232,424) to selective homonuclear polarization transfer under magic angle spinning is proposed. It is shown that, for a spinning speed fast enough to remove the maximum homonuclear dipolar coupling constant ω_Dinvolved, the flip-flop and flop-flop mechanisms are suitable for recoupling the spins with a chemical shift difference larger than ω_Dand a difference comparable to or smaller than ω_D, respectively. It is also shown that, for fast polarization transfer, the off-resonance frequencies should be much higher than the RF intensity in the flip-flop condition, while for the flop-flop condition, the off-resonance frequencies should be much lower than the RF intensity. Some one- and two-dimensional experiments are proposed by utilizing the capability of the R2TR method to abruptly switch on and off the recoupling condition, and are demonstrated for triply¹³C-enriched -alanine. The mixing time required for population transfer was found to be ca. 0.5 ms for the methine and methyl¹³C spins separated by 1.5 Å and ca. 5 ms for the methyl and the carboxyl carbons separated by 2.5 Å. The experimental results and theoretical simulations show that selective polarization transfer is achieved when the difference in the isotropic chemical shifts between the relevant pair of spins and a neighboring spin is more than 1000 Hz.     

NMR Frequency Shift under Sample Spinning

It is shown that, by using an RF coil of suitable shape, sample spinning around an axis parallel to the main field produces a shift of the NMR resonance frequency. This effect is experimentally verified on¹H and¹¹⁹Sn in liquid samples. A general discussion of sample spinning and frequency shift is given in connection with the rotating-frame picture. Several possible applications are described.     

稀土掺杂硼铝硅酸盐玻璃结构的NMR研究

在Sm₂O₃掺杂BaO-B₂O₃-Al₂O_3-SiO_2-Sm₂O₃(BBASS)玻璃系统的形成性能研究的基础上,借助MAS NMR以及差热测试(DTA)技术,研究了玻璃的结构特点以及玻璃组成、热处理条件等因素对玻璃结构的影响. 研究表明,在未掺稀土的BBAS玻璃结构中,硼氧多面体主要以[BO₃]、[BO₄]存在,铝氧多面体主要以[AlO₄]、[AlO₅]、[AlO₆]存在;随着BBAS中BaO含量的增加,硼氧三角体[BO₃]逐渐向[BO₄]转变,原先[AlO₄]、[AlO₅]、[AlO₆]共存的铝氧多面体结构逐渐转变为大量[AlO₄]和少量[AlO₅]共存的结构;稀土Sm³⁺具有较强的积聚作用,其能促使玻璃结构中的硼氧多面体形成巨大的网络结构;热处理对玻璃结构中硼氧多面体和铝氧多面体的配位结构影响不大.     

Pulsed Field Gradient Selection in Two-Dimensional Magic Angle Spinning NMR Spectroscopy of Dipolar Solids

The utility of gradient selection in MAS spectroscopy of dipolar solids is explored in two examples. In the first, rotor-synchronized gradients of appropriate strength and duration are applied to select¹H double-quantum coherences. The resulting DQ MAS spectrum of adamantane is compared with that acquired by the corresponding phase-cycling technique. As a second example, a¹H 2D exchange MAS experiment is performed on an elastomer sample. In this experiment, a gradient is applied to remove undesired coherences that would otherwise distort the spectrum for short mixing times. The diagonal-peak intensities in the resulting spectrum show a linear decrease with increasing mixing time indicating cross-relaxation by slow chain motions as the relevant process. Both types of experiments demonstrate the potential of gradient-selection techniques for MAS spectroscopy of dipolar solids.     

"Fast" Linear Prediction and Its Application to NMR Spectroscopy

A general "fast" linear-prediction (FLP) algorithm is presented. It determines P linear-prediction coefficients from M equations in ∼ 9MP + 27/2P² operations by using only ∼ 5(M + P) locations of memory. The procedure does not rely on any assumption about the signal to be analyzed, and within numerical accuracy, it produces the same results as the QR method. It is demonstrated that, with the FLP routine, it is feasible to determine more than 10,000 linear-prediction coefficients.     

Multiple-Rotor-Cycle QPASS Pulse Sequences: Separation of Quadrupolar Spinning Sidebands with an Application to La NMR

The quadrupolar phase-adjusted spinning sidebands (QPASS) pulse sequence has been recently demonstrated as a useful method for obtaining quadrupolar parameters with magic-angle spinning NMR. The sequence separates spinning sidebands by order in a two-dimensional experiment. A sheared projection of the 2D spectrum effectively yields the infinite spinning rate second-order quadrupolar powder pattern, which can be analyzed to determine quadrupolar coupling constants and asymmetry parameters. The RF power and spinning speed requirements of the original QPASS sequence make it an experimentally demanding technique. A new version of the sequence is demonstrated here and is shown to alleviate many problems associated with the original sequence. New solutions to the determining equations, based on the use of multiple rotor cycles in the QPASS sequence, lead to longer delays between the nine π pulses, provide less chance of pulse overlap, and allow for use of weaker RF field strengths that excite only the central quadrupolar transition. A three-rotor-cycle version of the new experiment is demonstrated on the ¹³⁹La nucleus.     

Spectral Editing in C MAS NMR under Moderately Fast Spinning Conditions

Novel procedures for the spectral assignment of peaks in high-resolution solid-state ¹³C NMR are discussed and demonstrated. These methods are based on the observation that at moderate and already widely available rates of magic-angle spinning (10–14 kHz MAS), CH and CH₂ moieties behave to a large extent as if they were effectively isolated from the surrounding proton reservoir. Dipolar-based analogs of editing techniques that are commonly used in liquid-state NMR such as APT and INEPT can then be derived, while avoiding the need for periods of homonuclear ¹H–¹H multipulse decoupling. The resulting experiments end up being very simple, essentially tuning-free, and capable of establishing unambiguous distinctions among CH, CH₂, and carbon sites. The principles underlying such sequences were explored using both numerical calculations and experimental measurements, and once validated their editing applications were illustrated on a number of compounds.     

DS算法在近红外光谱多元校正模型传递中的应用

模型传递问题是近红外光谱分析技术中一个关键的共性基础技术问题。模型传递通过在同一型号的两台仪器之间寻求一种变换关系,使得在一台仪器上建立起的模型能够应用于另外一台仪器样品光谱响应的预测。文章将直接校正(direct standardization,DS)算法应用于化学计量学多元校正中的模型传递,并研究了模型转换集样品的选择方法。在两台AOTF近红外光谱仪上进行了模型传递实验,首先采用Kennard/Stone算法选择转换集样品,然后采用DS方法进行模型传递。实验结果表明采用DS算法取得了较好的模型传递效果。DS算法不仅可以应用于不同仪器之间的光谱分析模型传递,而且该方法对于同一仪器的长时间漂移或者由于部件的更换、测量环境的改变等引入的光谱差异校正也是适用的。     

Application of High-Resolution NMR Spectroscopy in Analyzing Ionites

The ¹H and ¹³C NMR spectra of granulated carboxylic cationite MAC-3 and fibrous anionites FIBAN (A-5, A-7, and A-9) have been investigated. The dependence of the chemical shifts and times of spin–spin relaxation of the water protons and carbon nuclei of MAC-3 on the water content in a sample has been investigated. A comparison of the carbon spectra of polyelectolytes and FIBAN fibrous anionites swollen in water has been made. It is shown that the carbon spectra of the ionites swollen in water represent fairly narrow lines practically for all groups of the skeleton, and therefore the structure of the ionites can be analyzed using a high-resolution NMR.     

Application of NMR Spectroscopy and Imaging in Heterogeneous Biocatalysis

Heterogeneously catalyzed enzymatic glucose isomerization was considered as a model process to extend the application of nuclear magnetic resonance (NMR) and magnetic resonance imaging techniques to the studies of biocatalytic processes and heterogeneous biocatalysts. It has been demonstrated that the T ₂ times of glucose are different for its aqueous solution in the pores of an unmodified porous support and in a heterogeneous biocatalyst, comprising bacterial cells immobilized on the same support. This observation has been used to map the spatial distribution of the active component within a packed bed of biocatalyst in a model reactor. ¹³C NMR spectroscopy was applied to follow the progress of glucose isomerization catalyzed by the heterogeneous biocatalyst in a batch reactor. The utilization of proton spin decoupling and nuclear Overhauser effect was shown to be necessary to obtain high signal-to-noise ratio in the natural abundance ¹³C NMR spectra of a glucose–fructose syrup present in the packed bed of biocatalyst. The spectra thus obtained were suitable for the quantification of the glucose-to-fructose ratio achieved in the biocatalytic reaction.