液体核磁共振中由分子间偶极耦合和分子内标量耦合共同作用后的谱图裂分模式

在高极化多自旋液体样品中,同时存在着分子间偶极（D）耦合和分子内标量（J）耦合,它们的共同作用产生了一些原来观测不到的分子间多量子相干信号。而且,信号的裂分模式与只存在J耦合的多自旋体系中观测到的多量子相干信号的裂分模式不同。本文从理论和实验上研究了这些禁阻的共振峰及其独特的裂分模式。为了比较验证,我们以I2S3+X自旋体系为例,结合使用选择和非选择性的射频脉冲序列来获得分子间双量子相干信号的五种裂分模式。进而归纳出对IpSq+Xk (p, q, k = 1, 2, 3,…)自旋体系普适的裂分模式规则。并指出,它们中如(1:0:-1)的裂分模式会放大J耦合裂分,使得J耦合常数的测量更精确,特别在J耦合常数很小或不均匀场中的J耦合常数的测量中具有诱人的应用前景。结果表明理论预测,计算机模拟和实验观测结果三者吻合的很好。     

NMR detection using laser-polarized xenon as a dipolar sensor

Hyperpolarized (129)Xe can be used as a sensor to indirectly detect NMR spectra of heteronuclei that are neither covalently bound nor necessarily in direct contact with the Xe atoms, but coupled through long-range intermolecular dipole-dipole interactions. To reintroduce long-range dipolar couplings the sample symmetry has to be broken. This can be done either by using an asymmetric sample arrangement, or by breaking the symmetry of the spin magnetization with field gradient pulses. Experiments are performed where only a small fraction of the available (129)Xe magnetization is used for each point, so that a single batch of xenon suffices for the point-by-point acquisition of a heteronuclear NMR spectrum. Examples with (1)H as the analyte nucleus show that these methods have the potential to obtain spectra with a resolution that is high enough to determine homonuclear J couplings. The applicability of this technique with remote detection is discussed.     

Quantum Treatment of Intermolecular Multiple-Quantum Coherences with IntramolecularJCoupling in Solution NMR

A recently introduced density matrix picture for dipolar effects in solution NMR (1996,J. Chem. Phys.105,874) gave complete solutions for intermolecular multiple-quantum coherences for single-component samples without scalar couplings. This paper, for the first time, shows that this quantum picture can lead to explicit signal expressions for multicomponent samples of molecules with internal scalar couplings (here assumed to generate a first-order spectrum) and long-range dipolar couplings. Experimental observation of a triplet in the indirectly detected dimension for a heteronuclear CRAZED sequence (¹³CHCl₃sample, ZQ or 2Q coherences) gives clear evidence that the coupling is due to the intermolecular dipolar coupling. We also make comparisons with classical pictures which introduce the dipolar demagnetization field in multicomponent spin systems.     

Selective detection of hyperpolarized NMR signals derived from para-hydrogen using the Only Para-hydrogen SpectroscopY (OPSY) approach

A new family of NMR pulse sequences is reported for the recording of para-hydrogen enhanced NMR spectra. This Only Para-hydrogen SpectroscopY (OPSY) approach uses coherence selection to separate hyperpolarized signals from those of fully relaxed and thermally equilibrated protons. Sequence design, performance, practical aspects and applicability to other hyperpolarization techniques are discussed.     

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.     

High-resolution NMR of static samples by rotation of the magnetic field

Mechanical rotation of a sample at 54.7 degrees with respect to the static magnetic field, so-called magic-angle spinning (MAS), is currently a routine procedure in nuclear magnetic resonance (NMR). The technique enhances the spectral resolution by averaging away anisotropic spin interactions thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. It should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the B0 field (B0-MAS). Here, this principle is experimentally demonstrated in a static sample of solid hyperpolarized xenon at approximately 3.4 mT. By extension to moderately high fields, it is possible to foresee interesting applications in situations where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging.     

Sequentially acquired two-dimensional NMR spectra from hyperpolarized sample

A scheme capable of acquiring heteronuclear 2D NMR spectra of hyperpolarized sample is described. Hyperpolarization, the preparation of nuclear spins in a polarized state far from thermal equilibrium, can increase the NMR signal by several orders of magnitude. It presents opportunities to apply NMR spectroscopy to dilute samples that would otherwise yield insufficient signal. However, conventional 2D NMR spectroscopy, which is commonly applied for the determination of molecular structure, relies on the recovery of the initial polarization after each transient. For this reason, it cannot be applied directly to a sample that has been hyperpolarized once. With appropriately modified pulse schemes, two-dimensional NMR spectra an however be acquired sequentially by utilizing a small portion of the hyperpolarized signal in every scan, while keeping the remaining polarization for future scans. We present heteronuclear multi-quantum spectra of single hyperpolarized samples using this technique, and discuss different options for distributing the polarization among different scans. This robust method takes full advantage of Fourier NMR to resolve overlapping chemical shifts, and may prove particularly useful for the structural elucidation of compounds in mass-limited samples.     

An optimal strategy for recovering the deuterium (2H) quadrupolar interaction under magic-angle spinning NMR

By exploiting the homology in the form of the truncated high-field homonuclear dipole–dipole and quadrupole coupling Hamiltonians, we have previously demonstrated that a simple adaptation of a rotor-synchronized pulse sequence (DRAMA) used for the recovery of dipole–dipole couplings can also be used to resurrect quadrupole couplings (QUADRAMA). In the canonical implementation of these recovery pulse sequences, the couplings are not significantly scaled down from their static sample values. While such minimal scaling is of course desirable in the recovery of typical homonuclear dipolar couplings ( ≤ 2 kHz) and small quadrupole couplings, it is clearly not ideal for the recovery of the much larger quadrupole couplings (20–200 kHz) often encountered in solid-state ²H NMR. In such a case, some prior knowledge of the order of magnitude of the coupling is required to optimize the experimental conditions for QUADRAMA. In order to overcome this drawback, in this study, we have developed a general and optimized strategy for implementing the QUADRAMA technique which does not require any knowledge of the size of the coupling ν_Q. Experimental tests of the optimized protocol demonstrate that by judicious choices of a combination of scaling factors and recoupling times, ²H quadrupole couplings ranging over an order of magnitude from 3 to 42 kHz can be measured. Since this optimized protocol can reliably be used to recover couplings over a broad range, it expands the range of systems accessible to study by ²H NMR into a realm where static sample NMR and simple MAS NMR may fail.     

Spontaneous emission of NMR signals in hyperpolarized proton spin systems

Hyperpolarization of nuclear spins is gaining increasing interest as a tool for improving the signal-to-noise ratio of NMR and MRI. While in principle, hyperpolarized samples are amenable to the same or similar experiments as are used in conventional NMR, the large spin polarization may give rise to unexpected effects. Here, spontaneous emission of signal was observed from proton spin systems, which were hyperpolarized to negative spin temperature by dynamic nuclear polarization (DNP). An unexpected feature of these emissions is that, without any radio-frequency excitation, multiple beats arise that cannot be explained by the Bloch equations with radiation damping. However, we show that a simple modification to these equations, which takes into account an additional supply of hyperpolarized magnetization from a reservoir outside of the active detection region, can phenomenologically describe the observed signal. The observed effect demonstrates that even well-known mechanisms of spin evolution can give rise to unexpected effects when working with hyperpolarized samples, which may need to be addressed through the development of new experimental techniques.     

新型葫芦[6]脲纳米颗粒超极化129Xe"分子笼"研究

在相同浓度下,超极化¹²⁹Xe的核磁共振（NMR）灵敏度是传统质子NMR的10 000倍以上,但单原子Xe不具有靶向性,只有单一NMR信号.若超极化¹²⁹Xe与“分子笼”相结合,就能获得新的“笼”内Xe信号,为发展超极化¹²⁹Xe分子探针奠定基础.因此,构建新型的“分子笼”是发展新型超极化¹²⁹Xe分子探针的一个重要方向.葫芦[6]脲纳米颗粒的出现能改善以葫芦[6]脲为主体的¹²⁹Xe分子探针水溶性差、信号弱等缺点.本文构建了水溶性的葫芦[6]脲纳米颗粒,并发现其具有葫芦[6]脲/纳米颗粒内部两个“笼”内¹²⁹Xe信号.这一发现使这种纳米颗粒具有成为超极化¹²⁹Xe分子探针的潜力,能降低检测结果中假阳性和假阴性的发生率,值得更深入的探索和研究.     

Anisotropy of collagen fiber orientation in sheep tendon by 1H double-quantum-filtered NMR signals

The anisotropy of the angular distribution of collagen fibrils in a sheep tendon was investigated by 1H double-quantum (DQ) filtered NMR signals. Double-quantum build-up curves generated by the five-pulse sequence were measured for different angles between the direction of the static magnetic field and the axis of the tendon plug. Proton residual dipolar couplings determined from the DQ build-up curves in the initial excitation/reconversion time regime which mainly represent the bound water are interpreted in terms of a model of spin-1/2 pairs with their internuclear axes oriented on average along the fibril direction in the presence of proton exchange. The angular distribution of collagen fibrils around the symmetry axis of the tendon measured by the anisotropy of the residual dipolar couplings was described by a Gaussian function with a standard deviation of 12 degrees +/-1 degrees and with the center of the distribution at 4 degrees +/-1 degrees. The existence of this distribution is directly reflected in the finite value of the residual dipolar couplings at the magic angle, the value of the angular contrast, and the oscillatory behavior of the DQ build-up curves. The 1H residual dipolar couplings were also measured from the doublets recorded by the DQ-filtered signals. From the angular dependence of the normalized splitting the angular distribution of the collagen fibrils was evaluated using a Gaussian function with a standard deviation of 19 degrees +/-1 degrees and with the center of distribution at 2 degrees +/-1 degrees. The advantages and disadvantages of these approaches are discussed.     

Proton residual dipolar couplings by NMR magnetization exchange in cross-linked elastomers: determination and imaging.

Proton nuclear magnetic resonance (NMR) magnetization exchange is used to investigate residual dipolar couplings in a series of cross-linked poly(styrene-cobutadiene) elastomers. A new model for the dipolar unit is used for the evaluation of the signal decay in magnetization exchange experiments. It takes into account an extended residual dipolar coupling network along the polymer chain. It is shown that in the regime of short mixing times, information about the residual dipolar coupling between methine and methylene protons can be obtained which is not affected by other inter- and intramolecular dipolar couplings. The dynamic order parameter of methine-methylene protons is measured and correlated with cross-link density. This study certifies the quality of a filter for magnetization from residual dipolar couplings which exploit magnetization exchange. The filter can be used to generate contrast in NMR images of heterogeneous elastomers. The first proton NMR parameter image of a dynamic order parameter is presented for a phantom made from poly(styrene-cobutadiene) samples with different cross-link densities.     

Using solid-state 31P{19F} REDOR NMR to measure distances between a trifluoromethyl group and a phosphodiester in nucleic acids

REDOR is a solid-state NMR technique frequently applied to biological structure problems. Through incorporation of phosphorothioate groups in the nucleic acid backbone and mono-fluorinated nucleotides, 31P{19F} REDOR has been used to study the binding of DNA to drugs and RNA to proteins through the detection of internuclear distances as large as 13-14 A. In this work, 31P{19F} REDOR is further refined for use in nucleic acids by the combined use of selective placement of phosphorothioate groups and the introduction of nucleotides containing trifluoromethyl (-CF3) groups. To ascertain the REDOR-detectable distance limit between an unique phosphorous spin and a trifluoromethyl group and to assess interference from intermolecular couplings, a series of model compounds and DNA dodecamers were synthesized each containing a unique phosphorous label and trifluoromethyl group or a single 19F nucleus. The dipolar coupling constants of the various 31P and 19F or -CF3 containing compounds were compared using experimental and theoretical dephasing curves involving several models for intermolecular interactions.     

High-resolution, >1 GHz NMR in unstable magnetic fields

Resistive or hybrid magnets can achieve substantially higher fields than those available in superconducting magnets, but their spatial homogeneity and temporal stability are unacceptable for high-resolution NMR. We show that modern stabilization and shimming technology, combined with detection of intermolecular zero-quantum coherences (iZQCs), can remove almost all of the effects of inhomogeneity and drifts, while retaining chemical shift differences and J couplings. In a 25-T electromagnet (1 kHz/s drift, 3 kHz linewidth over 1 cm(3)), iZQC detection removes >99% of the remaining inhomogeneity, to generate the first high-resolution liquid-state NMR spectra acquired at >1 GHz.     

超极化气体氙-129的低场NMR测量

在自旋交换光泵过程中,多种参数可能会影响到最终可获得的超极化气体氙-129核自旋极化度.通过低场(0.002 T)核磁共振(NMR)系统研究了连续流动工作模式的自旋交换光泵过程,当混合工作气体流量为0.3 SLPM和0.5 SLPM时,实验测量得到最佳光泵泡工作温度;对于同位素富集和自然丰度的氙-129气体,核自旋极化度的建立时间分别为15 min和22 min.由于混合工作气体的压力以及组分会导致铷原子吸收线的频移和展宽,并且影响到其线型,实验通过低场NMR系统测量确定了用于自旋交换光泵的最佳激光工作波长.低场NMR测量为获得具有高核自旋极化度的超极化气体氙-129,并且能够用于人体肺部MRI研究提供了实验依据.     

Sample optimization and identification of signal patterns of amino acid side chains in 2D RFDR spectra of the alpha-spectrin SH3 domain

Future structural investigations of proteins by solid-state CPMAS NMR will rely on uniformly labeled protein samples showing spectra with an excellent resolution. NMR samples of the solid alpha-spectrin SH3 domain were generated in four different ways, and their (13)C CPMAS spectra were compared. The spectrum of a [u-(13)C, (15)N]-labeled sample generated by precipitation shows very narrow (13)C signals and resolved scalar carbon-carbon couplings. Linewidths of 16-19 Hz were found for the three alanine C(beta )signals of a selectively labeled [70% 3-(13)C]alanine-enriched SH3 sample. The signal pattern of the isoleucine, of all prolines, valines, alanines, and serines, and of three of the four threonines were identified in 2D (13)C-(13)C RFDR spectra of the [u-(13)C, (15)N]-labeled SH3 sample. A comparison of the (13)C chemical shifts of the found signal patterns with the (13)C assignment obtained in solution shows an intriguing match.     

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.     

Production of nitrogen-free, hyperpolarized 129Xe gas

129 Xe with a nuclear polarization far above the thermal equilibrium value (hyperpolarized) is used in NMR studies to increase sensitivity. Gaseous, adsorbed, or dissolved xenon is utilized in physical, chemical, and medical applications. With the aim in mind to study single-crystal surfaces by NMR of adsorbed hyperpolarized ¹²⁹Xe, three problems have to be solved. The reliable production of ¹²⁹Xe with highest nuclear polarization possible, the separation of the xenon gas from the necessary quench gas nitrogen without polarization loss, and the dosing/delivery of small amounts of polarized xenon gas to a sample surface. Here we describe an optical pumping setup that regularly produces xenon gas with a ¹²⁹Xe nuclear polarization of 0.7(±0.07). We show that a freeze–pump–thaw separation of xenon and nitrogen is feasible without a significant loss in xenon polarization. The nitrogen partial pressure can be suppressed by a factor of 400 in a single separation cycle. Dosing is achieved by using the low vapor pressure of a frozen hyperpolarized xenon sample. Received: 12 June 1998     

采用超导量子干涉组件在微特斯拉磁场下获取超极化~3He的核磁共振波谱和影像(英文)

该文以超导量子干涉元件研究光激发氦三极化气体的低磁场磁共振与造影术.使用圆偏极化雷色光将氦三气体极化,超导量子干涉元件磁共振与造影之测量是以磁通耦合方式来进行,超导量子干涉元件是用铋锶钙铜氧高溫超导罐来隔离环境噪音.此方法测得的磁共振信号与影像相较于直接将样品置于杜瓦瓶下方有较高的信噪比,当样品无法靠近感测元件时,此装置具高信噪比特色.磁通耦合方式的超导量子干涉元件低磁场磁共振与造影术及其氦三极化气体的肺部造影在学术与应用上是相当有趣.