Triple, quintuple and higher order multiple quantum MAS NMR of quadrupolar nuclei

The optimization of the coherence transfers involved in five, seven and nine-quantum versions of the recently discovered MQMAS technique, is analysed numerically. Data reported in this paper may serve as starting parameters for the experiment set up. An analysis of the intensity and resolution given by each type of experiment is performed, which confirms the need to use very high rf fields for MQ transfers. It follows that five-quantum is achievable rather easily but the use of seven and nine-quantum MAS experiments becomes increasingly difficult due to the demand for high rf power and decreasing sensitivity. The advantages of using the z-filter MQMAS method with respect to a two-pulse sequence are analysed. The method for qualitatively and quantitatively interpret the MQMAS spectra is described. The nature of the spinning side bands along the multiple quantum dimension is explained. It is shown that the rotor synchronization can be conveniently used to eliminate these side bands, but only for 3QMAS experiments. The use of the multiple-quantum method in combination with static samples and VAS, DAS and DOR techniques is finally discussed.     

The influence of NH3 -attaching on the NMR parameters in the zigzag BN nanotube

Two models of (10, 0) boron nitride nanotubes (BNNTs), perfect and Ammonia-attached, were studied in order to evaluate the influence of NH₃-attaching on the B-11 and N-15 nuclear magnetic resonance in the (10, 0) boron-nitride nanotube (BNNT) for the first time. At first, based on density functional theory (DFT) each of the structures was optimized using B3LYP/6-31G (d) model chemistry. At the next step, the chemical-shielding (CS) tensors were calculated using the B3LYP/6-31G (d, p) level of theory in both of the relaxed forms and were converted to experimentally measurable nuclear magnetic resonance (NMR) parameters, i.e. chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA). Our calculation revealed that in the NH₃-attached BNNT (the most stable model) the B atom chemically bonded to the NH₃ molecule has the largest chemical-shielding isotropic (CSI) and the smallest chemical-shielding anisotropic (CSA) values among the other boron nuclei. Additionally, the NMR parameters of those nuclei directly bonded to the boron dramatically change while those of the other B nuclei remain almost unchanged.     

直肠癌及直肠正常组织的IR和1H及31P NMR谱研究

对12例直肠组织的红外光谱图进行了测定,并对6例直肠癌变及正常组织的固体1H和31P NMR谱与另外6例直肠癌变及正常组织的液体1H和31P-NMR谱进行研究.结果表明,(1)直肠癌变和正常组织的红外光谱中存在明显差异.(2)在固体1H和31P NMR谱中,1H NMR谱上癌组织的脂肪酸与水相关的特征峰面积的比值I(5+6+7)/I(1+2+3+4)明显变低,依据I(5+6+7)/I(1+2+3+4)比值可区分癌变与正常组织.31P NMR谱上,癌变组织的最强峰的化学位移明显向高场偏移.(3)在液体1H和31P NMR谱中,1H NMR谱上癌组织中3.20处峰强较正常组织高,表明与癌变密切相关的胆碱类化合物包括游离的胆碱、磷酸胆碱及甘油磷酸胆碱等的量增大.31P NMR谱上,癌变组织的强度最大峰的化学位移分别为1.18,1.26,1.29,1.37,正常组织的化学位移为1.70和2.18,癌变组织的强度最大峰的化学位移明显向高场偏移.通过癌变与正常组织代谢物1H NMR谱峰强度和1P NMR谱峰化学位移的差异,可以区分直肠癌变和正常组织,核磁共振1H和31P NMR谱技术可能发展成为一种诊断直肠癌的新方法.     

Adsorption of CO gas molecules on zigzag BN/AlN nanoribbons for nano sensor applications

First-principle calculations have been performed to study the sensing of CO gas in various considered configurations. The adsorption of CO on zigzag BN nanoribbon (ZBNNR) and zigzag AlN nanoribbon (ZAlNNR) was modelled in five different possibilities. The effect of CO adsorption is to reduce the band gap in both types (BN/AlN) of the nanoribbons. Interestingly, a finite magnetic moment (0.96 ${\mu }_B$ for ZBNNR and 0.69 ${\mu }_B$ for ZAlNNR) has been obtained which depends upon the adsorption configuration of CO. Half-metallicity was also observed upon selective CO adsorption on ZAlNNR irrespective of the ribbon width. Present findings suggest that CO gas molecules could be detected through adsorption on BN/AlN nanoribbons via changes in electronic and/or magnetic properties.     

Study on ethyl groups with two different orientations in [N(C2H5)4]2CuBr4

The crystal structure and phase transition temperature of [N(C₂H₅)₄]₂CuBr₄ are studied using X-ray diffraction and differential scanning calorimetry (DSC); measurements revealed a tetragonal structure and the two phase transition temperatures T_C of 204 K and 255.5 K. The structural geometry near T_C is discussed in terms of the chemical shifts for ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups are distinguishable by the ¹³C NMR spectrum. The molecular motions are discussed in terms of the spin–lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The T_1ρ results reveal that the ethyl groups undergo tumbling motion, and furthermore that the ethyl groups are highly mobile.     

Potassium tetracyanoplatinate (II) trihydrate (K2Pt(CN)4·3H2O) studied by high resolution solid state C MAS NMR

Prudent analysis of the solid state ¹³C MAS NMR spectra of polycrystalline K₂Pt(CN)₄ · 3H₂O (KTCP)⁻ reveals that in crystals of this compound there are two types of carbon nuclei with slightly different ¹³C chemical shift tensors, contrary to what is found for the solution NMR spectrum and previous static powder NMR studies on this compound and the high resolution solid state NMR studies on other similar compounds. The ¹³C MAS spectra measured at different rotor spinning speeds are satisfactorily simulated though the use of a newly developed computer program based on a novel density matrix formulation. The present method is eminently successful even though the spectra are rather complicated because of (1) the relatively large anisotropies of the chemical shift tensors; (2) the high-order dipolar interactions between ¹³C and ¹⁴N nuclei because of the strong quadrupolar coupling constants of ¹⁴N nuclei; and (3) the indirect J-coupling between the ¹³C and ¹⁹⁵Pt. The principal elements as well as their orientations of the two ¹³C chemical shift tensors are evaluated from the spectral simulations.     

Proton-detected scalar coupling based assignment strategies in MAS solid-state NMR spectroscopy applied to perdeuterated proteins

Assignment of proteins in MAS (magic angle spinning) solid-state NMR relies so far on correlations among heteronuclei. This strategy is based on well dispersed resonances in the ¹⁵N dimension. In many complex cases like membrane proteins or amyloid fibrils, an additional frequency dimension is desirable in order to spread the amide resonances. We show here that proton detected HNCO, HNCA, and HNCACB type experiments can successfully be implemented in the solid-state. Coherences are sufficiently long lived to allow pulse schemes of a duration greater than 70 ms before incrementation of the first indirect dimension. The achieved resolution is comparable to the resolution obtained in solution-state NMR experiments. We demonstrate the experiments using a triply labeled sample of the SH3 domain of chicken α-spectrin, which was re-crystallized in H₂O/D₂O using a ratio of 1/9. We employ paramagnetic relaxation enhancement (PRE) using EDTA chelated Cu^II to enable rapid data acquisition.     

直肠组织的高分辨魔角旋转核磁共振波谱研究

文章对6例直肠癌变及正常组织进行高分辨魔角旋转核磁共振波谱研究,结果显示直肠癌变和正常组织的核磁共振氢谱存在明显差异。这可以通过特征峰面积与0.88处峰积分面积的比值上的差异看出：（1）在化学位移0.75～1.55之间,癌组织各种氨基酸[缬氨酸,异亮氨酸,亮氨酸]与脂肪酸甲基的比值（I₂/I₁）,癌组织乳酸盐与脂肪酸甲基的比值(I₄/I₁)都明显增大。（2）在化学位移1.55～2.90之间,癌变组织中亮氨酸、赖氨酸、异亮氨酸与脂肪酸甲基的比值（I₇/I₁）, 谷氨酸、谷氨酰胺、缬氨酸、琥珀酸与脂肪酸甲基的比值((I₉+I₁₁)/I₁)、天冬氨酸与脂肪酸甲基的比值((I₁₂+I₁₄)/I₁)都较正常组织明显增大。（3）在化学位移2.90～3.49之间,癌变组织氨基酸与脂肪酸甲基的比值（I₁₅/I₁）、胆碱类与脂肪酸甲基的比值((I₁₆+I₁₇)/I₁)、牛磺酸与脂肪酸甲基的比值((I₁₈+I₁₉)/I₁)都较正常组织明显增大。（4）在化学位移3.49～4.50之间,其他代谢物与脂肪酸甲基的比值（I₂₀/I₁）,以及甘油基与脂肪酸甲基的比值（I₂₂/I₁）在癌变组织中都有增大的趋势。（5）化学位移4.5～10之间,癌变组织的核苷酸发生了变化,癌变组织的不饱和脂肪酸与脂肪酸甲基的比值（I₂₄/I₁）明显减小。（6）在化学位移-8～0.75之间,癌变组织的谱峰有减少的趋势。通过上述分析可知,通过癌变与正常组织代谢物NMR谱峰的差异,可以区分癌变和正常组织。说明核磁共振波谱技术可能发展成为一种诊断直肠癌的新方法。     

核磁共振技术在生物组织中的应用

在生物组织内部存在大量的生化、生理反应以及分子的扩散运动,它们相互独立, 又相互关联,共同构成一个生化反应网络. 在病理条件下,某些生化反应过程会发生改变,代谢中间体可能会出现质和量的变化. 核磁共振 (NMR)可以对这些过程的研究提供非常有用的信息. 目前,有3种核磁共振波谱技术可以用于生物组织的研究：① 活体组织定域波谱技术;② 生物组织提取物的液体高分辨核磁共振技术;③ 离体组织的高分辨魔角旋转技术. 这些方法各有优劣,互为补充. 介绍了在生物组织的NMR研究方面的最新进展.     

Kinetics of H→P cross-polarization in human trabecular bone

Bloch-decay and cross-polarization (CP) ³¹P nuclear magnetic resonance (NMR) spectra of healthy human trabecular bone were acquired under magic-angle spinning (MAS) at 3 kHz. A single peak at 3.1 ppm was detected. Variable-contact time ¹H → ³¹P CP experiments revealed three signal components growing at various rates. The fast, moderate and slow components were assigned and assessed in P atom % to proton-rich (24%), hydroxyapatite (58%) and proton-deficient (18%) phosphate domains, respectively. Examination of CP kinetics is useful for the chemical characterization of bone tissue.     

Fast acquisition of multi-dimensional spectra in solid-state NMR enabled by ultra-fast MAS

The advantages offered by ultra-fast (>60 kHz) magic angle spinning (MAS) rotation for the study of biological samples, notably containing paramagnetic centers are explored.It is shown that optimal conditions for performing solid-state ¹³C NMR under 60 kHz MAS are obtained with low-power CW ¹H decoupling, as well as after a low-power ¹H,¹³C cross-polarization step at a double-quantum matching condition. Acquisition with low-power decoupling highlights the existence of rotational decoupling sidebands. The sideband intensities and the existence of first and second rotary conditions are explained in the framework of the Floquet–van Vleck theory.As a result, optimal ¹³C spectra of the oxidized, paramagnetic form of human copper zinc superoxide dismutase (SOD) can be obtained employing rf-fields which do not exceed 40 kHz during the whole experiment. This enables the removal of unwanted heating which can lead to deterioration of the sample. Furthermore, combined with the short ¹H T₁s, this allows the repetition rate of the experiments to be shortened from 3 s to 500 ms, thus compensating for the sensitivity loss due to the smaller sample volume in a 1.3 mm rotor. The result is that 2D ¹³C–¹³C correlation could be acquired in about 24 h on less than 1 mg of SOD sample.     

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of (127)I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of ⁷⁹Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T₁ exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of ¹²⁷I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.     

A new sequence for the MQMAS/HETCOR experiment

Two-dimensional (2D) multiple quantum MAS (magic angle spinning) spectroscopy has been combined with cross-polarisation to obtain a heteronuclear correlation spectrum between a quadrupolar spin-3/2 and a spin-1/2 nucleus. The advantage over the conventional correlation experiment is the increased resolution obtained in the multiple quantum dimension. Pure absorption 2D spectra can be obtained by implementing a zero quantum filter between the evolution of multiple quanta and the subsequent cross-polarisation step. The current experiment shows a considerable improvement in sensitivity compared to a previously introduced sequence.     

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.     

NMR and IR investigations of biologically active peptides in solution

Summary Nuclear-magnetic-resonance and infra-red spectroscopic features in solution have been investigated for the delineation of conformation and dynamics of the chemotactic peptide formyl-L-methionyl-L-leucyl-L-phenylalanine. The main conformation is unfolded and characterized by a certain degree of weak intermolecular hydrogen bounding. The steric hindrance of neighbouring side chains limits the motional averaging of the several possible conformers by excluding some of the rotamers. Dipolar and scalar connectivities, as detected by 2D NMR spectroscopy, point to a cis configuration of the H−N−C−H_α moiety in the case of leucine and trans in the cases of methionine and phenylalanine.

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Riassunto Per il chiarimento della conformazione e della dinamica molecolare del peptide chemotattico formil-L-metionil-L-leucil-L-fenilalanina sono stati studiati i parametri spettroscopici di risonanza magnetica nucleare ed infrarosso in soluzione. La conformazione principale è distesa e caratterizzata da un certo grado di deboli legami a idrogeno intermolecolari. L’impedimento sterico di catene laterali vicine limita la possibilità che i vari conformeri possibili siano mediati dai moti molecolari, escludendo alcuni rotameri. Le connettività dipolari e scalari, messe in evidenza dalla spettroscopia 2D NMR, suggeriscono una configurazione cis del residuo H−N−C−H_α nel caso della leucina e trans nei casi della metionina e della fenila lanina.

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Реэюме Исследуются ядерный магнитный резонанс и инфра-красные спектроскопические характеристики в растворе с целью описания конформации и динамики хемотактичного пептида формия-Л-метионил-Л-лейцил-Л-фенилаланина. Основная конформация явлется раэвернутой и характериэуется определенной степенью слабой межмолекулярной водородной связй. Стерическое припятствие для соседних боковых цепей ограничивает динамическое усреднение для некоторых конформаций, исключая некоторые ротамеры. Дипольнье и скалярные связности, зарегистрированные с помощью 2D ЯМР-спектроскопии, указывают на цис-конфигурацию H−N−C−H_α в случае лейцина и на транс-конфигурацию в случаях метионина и фенилаланина.

     

Cd MAS and static NMR study of halogenocadmate crystals

¹¹³Cd magic-angle spinning (MAS) and static NMR spectra are measured using 23 kinds of halogenocadmate crystals with known structures to determine the isotropic chemical shifts (δ_iso), chemical shift anisotropies (Δδ), and asymmetry parameters η and then to discuss the relationship among these NMR parameters and the halide-anion co-ordination environments around Cd²⁺. The δ_iso(MAS) values of halide-anion co-ordination polyhedra of Cd²⁺ largely change with the kind of halide-anion and the halide-anion co-ordination number. The |Δδ| and η in halogenocadmate crystals is largely dependent on the structure type of crystal, but is independent of the halide-anion co-ordination number of Cd²⁺.     

15N spin diffusion rate in solid-state NMR of totally enriched proteins: the magic angle spinning frequency effect

As demonstrated by means of the one-dimensional solid-state MAS exchange experiment (CODEX), the rate of the proton driven spin diffusion between backbone (15)N nuclei in totally enriched protein depends strongly on the magic angle spinning (MAS) frequency: spin diffusion at MAS frequency 16 kHz is about 4-5 times slower as compared to that at MAS frequency 1 kHz which is due to the averaging of the homo- and hetero-nuclear dipolar interactions by MAS. It is important that even at the highest MAS frequencies used in our experiments the spin diffusion rate is comparable or larger than typical values of the spin-lattice relaxation rates of backbone nitrogens in solid proteins. Thus, the precise quantitative analysis of (15)N T(1)'s in totally enriched solid proteins may lead to wrong quantitative results. On the other hand, the effectiveness of the (15)N-(15)N correlation and structure determination experiments making use of (15)N-(15)N distances can be increased by decreasing the MAS frequency as far as possible, which is counter intuitive to the commonly applied fast MAS conditions in order to reduce the dipolar-broadened line widths for increased spectral resolution.