Cross-polarization Magic Angle Spinning Experiments Research Articles

NMR spectroscopy-based analysis of gallstones of cancerous and benign gallbladders from different geographical regions of the Indian subcontinent.

Analysis of the chemical composition of gallstones is vital for the etiopathogenesis of gallstone diseases that can ultimately help in the prevention of its formation. In the present study, gallstones from seven different regions of India were analyzed to highlight the major difference in their composition. Also, gallstones of different pathological conditions i.e., benign (chronic cholecystitis, CC) and malignant gallbladder disease (gallbladder cancer GBC) were characterized. The type of polymorphs of cholesterol molecules was also studied to provide insight into the structure of gallstones. 1H solution state NMR spectroscopy 1D experiments were performed on a total of 94 gallstone (GS) samples collected from seven different geographical regions of India. Solid-State NMR spectroscopy 13C cross-polarization magic angle spinning (CPMAS) experiments were done on the 20 CC GS samples and 20 GBC GS samples of two regions. 1H NMR spectra from the solution state NMR of all the stones reveal that cholesterol was a major component of the maximum stones of the north India region while in south Indian regions, GS had very less cholesterol. 13C CPMAS experiments reveal that the quantity of cholesterol was significantly more in the GS of CC in the Lucknow region compared with GBC stones of Lucknow and Chandigarh. Our study also revealed that GS of the Lucknow region of both malignant and benign gallbladder diseases belong to the monohydrate crystalline form of cholesterol while GS of Chandigarh region of both malignant and benign gallbladder diseases exists in both monohydrate crystalline form with the amorphous type and anhydrous form. Gallstones have a complicated and poorly understood etiology. Therefore, it is important to understand the composition of gallstones, which can be found in various forms and clinical conditions. Variations in dietary practices, environmental conditions, and genetic factors may influence and contribute to the formation of GS. Prevention of gallstone formation may help in decreasing the cases of gallbladder cancer.

Studies of Organic Matter in Composting, Vermicomposting, and Anaerobic Digestion by 13C Solid-State NMR Spectroscopy

Composting, vermicomposting, and anaerobic digestion are three commonly applied processes for the transformation of organic waste into valuable products for soil amendment. The application of compost, vermicompost, and digestate to soil requires specific properties, such as maturity and stability, strongly related to the composition of organic matter. 13C solid-state Nuclear Magnetic Resonance (SSNMR) has often been applied to follow the transformation of organic matter during waste treatment processes, as well as to assess the quality of the produced amendments and the effectiveness of the treatments. Thanks to the possibility of associating the 13C chemical shift to different functional groups of biomacromolecules present in the waste feedstocks and in the final products, thorough characterizations of organic matter have been performed exploiting 13C cross-polarization magic angle spinning experiments, and semiquantitative descriptions of the evolution of the different groups during composting, vermicomposting and anaerobic digestion have been reported. Here, these studies are reviewed with the aim of highlighting the potential of the application of 13C SSNMR to these complex materials, as well as the critical issues and perspectives.

Acceleration of natural-abundance solid-state MAS NMR measurements on bone by paramagnetic relaxation from gadolinium-DTPA

Reducing the data collection time without affecting the signal intensity and spectral resolution is one of the major challenges for the widespread application of multidimensional nuclear magnetic resonance (NMR) spectroscopy, especially in experiments conducted on complex heterogeneous biological systems such as bone. In most of these experiments, the NMR data collection time is ultimately governed by the proton spin–lattice relaxation times (T1). For over two decades, gadolinium(III)-DTPA (Gd-DTPA, DTPA=Diethylene triamine pentaacetic acid) has been one of the most widely used contrast-enhancement agents in magnetic resonance imaging (MRI). In this study, we demonstrate that Gd-DTPA can also be effectively used to enhance the longitudinal relaxation rates of protons in solid-state NMR experiments conducted on bone without significant line-broadening and chemical-shift-perturbation side effects. Using bovine cortical bone samples incubated in different concentrations of Gd-DTPA complex, the 1H T1 values were calculated from data collected by 1H spin-inversion recovery method detected in natural-abundance 13C cross-polarization magic angle spinning (CPMAS) NMR experiments. Our results reveal that the 1H T1 values can be successfully reduced by a factor of 3.5 using as low as 10mM Gd-DTPA without reducing the spectral resolution and thus enabling faster data acquisition of the 13C CPMAS spectra. These results obtained from 13C-detected CPMAS experiments were further confirmed using 1H-detected ultrafast MAS experiments on Gd-DTPA doped bone samples. This approach considerably improves the signal-to-noise ratio per unit time of NMR experiments applied to bone samples by reducing the experimental time required to acquire the same number of scans.

High-resolution solid-state NMR study of isotactic polypropylenes

The high-resolution solid-state 13 C NMR spectra were recorded for metallocene (m) and Ziegler-Natta (ZN) iso- tactic polypropylenes (iPP) in pelletized form using cross polarization (CP) and magic angle spinning (MAS) techniques within the temperature range of 20-160°C. Besides the CP MAS experiments also the MAS 13 C NMR spectra (without CP), MAS 1 H NMR spectra and rotating frame spin-lattice relaxation times T1 ( 13 C) were measured at elevated temperatures. With the rise of temperature the splitting of CH2, CH and CH3 signals into two components was detected in 13 C NMR spec- tra and assigned to amorphous and crystalline phases. The temperature dependences of chemical shifts and integral intensi- ties obtained from the deconvoluted spectra provided information on the main chain and CH3 groups motions in amorphous and crystalline regions of studied samples. While T1 ( 13 C) values show that the rate of segmental motion in amorphous regions in m-iPP and ZN-iPP is virtually the same, larger linewidths in 13 C and 1 H NMR spectra indicate somewhat larger restraints of the motion in amorphous regions of ZN-iPP.

ChemInform Abstract: High Resolution 31P Solid State NMR in Phosphorus-Transition Metal Compounds

Abstract The use of a high resolution solid state 31 P NMR technique to investigate the coordination of phopshorus ligands bound to transition metal atoms is reviewed using examples which include information on molecular structure and crystallographic sites in organometallic solids, and characterization of supported transition metal complexes on surfaces. Moreover 31 P CPMAS (cross polarization magic angle spinning) experiments can afford a wealth of information such as chemical shift anisotropy, isotropic coupling constants with quadrupolar nuclei and quadrupole coupling constants that are often obscured in solution state due to the rapid isotropic motion of the molecules.

Distribution and mobility of phosphates and sodium ions in cheese by solid‐state 31P and double‐quantum filtered 23Na NMR spectroscopy

The feasibility of solid-state magic angle spinning (MAS) (31)P nuclear magnetic resonance (NMR) spectroscopy and (23)Na NMR spectroscopy to investigate both phosphates and Na(+) ions distribution in semi-hard cheeses in a non-destructive way was studied. Two semi-hard cheeses of known composition were made with two different salt contents. (31)P Single-pulse excitation and cross-polarization MAS experiments allowed, for the first time, the identification and quantification of soluble and insoluble phosphates in the cheeses. The presence of a relatively 'mobile' fraction of colloidal phosphates was evidenced. The detection by (23)Na single-quantum NMR experiments of all the sodium ions in the cheeses was validated. The presence of a fraction of 'bound' sodium ions was evidenced by (23)Na double-quantum filtered NMR experiments. We demonstrated that NMR is a suitable tool to investigate both phosphates and Na(+) ions distributions in cheeses. The impact of the sodium content on the various phosphorus forms distribution was discussed and results demonstrated that NMR would be an important tool for the cheese industry for the processes controls.

Solid-State NMR Spectroscopy Detects Interactions between Tryptophan Residues of the E. coli Sugar Transporter GalP and the α-Anomer of the d-Glucose Substrate

An experimental approach is described in which high resolution 13C solid-state NMR (SSNMR) spectroscopy has been used to detect interactions between specific residues of membrane-embedded transport proteins and weakly binding noncovalent ligands. This procedure has provided insight into the binding site for the substrate D-glucose in the Escherichia coli sugar transport protein GalP. Cross-polarization magic-angle spinning (CP-MAS) SSNMR spectra of GalP in its natural membrane at 4 degrees C indicated that the alpha- and beta-anomers of D-[1-(13)C]glucose were bound by GalP with equal affinity and underwent fast exchange between the free and bound environments. Further experiments confirmed that by lowering the measurement temperature to -10 degrees C, peaks could be detected selectively from the substrate when restrained within the binding site. Dipolar-assisted rotational resonance (DARR) SSNMR experiments at -10 degrees C showed a selective interaction between the alpha-anomer of D-[1-(13)C]glucose and 13C-labels within [13C]tryptophan-labeled GalP, which places the carbon atom at C-1 in the alpha-anomer of D-glucose to within 6 A of the carbonyl carbon of one or more tryptophan residues in the protein. No interaction was detected for the beta-isomer. The role of tryptophan residues in substrate binding was investigated further in CP-MAS experiments to detect D-[1-(13)C]glucose binding to the GalP mutants W371F and W395F before and after the addition of the inhibitor forskolin. The results suggest that both mutants bind D-glucose with similar affinities, but have different affinities for forskolin. This work highlights a useful general experimental strategy for probing the binding sites of membrane proteins, using methodology which overcomes the problems associated with the unfavorable dynamics of weak ligands.

The domain structure and mobility of semi-crystalline poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate- co-3-hydroxyvalerate): A solid-state NMR study

Solid-state NMR techniques have been employed to investigate the domain structure and mobility of the bacterial biopolymeric metabolites such as poly(3-hydroxybutyrate) (PHB) and its copolymers poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) containing 2.7 mol% (PHBV2.7) and 6.5 mol% (PHBV6.5) 3-hydroxyvalerate. Both single-pulse excitation with magic-angle spinning (SPEMAS) and cross-polarization magic-angle spinning (CPMAS) 13C NMR results showed that these biopolymers were composed of amorphous and crystalline regions having distinct molecular dynamics. Under magic-angle spinning, 1H T 1 ρ and 13C T 1 showed two processes for each carbon. Proton relaxation-induced spectral editing (PRISE) techniques allowed the neat separation of the 13C resonances in the crystalline regions from those in the amorphous ones. The proton spin–lattice relaxation time in the tilted rotating frame, H T 1 ρ T , measured using the Lee–Goldburg sequence with frequency modulation (LGFM) as the spin–locking scheme, was also double exponential and significantly longer than 1H T 1 ρ . The difference between H T 1 ρ T for the amorphous and crystalline domains was greater than that of 1H T 1 ρ . Our results showed that the H T 1 ρ T differences could be exploited in LGFM–CPMAS experiments to separate the signals from two distinct regions. 1H spin-diffusion results showed that the domain size of the mobile components in PHB, PHBV2.7 and PHBV6.5 were about 13, 24 and 36 nm whereas the ordered domain sizes were smaller than 76, 65 and 55 nm, respectively. The results indicated that the introduction of 3-hydroxyvalerate into PHB led to marked molecular mobility enhancement in the biopolymers.

Stochastic molecular motions in the nematic, smectic-A, and solid phases ofp,p′-di-n-heptyl-azoxybenzene as seen by quasielastic neutron scattering andC13cross-polarization magic-angle-spinning NMR

Molecular rotational dynamics in p,p'-di-n-heptyl-azoxybenzene was studied by means of quasielastic neutron scattering (QENS) and 13C cross-polarization magic-angle-spinning (CPMAS) NMR. Fast reorientation of the hydrogen nuclei was observed by QENS in the two liquid crystalline (LC) phases nematic and smectic A, as well as in the crystalline phase. The latter could not be restricted to the -CH3 rotations alone, and a clear indication was found of some other reorientation motions persisting in the crystal. Two Lorentz-type components convoluted with the resolution function gave an excellent fit to the QENS spectra in both LC phases. The narrow (slow) component was attributed to the reorientation of the whole molecule around the long axis. The corresponding characteristic time of approximately 130 ps agreed well with the values obtained in recent dielectric relaxation and 2H NMR studies. The full width at half maximum of the broader (fast) component shows a quadratic Q dependence (Q is the momentum transfer). Hence the corresponding motions could be described by a stretched exponential correlation function and were interpreted as various "crankshaft-type" motions within the alkyl tails. The 13C CPMAS experiments fully corroborated the QENS results, sometimes considered ambiguous in complex systems.

Surface dynamics of bacteriorhodopsin as revealed by (13)C NMR studies on [(13)C]Ala-labeled proteins: detection of millisecond or microsecond motions in interhelical loops and C-terminal alpha-helix.

We have recorded (13)C NMR spectra of [2-(13)C]-, [1-(13)C]-, [3-(13)C],- and [1,2,3-(13)C(3)]Ala-labeled bacteriorhodopsin (bR), and its mutants, A196G, A160G, and A103C, by means of cross polarization-magic angle spinning (CP-MAS) and dipolar decoupled-magic angle spinning (DD-MAS) techniques, to reveal the conformation and dynamics of bR, with emphasis on the loop and C-terminus structures. The (13)C NMR signals of the loop (C-D, E-F, and F-G) regions were almost completely suppressed from [2-(13)C]-, [1-(13)C]Ala-, and [1-(13)C]Gly-labeled bR, due to the presence of conformational fluctuation with correlation times of 10(-4) s that interfered with the peak-narrowing by magic angle spinning. The observation of such suppressed peaks for specific residues provides a unique means of detecting intermediate frequency motions on the time scale of ms or micros in the surface loops of membrane proteins. Instead, the three well-resolved (13)C CP-MAS NMR signals of [2-(13)C]Ala-bR, at 50.38, 49.90, and 47.96 ppm, were ascribed to the C-terminal alpha-helix previously proposed from the data for [3-(13)C]Ala-bR: the former two peaks were assigned to Ala 232 and 238, in view of the results of successive proteolysis experiments, while the highest-field peak was ascribed to Ala 235 prior to Pro 236. Even such (13)C NMR signals were substantially broadened when (13)C NMR spectra of fully labeled [1,2,3-(13)C]Ala-bR were recorded, because the broadening and splitting of peaks due to the accelerated transverse relaxation rate caused by the increased number of relaxation pathways through a number of (13)C-(13)C homo-nuclear dipolar interactions and scalar J couplings, respectively, are dominant among (13)C-labeled nuclei. In addition, approximate correlation times for local conformational fluctuations of different domains, including the C-terminal tail, C-terminal alpha-helix, loops, and transmembrane alpha-helices, were estimated by measurement of the spin-lattice relaxation times in the laboratory frame and spin-spin relaxation times under the conditions of cross-polarization-magic angle spinning, and comparative study of suppressed specific peaks between the CP-MAS and DD-MAS experiments.

Structural properties of sodium phosphate glasses from 23Na → 31P cross-polarization NMR

One-dimensional heteronuclear correlation solid-state nuclear magnetic resonance (NMR) experiments are utilized to determine local structural changes in sodium phosphate glass compositions with ratios of Na/P of 0.25, 0.78, 1 and 1.3. Glasses containing only Q 2 (metaphosphate composition), Q 2 and Q 1 (between meta- and pyrophosphate compositions), and Q 3 and Q 2 (ultraphosphate region) have been investigated. 23Na → 31P cross-polarization magic-angle-spinning (CPMAS) NMR detects direct interactions between distinct phosphorus sites and sodium nuclei in the second coordination sphere. Variable-contact CPMAS experiments provide additional information about association of 23Na nuclei with specific phosphate sites. Time constants describing the transfer of magnetization from sodium to phosphorus nuclei are found to be affected by sodium ion concentration in ultraphosphate glasses, and an important finding of these investigations is the degree of correlation of sodium ions with Q 3 sites in ultraphosphate glasses.

13C Solid-State NMR Determination of Cross-Linking Degree in Superabsorbing Cellulose-Based Networks

13C solid-state NMR analysis has been performed on microporous superabsorbing networks based on hydroxyethylcellulose and sodium salt of carboxymethylcellulose in order to determine the chemically effective cross-linking degree. Microporous samples characterized by two different degrees of cross-link have been analyzed by performing 13C cross-polarization magic angle spinning (CP-MAS) solid-state spectra and single-pulse excitation (SPE) experiments. Cross-linking degrees, expressed as number of cross-links per monomer, have been evaluated from the area of resonance due to methylene carbon atoms present on reacted cross-linker molecule (divinyl sulfone) and area of resonances due to anomeric carbon atoms C-1 of the polysaccharide rings. Optimization procedures for both CP-MAS and SPE experiments are also reported.

Mapping aluminum/phosphorus connectivities in aluminophosphate glasses

Two-dimensional 27Al/ 31P cross-polarization magic-angle spinning (CPMAS) NMR experiments are performed on sodium aluminophosphate glasses in the xNaAlO 2·(1− x)NaPO 3 compositional range, providing direct detection of particular aluminum species connected to specific phosphorus species. In particular, the spectral-editing capabilities of two-dimensional experiments allow the elucidation of phosphorus resonances that are strongly correlated to four-, five- and six-coordinate aluminum species. Results from two-dimensional contour plots and projections of spectra along the 31P dimension allow the assignment of phosphate structures that cannot be directly assigned from one-dimensional 31P MAS spectra. Q 1 0, Q 3 0 and Q 2 1 species have all been identified in these aluminophosphate glasses and tetrahedral aluminum species are bonded to each of these phosphate moieties. Changes in the phosphate/aluminate ordering are also monitored by inspection of the phosphorus subspectra of the two-dimensional CPMAS experiments, demonstrating a change in aluminum/phosphorus connectivity correlated to changes in molar composition.

In situ solid-state NMR studies of ethanol photocatalysis: characterization of surface sites and their reactivities

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of ethanol (C 2H 5OH) over two TiO 2-based catalysts, Degussa P-25 powder and a monolayer TiO 2 catalyst dispersed on porous Vycor glass. Two adsorption sites were observed for ethanol, a chemisorbed species identified as an ethoxide species, and a hydrogen-bonded species. The Ti-bound ethoxide was found to be the more photocatalytically reactive species. 13 C magic-angle spinning (MAS) experiments revealed that long-lived intermediates, including acetaldehyde (CH 3CHO), acetic acid, formic acid, and acetate were observed under dry conditions and in the presence of molecular oxygen. Similar reaction intermediates form on the surfaces of both catalysts. 13 C cross-polarization MAS experiments allowed us to identify and follow the evolution of surface-bound species during UV irradiation. Acetate, which forms from mobile acetic acid, appears to be bound to the non-irradiated surfaces of the powdered TiO 2 catalyst, and was also observed on the surface of TiO 2/PVG catalyst, and further degradation was not possible. The presence of molecular oxygen was found to be essential for photooxidation to proceed.

Deuterium Cross-Polarization Magic-Angle Spinning

In this publication a theoretical model is presented that describes cross-polarization magic-angle spinning (CPMAS) NMR experiments on a spin systemS(1)IN, consisting of a set ofNabundant homonuclear spins withI= 12 coupled to a single rare spin withS= 1. The spin evolution during this magic-angle spinning experiment is described using Floquet theory. The model is an extension of the formalism that was recently introduced to describe CPMAS ofS(12)INspin systems. First, experimental results of2H CPMAS experiments on partially deuterated dimethyl sulfone and malonic acid are shown. The rotational-echo intensities of the2H free-induction-decay signals were monitored and plotted as a function of the difference between the intensities of the RF fields applied on the deuterons and the protons during the mixing time. Then the Floquet description of a spin system withS= 1 is presented in order to enable the introduction of the Floquet model for CPMAS NMR. The Floquet Hamiltonian of the rotating quadrupolar spin is defined and the difference between spin locking in the rotating frame and in Floquet space is discussed. This is followed by a description of the spin evolution of theS(1)INsystem during CPMAS experiments. The modified Hartmann–Hahn conditions for these experiments are derived and a methodology for calculating the cross-polarization S-spin signal intensities is demonstrated. The discussion is restricted to spin-1 nuclei with relatively small quadrupole interactions and is directed toward2H CPMAS. S-spin signal intensities as a function of mismatched Hartmann–Hahn conditions are evaluated for powder samples with quadrupolar frequencies of 40 and 120 kHz.

A study of 13C-enriched chemical vapor deposited diamond film by means of 13C nuclear magnetic resonance, electron paramagnetic resonance, and dynamic nuclear polarization

13C nuclear magnetic resonance (NMR), with and without dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) measurements are presented on two 13C-enriched chemical vapor deposited (CVD) diamond films. The samples were prepared by the decomposition in a microwave discharge of a gas mixture of CH4 and H2, with and without O2. 13C NMR spectra, obtained with or without magic-angle spinning (MAS) both at a field of 14 T using direct polarization (DP) and at 3.5 T via cross polarization (CP), are presented. Because the samples contain both nuclear and (unpaired) electron spins, irradiation of the sample with microwaves with a frequency at or near the electron Larmor frequency leads to an enhancement of the nuclear spin polarization via the effect referred to as dynamic nuclear polarization (DNP). This enhancement (at a field of 1.4 T), combined with the effects of isotopic enrichment (14% 13C), makes it possible to measure 13C DNP–CP–MAS spectra within a couple of hours, compared to almost 20 h for the CP–MAS spectra taken at high field without DNP. Data based on DNP–CP–MAS experiments with variable CP-match time or dipolar-dephasing time are shown and discussed in terms of structural models. The cross-polarization signals are rationalized in terms of estimates of the chemical shifts of hydrogenated diamond surfaces; this approach indicates that most of the hydrogens in the polycrystalline sample are at the intergrain boundaries.

High resolution 31P solid state NMR in phosphorus-transition metal compounds

The use of a high resolution solid state 31P NMR technique to investigate the coordination of phopshorus ligands bound to transition metal atoms is reviewed using examples which include information on molecular structure and crystallographic sites in organometallic solids, and characterization of supported transition metal complexes on surfaces. Moreover 31P CPMAS (cross polarization magic angle spinning) experiments can afford a wealth of information such as chemical shift anisotropy, isotropic coupling constants with quadrupolar nuclei and quadrupole coupling constants that are often obscured in solution state due to the rapid isotropic motion of the molecules.

NMR lineshapes of 13C nuclei directly bonded to deuterium in solid state cross polarization magic-angle spinning experiments

The solid state 13C NMR spectral lineshape of a carbon atom directly bonded to a quadrupolar nucleus can take the form of an asymmetric multiplet in magic-angle spinning experiments. In this study, we have obtained spectra at various B 0 field strengths of a sugar molecule deuterated at a specific position. The spectrum of this molecule, acquired using the dipolar-dephasing pulse sequence, was much simplified, containing essentially only signals assigned to the deuterated carbon. Lineshape simulations were found to be the most successful for spectra acquired at higher magnetic field strengths.