Sensitivity improvement of 1H-15N cross-polarization at high MAS frequency applied to NMR structural characterization of organic solids

(15)N CP/MAS solid state NMR should be a method of choice to obtain essential structural information on organic materials containing nitrogen atoms. However, the technique is generally not selected for the characterization of non-labelled chemical compounds, which represents the most common situation encountered by chemists. Actually, due to the poor sensitivity of (15)N the method is time-consuming and a very fine calibration is often a prerequisite to reach a sufficient signal/noise. The main drawback comes from the weakness of (15)N-(1)H dipolar couplings which leads to a splitting of the static Hartman Hahn condition into very narrow sideband conditions under MAS. Practically, it is more difficult to obtain a high enough CP transfer level on (15)N for the entire spectrum than on other more conventional nuclei like (13)C. An experimental investigation of the CP efficiency using the ramp and adiabatic CP transfer experiments is here proposed. Preliminary adjustments of experimental settings were first made on an (15)N-labeled substituted heterocyclic model system, and then applied to several other organic compounds. Particular attention was paid to the detection of non-protonated nitrogen atoms with a significant chemical shift anisotropy, which represented the least favourable case. It was experimentally demonstrated that, for these atoms, the adiabatic passage provided a much higher transfer level than the more conventional ramp sequence leading to an enhancement factor of up to 3.5 at a MAS frequency of 30 kHz. The resulting sensitivity rendered possible the detection of non-protonated nitrogen atoms at natural abundance with 2.5-mm rotors at 9.4 T.     

Enhancing NMR Sensitivity of Natural‐Abundance Low‐γ Nuclei by Ultrafast Magic‐Angle‐Spinning Solid‐State NMR Spectroscopy

Although magic‐angle‐spinning (MAS) solid‐state NMR spectroscopy has been able to provide piercing atomic‐level insights into the structure and dynamics of various solids, the poor sensitivity has limited its widespread application, especially when the sample amount is limited. Herein, we demonstrate the feasibility of acquiring high S/N ratio natural‐abundance ¹³C NMR spectrum of a small amount of sample (≈2.0 mg) by using multiple‐contact cross polarization (MCP) under ultrafast MAS. As shown by our data from pharmaceutical compounds, the signal enhancement achieved depends on the number of CP contacts employed within a single scan, which depends on the T_1ρ of protons. The use of MCP for fast 2D ¹H/¹³C heteronuclear correlation experiments is also demonstrated. The significant signal enhancement can be greatly beneficial for the atomic‐resolution characterization of many types of crystalline solids including polymorphic drugs and nanomaterials.     

13C nuclear overhauser polarization-magic-angle spinning nuclear magnetic resonance spectroscopy in uniformly 13C-labeled solid proteins

A recently introduced (13)C polarization technique based on the nuclear Overhauser effect in rotating solid (nuclear Overhauser polarization-magic-angle spinning, NOP-MAS) (Takegoshi, K.; Terao, T. J. Chem. Phys. 2002, 117, 1700-1707) is applied to uniformly (13)C, (15)N-labeled proteins. NOP enhancement factors per scan of 1.5 approximately 2.0 are obtained, while that by cross polarization (CP) is less than 1.0. We show that uniform enhancement of all (13)C signals by CP is difficult to attain, while it is easily achieved by NOP, thus enabling quantitative comparison of signal intensities. NOP is easy to carry out under fast MAS and works well even for somewhat mobile molecules, for which CP does not work. Moreover, in labeled protein samples containing nonlabeled additives, NOP can eliminate the latter signals. For these features, NOP is superior to CP in many uniformly (13)C labeled proteins.     

In situ 13C DEPT-MRI as a tool to spatially resolve chemical conversion and selectivity of a heterogeneous catalytic reaction occurring in a fixed-bed reactor

The distortionless enhancement by polarisation transfer (DEPT) nuclear magnetic resonance (NMR) technique, combined with magnetic resonance imaging (MRI), has been used to provide the first in situ spatially-resolved and quantitative measurement of chemical conversion and selectivity within a fixed-bed reactor using natural abundance 13C NMR.     

Characterization of Brain Metabolism by Nuclear Magnetic Resonance

The noninvasive, quantitative ability of nuclear magnetic resonance (NMR) spectroscopy to characterize small molecule metabolites has long been recognized as a major strength of its application in biology. Numerous techniques exist for characterizing metabolism in living, excised, or extracted tissue, with a particular focus on ¹H-based methods due to the high sensitivity and natural abundance of protons. With the increasing use of high magnetic fields, the utility of in vivo ¹H magnetic resonance spectroscopy (MRS) has markedly improved for measuring specific metabolite concentrations in biological tissues. Higher fields, coupled with recent developments in hyperpolarization, also enable techniques for complimenting ¹H measurements with spectroscopy of other nuclei, such as ³¹P and ¹³C, and for combining measurements of metabolite pools with metabolic flux measurements. We compare ex vivo and in vivo methods for studying metabolism in the brain using NMR and highlight insights gained through using higher magnetic fields, the advent of dissolution dynamic nuclear polarization, and combining in vivo MRS and ex vivo NMR approaches.     

Suitability of Different 13C Solid-state NMR Techniques in the Characterization of Humic Acids

Qualitative and quantitative analyses of humic acids (HAs) with five different ¹³C solid-state NMR techniques were assessed using HAs of various origins and locations. The NMR techniques compared are: (1) direct polarization/magic angle spinning (DP/MAS) at 13 kHz, (2) conventional cross polarization (CP)/MAS at 5 kHz, (3) ramp-CP/MAS at 8 kHz, (4) CP/total sideband suppression (TOSS) at 4.5 kHz, and (5) DP/MAS corrected by CP/spin-lattice relaxation with TOSS. The spectra from the five techniques were first compared qualitatively. Then, each spectrum was divided into eight regions for quantitative evaluation. DP/MAS spectra were used as quantitative references. Ramp-CP/MAS and CP/TOSS spectra gave consistently better results than those of the conventional CP/MAS spectra at a ¹³C frequency of 75 MHz, which were incorrect due to spinning sidebands. CP/MAS at low magnetic fields (22.6 and 50.6 MHz ¹³C frequency) indicated improved integration results but lower resolution. Correction factors calculated by comparison with DP/MAS will be useful to convert the non-quantitative peak areas in the CP/TOSS and ramp-CP/MAS spectra into more quantitative results.     

Hyperpolarized 13C NMR Spectroscopy of Urine Samples at Natural Abundance by Quantitative Dissolution Dynamic Nuclear Polarization

Hyperpolarized nuclear magnetic resonance (NMR) offers an ensemble of methods that remarkably address the sensitivity issues of conventional NMR. Dissolution Dynamic Nuclear Polarization (d-DNP) provides a unique and general way to detect ¹³C NMR signals with a sensitivity enhanced by several orders of magnitude. The expanding application scope of d-DNP now encompasses the analysis of complex mixtures at natural ¹³C abundance. However, the application of d-DNP in this area has been limited to metabolite extracts. Here, we report the first d-DNP-enhanced ¹³C NMR analysis of a biofluid -urine- at natural abundance, offering unprecedented resolution and sensitivity for this challenging type of sample. We also show that accurate quantitative information on multiple targeted metabolites can be retrieved through a standard addition procedure.     

液晶型聚氨酯弹性体的固体高分辨核磁共振研究

采用固体高分辨＾１３Ｃ核磁共振谱以及溶液碳谱、氢谱的方法对以聚四氢呋喃（ＰＴＭＯ）为软段、４，４＇－二苯基甲烷二异氰酸酯（ＭＤＩ）为硬段、４，４＇－二羟已氧基联苯（ＨＢ６）为扩链剂的液晶型聚氨酯弹性体的相态结构、分子运动、氢键相互作用等问题进行了研究。探讨了样品的化学结构与上述问题间的关系。     

In situ 13C solid-state polarization transfer NMR to follow starch transformations in food

Convenience food products tend to alter their quality and texture while stored. Texture-giving food components are often starch-rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ ¹³C solid-state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments—the time scale of reorientation of the CH-bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing ¹³C solid-state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the ¹³C chemical shift. In conjunction with ¹H solid-state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch-based products and commercial pasta as a function of temperature and storage.     

乙烯-丙烯酸共聚物非晶区结构和分子运动的固体核磁共振研究

利用固体高分辨1 3CCP MAS和变温固体质子宽线技术对乙烯 丙烯酸共聚物 (EAA)非晶区的结构和分子运动进行了研究 ,结果发现 ,非晶区中羧基之间可以形成氢键 ,其数量随着共聚物中丙烯酸共聚单体含量的增加而增加 ,使得共聚物中非晶区分子运动能力随非晶区相对含量的增加而减弱 ,这是一种与一般的乙烯共聚物相反的变化趋势 .通过交叉极化方法间接测量了非晶区中乙烯链段的1 H自旋 自旋弛豫时间 (T2 ) ,表明非晶区中乙烯链段的运动同样受到氢键的束缚 .随着温度的升高 ,羧基之间的氢键发生解离 ,非晶区的柔性增强     

乙烯-乙烯醇共聚物的固体核磁共振研究

乙烯 (Ｅ) /乙烯醇 (Ｖ)共聚物 (ＥＶＯＨ)为结晶性高聚物 ,作为膜材料有着广泛的用途 .在该体系中 ,不仅存在复杂的化学和物理结构 ,如序列分布、立构规整性和共晶结构 ,还存在复杂的氢键相互作用 ,是研究化学结构、聚集态结构和氢键相互作用之间关系的代表性体系 .通过ＤＳＣ[1 ] 、Ｘ 射线衍射[2 ] 、固体高分辨核磁共振碳谱 (1 3Ｃ ＣＰ/ＭＡＳ ＮＭＲ) [3～ 7] 等不同的研究方法 ,前人对ＥＶＯＨ体系及与之直接相关的乙烯醇均聚物(ＰＶＡ)的熔融温度、结晶度以及结晶结构等问题进行了大量研究 .1 984年Ｔｅｒａｏ等[6] 首先报道了在固…     

Structural characterization of vulcanized natural rubber by latex state 13C NMR spectroscopy

Structural characterization of vulcanized natural rubber was performed by high‐resolution latex‐state ¹³C NMR spectroscopy. The vulcanized natural rubber latex was prepared by vulcanization of high ammonia natural rubber latex with sulfur and sodium di‐n‐butyldithiocarbamate as vulcanizing agents. High resolution was attained for latex‐state ¹³C NMR spectroscopy even after vulcanization of the rubber latex, as is evident from no background in spectrum and narrow half width of signals, which were independent of vulcanization time. Small signals at 44 and 58 ppm in the carbon region were assigned by measurements of both distortionless enhancement by polarization transfer (DEPT) and attached proton test (APT) to secondary, tertiary, and quaternary carbons of crosslinking points. The assignment was proved by high‐resolution solution‐state NMR spectroscopy of vulcanized liquid cis‐1,4‐polyisoprene as a model, in which DEPT, APT, 2‐dimensional ¹H‐¹³C correlation (HETCOR), and 2‐dimensional heteronuclear multiple bond correlation (HMBC) measurements were applied. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1003–1009, 2007     

Line shapes in CP/MAS (13)C NMR spectra of cellulose I

The CP/MAS (13)C NMR line shape of cellulose I has been qualitatively analyzed by direct simulations using the Ornstein-Uhlenbeck stochastic process and the Kubo model. Both approaches describe a anhydroglucose C4 carbon as a oscillator with fluctuating Larmor frequency. The NMR resonance frequency is written omega=omega +omega(t), where the fluctuating part with zero mean was modelled as a stationary Markov diffusion process. The simulation results both motivates the use of multiple line shapes when fitting CP/MAS (13)C NMR spectra recorded on cellulose I and gives some insights into why signals from crystalline cellulose I give rise to Lorentzian line shapes.     

Application of 1D BIRD or X-filtered DEPT long-range C-C relay for detection of proton and carbon via four bonds and measuring long-range 13C-13C coupling constants

We propose the 13C-detecting 1D DEPT long-range C-C relay to detect super long-range H-C connectivity via four bonds (1H-13C-X-X-13C, X represents 12C or heteronuclear). It is derived from the DEPT C-C relay which detects the H-C correlations via two bonds (1H-13C-13C) by setting the delays for J(CC) in the C-C relay sequence to the (LR)J(CC). This sequence gives correlation signals split by small (LR)J(CC), which seriously suffers from residual center signal. The unwanted signal is due to long-range C-H couplings ((LR)J(CH)). The expected relayed magnetization transfer 1J(CH) --> (LR)J(CC) occurs in the 1H-13C-X-(X)-13C isotopomer, whereas the unwanted signal of (LR)J(CH) comes from 1H-12C-(X)-13C isotopomers, whose population is 100 times larger than that of the 1H-13C-X-(X)-13C isotopomer. The large dispersive line of this unwanted center signal would be a fatal problem in the case of detecting small (LR)J(CC) couplings. This central signal could be removed by an insertion of BIRD pulse or X-filter. DEPT spectrum editing solved a signal overlapping problem and enabled accurate determination of particular (LR)J(CC) values. We demonstrate here the examples of structure determination using connectivity between 1H and 13C via four bonds, and the application of long-range C-C coupling constants to discrimination of stereochemical assignments.     

Probing the Double Bond and Phase Properties of Natural Lipid Dispersions by Cross Polarization/Magic Angle Spinning 13C NMR

Solid-state high resolution ¹³C NMR spectra of bovine brain sulfatide, egg sphingomyelin, egg phosphatidylcholine, and cholesterol/phosphatidylcholine dispersions were obtained by the cross-polarization/magic angle spinning (CP/MAS) method as functions of contact time, temperature and composition. Quantitative analysis of the chemical shift and linewidth data indicate that in the liquid-crystalline state the hydrocarbon chains of sulfatide are packed less orderly than those of sphingomyelin, and that there are two or more conformers for the ceramide residues of natural sphingolipids. The fatty acid double bond showed higher mobility than that of sphingosine as monitored by the signal intensity profiles as a function of contact time. The same approach was used to study the coexisting Lα (liquid-crystalline phase) and β (cholesterol-rich characterized phase) two-phase region of cholesterol/phosphatidylcholine mixtures. Without isotope enrichment, structural and phase properties of natural lipid dispersions can be obtained easily by monitoring the chemical shift, the signal intensity and the linewidth of ^l3C NMR spectra.     

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.     

Pre-irradiation induced grafting of styrene into crosslinked and non-crosslinked polytetrafluoroethylene films for polymer electrolyte fuel cell applications. II: Characterization of the styrene grafted films

Crosslinked and non-crosslinked polytetrafluoroethylene films (RX-PTFE and V-PTFE films, respectively) were irradiated by γ-ray and then grafted with styrene in liquid phase. Microscope FT-IR spectroscopy, TGA, solid state ¹³C CP/MAS and high resolution HS/MAS NMR spectroscopy, wide-angle X-ray diffraction (WAXD) study were used to get the structural information of the styrene grafted RX-PTFE and V-PTFE films. From microscope FT-IR spectra of the grafted RX-PTFE films, the “grafting front mechanism” was proved. TGA analysis showed that the grafted films have a small degradation step and two main degradation steps. In the ¹³C CP/MAS NMR spectra of the non-grafted films, there are no signal due to the absence of the hydrogen atom. While in the spectra of the grafted films, there are signals attributed to the polystyrene grafts. In the ¹³C HS/MAS NMR spectra of the grafted films, the relative intensity of the peaks attributed to the polystyrene grafts increased while the relative intensity of the peak attributed to PTFE matrix decreased with the increase in the DOG. From WAXD patterns, the intensity of the crystalline peak decrease with the increase in the DOG. The grafted films were sulfonated by chlorosulfonic acid and the results of highest IEC value exceeded 3.0. Those results will be reported in the near future.     

Natural Abundance 15N NMR by Dynamic Nuclear Polarization: Fast Analysis of Binding Sites of a Novel Amine‐Carboxyl‐Linked Immobilized Dirhodium Catalyst

A novel heterogeneous dirhodium catalyst has been synthesized. This stable catalyst is constructed from dirhodium acetate dimer (Rh₂(OAc)₄) units, which are covalently linked to amine‐ and carboxyl‐bifunctionalized mesoporous silica (SBA‐15?NH₂?COOH). It shows good efficiency in catalyzing the cyclopropanation reaction of styrene and ethyl diazoacetate (EDA) forming cis‐ and trans‐1‐ethoxycarbonyl‐2‐phenylcyclopropane. To characterize the structure of this catalyst and to confirm the successful immobilization, heteronuclear solid‐state NMR experiments have been performed. The high application potential of dynamic nuclear polarization (DNP) NMR for the analysis of binding sites in this novel catalyst is demonstrated. Signal‐enhanced ¹³C CP MAS and ¹⁵N CP MAS techniques have been employed to detect different carboxyl and amine binding sites in natural abundance on a fast time scale. The interpretation of the experimental chemical shift values for different binding sites has been corroborated by quantum chemical calculations on dirhodium model complexes.     

Coupling NMR to NOM

This work itemizes and critically assesses several 1D and multi-dimensional nuclear magnetic resonance (NMR) techniques, in both the liquid (solvent suppression, APT, DEPT, INEPT, COSY, TOCSY, HSQC, HMQC, HMBC, NOESY, ROESY and others) and solid states (DP, SACP, RAMP-CP, CP-TOSS, MQ-DEPT, 2D ¹H–¹³C HETCOR and others), which are relevant to the characterization of natural organic matter (NOM). The pros and cons of many of the discussed techniques are compared in an effort to provide guidance to the most beneficial utilization of these NMR instrumental techniques for researchers interested in gaining insight into various aspects of NOM.Abbreviations 1D One dimensional - 2D Two dimensional - APT Attached proton test - BIRD Bilinear rotation decoupling - CP Cross polarization - COSY Correlation spectroscopy - CSA Chemical shift anisotropy - DEPT Distortionless enhancement by polarization transfer - DMSO Dimethyl sulfoxide - DOSY Diffusion ordered spectroscopy - DP Direct polarization - DQ Double quantum - FID Free induced decay - FT Fourier transform - FT-ICR-MS Fourier transform-ion cyclotron resonance-mass spectroscopy - HETCOR Heteronuclear correlation - HH Hartmann–Hahn - HMBC Heteronuclear multiple bond correlation - HMQC Heteronuclear multiple quantum coherence - HSQC Heteronuclear single quantum coherence - INEPT Insensitive nuclei enhanced by polarization transfer - LR-COSY Long-range COSY - MAS Magic-angle spinning - MQ Multiple quantum - MS Mass spectroscopy - NMR Nuclear magnetic resonance - NOE Nuclear Overhauser enhancement - NOESY Nuclear Overhauser enhanced spectroscopy - NOM Natural organic matter - PASS Phase adjustment of spinning sidebands - RAMP Ramped amplitude - RESTORE Restoration of spectra via T _CH and T one rho (T _1H) editing - r.f. Radio frequency - ROESY Rotating frame Overhauser enhancement spectroscopy - SACP Single amplitude cross polarization - SOM Soil organic matter - SS Spinning sideband - TMS Tetramethylsilane - TOCSY Total correlation spectroscopy - TOSS Total suppression of sidebands - TPPM Two-pulse phase modulation - VCT Variable contact time - VSL Variable spin lock - WATERGATE Water suppression by gradient tailored excitation     

Solid state NMR studies of syndiotactic polystyrene/ethylbenzene and poly(ethylene oxide)/LiCF3SO3 molecular complexes

Solvent dynamics and polymer-solvent interactions in syndiotactic (s) polystyrene (PS)/ethylbenzene (PhEt) clathrates, as well as polymer-salt interactions in the poly(ethylene oxide) (PEO)/LiCF₃SO₃ complex, were characterized by solid state ¹H and ¹³C NMR. ¹H static and ¹H MAS NMR spectra have shown that PhEt molecules in s-PS clathrates retain relatively large, but spatially anisotropic mobility. ¹³C CP/MAS (cross polarization/magic angle spinning) spectra and CP dynamics measured for s-PS-dg/PhEt system indicate that at least a part of PhEt molecules are intercalated between phenyl rings of s-PS. ¹³C CP/MAS NMR spectra show that PEO carbons in complex with LiCF₃SO₃ are more shielded in comparison to neat crystalline PEO. The results (distances) obtained from CP dynamics are in agreement with the published crystal structure of the PEO/LiCF₃SO₃ complex. ¹³C spin-lattice relaxation time measurements have shown that the mobility of PEO in the complex is lower than that in neat crystalline PEO.