Fast access to residual dipolar couplings by single-scan 2D NMR in oriented media

Ultrafast 2D NMR for the analysis of complex mixtures

2D NMR is extensively used in many different fields, and its potential for the study of complex biochemical or chemical mixtures has been widely demonstrated. 2D NMR gives the ability to resolve peaks that overlap in 1D spectra, while providing both structural and quantitative information. However, complex mixtures are often analysed in situations where the data acquisition time is a crucial limitation, due to an ongoing chemical reaction or a moving sample from a hyphenated technique, or to the high-throughput requirement associated with large sample collections. Among the great diversity of available fast 2D methods, ultrafast (or single-scan) 2D NMR is probably the most general and versatile approach for complex mixture analysis. Indeed, ultrafast NMR has undergone an impressive number of methodological developments that have helped turn it into an efficient analytical tool, and numerous applications to the analysis of mixtures have been reported. This review first summarizes the main concepts, features and practical limitations of ultrafast 2D NMR, as well as the methodological developments that improved its analytical potential. Then, a detailed description of the main applications of ultrafast 2D NMR to mixture analysis is given. The two major application fields of ultrafast 2D NMR are first covered, i.e., reaction/process monitoring and metabolomics. Then, the potential of ultrafast 2D NMR for the analysis of hyperpolarized mixtures is described, as well as recent developments in oriented media. This review focuses on high-resolution liquid-state 2D experiments (including benchtop NMR) that include at least one spectroscopic dimension (i.e., 2D spectroscopy and DOSY) but does not cover in depth applications without spectral resolution and/or in inhomogeneous fields.

2 H QUOSY 2D-NMR Experiments in Weakly Aligning Systems: From the Conventional to the Ultrafast Approach

We describe three anisotropic ultrafast (UF) QUadrupolar Ordered SpectroscopY (QUOSY) 2D-NMR experiments (referred to as ADUF 2D NMR spectroscopy) designed for recording the ² H homonuclear 2D spectra of weakly aligned (deuterated) solutes in sub-second experiment times. These new ADUF 2D experiments derive from the Q-COSY, Q-resolved and Q-DQ 2D pulse sequences (J. Am. Chem. Soc. 1999, 121, 5249) and allow the correlation between the two components of each quadrupolar doublet, and then their assignment on the basis of ² H chemical shifts. The UF 2D pulse sequences are analyzed by using the Cartesian spin-operator formalism for spin I=1 nuclei with a small quadrupolar moment. The optimal experimental/practical conditions as well as the resolution, sensitivity and quantification issues of these ADUF 2D experiments are discussed on comparison to their conventional 2D counterparts and their analytical potentialities. Illustrative ADUF 2D experiments using deuterated achiral/prochiral/chiral solutes in poly-γ-benzyl-L-glutamate based chiral liquid crystals are presented, as well as the first examples of natural abundance deuterium (ANADUF) 2D spectrum using 14.1 T magnetic field and a basic gradient unit (53 G.cm^-1 ) in oriented solvents.

Multinuclear NMR in polypeptide liquid crystals: Three fertile decades of methodological developments and analytical challenges

NMR spectroscopy of oriented samples makes accessible residual anisotropic intramolecular NMR interactions, such as chemical shift anisotropy (RCSA), dipolar coupling (RDC), and quadrupolar coupling (RQC), while preserving high spectral resolution. In addition, in a chiral aligned environment, enantiomers of chiral molecules or enantiopic elements of prochiral compounds adopt different average orientations on the NMR timescale, and hence produce distinct NMR spectra or signals. NMR spectroscopy in chiral aligned media is a powerful analytical tool, and notably provides unique information on (pro)chirality analysis, natural isotopic fractionation, stereochemistry, as well as molecular conformation and configuration. Significant progress has been made in this area over the three last decades, particularly using polypeptide-based chiral liquid crystals (CLCs) made of organic solutions of helically chiral polymers (as PBLG) in organic solvents. This review presents an overview of NMR in polymeric LCs. In particular, we describe the theoretical tools and the major NMR methods that have been developed and applied to study (pro)chiral molecules dissolved in such oriented solvents. We also discuss the representative applications illustrating the analytical potential of this original NMR tool. This overview article is dedicated to thirty years of original contributions to the development of NMR spectroscopy in polypeptide-based chiral liquid crystals.

Improvements, extensions, and practical aspects of rapid ASAP-HSQC and ALSOFAST-HSQC pulse sequences for studying small molecules at natural abundance

Previously we introduced two novel NMR experiments for small molecules, the so-called ASAP-HSQC and ALSOFAST-HSQC (Schulze-Sünninghausen et al., 2014), which allow the detection of heteronuclear one-bond correlations in less than 30s at natural abundance. We propose an improved symmetrized pulse scheme of the basic experiment to minimize artifact intensities and the combination with non-uniform sampling to enable the acquisition of conventional HSQC spectra in as short as a couple of seconds and extremely ¹³C-resolved spectra in less than ten minutes. Based on steady state investigations, a first estimate to relative achievable signal intensities with respect to conventional, ASAP-, and ALSOFAST-HSQC experiments is given. In addition, we describe several extensions to the basic pulse schemes, like a multiplicity-edited version, a revised symmetrized CLIP-ASAP-HSQC, an ASAP-/ALSOFAST-HSQC sequence with broadband BIRD-based ¹H,¹H decoupling, and a symmetrized sequence optimized for water suppression. Finally, RF-power considerations with respect to the high duty cycle of the experiments are given.

Perspectives in the application of residual dipolar couplings in the structure elucidation of weakly aligned small molecules

This perspective article aims to review the general methodology in the application of residual dipolar couplings (RDCs) in the structure elucidation of small molecules and give the author's view on challenges for future applications. Recent improvements in the availability of alignment media, new pulse sequences for the measurement of couplings and improvements in the analysis software have garnered widespread interest in the technique. However, further generalization is needed in order to make RDC analysis into a truly "routine" method. Copyright © 2016 John Wiley & Sons, Ltd.

Detection of quadrupolar nuclei by ultrafast 2D NMR: exploring the case of deuterated analytes aligned in chiral oriented solvents

Anisotropic ²H ultrafast (ADUF) 2D NMR spectroscopy for studying deuterated analytes dissolved in chiral liquid crystals is investigated and its analytical possibilities are evaluated.

CLIP-ASAP-HSQC for fast and accurate extraction of one-bond couplings from isotropic and partially aligned molecules

Fast measurement of heteronuclear one-bond couplings, a class of NMR parameters valuable for structure elucidation, is highly desirable, especially if samples undergo chemical reactions or dynamic processes are observed. Methods presented so far face severe limitations in terms of resolution, accessible bandwidth, and sensitivity. We present the CLean InPhase-Acceleration by Sharing Adjacent Polarization-HSQC (CLIP-ASAP-HSQC) pulse sequence that allows fast acquisition of spectra with clean inphase multiplets in about 25 s. The performance in terms of accurate extraction of one-bond couplings is demonstrated on three test samples including partially aligned molecules.

Spatial reorientation experiments for NMR of solids and partially oriented liquids

Motional reorientation experiments are extensions of Magic Angle Spinning (MAS) where the rotor axis is changed in order to average out, reintroduce, or scale anisotropic interactions (e.g. dipolar couplings, quadrupolar interactions or chemical shift anisotropies). This review focuses on Variable Angle Spinning (VAS), Switched Angle Spinning (SAS), and Dynamic Angle Spinning (DAS), all of which involve spinning at two or more different angles sequentially, either in successive experiments or during a multidimensional experiment. In all of these experiments, anisotropic terms in the Hamiltonian are scaled by changing the orientation of the spinning sample relative to the static magnetic field. These experiments vary in experimental complexity and instrumentation requirements. In VAS, many one-dimensional spectra are collected as a function of spinning angle. In SAS, dipolar couplings and/or chemical shift anisotropies are reintroduced by switching the sample between two different angles, often 0° or 90° and the magic angle, yielding a two-dimensional isotropic-anisotropic correlation spectrum. Dynamic Angle Spinning (DAS) is a related experiment that is used to simultaneously average out the first- and second-order quadrupolar interactions, which cannot be accomplished by spinning at any unique rotor angle in physical space. Although motional reorientation experiments generally require specialized instrumentation and data analysis schemes, some are accessible with only minor modification of standard MAS probes. In this review, the mechanics of each type of experiment are described, with representative examples. Current and historical probe and coil designs are discussed from the standpoint of how each one accomplishes the particular objectives of the experiment(s) it was designed to perform. Finally, applications to inorganic materials and liquid crystals, which present very different experimental challenges, are discussed. The review concludes with perspectives on how motional reorientation experiments can be applied to current problems in chemistry, molecular biology, and materials science, given the many advances in high-field NMR magnets, fast spinning, and sample preparation realized in recent years.

Fast hybrid multi-dimensional NMR methods based on ultrafast 2D NMR

Conventional multi-dimensional (nD) NMR experiments are characterized by inherent long acquisition durations, while ultrafast (UF) NMR makes it possible to reduce to a few hundreds of milliseconds the overall acquisition duration of a complete nD NMR dataset. Although extremely promising for a number of specific applications, the UF strategy suffers from significant limitations compared with its conventional counterpart. The main limitations concern the sensitivity, the resolution, and the accessible spectral width. However, when the targeted applications are compatible with an acquisition duration between a few seconds and a few minutes, hybrid UF techniques can be used to improve the performance of UF nD NMR while remaining faster than conventional acquisitions. Much better results in terms of signal-to-noise ratio can be achieved with the multi-scan single-shot approach or with interleaved acquisitions. Even more, for the same experimental duration, and in the case of homonuclear 2D NMR, the multi-scan single-shot approach has a much higher precision than conventional 2D NMR. Interleaved 2D NMR overcomes the drawbacks of single-scan UF NMR in terms of spectral width and provides spectra for which the quality is not significantly different from that obtained with conventional 2D NMR. Finally, high spectral qualities have been demonstrated from hybrid conventional/UF 3D approaches capable of recording a whole 3D spectrum in the time needed to record a conventional 2D spectrum. This mini-review aims at describing the principles, the recent advances and the latest applications of these hybrid techniques. Copyright © 2015 John Wiley & Sons, Ltd.

Quantitative 2D liquid-state NMR

Two-dimensional (2D) liquid-state NMR has a very high potential to simultaneously determine the absolute concentration of small molecules in complex mixtures, thanks to its capacity to separate overlapping resonances. However, it suffers from two main drawbacks that probably explain its relatively late development. First, the 2D NMR signal is strongly molecule-dependent and site-dependent; second, the long duration of 2D NMR experiments prevents its general use for high-throughput quantitative applications and affects its quantitative performance. Fortunately, the last 10 years has witnessed an increasing number of contributions where quantitative approaches based on 2D NMR were developed and applied to solve real analytical issues. This review aims at presenting these recent efforts to reach a high trueness and precision in quantitative measurements by 2D NMR. After highlighting the interest of 2D NMR for quantitative analysis, the different strategies to determine the absolute concentrations from 2D NMR spectra are described and illustrated by recent applications. The last part of the manuscript concerns the recent development of fast quantitative 2D NMR approaches, aiming at reducing the experiment duration while preserving - or even increasing - the analytical performance. We hope that this comprehensive review will help readers to apprehend the current landscape of quantitative 2D NMR, as well as the perspectives that may arise from it.

New practical tools for the implementation and use of ultrafast 2D NMR experiments

Ultrafast (UF) 2D NMR is a very promising methodology enabling the acquisition of 2D spectra in a single scan. In the last few years, the analytical performance of UF 2D NMR has been highly increased, consequently maximizing its range of applications. However, its implementation and use by non-specialists are far from being straightforward, because of the specific acquisition and processing procedures and parameters characterizing UF NMR. To make this methodology implementable and applicable by non-specialists, we developed a simple routine capable of translating conventional parameters (spectral widths and transmitter frequencies) into specific UF parameters (gradient and chirp pulse parameters). This macro was subsequently implemented in a Web page, which is available for external users. Although the algorithm was designed for two widely used 2D experiments, COSY and HSQC, it can easily be extended to any other pulse sequence. The robustness of this routine was verified successfully on a variety of small molecules. We believe that this tool will eliminate much of the technical difficulties related to UF 2D NMR and will make the technique accessible to a wider audience of organic and analytical chemists.