Enhanced chromatographic NMR with polyethyleneglycol. A novel resolving agent for diffusion ordered spectroscopy

Convection compensation in 3D iDOSY-HMBC 1H-13C-correlation experiments

3D iDOSY-HMBC (3D incorporated Diffusion Ordered SpectroscopY-Heteronuclear Multiple Bond Correlation) pulse sequences were modified to incorporate convection compensation element. No additional delays were required, and convection compensation was directly constructed within the existing delay periods in 3D iDOSY-HMBC pulse sequence. Convection compensation was achieved by pulsed field gradient double echo, thus avoiding the intensity loss normally related to stimulated echo methods. The incorporated convection compensation element improves the usability of 3D iDOSY-HMBC. In case of elevated temperatures, thermal convection leads to loss of resolution in DOSY-dimension or even to severe decrease of signal intensity, thus making convection uncompensated 3D iDOSY-HMBC infeasible. Aforementioned problems can be circumvented via utilization of the presented convection compensated 3D iDOSY-HMBC-versions.

ViscY NMR experiments in phosphoric acid as a viscous solvent for individualization of small molecules within mixtures by spin diffusion

The analysis of small molecules within complex mixtures is a particularly difficult task when dealing with the study of metabolite mixtures or chemical reaction media. This issue has fostered in recent years an active search for effective and practical solutions. In this context, the ViscY NMR approach has been recently proposed. ViscY collectively designates the NMR experiments that take advantage of spin diffusion in highly viscous solvents or solvent blends for the individualization of the NMR spectra of small molecule mixture components. Two viscous media were prepared from ortho-phosphoric acid (85%) solution by dilution with either D2O or DMSO-d6, thus providing solvent blends with slightly different polarities in which all liquid-state NMR experiments can be carried out easily. Two mixtures, one of four structurally close dipeptides and one of four low-polarity phosphorus-containing compounds, were used for the method assessment, using ViscY experiments such as homonuclear selective 1D and 2D 1H NOESY experiments, heteronuclear 2D 1H-15N/1H-31P HSQC-NOESY and 1H-13C/1H-15N/1H-31P NOAH experiments.

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

Incorporated diffusion ordered heteronuclear multiple bond correlation spectroscopy, 3D iDOSY-HMBC. Merging of diffusion delay with long polarization transfer delay of HMBC

3D iDOSY-HMBC pulse sequences allow the simplification of HMBC data of mixtures via separation in the diffusion domain. The presented methods utilize incorporated DOSY approach, iDOSY, where the existing delays of the basic pulse sequence are utilized for diffusion attenuation. In the simplest form of the proposed 3D iDOSY-HMBC sequences, no extra delays or RF-pulses were required, only two diffusion gradients were added within HMBC polarization transfer delay.

Diffusion measurements with continuous hydrogenation in PHIP

DOSY is a powerful spectroscopic NMR technique that resolves components in mixtures through the evaluation of different diffusion coefficients. The application of DOSY to dilute mixtures is hampered by the low signal to noise ratios (SNR), leading to long acquisition times. The use of PHIP may resolve this issue as long as reproducible signals are obtained in order to perform 2D experiments. Here we show that the use of hollow membranes and adequate gas flow produce constant polarization for a time-span that enables the acquisition of 2D experiments. A pressure gradient is evidenced by the presence of convection, which is accounted for by using a DPGSE sequence. The influence of J-coupling evolution during the sequence is studied both numerically and experimentally, to determine the optimum echo-time. The applicability of the method for samples with poor SNR is explored by setting the reaction rate to achieve a low intensity of polarized signals.

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.

Chromatographic NMR spectroscopy: the effect of hollow silica microspheres on magnetic field inhomogeneities and resonance lineshapes

We provide a description of the line broadening phenomenon observed in chromatographic NMR spectra with hollow silica microspheres.

Polar mixture analysis by NMR under spin diffusion conditions in viscous sucrose solution and agarose gel

A viscous sucrose solution is used for the first time to access individual NMR spectra of mixture components under spin diffusion conditions.

Combined maximum-quantum and DOSY 3D experiments provide enhanced resolution for small molecules in mixtures

We illustrate here as the combination of high-order maximum-quantum (MaxQ) and Diffusion-Ordered SpectroscopY (DOSY) NMR experiments in a 3D layout allows superior resolution for crowded NMR spectra. Non-uniform sampling (NUS) allows compressing the experimental time effectively to reasonable durations. Because diffusion effects were encoded within multiple-quantum coherences, increased sensitivity to magnetic field gradients is observed, requiring compensation for convection effects. The experiment was demonstrated on the spectra of a mix of small polyaromatic molecules. Specifically, in the case analyzed, the experiment provided an extreme simplification through the MaxQDOSY-MaxQ projection plane that presents one peak per molecule. Copyright © 2016 John Wiley & Sons, Ltd.

Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible matrix: a vanishing surfactant

Adding salt to a surfactant solution gives an “invisible matrix” for mixture analysis by diffusion-ordered NMR (DOSY).

Pulsed-field gradient nuclear magnetic resonance measurements (PFG NMR) for diffusion ordered spectroscopy (DOSY) mapping

The advent of Diffusion Ordered SpectroscopY (DOSY) NMR has enabled diffusion coefficients to be routinely measured and used to characterize chemical systems in solution. Indeed, DOSY NMR allows the separation of the chemical entities present in multicomponent systems and provides information on their intermolecular interactions as well as on their size and shape.

Matrix-assisted diffusion-ordered spectroscopy: choosing a matrix

Diffusion-ordered spectroscopy (DOSY) is an important technique for separating the NMR signals of the components in a mixture, and relies on differences in diffusion coefficient. Standard DOSY experiments therefore struggle when the components of a mixture are of similar size, and hence diffuse at similar rates. Fortunately, the diffusion coefficients of solutes can be manipulated by changing the matrix in which they diffuse, using matrix components that interact differentially with them, a technique known as matrix-assisted DOSY. In the present investigation, we evaluate the performance of a number of new, previously used, and mixed matrices with an informative test mixture: the three positional isomers of dihydroxybenzene. The aim of this work is to present the matrix-assisted DOSY user with information about the potential utility of a set of matrices (and combinations of matrices), including ionic and non-ionic surfactants, complexing agents, polymers, and mixed solvents. A variety of matrices improved the diffusion resolution of the signals of the test system, with the best separation achieved by mixed micelles of sodium dodecyl sulfate and cetyl trimethylammonium bromide. The use of mixed matrices offers great potential for the analyst to tailor the matrix to a particular sample under study. © 2016 The Authors Magnetic Resonance in Chemistry Published by John Wiley & Sons, Ltd.

Flavonoid glycosides from Persea caerulea. Unraveling their interactions with SDS-micelles through matrix-assisted DOSY, PGSE, mass spectrometry, and NOESY

Two flavonoid glycosides derived from rhamnopyranoside (1) and arabinofuranoside (2) have been isolated from leaves of Persea caerulea for the first time. The structures of 1 and 2 have been established by ¹ H NMR, ¹³ C NMR, and IR spectroscopy, together with LC-ESI-TOF and LC-ESI-IT MS spectrometry. From the MS and MS/MS data, the molecular weights of the intact molecules as well as those of quercetin and kaempferol together with their sugar moieties were deduced. The NMR data provided information on the identity of the compounds, as well as the α and β configurations and the position of the glycosides on quercetin and kaempferol. We have also explored the application of sodium dodecyl sulfate (SDS) normal micelles in binary aqueous solution, at a range of concentrations, to the diffusion resolution of these two glycosides, by the application of matrix-assisted diffusion ordered spectroscopy (DOSY) and pulse field gradient spin echo (PGSE) methodologies, showing that SDS micelles offer a significant resolution which can, in part, be rationalized in terms of differing degrees of hydrophobicity, amphiphilicity, and steric effects. In addition, intra-residue and inter-residue proton-proton distances using nuclear Overhauser effect build-up curves were used to elucidate the conformational preferences of these two flavonoid glycosides when interacting with the micelles. By the combination of both diffusion and nuclear Overhauser spectroscopy techniques, the average location site of kaempferol and quercetin glycosides has been postulated, with the former exhibiting a clear insertion into the interior of the SDS-micelle, whereas the latter is placed closer to the surface. Copyright © 2016 John Wiley & Sons, Ltd.

Silica sol assisted chromatographic NMR spectroscopy for resolution of trans- and cis-isomers

Chromatographic NMR spectroscopy can separate the mixtures of species with significantly different molecular size, but generally fails for isomeric species. Herein, we reported the resolution of trans- and cis-isomers and their structural analogue, which are different in molecular shapes, but similar in mass, were greatly enhanced in the presence of silica sol. The mixtures of maleic acid, fumaric acid and succinic acid, and the mixtures of trans- and cis-1,2-cyclohexanedicarboxylic acids, were distinguished by virtue of their different degrees of interaction with silica sol. Moreover, we found mixed solvents could improve the spectral resolution of DOSY spectra of mixtures.

Matrix-assisted diffusion-ordered spectroscopy

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

Molecular signal suppression by in situ microextraction in nuclear magnetic resonance spectroscopy

The detailed characterization of complex mixtures by NMR is often hampered by the presence of signals from uninformative compounds, the resonances of which overlap with those of the molecules of interest. We provide here a proof of principle for an approach to NMR signal suppression in complex samples using Molecularly Imprinted Polymers (MIPS). Addition of a few milligrams of polymer to a solution traps the target molecule in typical micromolar to millimolar concentration, thus achieving in situ signal suppression, without altering any other spectral features. This method minimized any manipulation or perturbation of the spectrum and was applied to a complex mixture of known compounds and to a plant extract, in both cases spiked with a compound (bisphenol A), which was subsequently removed by selective binding to a complementary MIP. What is described in this report is comparable with microextraction and may in due course be applied to a large number of analytical challenges.

Polydimethylsiloxane: a general matrix for high-performance chromatographic NMR spectroscopy

The detection and structural characterization of the components of a mixture is a challenging task. Therefore, the development of a facile and general method that enables both the separation and the structural characterization of the components is desired. Diffusion-ordered NMR spectroscopy (DOSY) with the aid of a matrix is a promising tool for this purpose. However, because the currently existing matrices only separate limited components, the application of the DOSY technique is restricted. Herein we introduce a new versatile matrix, poly(dimethylsiloxane), which can fully separate many mixtures of different structural types by liquid-state NMR spectroscopy. With poly(dimethylsiloxane), liquid-state chromatographic NMR spectroscopy could become a general approach for the structural elucidation of mixtures of compounds.

Evaluation of the separation performance of polyvinylpyrrolidone as a virtual stationary phase for chromatographic NMR

Polyvinylpyrrolidone (PVP) was used as a virtual stationary phase to separate p-xylene, benzyl alcohol, and p-methylphenol by the chromatographic NMR technique. The effects of concentration and weight-average molecular weight (Mw) of PVP, solvent viscosity, solvent polarity, and sample temperature on the resolution of these components were investigated. It was found that both higher PVP concentration and higher PVP Mw caused the increase of diffusion resolution for the three components. Moreover, the diffusion resolution did not change at viscosity-higher solvents. Moreover, the three components showed different resolution at different solvents. As temperature increased, the diffusion resolution between p-xylene and benzyl alcohol gradually increased, and the one between p-xylene and p-methylphenol slightly increased from 278 to 298 K and then decreased above 298 K. It was also found that the polarity of the analytes played an important role for the separation by affecting the diffusion coefficient.

Resolving complex mixtures: trilinear diffusion data

Complex mixtures are at the heart of biology, and biomacromolecules almost always exhibit their function in a mixture, e.g., the mode of action for a spider venom is typically dependent on a cocktail of compounds, not just the protein. Information about diseases is encoded in body fluids such as urine and plasma in the form of metabolite concentrations determined by the actions of enzymes. To understand better what is happening in real living systems we urgently need better methods to characterize such mixtures. In this paper we describe a potent way to disentangle the NMR spectra of mixture components, by exploiting data that vary independently in three or more dimensions, allowing the use of powerful algorithms to decompose the data to extract the information sought. The particular focus of this paper is on NMR diffusion data, which are typically bilinear but can be extended by a third dimension to give the desired data structure.

Natural product mixture analysis by matrix-assisted DOSY using Brij surfactants in mixed solvents

Brij surfactant micelles in mixed solvent systems aid resolution of natural product NMR signals in diffusion-ordered spectroscopy.

Chromatographic NMR with size exclusion chromatography stationary phases

Chromatographic NMR describes the use of stationary phases or solvent additives, such as polymers, to modify the diffusion properties of analyte molecules and thereby improve the observed resolution in the diffusion domain. This paper demonstrates similar ideas using size exclusion chromatographic media and characterises the changes in the observed diffusion coefficient using a series of polymer molecular weight reference standards of known polydispersity. The results are interpreted in terms of a simple description of the size exclusion phenomena.

Matrix-assisted diffusion-ordered spectroscopy: application of surfactant solutions to the resolution of isomer spectra

The component spectra of a mixture of isomers with nearly identical diffusion coefficients cannot normally be distinguished in a standard diffusion-ordered spectroscopy (DOSY) experiment but can often be easily resolved using matrix-assisted DOSY, in which diffusion behaviour is manipulated by the addition of a co-solute such as a surfactant. Relatively little is currently known about the conditions required for such a separation, for example, how the choice between normal and reverse micelles affects separation or how the isomer structures themselves affect the resolution. The aim of this study was to explore the application of sodium dodecyl sulfate (SDS) normal micelles in aqueous solution and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in chloroform, at a range of concentrations, to the diffusion resolution of some simple model sets of isomers such as monomethoxyphenols and short chain alcohols. It is shown that SDS micelles offer better resolution where these isomers differ in the position of a hydroxyl group, whereas AOT aggregates are more effective for isomers differing in the position of a methyl group. For both the normal SDS micelles and the less well-defined AOT aggregates, differences in the resolution of the isomers can in part be rationalised in terms of differing degrees of hydrophobicity, amphiphilicity and steric effects.

Glycerol and glycerol carbonate as ultraviscous solvents for mixture analysis by NMR

NMR of weakly polar analytes in an apolar ultraviscous solvent has recently been proposed for mixture analysis as a pertinent alternative to the DOSY experiment. The present article reports the first use of glycerol and glycerol carbonate as polar solvents for the NMR analysis of a model mixture of dipeptides. This work demonstrates the high potentiality of these solvents for the analysis of mixtures made of polar and potentially bioactive compounds. Medium-sized molecules slowly reorient in glycerol and glycerol carbonate under particular temperature conditions, so that solute resonances may show spin diffusion in NOESY spectra, thus opening the way to mixture analysis. Glycerol and glycerol carbonate have turned out to be ultraviscous solvents of choice for the individualization of four structurally close mixed dipeptides: Leu-Val, Leu-Tyr, Gly-Tyr and Ala-Tyr by means of 1D and 2D NOESY experiments. Selective sample excitation and signal detection were implemented to eliminate the intense proton signals of the non-deuterated solvents. Moreover, the recording of a multiplet selective 2D NOESY-TOCSY has shown that the analytical power of NMR in highly viscous solvents is not limited to the extraction of mixture component 1D subspectra but may also yield some supplementary information about atom connectivity within components.

Resolution of a nonionic surfactant oligomeric mixture by means of DOSY with inverse micelle assistance

DOSY is a recognized, efficient technique in the analysis of mixtures. It relies on the differences in self-diffusion coefficients, which are determined by the molecular size. Nowadays, efforts are directed towards devising matrices able to interact with the components of the mixture with differential affinity, and therefore capable to interfere with the diffusion processes and to display resolving power towards species of close, or even equal molecular weight, like isomers. Usually, commercial nonionic surfactants are mixtures of oligomeric species, since the head group, which is a short polyoxyehtylene chain, is somewhat polydisperse. The embedment of Igepal CA-520, 5 polyoxyethylene iso-octylphenyl ether, in an inverse microemulsion led to the separation of (1)H signals of the various oligomeric components. This ensued from the differential partitioning between the oil and the surface of the inverse micelles, which depends on the ethyleneoxide number (EON) of the head groups. Thus, it was possible to ascertain that the length distribution of the polyethyleneoxide chains is ingood agreement with the Poisson distribution theoretically predicted for the polymerization of ethylene oxide. The DOSY spectrum contributed to the assignment of the signals and afforded the partition degree, between the two environments, for each individual oligomeric species, providing further insight into nonionic inverse microemulsions, at present widely employed reaction media in the nanotechnological syntheses.