Probing Liquid-Liquid Interfaces with Spatially Resolved NMR Spectroscopy

Efficient method for in situ agitation of liquids directly inside NMR spectrometer

The objective of the method is to allow agitation and fast homogenization of liquid systems in NMR tubes, directly inside the NMR spectrometer. The setup makes it possible to record spectra of samples that are macroscopically not stable, as dispersions of large particles. It makes also possible to fasten the homogeneization of liquid during a reaction or a phase transition. In the present paper, the method has been evaluated using homogeneous liquid extraction (HLLE). This configuration can also be used to introduce gases in different systems to perform various types of experiments. The set up consists in a Teflon tube inserted in the NMR tube bringing gas that yields agitation by bubbling. The gas flow is tuned using an electronically operated valve connected to gas line and to the NMR console. The method details how to reach proper homogenization without any perturbation, as liquid leaks.•An easy method for agitation of liquids inside NMR spectrometers.•The set up can be used for the insertion of gases in the NMR tube inside the spectrometer.•The method allows the study of the mixing of biphasic systems by NMR techniques.

Molecular Insights into the Salinity Effects on Movability of Oil-Brine in Shale Nanopore-Throat Systems

Low-salinity flooding has been well recognized as a promising strategy to increase shale oil recovery, but the underlying mechanism remains unclarified, especially for complex nanopore networks filled with oil-brine fluids. In this study, the pressure-driven flow of an oil-brine fluid with varying salinities in shale nanopore-throat channels was first investigated based on molecular dynamics simulations. The critical pressure driving oil to intrude into a nanothroat filled with brine of varying salinities was determined. Simulation results indicate that the salinity of brine exhibits great effects on the movability of oil, and low salinity favors the increase of oil movability. Further analysis of the interactions between fluid and pore walls as well as the displacement pressures reveals dual effects of brine salinity on oil transportation in a nanopore-throat. On the one hand, hydrated cations anchoring onto throat walls enlarge the effective flow width in the throat before the hydration complexes reach the maximum. On the other hand, the interfacial tension between oil and brine increases with the brine salinity, which increases the capillary resistance and leads to a higher displacement pressure. These findings highlight the effects of brine salinity on oil movability in a nanopore-throat, which will promote the understanding of oil accumulation and dissipation in petroleum systems, as well as help to develop enhanced oil recovery.

A one-shot double-slice selection NMR method for biphasic systems

We propose a new and robust one-shot double-slice selection experiment to detect 1H NMR signals of biphasic systems simultaneously. The resultant spectrum contains opposite-phased peaks representing the chemical species from the two phases, respectively.

Temporal and spatial characterisation of protein liquid-liquid phase separation using NMR spectroscopy

Liquid-liquid phase separation (LLPS) of protein solutions is increasingly recognised as an important phenomenon in cell biology and biotechnology. However, opalescence and concentration fluctuations render LLPS difficult to study, particularly when characterising the kinetics of the phase transition and layer separation. Here, we demonstrate the use of a probe molecule trifluoroethanol (TFE) to characterise the kinetics of protein LLPS by NMR spectroscopy. The chemical shift and linewidth of the probe molecule are sensitive to local protein concentration, with this sensitivity resulting in different characteristic signals arising from the dense and lean phases. Monitoring of these probe signals by conventional bulk-detection ¹⁹F NMR reports on the formation and evolution of both phases throughout the sample, including their concentrations and volumes. Meanwhile, spatially-selective ¹⁹F NMR, in which spectra are recorded from smaller slices of the sample, was used to track the distribution of the different phases during layer separation. This experimental strategy enables comprehensive characterisation of the process and kinetics of LLPS, and may be useful to study phase separation in protein systems as a function of their environment.

Highly Efficient Determination of Complex NMR Multiplet Structures in Inhomogeneous Magnetic Fields

Proton-proton scalar (J) coupling plays an important role in disentangling molecular structures and spatial conformations. But it is challenging to extract J coupling networks from congested ¹H NMR spectra, especially in inhomogeneous magnetic fields. Herein, we propose a general liquid NMR protocol, named HR-G-SERF, to implement highly efficient determination of individual J couplings and corresponding coupling networks via simultaneously suppressing effects of spectral congestions and magnetic field inhomogeneity. This method records full-resolved 2D absorption-mode spectra to deliver great convenience for multipet analyses on complex samples. More meaningfully, it is capable of disentangling multiplet structures of biological samples, that is, grape sarcocarp, despite of its heterogeneous semisolid state and extensive compositions. In addition, a modification, named AH-G-SERF, is developed to compress experimental acquisition and subsequently improve unit-time SNR, while maintaining satisfactory spectral performance. This accelerated variant may further boost the applicability for rapid NMR detections and afford the possibility of adopting hyperpolarized substances to enhance the overall sensitivity. Therefore, this study provides a promising tool for molecular structure elucidations and composition analyses in chemistry, biochemistry, and metabonomics among others.

Impact of the chemical structure on the distribution of neuroprotective N-alkyl-9H-carbazoles at octanol/water interfaces

All-atom molecular dynamics (MD) simulations were performed on distribution and agglomeration dynamics of neuroprotective N-(3-anilinopropyl)-9H-carbazoles at octanol/water interfaces.

High-Resolution Probing of Heterogeneous Samples by Spatially Selective Pure Shift NMR Spectroscopy

Liquid NMR spectroscopy generally encounters two major challenges for high-resolution measurements of heterogeneous samples, namely, magnetic field inhomogeneity caused by spatial variations in magnetic susceptibility and spectral congestion induced by crowded NMR resonances. In this study, we demonstrate a spatially selective pure shift NMR approach for high-resolution probing of heterogeneous samples by suppressing effects of field inhomogeneity and J coupling simultaneously. A Fourier phase encoding strategy is proposed and implemented for spatially selective pure shift experiments to enhance signal intensity and further boost the applicability. The spatially selective pure shift method can serve as an effective tool for high-resolution probing of heterogeneous samples, thus presenting interesting prospects for extensive applications in the fields of chemistry, physics, biology, and food science.

Spatial encoding and spatial selection methods in high-resolution NMR spectroscopy

A family of high-resolution NMR methods share the common concept of acquiring in parallel different sub-experiments in different spatial regions of the NMR tube. These spatial encoding and spatial selection methods were for the most part introduced independently from each other and serve different purposes, but they share common ingredients, often derived from magnetic resonance imaging, and they all benefit from a greatly improved time-efficiency. This review article provides a description of several spatial encoding and spatial selection methods, including single-scan multidimensional experiments (ultrafast 2D NMR, DOSY, Z spectroscopy, inversion recovery and Laplace NMR), pure shift and selective refocusing experiments (including Zangger-Sterk decoupling, G-SERF and PSYCHE), a Z filter, and fast-pulsing slice-selective experiments. Some key elements for spatial parallelisation are introduced and when possible a common framework is used for the analysis of each method. Sensitivity considerations are discussed, and a selection of applications is analysed to illustrate which questions can be answered thanks to spatial encoding and spatial selection methods, and discuss the perspectives for future developments and applications.

Enhancing the detection of edges and non-differentiable points in an NMR spectrum using delayed-acquisition

Delayed-acquisition, which is a common technique for improving spectral resolution in Fourier transform based spectroscopies, typically relies upon differences in T₂ relaxation rates that are often due to underlying differences in dynamics and/or complexities of the spin systems being studied. After an acquisition delay, the broad signals from fast T₂-relaxing species are more suppressed relative to the sharp signals from slow T₂-relaxing species. In this paper, an alternative source of differential "dephasing" under delayed-acquisition is demonstrated that is based solely upon the mathematical properties of the line shape and is independent of the underlying spin dynamics and/or complexity. Signals associated with frequencies where the line shape either changes sharply and/or is non-differentiable at some finite order dephase at a much slower rate than those signals associated with frequencies where the line shape is smooth. Experiments employing delayed-acquisition to study interfaces in biphasic samples, to measure spatially-dependent longitudinal relaxation, and to highlight sharp features in NMR spectra are presented.

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples

The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software.

Study of liquid-liquid interfaces by an easily implemented localized NMR sequence

To selectively extract heavy metals from solutions containing fission products, it is essential to optimize the liquid-liquid extraction processes. Such an objective requires improving the fundamental knowledge of the different mechanisms that are involved in these processes. In that respect, we propose a localized NMR sequence named LOCSY to assess the concentration profiles of different species involved in these processes. One of the goals of this sequence is to study the products as close as possible to the liquid-liquid interface with the help of a standard NMR spectrometer of chemistry labs. The one-dimensional spatial localization along the NMR tube is obtained by a discrete stepping of the frequency-selective excitation pulses under a pulsed field gradient. Specific data processing has been developed to obtain the 1D NMR spectra as a function of the vertical position in the NMR tube. The LOCSY sequence has been tested and evaluated on three different systems: (i) a cylindrical phantom inserted in the NMR tube containing 4-methylsalicylic acid solution, (ii) D(2)O/olive oil biphasic system, and (iii) the dissolution of solid saccharose in D(2)O. These examples illustrate potential applications of the LOCSY sequence, particularly the possibility to measure concentration profiles and to study phenomena such as diffusion, provided the dynamic range is compatible with NMR timescale and sensitivity.