Rotor synchronization of radiofrequency and gradient pulses in high-resolution magic angle spinning NMR

Matrix-assisted DOSY

The analysis of mixtures by NMR spectroscopy is challenging. Diffusion-ordered NMR spectroscopy enables a pseudo-separation of species based on differences in their translational diffusion coefficients. Under the right circumstances, this is a powerful technique; however, when molecules diffuse at similar rates separation in the diffusion dimension can be poor. In addition, spectral overlap also limits resolution and can make interpretation challenging. Matrix-assisted diffusion NMR seeks to improve resolution in the diffusion dimension by utilising the differential interaction of components in the mixture with an additive to the solvent. Tuning these matrix-analyte interactions allows the diffusion resolution to be optimised. This review presents the background to matrix-assisted diffusion experiments, surveys the wide range of matrices employed, including chromatographic stationary phases, surfactants and polymers, and demonstrates the current state of the art.

HR-MAS NMR Based Quantitative Metabolomics in Breast Cancer

High resolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) spectroscopy is increasingly used for profiling of breast cancer tissue, delivering quantitative information for approximately 40 metabolites. One unique advantage of the method is that it can be used to analyse intact tissue, thereby requiring only minimal sample preparation. Importantly, since the method is non-destructive, it allows further investigations of the same specimen using for instance transcriptomics. Here, we discuss technical aspects critical for a successful analysis—including sample handling, measurement conditions, pulse sequences for one- and two dimensional analysis, and quantification methods—and summarize available studies, with a focus on significant associations of metabolite levels with clinically relevant parameters.

Size-exclusion chromatographic NMR under HR-MAS

The addition of stationary phases or sample modifiers can be used to modify the separation achievable in the diffusion domain of diffusion NMR experiments or provide information on the nature of the analyte-sample modifier interaction. Unfortunately, the addition of insoluble chromatographic stationary phases can lead to line broadening and degradation in spectral resolution, largely because of differences in magnetic susceptibility between the sample and the stationary phase. High-resolution magic angle spinning (HR-MAS) techniques can be used to remove this broadening. Here, we attempt the application of HR-MAS to size-exclusion chromatographic NMR with limited success. Observed diffusion coefficients for polymer molecular weight reference standards are shown to be larger than those obtained on static samples. Further investigation reveals that under HR-MAS it is possible to obtain reasonably accurate estimates of diffusion coefficients, using either full rotor synchronisation or sophisticated pulse sequences. The requirement for restricting the sample to the centre of the MAS rotor to ensure homogeneous magnetic and RF fields is also tested. Copyright © 2016 John Wiley & Sons, Ltd.

Matrix-assisted diffusion-ordered spectroscopy

Matrix-assisted diffusion-ordered NMR spectroscopy has the potential to transform mixture analysis by DOSY NMR.

Assessing Susceptibility to Epilepsy in Three Rat Strains Using Brain Metabolic Profiling Based on HRMAS NMR Spectroscopy and Chemometrics

The possibility that a metabolomic approach can inform about the pathophysiology of a given form of epilepsy was addressed. Using chemometric analyses of HRMAS NMR data, we compared several brain structures in three rat strains with different susceptibilities to absence epilepsy: Genetic Absence Epilepsy Rats from Strasbourg (GAERS), Non Epileptic Control rats (NEC), and Wistar rats. Two ages were investigated: 14 days postnatal (P14) before the onset of seizures and 5 month old adults with fully developed seizures (Adults). The relative concentrations of 19 metabolites were assessed using (1)H HRMAS NMR experiments. Univariate and multivariate analyses including multiblock models were used to identify the most discriminant metabolites. A strain-dependent evolution of glutamate, glutamine, scyllo-inositol, alanine, and glutathione was highlighted during cerebral maturation. In Adults, data from somatosensory and motor cortices allowed discrimination between GAERS and NEC rats with higher levels of scyllo-inositol, taurine, and phosphoethanolamine in NEC. This epileptic metabolic phenotype was in accordance with current pathophysiological hypothesis of absence epilepsy (i.e., seizure-generating and control networks) and putative resistance of NEC rats and was observed before seizure onset. This methodology could be very efficient in a clinical context.

Therapeutic and preventive effects of methylene blue on Alzheimer's disease pathology in a transgenic mouse model

Methylene blue (MB) belongs to the phenothiazinium family. It has been used to treat a variety of human conditions and has beneficial effects on the central nervous system in rodents with and without brain alteration. The present study was designed to test whether chronic MB treatment taken after (therapeutic effect) or before (preventive effect) the onset of beta-amyloid pathology influences cognition in a transgenic mouse model (APP/PS1). In addition, the present study aims at revealing whether these behavioral effects might be related to brain alteration in beta-amyloid deposition. To this end, we conducted an in vivo study and compared two routes of drug administration, drinking water versus intraperitoneal injection. Results showed that transgenic mice treated with MB orally or following intraperitoneal injection were protected from cognitive impairments in a variety of social, learning, and exploratory tasks. Immunoreactive beta-amyloid deposition was significantly reduced in the hippocampus and adjacent cortex in MB-treated transgenic mice. Interestingly, these beneficial effects were observed independently of beta-amyloid load at the time of MB treatment. This suggests that MB treatment is beneficial at both therapeutic and preventive levels. Using solid-state High Resolution Magic Angle Spinning Nuclear Magnetic Resonance (HRMAS-NMR), we showed that MB administration after the onset of amyloid pathology significantly restored the concentration of two metabolites related to mitochondrial metabolism, namely alanine and lactate. We conclude that MB might be useful for the therapy and prevention of Alzheimer's disease. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

NMR methods for unravelling the spectra of complex mixtures

The main methods for the simplification of the NMR of complex mixtures by selective attenuation/suppression of the signals of certain components are presented. The application of relaxation, diffusion and PSR filters and other techniques to biological samples, pharmaceuticals, foods, living organisms and natural products are illustrated with examples.

High-resolution magic-angle-spinning NMR spectroscopy for metabolic profiling of intact tissues

Metabolic profiling, metabolomic and metabonomic studies require robust study protocols for any large-scale comparisons and evaluations. Detailed methods for solution-state NMR spectroscopy have been summarized in an earlier protocol. This protocol details the analysis of intact tissue samples by means of high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy and we provide a detailed description of sample collection, preparation and analysis. Described here are (1)H NMR spectroscopic techniques such as the standard one-dimensional, relaxation-edited, diffusion-edited and two-dimensional J-resolved pulse experiments, as well as one-dimensional (31)P NMR spectroscopy. These are used to monitor different groups of metabolites, e.g., sugars, amino acids and osmolytes as well as larger molecules such as lipids, non-invasively. Through the use of NMR-based diffusion coefficient and relaxation times measurements, information on molecular compartmentation and mobility can be gleaned. The NMR methods are often combined with statistical analysis for further metabonomics analysis and biomarker identification. The standard acquisition time per sample is 8-10 min for a simple one-dimensional (1)H NMR spectrum, giving access to metabolite information while retaining tissue integrity and hence allowing direct comparison with histopathology and MRI/MRS findings or the evaluation together with biofluid metabolic-profiling data.

Application of HR-MAS NMR in the solid-phase synthesis of a glycopeptide using Sieber amide resin

The solid-phase synthesis (SPS) of a structurally complex glycopeptide, using Sieber amide resin, was monitored by high resolution magic angle spinning NMR, demonstrating the further application of this technique. A synthetic peptidoglycan derivative, a precursor of a biologically active PGN, known to be involved in the cellular recognition, was prepared by SPS. The synthesis involved the preparation of an N-alloc glucosamine moiety and the synthesis of a simple amino acid sequence L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. Last step consisted the coupling, on solid-phase, of the protected muramyl unit to the peptide chain. Proton spectra with good suppression of the polystyrene signals in swollen resin samples were obtained in DMF-d(7) as a solvent and by using a nonselective 1D TOCSY/DIPSI-2 scheme, thus allowing to follow the SPS without losses of compound and cleavage from the resin. The assignment of the proton spectra of the resin-bound amino acid sequence and of the bound glycopeptide was achieved through the combination of MAS COSY, TOCSY and NOESY.

Quantification of choline- and ethanolamine-containing metabolites in human prostate tissues using 1H HR-MAS total correlation spectroscopy

A fast and quantitative 2D high-resolution magic angle spinning (HR-MAS) total correlation spectroscopy (TOCSY) experiment was developed to resolve and quantify the choline- and ethanolamine-containing metabolites in human prostate tissues in approximately 1 hr prior to pathologic analysis. At a 40-ms mixing time, magnetization transfer efficiency constants were empirically determined in solution and used to calculate metabolite concentrations in tissue. Phosphocholine (PC) was observed in 11/15 (73%) cancer tissues but only 6/32 (19%) benign tissues. PC was significantly higher (0.39 +/- 0.40 mmol/kg vs. 0.02 +/- 0.07 mmol/kg, z = 3.5), while ethanolamine (Eth) was significantly lower in cancer versus benign prostate tissues (1.0 +/- 0.8 mmol/kg vs. 2.3 +/- 1.9 mmol/kg, z = 3.3). Glycerophosphocholine (GPC) (0.57 +/- 0.87 mmol/kg vs. 0.29 +/- 0.26 mmol/kg, z = 1.2), phosphoethanolamine (PE) (4.4 +/- 2.2 mmol/kg vs. 3.4 +/- 2.6 mmol/kg, z = 1.4), and glycerophosphoethanolamine (GPE) (0.54 +/- 0.82 mmol/kg vs. 0.15 +/- 0.15 mmol/kg, z = 1.8) were higher in cancer versus benign prostate tissues. The ratios of PC/GPC (3.5 +/- 4.5 vs. 0.32 +/- 1.4, z = 2.6), PC/PE (0.08 +/- 0.08 vs. 0.01 +/- 0.03, z = 3.5), PE/Eth (16 +/- 22 vs. 2.2 +/- 2.0, z = 2.4), and GPE/Eth (0.41 +/- 0.51 vs. 0.06 +/- 0.06, z = 2.6) were also significantly higher in cancer versus benign tissues. All samples were pathologically interpretable following HR-MAS analysis; however, degradation experiments showed that PC, GPC, PE, and GPE decreased 7.7 +/- 2.2%, while Cho+mI and Eth increased 18% in 1 hr at 1 degrees C and a 2250 Hz spin rate.

Methods for acquisition and assignment of multidimensional high-resolution magic angle spinning NMR of whole cell bacteria

High-resolution magic-angle spinning (HR-MAS) NMR was developed in late 1990s, and it has evolved quickly for the study of a variety of biological matrixes. Recently, it has been used as an effective means to study the cell wall structures of intact bacteria. (1)H-(13)C heteronuclear single quantum coherence (HSQC) HR-MAS NMR can provide rapid analysis of the cell wall structure in live bacterial cells, thus allowing observation of drug effects, gene mutation, species differentiation, and environmental effects. However, this rapid analysis is dependent on having an established framework of HR-MAS NMR experiments and a detailed assignment of the whole-cell NMR spectra. This study examines parameters and describes strategies for the effective application of 2D and 3D HR-MAS NMR techniques to assign and study bacterial cell wall structures using Mycobacterium smegmatis as a model organism. Important parameters for successful whole-cell HR-MAS NMR studies, including pulse sequences, rotor synchronization, acquisition times, labeling strategies, temperature, number of cells, and cell viability, are described. A four-prong approach is presented for assignment of the complex whole-cell spectra, including the use of 3D HCCH-TOCSY and HCCH-COSY HR-MAS NMR.

Concentration profiling in rat tissue by high-resolution magic-angle spinning NMR spectroscopy: investigation of a model drug

The utility of high-resolution magic-angle spinning (HR-MAS) NMR for studying drug delivery in whole tissues was explored by dosing female Sprague-Dawley rats with topical or injectable benzoic acid (BA). In principle, HR-MAS NMR permits the detection of both intra- and extracellular compounds. This is an advantage over the previous detection of topically applied BA using microdialysis coupled to HPLC/UV as microdialysis samples only the extracellular space. Skin and muscle samples were analyzed by (1)H HR-MAS NMR, and BA levels were determined using an external standard solution added to the sample rotor. One to two percent of the BA topical dose was detected in the muscle, showing that BA penetrated through the dermal and subcutaneous layers. Since BA was not detected in the muscle in the microdialysis studies, the NMR spectra revealed the intracellular localization of BA. The amount of BA detected in muscle after subcutaneous injection correlated with the distance from the dosing site. Overall, the results suggest that HR-MAS NMR can distinguish differences in the local concentration of BA varying with tissue type, dosage method, and tissue proximity to the dosing site. The results illustrate the potential of this technique for quantitative analysis of drug delivery and distribution and the challenges to be addressed as the method is refined.

Selective intracellular accumulation of the major metabolite issued from the activation of the prodrug ethionamide in mycobacteria

BACKGROUND

Ethionamide is one of the most widely used drugs for the treatment of multidrug-resistant tuberculosis (MDR-TB). Like isoniazid, and pyrazinamide, ethionamide is a prodrug that needs to be activated by a mycobacterial enzyme. Activation pathways of prodrugs are generally problematic to uncover as they produce intermediates potentially difficult to characterize, to purify and that might prove unstable outside of their cellular context.

OBJECTIVES AND METHODS

We have used high resolution magic angle spinning-NMR (HRMAS-NMR) to follow ethionamide activation directly within living mycobacterial cells.

RESULTS

Data indicated that the intracellular metabolization of ethionamide strictly depends on the presence of the monooxygenase EthA and that EthA-dependent activation of ethionamide is coupled to a precise molecular sorting mechanism of the ethionamide metabolites. We found that the previously identified ethionamide metabolite 2-ethyl-4-hydroxymethylpyridine is produced in substantial amounts by the ethionamide-treated mycobacteria and that it is present exclusively outside of the bacteria. In contrast, the still unidentified ethionamide metabolite ETH* is the only ethionamide derivative detected within the bacterial cell. Moreover, ETH* appears to be unable to cross the bacterial envelope and consequently accumulates within the cytoplasm of the ethionamide-treated mycobacteria.

CONCLUSIONS

These results strongly suggest that ETH* is the active antimycobacterial ethionamide derivative and open new perspectives for the understanding of the mode of action of prodrugs.

Applications of variable-angle sample spinning experiments to the measurement of scaled residual dipolar couplings and 15N CSA in soluble proteins

NMR spectra of ubiquitin in the presence of bicelles at a concentration of 32% w/v have been recorded at 700 MHz under sample spinning conditions at the magic angle (54.7 degrees ) and at an angle of 45.5 degrees . At the magic angle, the 1H-15N HSQC spectrum of ubiquitin in bicelles is virtually indistinguishable from the one recorded on the protein in solution. Spinning the sample at the magic angle creates an isotropic environment with no preferred bicelle orientations, thus allowing the determination of scalar coupling constants. For an angle of rotation of 45.5 degrees , the bicelles orient with their normal perpendicular to the spinning axis leading to the observation of strong residual dipolar couplings and chemical shift variations of the 15N resonances.

Monitoring of the ethionamide pro-drug activation in mycobacteria by 1H high resolution magic angle spinning NMR

In this study, we use HRMAS NMR as a non-invasive technique to monitor the in vivo metabolism of a xenobiotic. The antituberculosis Ethionamide is a pro-drug that has to be activated in mycobacteria before inhibiting its cellular target. The use of (1)H HRMAS NMR has allowed to detect a metabolite (ETH*) of the drug directly in living bacteria, even with a spectrometer operating at the relatively low magnetic field of 300MHz. We show that metabolism monitoring of an unlabelled drug at a therapeutically relevant concentration as low as 5mug/ml is within reach of the technique. (1)H HRMAS NMR in combination with diffusion filtering leads to the conclusion that the metabolite is located inside the intact cells. The comparison of the metabolite NMR signature with that of synthetic molecules proves the non-identity of ETH* with the ETH derivatives described previously in the literature.

Field modulation effects induced by sample spinning: application to high-resolution magic angle spinning NMR

High-resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample and on pulse sequences originally developed for liquid state NMR. In most cases the outcome of the experiment is conform to what is expected from high-resolution liquid state NMR spectroscopy. However in some instances, experiments run under MAS can produce some very puzzling results. After reviewing the basic hardware which is at the heart of HRMAS spectroscopy, we show that the origin of this behavior lies in the natural time-dependence of some physical quantities imparted by the rotation. We focus in particular on the effects of B1 inhomogeneities on the nutation, the (90 degrees)+x-t-(90 degrees )-x and the MLEV16 experiments. Different models of radiofrequency distribution of B1 fields in a solenoidal coil are derived from simple geometrical considerations. These models are shown by NMR spin dynamics calculations to reproduce the experimental NMR results. They are also consistent with electromagnetic simulations of the B1 field distribution inside a solenoidal coil.

Development of a high-resolution magic-angle spinning nuclear magnetic resonance identity assay of the capsular polysaccharide from Haemophilus influenzae type b present in cetavlon precipitate

We describe the use of high-resolution magic-angle spinning nuclear magnetic resonance to control the identity of the capsular polysaccharide from Haemophilus influenzae type b (Hib) present in the cetavlon precipitate. This step is one of the earliest in the purification of this polysaccharide, which is further used in the production of Hib polysaccharide-protein conjugate vaccine. The effects of sample procedure and magnetic field strength have been investigated. Since this assay is rapid and simple, it may represent a useful technique for characterization of polysaccharides present in complex and insoluble matrices. Moreover, it allows a rapid evaluation of the structure of the produced polysaccharides very early on during the production process and is as such an essential analytical tool before starting the purification process.

Improved signal to noise in high-resolution magic angle spinning total correlation spectroscopy studies of prostate tissues using rotor-synchronized adiabatic pulses

A rotor-synchronized WURST-8 adiabatic pulse scheme was compared to the conventional MLEV-17 hard pulse scheme for isotropic mixing in total correlation spectroscopy (TOCSY) studies of intact human prostate tissues under high-resolution magic angle spinning (HR-MAS) conditions. Both mixing schemes were extremely sensitive to the rotational resonance condition and dramatic reductions in signal to noise were observed when pulse durations deviated from 1/(spin rate). A significant increase in cross-peak intensities was observed using rotor-synchronized WURST-8 adiabatic pulses versus those observed using the rotor-synchronized MLEV-17 hard pulse scheme in both solution and tissue. In tissue, absolute signal intensities ranged from 1.5x to 10.5x greater (average: 4.75x) when WURST-8 was used in place of MLEV-17. Moreover, the difference was so dramatic that several metabolite cross peaks observed using WURST-8 pulses were not observed using MLEV-17 pulses, including cross peaks corresponding to many of the choline- and ethanolamine-containing metabolites. Due to the complex modulation of TOCSY cross peaks for multiply coupled spins and the shorter T(2) relaxation times of tissue metabolites, maximum cross-peak intensities occurred at shorter mixing times than predicted by theory. In summary, a WURST-8 adiabatic mixing scheme produced significantly greater absolute cross-peak signal intensities than MLEV-17 hard pulse mixing, and maximum cross-peak intensity versus mixing time must be established for specific spin systems and T(2) relaxation times.

Resin dynamics contributes to the NMR line broadening of organic molecules grafted onto a polystyrene resin

Despite the use of high resolution magic angle spinning NMR, the NMR linewidth of anchored molecules on the commonly used Merrifield solid phase resins remains larger than that of the corresponding molecules in solution. We investigate the different mechanisms that might be at the origin of this line broadening. Experimentally, we use the CPMG method to determine the (15)N relaxation times of a tethered tripeptide and show that the slow resin dynamics significantly contributes to the transverse relaxation.